Thoughts for a low-power 60Hz 120Vrms signal generator please

[berkeman]

For a metrology (power metering) calibration test fixture at work, I'm looking for a low-cost way to generate an isolated 120Vrms (or 240Vrms) sine wave to feed into the high-impedance mesasurement circuitry.

The first test fixture used a $40k AC Mains generator instrument driving a 1kW load. As we worked through the project, it became apparent that we could handle the current sensing part of the power measurement separately, and only needed a low-current 120Vrms source to drive the voltage input of the metrology circuit (input impedance is around 5kOhms).

I've tried a nice little BESTEK "pure sine" inverter from 12V-->120Vrms, but it has some strange bistable behavior switching between two slightly different output voltages every 30 seconds or so (pretty hard to automate in a 0.05% accurate metrology calibration setup).

I'm starting to look into high-voltage opamps to take a low voltage signal generator output (around 3Vpp) and amplify it up to 120Vrms, but I'd be interested in any ideas that you all might have. I've also looked at using a step-up transformer to go from about 5Vrms to 120Vrms, but the transformers I've found so far have low input unductance that can't be driven by a 50 Ohm signal generator...

 

[anorlunda]

Why not just a 120V:120V isolation transformer driven by AC mains?

This one handles 150 VA, 120V to 120V, $100.

https://www.zoro.com/bk-precision-i...bb7b61de44ae15d215da0158550c3fb2&gclsrc=3p.ds

 

[berkeman]

Why not just a 120V:120V isolation transformer driven by AC mains?

It's a matter of accuracy and variation versus time. The BESTEK is very steady, except for the bistable problem. My expensive AC mains generator is still jittering more than I like. I tested our lab AC Mains feed near the end of today, and it was varying around 0.1% continuously, which is way too much.

I've tried to use a signal generator plus step-up transformer, but the 50 Ohm output signal generator can't drive the typical 5Vrms-120Vrms step-up transformer.

 

[dlgoff]

For a metrology (power metering) calibration test fixture at work, I'm looking for a low-cost way to generate an isolated 120Vrms (or 240Vrms) sine wave to feed into the high-impedance mesasurement circuitry.

The first test fixture used a $40k AC Mains generator instrument driving a 1kW load. As we worked through the project, it became apparent that we could handle the current sensing part of the power measurement separately, and only needed a low-current 120Vrms source to drive the voltage input of the metrology circuit (input impedance is around 5kOhms).

I've tried a nice little BESTEK "pure sine" inverter from 12V-->120Vrms, but it has some strange bistable behavior switching between two slightly different output voltages every 30 seconds or so (pretty hard to automate in a 0.05% accurate metrology calibration setup).

I'm starting to look into high-voltage opamps to take a low voltage signal generator output (around 3Vpp) and amplify it up to 120Vrms, but I'd be interested in any ideas that you all might have. I've also looked at using a step-up transformer to go from about 5Vrms to 120Vrms, but the transformers I've found so far have low input unductance that can't be driven by a 50 Ohm signal generator...

View attachment 282062

Maybe look into crystal controlled vacuum tube oscillators?

 

[Averagesupernova]

First thing that comes to mind is audio amp driving 70 volt line transformer used in PA systems. Depending on phase relationships, maybe can arrange feedback to an opamp stage that drives the audio amp.

 

[Tom.G]

First thing that comes to mind is audio amp driving 70 volt line transformer used in PA systems. Depending on phase relationships, maybe can arrange feedback to an opamp stage that drives the audio amp.

And to follow thru on that:
Add another 70V line audio transformer and wire its secondary in series with the existing 70V output.

Audio amps with 70V line output are less common than those that drive speakers. You can use two transformers designed for driving speakers from a 70V line. Connect the low impedance secondaries in parallel to the amp speaker output, and the 70V primaries in series for 120V-140V output.

To keep distortion down, I recommend that the audio amp power rating be at least two to three times your expected load. The transformers can be rated at twice the expected load.

