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

Mentor
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.... ## Answers and Replies Mentor 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 Science Advisor Gold Member 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....

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Maybe look into crystal controled vacuum tube oscillators?

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Averagesupernova
Gold Member
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.

berkeman, DaveE and Tom.G
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.

berkeman
DaveE
Gold Member
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.

berkeman
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.

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

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

berkeman
Mentor
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...

Averagesupernova, DaveE and hutchphd
Tom.G
I'm able to resonate that winding with about 4μF of X2 capacitance.
I suggest low Q, and temperature stable caps.

Mentor
I suggest low Q, and temperature stable caps.

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.)

Tom.G
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? Mentor 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 Homework Helper Gold Member 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....

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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%? 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. Mentor 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. Averagesupernova 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 Science Advisor 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. Mentor 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. Last edited: tech99, hutchphd, DaveE and 2 others DaveE Science Advisor Gold Member 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!

- 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.

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