# DC12V 12A car window motor - what spec for a 220V PSU?

jim hardy
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Found a dimmer switch rated 250V AC 40 - 250W
Can this be used?

It will be an experiment with i'd estimate 60% chance of success.
It'll either work fine or wreck the dimmer, so if it's one you already have and don't mind sacrificing...

I have an extension cord with a lamp dimmer & receptacle in a box at one end just for experiments like this. One side of receptacle gets full voltage, other side is through dimmer. In US that's easy because our receptacles provide for separate feeds.

Try it with a lamp first . Then set it to minimum , plug in the charger and bring voltage up slowly while listening for unusual humming in your transformer.
Asymmetry in the dimmer output will make the transformer hum and run hot but it should self-announce by those symptoms..
You might feel a little line frequency vibration in your motor but that's expected.

Keep this thing safe. PF is wary about inexperienced people working with line voltage.
Be sure you put the dimmer and receptacle in an outlet box and all connections are safe from tiny fingers in your household that are curious...
Your work in post #29 looks pretty good so i dont mind suggesting you give it a try.
I think over there you have fused appliance cords - one of those sounds like a good safety feature. I don't like 240 volts , the sparks are too big for me.......

old jim

marcophys
Thanks Jim for sharing that idea.
Yes I'm fine with wiring and general electricity... it's the development theory that I lack - I always have to do a lot of reading

Life is a constant learning process; and even when we know something in an associated field... we don't know all it's implications.
Here's a perfect example, just rediscovered now.

Q. Why do we need control over our electrical supply?
A. Because all the French cook at midday

We know this, but yet, it still can catch us (me) out.
I wanted to test densification at the newly achieved low speed.
I was all set to go, and hit the switch.
All I got was a pleasant hum, and a turntable that wasn't turning.

At least we can see that we have discovered the limits of motor operation - and we are on them.

This also explains why the test measurements are inconsistent.
French electricity, in all respects, is to a low standard.
Momentary power outages are frequent, especially if we have a bit of rain.

Turntable speeds
We have a 4 speed system - 2 with the motor and 2 with the PSU (battery charger)
Code:
                 RPM

Charger Normal      Charger Quick

Motor               Motor
Slow      Fast      Slow      Fast

11        19        30        42
Because the slowest speed is on the limit, it needs to rise to perhaps 14.
For this reason, densification was tested at 19 rpm

The result was almost perfect, or perhaps perfect depending upon the desired finish.
There was a very slight shine, yet even speckling of the iron surface.

Clearly, everything is dependent upon diameter, but with the current cylinder, 14 would be ideal.

Next job: The dimmer switch.

jim hardy
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Next job: The dimmer switch.
I hope it works . I never used a dimmer with a battery charger, might try it out if i can find my test box. Quit using it when i got a fine ten amp Variac...

Dimmer controlled socket

I found a BS1363 MK socket with switch and indicator lamp (there is a God).
It has seen good service, and it intends to keep on giving.
(It's face could benefit from surface enhancement )

The dimmer is 240V AC 40-250W

The live mains supply lead passes through the dimmer.
The earth and neutral leads pass direct to the socket.

The switch and socket were mounted on a convenient insulation panel:

Dimmer Control Test

Battery charger connected, the dimmer was wound up.
A quiet buzz could be heard.

The turntable is connected to the charger via 10m 1.5 sq mm cable.

Test 1
On normal or low charge setting - with the dimmer at maximum output
Slowest motor setting (clockwise)

3V DC is seen at the motor.
16 rpm is achieved
The motor hums

There is zero adjustment available on the dimmer at these settings.
Any reduction stops the motor.

Test 2
On normal or low charge setting
Fastest motor setting (anti-clockwise)

12 rpm was achieved
2.5V DC is seen at the motor

The fast motor setting is the best (for whatever reason - it's still a mystery to us).

Working at these minimum levels, may be highlighting any fluctuations in the mains supply.
The motor seemed to initially struggle, and then settled down and ran well.

Test 3
On high charge setting (!)
Fastest motor setting (anti-clockwise)

10 rpm was achieved
2.V DC is seen at the motor

Note: At this power setting, the turntable will not start unassisted
14 rpm @ 2.5V DC appears to be an acceptable minimum or thereabouts, without supply voltage regulation

Conclusions
The slow motor setting (clockwise) can now be dispensed with.
Anti clockwise runs better, with barely any variation of speed within 1 rotation (must be worm drive associated).

Jim's dimmer switch mod has proved successful in providing motor speed control.
I note Jim, that you suggested that the transformer would run hot, and forewarned of the motor hum.