If waveform distortion or amplitude stability becomes a problem, you may be able to design-in some negative feedback from the 120V side to the amplifier input.

Let us know how it works out.

Cheers,
Tom

p.s. There is a regular visitor to this site (don't recall his name) that has been designing and building high-end audio amplifiers. Perhaps someone recalls who it is and directs him to this thread.

 

[DaveE]

For low volume (i.e. high cost) instrumentation, I like your HV op-amp idea. Look at Apex Microtechnology, they have lots of stuff that can do this. At 3W out you can deal with a simple linear approach. Then it's easy with feedback to provide exactly what you want at the output.

Like this one PA88 (chosen mostly at random). I used their stuff many years ago for piezo drivers and was quite happy with them. Better yet, call one of their app engineers and ask them. They may have a circuit already, since your application sounds pretty common.

 

[Baluncore]

I've tried to use a signal generator plus step-up transformer, but the 50 Ohm output signal generator can't drive the typical 5Vrms-120Vrms step-up transformer.

Use an audio power amplifier module between the sig gen and the step up transformer.
70 ohm transformers are not needed.

 

[Rive]

I too think that a very simple audio amplifier would be able to match the impedance between the signal generator and the transformer.

 

[berkeman]

Thanks very much for all of the helpful replies and ideas. You've given me a lot to try this week. Much appreciated!

 

[dlgoff]

Thanks very much for all of the helpful replies and ideas. You've given me a lot to try this week. Much appreciated!

Let us know what you've tried and how well it worked.

 

[tech99]

You might find a small 5V mains transformer will work if you resonate the 120V winding with a capacitor.

 

[berkeman]

You might find a small 5V mains transformer will work if you resonate the 120V winding with a capacitor.

Thanks, this is the most promising avenue so far. Based on my measured 120V winding inductance, I'm able to resonate that winding with about 4μF of X2 capacitance. That makes the input impedance at the 5V winding about 4 Ohms for the 15VA AC transformer that I'm using right now. That's in the right place for an audio power amp. More in a bit...

 

[Tom.G]

I'm able to resonate that winding with about 4μF of X2 capacitance.

I suggest low Q, and temperature stable caps.

Also consider load reactance. If the load has/is a rectifier/filter it could lead to much fun.

 

[berkeman]

I suggest low Q, and temperature stable caps.

Also consider load reactance. If the load has/is a rectifier/filter it could lead to much fun.

Thankfully the load is just the 5kOhm resistive input impedance of the metrology voltage measurement circuit. The Mfg Test floor is pretty well temperature controlled as well. (The old factory there in China not so much -- inside temperature was close to the outside temperature year-round. Yikes.)

 

[cmb]

For a metrology (power metering) calibration test fixture at work, I'm looking for a low-cost way to generate an isolated 120Vrms (or 240Vrms) sine wave to feed into the high-impedance mesasurement circuitry.

The first test fixture used a $40k AC Mains generator instrument driving a 1kW load. As we worked through the project, it became apparent that we could handle the current sensing part of the power measurement separately, and only needed a low-current 120Vrms source to drive the voltage input of the metrology circuit (input impedance is around 5kOhms).

I've tried a nice little BESTEK "pure sine" inverter from 12V-->120Vrms, but it has some strange bistable behavior switching between two slightly different output voltages every 30 seconds or so (pretty hard to automate in a 0.05% accurate metrology calibration setup).

I'm starting to look into high-voltage opamps to take a low voltage signal generator output (around 3Vpp) and amplify it up to 120Vrms, but I'd be interested in any ideas that you all might have. I've also looked at using a step-up transformer to go from about 5Vrms to 120Vrms, but the transformers I've found so far have low input unductance that can't be driven by a 50 Ohm signal generator...

View attachment 282062

More information required if you want a good answer;-

What accuracy on current?
What accuracy on voltage?
Permissible total harmonic content?
Output impedance?
Efficiency?

If you just want a high voltage signal of generically approximate accuracy defined by other inputs and not bothered about efficiency, then you can use a low voltage op amp with a voltage divider taking in negative feedback from the circuit output voltage, where the circuit output is at the collector of a bipolar whose base is driven by the op amp output and where a resistor is put between the collector and your input 120V DC. The positive of the op amp takes in a 60Hz signal from a sig gen. The accuracy of volt and signal is then the accuracy of those input components.

At this frequency, a Darlington would be a good way to use a low current output op amp.

If you want a precision instrument, first (additional) question there would be do you want to also generate the waveform yourself, or can you use a reference signal generator?

 

[berkeman]

More information required if you want a good answer;-

>What accuracy on current?
>What accuracy on voltage?
The product is rated at power accuracy <= 0.5%, but Marketing has made noises about selling a version that is rated at <=0.1% power accuracy. My pass/fail limit right now is <0.05%.

>Permissible total harmonic content?
I'd like to keep the harmonic content low (not sure what number) to be sure that I don't compromise the accuracy of the measurement in the Mfg Test. I don't have a good feel for how the metering IC handles harmonic content (in terms of still giving an accurate RMS measurement)

>Output impedance?
The 120Vrms signal is driving the 5kOhm resistive input impedance of the metrology voltage measurement circuit

>Efficiency?
Not a big deal, but if the generator gets pretty hot, that may be an issue.

do you want to also generate the waveform yourself, or can you use a reference signal generator?

I'd prefer to use a signal generator. I'm currently using an HP 33120 AFG as the source, and typical max output voltage for a sine wave is 10Vpp into a 50 Ohm load (so 20Vpp into a high impedance load).

 

[rude man]

It's a matter of accuracy and variation versus time. The BESTEK is very steady, except for the bistable problem. My expensive AC mains generator is still jittering more than I like. I tested our lab AC Mains feed near the end of today, and it was varying around 0.1% continuously, which is way too much.

I've tried to use a signal generator plus step-up transformer, but the 50 Ohm output signal generator can't drive the typical 5Vrms-120Vrms step-up transformer.

For a metrology (power metering) calibration test fixture at work, I'm looking for a low-cost way to generate an isolated 120Vrms (or 240Vrms) sine wave to feed into the high-impedance mesasurement circuitry.

The first test fixture used a $40k AC Mains generator instrument driving a 1kW load. As we worked through the project, it became apparent that we could handle the current sensing part of the power measurement separately, and only needed a low-current 120Vrms source to drive the voltage input of the metrology circuit (input impedance is around 5kOhms).

I've tried a nice little BESTEK "pure sine" inverter from 12V-->120Vrms, but it has some strange bistable behavior switching between two slightly different output voltages every 30 seconds or so (pretty hard to automate in a 0.05% accurate metrology calibration setup).

I'm starting to look into high-voltage opamps to take a low voltage signal generator output (around 3Vpp) and amplify it up to 120Vrms, but I'd be interested in any ideas that you all might have. I've also looked at using a step-up transformer to go from about 5Vrms to 120Vrms, but the transformers I've found so far have low input unductance that can't be driven by a 50 Ohm signal generator...

View attachment 282062

Best bet is what I think post

For a metrology (power metering) calibration test fixture at work, I'm looking for a low-cost way to generate an isolated 120Vrms (or 240Vrms) sine wave to feed into the high-impedance mesasurement circuitry.

The first test fixture used a $40k AC Mains generator instrument driving a 1kW load. As we worked through the project, it became apparent that we could handle the current sensing part of the power measurement separately, and only needed a low-current 120Vrms source to drive the voltage input of the metrology circuit (input impedance is around 5kOhms).

I've tried a nice little BESTEK "pure sine" inverter from 12V-->120Vrms, but it has some strange bistable behavior switching between two slightly different output voltages every 30 seconds or so (pretty hard to automate in a 0.05% accurate metrology calibration setup).