From this... are we considering this to be a simple test... and at some point a more suitable motor control should be implemented.
Either way, we have learned a great deal.

Drawbacks
The only drawback derives from the fluctuating mains supply.
I've yet to monitor this but I guess it could easily range from 220V to 250V.

Is it the case that a mains supply voltage regulator is required?
Example - (this was at the top of the ebay page)
http://www.ebay.co.uk/itm/Voltage-R...immer-Thermostat-AC-220V-4000W-/282425934168?

or this one with a fan:
http://www.ebay.co.uk/itm/AC-220V-4...or-Speed-Control-Controller-Fan/192027019684?

Would something like this always give a stable output, if set to lower than the minimum voltage supply?
In effect, a dimmer, but with known voltage output, and perhaps no negative 'buzzing' overheating effects?
Or is this cheap kit missing what is required?

I'm guessing that the PWM controller system would still require a DC voltage regulator, to enable standardised settings for rpm?

I'd be quite happy using Jims dimmer switch concept... it's just a question of whether it is fundamentally flawed for long term use.
(and even if this is so... it has proved invaluable for test purposes)

jim hardy
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I note Jim, that you suggested that the transformer would run hot,
Thanks for the feedback !!!

Feel of it.
What worries me is your dimmer looks to be second hand and voltage to the motor seems to have dropped.

Asymmetric power from the dimmer will apply some DC to the charger's transformer . It should NOT be operated with THAT condition. It will hum loudly and get very hot.

Those are the easiest symptoms to see.

If you can measure voltage across the charger's primary winding (and i dont see a convenient place in your pictures)
read it with a DMM set for DC about 50 or 100 volts scale. More than a couple volts DC is too much,

Why , you'll naturally ask ?
We're handing the dimmer a largely inductive load now instead of the resistive one it's designed for.
So double check your transformer for buzzing, running hot, and best test is for DC content in the AC applied to its primary.
Most DMM's when set for DC will filter out the AC so long as you're not overranging them drastically. Don't try to measure DC content of a 240 AC wave on the 2 volt DC scale, but 100 volt scale should work and 50 might.

You need to have less than 1% DC content.

The heart of a lamp dimmer is an electronic switch known as a 'thyristor" .
Asymmetric firing of thyristors used to burn up relay coils in my nuke plant. They'd check fine on the bench at room temperature but when the thyristors got to operating temperature (~110F) they developed DC content. We found it first with oscilloscope and current probe, later learned that trick with the DMM set for DC.

So that's why i am sensitive , perhaps overly , but better safe than you know ....
I'll feel better if you run it a while and feel of the charger's transformer .
Measurement of DC content would put a number on that qualitative test and be very reassuring.

I'd hate to be the guy who led you to burn up a battery charger because of a defective lamp dimmer.

Learn from my hard knocks it's easier that way, and share yours ? That's what we do here, share our experience.

old jim

marcophys
What worries me is your dimmer looks to be second hand and voltage to the motor seems to have dropped.
The dimmer is second hand... it was used to power a normal bulb in the 90's - probably 100W.
The voltage to the motor definitely dropped... it dropped by an additional 25m 1.5 sq mm cable.

IE. We achieved similar conditions by replacing the cable with the dimmer switch on maximum setting.

Asymmetric power from the dimmer will apply some DC to the charger's transformer . It should NOT be operated with THAT condition. It will hum loudly and get very hot.

Those are the easiest symptoms to see.
Forgive me, but I'm finding this statement ambiguous.

Are you stating that, fundamentally, the dimmer WILL apply some DC to the chargers transformer
,,, and therefore... regardless of the useful tests, it should NOT be used on a day to day basis?
OR
Is this warning only applicable if the transformers hums loudly?

Either way... my feedback is that the transformer exhibits a very low noise buzz.
You can hear it, but it's not much.

If you can measure voltage across the charger's primary winding (and i don't see a convenient place in your pictures)
read it with a DMM set for DC about 50 or 100 volts scale. More than a couple volts DC is too much,

Why , you'll naturally ask ?
We're handing the dimmer a largely inductive load now instead of the resistive one it's designed for.
So double check your transformer for buzzing, running hot, and best test is for DC content in the AC applied to its primary.
Most DMM's when set for DC will filter out the AC so long as you're not overranging them drastically. Don't try to measure DC content of a 240 AC wave on the 2 volt DC scale, but 100 volt scale should work and 50 might.

You need to have less than 1% DC content.
From this, I believe that you are asking me to meter the live and neutral at the MK socket (post dimmer switch).