I'm starting to look into high-voltage opamps to take a low voltage signal generator output (around 3Vpp) and amplify it up to 120Vrms, but I'd be interested in any ideas that you all might have. I've also looked at using a step-up transformer to go from about 5Vrms to 120Vrms, but the transformers I've found so far have low input unductance that can't be driven by a 50 Ohm signal generator...

View attachment 282062

The suggested idea of a low-voltage op amp driving a 120V buffer seems best bet. Both are commerially available from sources like analog devices, burr-brown et al. In fact, I believe straight 120V op amps are also available, probably in hybrid rather than integrated circuit form.

If going the buffer route you have to consider overall stability.

You of course also would need at least one 140V or so dc supply. Preferably two.

Are you confident your sig gen performs +/= 0.1%?

 

[cmb]

At 120V and 5kOhm load it might be easier to use a switching FET to ensure there aren't any odd instabilities, in the sort of way a PWM works. It can be done without digital parts, I'll see if I can explain in a LT Spice drawing.

 

[berkeman]

You of course also would need at least one 140V or so dc supply. Preferably two.

The 120Vrms output is 340Vpp, so I'd need more like +/-200Vdc power supplies and a 400V opamp...

Are you confident your sig gen performs +/= 0.1%?

The accuracy of the sine wave signal generator source isn't so important in this case, as long as it doesn't drift too much (more than 0.01%) over the 20-30 second calibration interval. I use a 6-digit DVM to read the actual AC Mains input to the metering device, and that DVM needs to be accurate and calibrated.

 

[cmb]

In an age before semi-conductors a long time ago (or not so long ago, depending on your perspective) there was a class of electrical machines called rotary converters. These consisted of a motor driving a generator. This facilitated the conversion of DC to AC, AC to DC, AC of one frequency/voltage to AC of another frequency/voltage, power factor correction, 1ph to 3ph, etc., etc..

Amongst these was a device called a rotary accumulator, which to all effect was an AC capacitor. This consisted of a motor and a generator operating at the same voltage and frequency, but the one isolated the circuits from the other and, due to the mechanical inertia of a rotating mass between them, fully solved all manner of problems with mains line quality and filtering issues.

This would be the device that could have been easily recommended for you in this application, but I have not seen one for 20 years, and it was an obsolete antique even back then.

Funnily enough, it was attached to an old electrical test chamber and no-one in the company new what it was meant to do, so it hadn't been used/run in decades. That sort of grey-beard knowledge was (and is becoming) long lost.

.. you could make your own if it was important for your company, a couple of relatively inexpensive motors and a handy workshop should be able to knock one up for a few hundred bucks. It will also filter out all mains noise, which might actually be (at least in part) what your DVM is picking up, rather than voltage deviations on the fundamental frequency.

 

[Baluncore]

This would be the device that could have been easily recommended for you in this application, but I have not seen one for 20 years, and it was an obsolete antique even back then.

I can let you have one from my collection. WWII saw many different rotary converters in use. The Japanese made a nice 12 VDC to 120 VAC unit, used later on fishing boats. The problem would be that you need to regulate the field current more accurately, and spin it at precisely 60 Hz.

 

[berkeman]

a class of electrical machines called rotary converters. These consisted of a motor driving a generator.

Yeah, we used to have a large high-power motor-generator at work to make clean 50Hz power for lots of Euro module testing. That was on my list of things to look into.

Let us know what you've tried and how well it worked.

I prototyped the tip from @tech99 today, and I think it will work with some changes to my initial try. The 5kOhm load that I'm driving takes around 3W to drive at 120Vrms, so my initial setup used a 15W audio amp to drive a 5Vrms:120Vrms (15VA) transformer. I tuned the output capacitance (using X2 caps) to resonate at 60Hz to increase the impedance that the audio amplifier was driving.

It worked pretty well, but my initial guess of a 15VA transformer sizing turned out to be a bit too small and it saturated at higher voltage drive levels (probably due to the low Power Factor using the output tuning). I ordered a larger VA transformer today and hopefully will be able to test it Monday or Tuesday.

Thanks again to everyone for the help on this real-world design challenge.

 

[DaveE]

In an age before semi-conductors a long time ago (or not so long ago, depending on your perspective) there was a class of electrical machines called rotary converters. These consisted of a motor driving a generator.

Sometimes old tech is the best.

My previous employer made big lasers (OK, all of the lasers, actually). One required 3Φ 480V 50KW of input power and was a favorite* of ultra high-tech. A customer that made the highest of high end semiconductor inspection machines used it and always specified to their customers that they had to supply it from a motor generator. Partly to avoid power quality issues and partly to achieve state of the art performance from the laser. This is the sort of thing you can do when your machine costs ~$1M, but can also do things no one else can. Most customers just ran it from the mains supply, but, when you need the best, a dedicated generator is hard to beat.

I've been away for a decade, I wonder if that's still how the best ICs are made.

* Not a favorite, a necessary evil. I used to say of all of our Ion laser customers that if they could do it any other way they would. No one wants a big Ion laser, they need it, and then have to put up with all of the associated crap required.

Still, wouldn't it be funny to show up with a little 10W motor-flywheel-generator setup for the factory!

 

[AlexCaledin]

- it seems, this thing is too often forgotten (I built such things myself):

- though typically used for fluorescent lamps, it's a good way to obtain any sinusoidal voltage.
The frequency is determined by the resonant "tank" of the transformer inductance (stabilized by air gap) and the capacitor between the BJT collectors - while the tank amplitude is strictly 12V multiplied by Pi.
Very important thing here, it's the input inductor - it makes the resonant oscillation fed by current rather than voltage - but still keeps the voltage amplitude firmly determined - interesting indeed.

 

[Rive]

- it seems, this thing is too often forgotten:

I really doubt that would produce sine wave.

 

[Baluncore]

I really doubt that would produce sine wave.

Correct.
Notice the heater filaments are not powered as the starter is not present.
It produces a ±200 V spiky output designed to strike a florescent tube.
Amplitude is input voltage dependent.
The load dependent frequency is tens of kHz.

 

[cmb]

It could be filtered but the issue is that there is no feedback to control the output. Any circuit will have some variability, especially with magnetic components in it, so the key for 0.01% is, pretty much, something with feedback.

With feedback you can make the output almost as good as the input quality. If not, then unlikely.

A device like a rotary converter might work because it presents a very stiff low supply impedance (like a battery to DC, if you like).

 

[AlexCaledin]

awwww I do admonish you) try to think again...
. . . . . . . . . . . . . . . . . .
- it can be quartz-stabilized; and using MOSFETs (to say nothing of adding the feedback!), the voltage is made stabile enough, too.

Yes, the sinus can be wrecked easily, by means of using a real bad transformer with a heavy non-linear load.

 

[Rive]

It produces a ±200 V spiky output designed to strike a florescent tube.

At resonant frequency it's still decent 'low harmonics' I guess... But as a signal source for precise measurement? ...

 

[eq1]

240Vrms into 5Kohm is ~12W. So it if were me I would drive the primary side with a 20W low voltage amplifier into a transformer with ~20x gain or something. Have the amplifier reference be a sine from an AD9837. You can put a peak hold on the secondary side of the transformer and use it to control the voltage gain on the low voltage amplifier. With the AD9837 you'll get a very precise frequency and with the output peak-hold controlling the input peak into the transformer you'll get a precise output level too. It shouldn't be too hard to power the whole board from a standard 60W power brick.

The whole board is a just a hodge podge of a couple of typical application notes so it should be easy to throw together.

Since this is for a test fixture I wouldn't go with the tuned resonator approach unless there is a tech that can keep the fixture itself in calibration. Keeping the fixture from drifting is going to be no fun.

 

[DaveE]

I wouldn't go with the tuned resonator approach

I wouldn't go with a high Q resonator, because of the alignment issues you raised. But a low Q resonance could be useful as a BPF.