I can confirm that your eyesight is still good
Absaar has cunningly masked the primary leads with a particularly delicate plastic enclosure, that has been glued together.
But the plug flex is only 1.5m.
Therefore I can disconnect the dimmer panel - open the plug, and insert it, with the exposed pins available.

With the dimmer reconnected, I can sample the modified supply to the transformer.
Yes?

The heart of a lamp dimmer is an electronic switch known as a 'thyristor" .
Asymmetric firing of thyristors used to burn up relay coils in my nuke plant. They'd check fine on the bench at room temperature but when the thyristors got to operating temperature (~110F) they developed DC content. We found it first with oscilloscope and current probe, later learned that trick with the DMM set for DC.

So that's why i am sensitive , perhaps overly , but better safe than you know ....
I'll feel better if you run it a while and feel of the charger's transformer .
Measurement of DC content would put a number on that qualitative test and be very reassuring.
I have another dimmer switch... it will be transferable.

On with the testing....................

Notes:
I wish that I understood this transformer .

Measuring the Dimmed live feed
.... at DC setting 200V

The reading is up and down, varying from a peak of 1.3V DC to 0.1V DC.
This is a repeating cycle, that hovers mid range then goes up... and then down.
... but max is always momentarily 1.3V DC.

Therefore well under the 2V DC that you warned about.

Tom.G
We're handing the dimmer a largely inductive load now instead of the resistive one it's designed for.
So double check your transformer for buzzing, running hot, and best test is for DC content in the AC applied to its primary.
Just add an incandescent lamp (60W - 100W ?) as a parallel load for the dimmer. That reduces the inductive load phase shift and makes the dimmer behave much better.

marcophys
jim hardy
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Are you stating that, fundamentally, the dimmer WILL apply some DC to the chargers transformer
,,, and therefore... regardless of the useful tests, it should NOT be used on a day to day basis?
OR
Is this warning only applicable if the transformers hums loudly?
The latter

Either way... my feedback is that the transformer exhibits a very low noise buzz.
You can hear it, but it's not much.
That's expected. Instead of a smooth sinewave it is receiving a 'chopped' approximate sinewave with sharp leading edges.

I think you'll be okay.

Forgive me, but I'm finding this statement ambiguous.
I was worried about the reduced voltage but you explained it as extra cable.
Sorry for the confusion . I shoulda said "If you have there an asymmetric behaving dimmer, hum and transformer heating will be the most obvious symptoms ."

The reading is up and down, varying from a peak of 1.3V DC to 0.1V DC.
This is a repeating cycle, that hovers mid range then goes up... and then down.
... but max is always momentarily 1.3V DC.

If the polarity sign on your meter shows for that measurement sometimes + and sometimes - then you have very good symmetry and no worries at all.
If it was always same polarity, feel the windings and the core after a few tens of minutes or an hour of operation.with no load on it.
If they're not uncomfortably hot to the touch i think you're fine.

Asymmetry puts a small amount of DC into the transformer. That causes the core to approach saturation heating it
and more current through the primary winding heating it too. Secondary winding will be heated only by thermal conduction outward from core and primary which is slow.
Those effects will be most noticeable near maximum "brightness" .

I'm oversensitized to this, probably.
As i said we had trouble with asymmetric thyristors in 1970's.
Burning up a hundred dollar relay wasn't an issue.
But when that relay tripped the nuke plant it cost my employer a literal million bucks for fuel oil to replace the megawatts we didn't make that day.
So i have a lot of my own "midnight oil" invested in tracking down what was killing those relay coils . Taught me a lot about inductance...

Thanks for tolerating an 'old fire-horse' . We've found no smoke but diligently checked for a potential cause of it.
If you build these motor drives for your friends , make that DC measurement and pick out dimmers with minimal DC content.
Probably the lamp dimmer people won't test their products for that parameter because a little DC is of no consequence to an incandescent lamp.

Congratulations for your inventive project . We learn SO MUCH MORE by doing than by just reading about doing , eh?

old jim

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marcophys
Just add an incandescent lamp (60W - 100W ?) as a parallel load for the dimmer. That reduces the inductive load phase shift and makes the dimmer behave much better.
Wiring a 40W lamp in parallel to the Transformer feed
Haha... thanks Tom... it worked!
The buzzing dropped substantially - barely perceptible (as you'll see in the video below)

You've just jumped the development process... thanks for contributing

If the polarity sign on your meter shows for that measurement sometimes + and sometimes - then you have very good symmetry and no worries at all.
Metering the DC voltage
The video shows a number of DC voltage tests.
YouTube allows the video speed to be reduced... but even at normal speed, the +ve -ve cycle is evident.