If the system is pulled towards a high Q resonance (i.e. self alignment) then high Q may be good to increase the impedance at resonance. But it's pointless if the Q is low or if you miss the resonant frequency.

 

[AlexCaledin]

- here is that sine source simulated, to show the output wave and its amplitude being just Pi*V1:

V3 is simple 100 Hz square pulse; in reality it's made of the transformer signal;

L2 is 1 Henry;

(the L1R1 can be removed, it's to obtain the stationary oscillation sooner)

The R4 is representing the load. The 100Hz voltage across it:

The maximum is just Pi*10V:

 

[Baluncore]

- here is that sine source simulated, to show the output wave and its amplitude being just Pi*V1:

Which sine source is that?
That is a completely different circuit to the one in post #25.

 

[Baluncore]

- ah, actually same, it may be built in a variety of variants

You must be kidding yourself.

 

[eq1]

With the AD9837 you'll get a very precise frequency and with the output peak-hold controlling the input peak into the transformer you'll get a precise output level too.

I was thinking about this while drinking my morning coffee and realized the peak-hold solution is way too hard. One could make the hold cap leak so the peak doesn't need to be refreshed, hence no need for a reset pulse, but the AD9837 is giving a pure sine and the THD of the VGA and power amplifier will be very low at 10s of Hz (with any part made after 2010) so the measurement of the output can be to just integrate this sine and compare that to a reference. This would provide a higher bandwidth on the control and is easier to implement. The easiest, and most accurate way, is to just buy an amplifier that measures Vrms directly. Or, rectify the output and integrate that with a time constant that's slow compared to 60Hz, not Vrms but good enough for a known pure sine. Diode drop is small compared to 120V so even that should still be a quite accurate measurement w/o much effort.

 

[berkeman]

I prototyped the tip from @tech99 today, and I think it will work with some changes to my initial try. The 5kOhm load that I'm driving takes around 3W to drive at 120Vrms, so my initial setup used a 15W audio amp to drive a 5Vrms:120Vrms (15VA) transformer. I tuned the output capacitance (using X2 caps) to resonate at 60Hz to increase the impedance that the audio amplifier was driving.

It worked pretty well, but my initial guess of a 15VA transformer sizing turned out to be a bit too small and it saturated at higher voltage drive levels (probably due to the low Power Factor using the output tuning). I ordered a larger VA transformer today and hopefully will be able to test it Monday or Tuesday.

As an update, even with a 40VA 5V:120Vrms transformer, at the higher drive levels I was getting a 3rd harmonic instability. I'm thinking it's an issue with parasitics in this line of transformers, but thankfully I came up with a way around it for now. I've added a simple isolation transformer at the output, and used a 110V:220Vrms version to get voltage doubling. That let's me run the 5V:120Vrms transformer at lower levels to avoid the 3rd harmonic problems. It's running on the bench now, and has low jitter like I wanted. I'll use it for a test production run this weekend to see how it holds up.

AD9837

That's an interesting part. It looks like it may have low enough noise that I could use it in this fixture. I'll read over the datasheet and maybe order one of their Eval Boards (Digikey has them in stock). Thanks for the idea.

 

[berkeman]

Have the amplifier reference be a sine from an AD9837. You can put a peak hold on the secondary side of the transformer and use it to control the voltage gain on the low voltage amplifier. With the AD9837 you'll get a very precise frequency and with the output peak-hold controlling the input peak into the transformer you'll get a precise output level too.

Could you say more about how you would level the output voltage from this sine generator IC using feedback? Are you thinking of adding a variable gain stage between this IC and the audio power amp circuit? The datasheet for this IC lists 200ppm/C as the temperature coefficient on the output voltage (0.02%/C), so I'd definitely need to do something to level the output amplitude. Thanks.

 

[Tom.G]

A slightly orthogonal approach:
If the current sensing does not need to reproduce the current waveform. (or maybe even if it does.)