Video Notes (Important)
The video shows a very noisy motor that masks the 'hum'.

This was just bad luck due to the speed at max dimmer - a speed that happened to resonate.
The noise resonates through the metal grill (dropped in position) and is amplified by the cabinet.
This was probably due to the gearing.

When making a video, one is time limited, when attempting a single take (which I was doing).
A better test would be to have slowed the motor down (to move out of resonance range).

I did this after making the video.
Here's the big news!

At reduced dimmer setting:

10 revolutions in 25 seconds NO LAMP (2nd test 27 sec)
10 revolutions in 21 seconds WITH LAMP (2nd test 22 sec)​

There is also a small reduction in motor hum.

Therefore, not only does a parallel lamp aid the transformer...
... it also aids the motor.

Having repeated the tests... there is a very clear difference between 'with and without' the lamp.
This means that the secondary coil is affected by the lamp - fact!

4 x 25 seconds = 100 seconds (40 revolutions) - without lamp
4 x 21 seconds = 84 seconds (40 revolutions) - with lamp

Therefore, by adding the lamp we see a 16% improvement in motor efficiency.

Note:
No change in efficiency is witnessed by increasing the lamp wattage to 100W.
Therefore, at this moment in time, it may be possible for those of you with the theoretical knowledge to consider a suitable replacement.

I'm thinking... a simple +ve -ve component bridge within the MK socket, that would replace the bulk of the lamp.

Ha!
This project keeps on giving

Here's the video
I would definitely consider slowing the video down to 0.5 speed (at appropriate moments).
This allows you to see the flipping between +ve & -ve DC that Jim mentioned.

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jim hardy
jim hardy
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This project keeps on giving
Bravo !

jim hardy
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Therefore, at this moment in time, it may be possible for those of you with the theoretical knowledge to consider a suitable replacement.
That TomG is a very clever fellow.
I'm still trying to figure out just what his lamp did to alleviate the humming.

Come to think of it,
our thyristor relay drivers in the plant had resistors across their output..... i'd forgotten that.

Nice project, glad you're having fun..

Yes, TBH I was thinking about a resistor bridging the dimmed output (resistive load).

100W or 40W lamp seemed to make no difference.
Perhaps if the wattage reduced significantly... I think that I have an 8W lamp - or used to have, as it hasn't turned up (and I've been looking).

Or perhaps it simply requires a resistive load, regardless of it's resistance.

The ideal place to locate a small component, from a topology perspective, would be in the MK socket outlet.

The question - is it possible vis a vis heat?

It's a metal enclosure, with sufficient space for careful routing of the cables.

If it is considered possible... what might be a suitable resistance range, to place between the dimmed live and neutral?

It seems that I would need one rated at 400v.

If:
Current I = V/R 240/1000 = 0.24 Amps

Power consumed P = I2R = 0.242 x 1000 = 57.6 W

10K Ω would consume 5.76 Watts.
100k Ω would consume 0.576 Watts​

Is this correct?

I wonder if the resistance could be even higher - I don't understand the mathematical relationship between the resistive load, and the disharmony in the dimmed AC supply.

jim hardy
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Aha i remember now.

It's "holding current" for the thyristor.
Holding current is the minimum current required to hold a thyristor in its conducting state. It will be tens of milliamps.
When current drops below that amount it will turn back off unless there's still a gate signal applied.
.
This is a basic lamp dimmer

(Triac is the name for a bidirectional thyristor)

Capacitor C delivers a brief firing pulse to the Triac's gate terminal .

Load current through the lamp must rise to Triac's "holding current" before the end of that firing pulse.
Inductive load like a motor will postpone rise of load current because of its inductance. So the triac might turn back off .
Resistive load does not postpone current rise. So some resistance in parallel with your transformer primary will help assure reliable turn-on of the Triac.

I suspect that's what was happening when you saw those 1.3 volt DC readings, Triac misfires.

More detail here
http://www.st.com/content/ccc/resou...df/jcr:content/translations/en.CD00003853.pdf
and here http://www.st.com/content/ccc/resou...df/jcr:content/translations/en.CD00003867.pdf

and here's a datasheet for a 4 amp Triac you might well find in a lamp dimmer:
http://www.st.com/content/ccc/resou...df/jcr:content/translations/en.CD00002887.pdf
Its holding current is just 15 milliamps.

I'd think a 50 milliamp lamp might work , that'd be only about ten watts? An outdoor Christmas tree lamp? You could hide that inside the battery charger.