Rather than using a high purity and stable 120VAC, can you use the ratio between the current reading output and the 120VAC supply? Stated another way, the transducing device is reporting the percentage of the 120VAC excitation as the output.

(bridge measurement or ratiometer concept)

Cheers,
Tom

p.s. Those X2 caps you are using for a tank, it is the next character in the part No. that specifies the temperature stability. See: 'Class II' at
https://ec.kemet.com/blog/mlcc-dielectric-differences/

The site also has a simulator (K-SIM) showing temperature curves.

 

[cmb]

@berkeman, IMHO I think any solution to achieve 0.01% is really not a simple project.

The proposal to use an AF amp into a audio transformer is exactly how we do it (I speak in the past tense having been a principal engineer in ISO17025 testing). We'd then quote a measurement error of a % or two.

The act of measuring this to provide a feedback loop is a main complication.

TBH, having thought a bit more about it, you might find value in the principle of the saturation amplifier. Given you can just run mains and 'modify it a bit', this is, on reflection, probably your optimum.

https://en.wikipedia.org/wiki/Saturable_reactor

The variable resistor here can be a Darlington (high current capability) fed by a precision op amp. You have to peak-detect your last peak, and smooth the transitions of the transistor, however that could be quite simple as the appropriate means would be to shift the AC to >0, smooth it with RC via a peak-detect diode, and selection of those components will then average out the last few peaks.

In theory you could get the monitoring circuit to feed the transistor and use all passives (and one transistor) only.

Now ... where to buy transformers for saturation reactors? You might find an AC isolation transformer with an additional low voltage secondary winding, this might work.

IIRC, it was popularly used as a 'magnetic amplifier' in the 20's early 30's I believe, particularly in Germany for some reason, maybe they had difficulty making or importing valves?

(Personally, I'd still look at a MHz rate FET-switching voltage follower with a precision comparator to an input signal.)

Good luck with whatever you do. If you want any specific circuits from me, let me know.

 

[Tom.G]

Another possibility is a ferroresonant transformer, i.e. a constant voltage transformer such as supplied by Sola (now part of Emerson Electric).

They are frequency sensitive though, ±1% frequency yields ±0.25% voltage change, so drive it with a stable oscillator - not the power line.

Capacitive loads confuse them and I don't know about output waveform, better check if important. Also, they are designed to run hot in normal operation due to designed core saturation

Here is an data sheet and synopsis:
https://www.emerson.com/documents/a...nusoidal-transformers-solahd-en-us-163820.pdf

Cheers,
Tom

 

[eq1]

Could you say more about how you would level the output voltage from this sine generator IC using feedback? Are you thinking of adding a variable gain stage between this IC and the audio power amp circuit?

I can only use text right now so I'll do my best. I was thinking the signal chain would look like this:

AD9837 -> VGA(e) -> PowerAmp -> Non Isolating Transformer -> 5K load -> Rectifier -> Integrator -> subtract reference -> e

e is the error signal which gets fed back into the variable gain amplifier (VGA). Design the VGA so positive e lowers gain. Set the integrator with the time constant of maybe 10 periods of the AD9837 signal. That should be sufficient to track out error introduced from the environment and be stable. The VGA should probably be able to respond to e faster than the integrator can change its output. So for 60Hz AD9837 maybe integrate at 6Hz and a VGA that can respond to e w/o attenuation at 600Hz. That should be safe without doing any math or designing actual circuits. Then you can support 50Hz/60Hz too. :)

Not totally sure the best way to do circuit startup. Maybe slowly ramp the VGA in an open loop way to give the integrator it's warmup time. Then after a timeout switch to e. That's probably the simplest.

The reference is the expected average value of the absolute value of the sine with 120Vrms or 240Vrms depending on the mode the user selects. If you want to really dial it in, design the reference to be tweaked with a pot so you can hand calibrate out the sensor offset (static error from things like the rectifier, etc.)

The output sensor (rectifier+integrator) is not measuring rms directly, but since the AD8937 is only giving a single tone, and distortion can be minimized by design in the signal chain, I expect it to be sufficient.

 

[PaulWDent]

It's a matter of accuracy and variation versus time. The BESTEK is very steady, except for the bistable problem. My expensive AC mains generator is still jittering more than I like. I tested our lab AC Mains feed near the end of today, and it was varying around 0.1% continuously, which is way too much.

I've tried to use a signal generator plus step-up transformer, but the 50 Ohm output signal generator can't drive the typical 5Vrms-120Vrms step-up transformer.

Then drive it with a unity gain op-amp! e.g
This seems a very trivial problem

Thanks, this is the most promising avenue so far. Based on my measured 120V winding inductance, I'm able to resonate that winding with about 4μF of X2 capacitance. That makes the input impedance at the 5V winding about 4 Ohms for the 15VA AC transformer that I'm using right now. That's in the right place for an audio power amp. More in a bit...

Your 4 ohms can't be right. With no load on the secondary it would be much higher.
I think you said the load at 120v was 5k ohms. So let's do a calculation: The power you need is 120^2
5000. That's 2.88 watts. You don't usually get that much power out of a signal generator so you do need an amplifier.
You need to build a unity gain amplifier using a rail -to-rail op amp followed by a complementary pnp/npn emitter follower using beefy transistors. Then use your capacitor-tuned step up transformer.

 

[PaulWDent]

As an update, even with a 40VA 5V:120Vrms transformer, at the higher drive levels I was getting a 3rd harmonic instability. I'm thinking it's an issue with parasitics in this line of transformers, but thankfully I came up with a way around it for now. I've added a simple isolation transformer at the output, and used a 110V:220Vrms version to get voltage doubling. That let's me run the 5V:120Vrms transformer at lower levels to avoid the 3rd harmonic problems. It's running on the bench now, and has low jitter like I wanted. I'll use it for a test production run this weekend to see how it holds up.
That's an interesting part. It looks like it may have low enough noise that I could use it in this fixture. I'll read over the datasheet and maybe order one of their Eval Boards (Digikey has them in stock). Thanks for the idea.

It's not the VA that needs to ne higher - it's voltage. Use a 240v winding for 120v if you want to operate far from saturation. It's volt-seconds per turn that determine the core magnetic field, not output current. Mains teansgormers sre designed with only 10% margin to saturation and it sounds like you need to be far from that. You might also like to know that transformers take an inrush current when first switching on the power because if you switch power on at a voltage zero crossing the first half cycle integrates to give double the steady-state volt-seconds. So operating with a 2:1 voltage derating is definitely what you need.

 

[berkeman]

It's not the VA that needs to ne higher - it's voltage. Use a 240v winding for 120v if you want to operate far from saturation. It's volt-seconds per turn that determine the core magnetic field, not output current. Mains teansgormers sre designed with only 10% margin to saturation and it sounds like you need to be far from that. You might also like to know that transformers take an inrush current when first switching on the power because if you switch power on at a voltage zero crossing the first half cycle integrates to give double the steady-state volt-seconds. So operating with a 2:1 voltage derating is definitely what you need.

Please repost your reply without the typos. Maybe wait until morning?

 

[cmb]

Then drive it with a unity gain op-amp! e.g
This seems a very trivial problem

Would you please name the 120V [3-nines-accurate] calibrated signal generator you have in mind, and then the 120V op amp, by serial number/manufacturer?

Thanks. Should be very trivial.

 

[Windadct]

Would KWHM test equipment work - since you indicated it was a power meter.

Ref : https://www.tescometering.com/

 

[cmb]

Would you please name the 120V [3-nines-accurate] calibrated signal generator you have in mind, and then the 120V op amp, by serial number/manufacturer?

Thanks. Should be very trivial.

Sorry, I just realized my error but timed out on editing ... One needs a 350V signal generator and op-amp (we're talking peak-peak here).

Question again to @PaulWDent . Thanks.