What does @Tom.G think ?

old jim

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Asymptotic, krater and marcophys
Tom.G
I'd think a 50 milliamp lamp might work , that'd be only about ten watts? An outdoor ? You could hide that inside the battery charger.

What does @Tom.G think ?
He thinks highly of it. Might need two Christmas tree lamps, in the USA they are either 4 or 7 Watts each.

Asymptotic and jim hardy
jim hardy
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Thanks Tom.

Hmm an automotive lamp on the 12 volt side, in parallel with the motor, might work. I think a brake light is about 25 watts ?

1156 is single filament, 1157 is dual one about 6 watts for taillight one about 26. for brake ..
At reduced voltage they last a long time.

marcophys
Great.
Two lamps in the battery charger enclosure would indicate that it was on!

However, I am interested to understand the difference between a lamp filament, and a resistor.
IE. is the resistor idea wrong?

The reason being that I don't have a small 240V lamp.
Neither do I have the resistors.

It is looking like we have arrived at that point in the project where something must be bought

So I can source a lamp and select it as small as possible (for easy location).
Or I can source the required resistors.

The dimmed mains supply
@ slow motor rotation = circa 117V AC
@ maximum output = 193V AC

Therefore the operating AC voltage range will be approximately 115V - 130V

The DC voltage
At the motor is in the 2V - 3V range.
At the transformer it is 3V - 4V range
(It's dropping 1V conveniently)

From this we could estimate the resistors required, or suitable lamps from what is available.
The resistors being easier to specify - If they would work as well as the lamps.
Also, they could be installed very easily into the system.
They might also have less likelihood of failure.

The lamps aren't a major problem, but resistors seem to offer a path worth examining.

Obviously if they can't work, I'll simply research the lamps.

jim hardy
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I had thought the lamps would be a lot easier to acquire. Here every Walmart and auto parts store has lamps and sockets on the shelf , resistors one has to order.

A lamp filament is just a resistor. But since resistance in most substances is a function of temperature, and since the filament goes from room temperature to maybe 4,500 degF, there's quite a resistance change.
Resistance cold is typically 1/10th what it is hot.

You know that a 40 watt lamp worked okay. Can you try a 20 ?

Power = E2 / R
a 40 watt 230 volt lamp is nominally
40 = 2302 / R so R = 1322 ohms , probably somewhat less at your 130 volts.
20 watts would be twice that many of course.

A 40 watt resistor will cost a lot more than a light bulb.

I only leapt at automotive lamps because they're so ubiquitous. Spend a few minutes in a hardware store perusing bulbs and sockets.
There exist small halogen 6 and 12 volt bulbs for outdoor, undercounter and reading lamps , from just a few to 100 watts. I use them for troubleshooting car electrical problems. A dead lamp salvaged from a thrift shop will provide a socket .
On the low side of your transformer you'd divide those ohms above by square of the turns ratio. 240V in 12V out is ratio of about 20...
1322 / 202 = about 3.3 ohms

6 volt 30 watts ? 36/30 is only ~1.2 ohm, sounds low and might blow if dimmer ever got turned all the way up.

12 volts 50 watts 144/50 = 2.9 ohms , not bad.....
At your 3 volts or so it'll just glow orange.

You already figured out how to calculate resistance and power. There's how to figure your resistance for the high and low sides.
.
Keep on experimenting. Lamps are good for a cheap power resistor where you don't need precision.

have fun, foremost.

old jim

Asymptotic and marcophys
Thanks Jim!
Yes... development engineering is most definitely fun.
Fine to take a drawing and bring it to life... that hits a different pleasure receptor.
... but creating something, and solving the problems that are encountered, is simply unbeatable.

The final stages are upon us.
The turntable and control system is now operational.
We are left only with the desire to develop the parallel resistance, to improve the package.

On this matter... I am re-examining the concept of 'modding' the transformer.
... by placing the resistance loads in association with the input control system, and the output to the motor.

This will leave the battery charger untouched.

Applying a lamp to the DC side seems to make no difference whatsoever.
I tried using a 24V 50mA lamp.
It glowed.

jim hardy
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I tried using a 24V 50mA lamp.
That's only 2.5 ma at the primary where the dimmer is.
Transformer steps primary voltage DOWN from 230-ish to 12, and current UP from 2.5 ma to 50 ma.

You'll need a 12 volt two to four amp lamp..
Wattage comparable to what works on primary side.
Can you borrow one from an automobile? Brake, turn signal or headlamp ? Dome light won't do.

Don't despair , this is Edison's '99% perspiration' complement to his '1% inspiration' .

Mother Nature likes to see us sweat.

old jim

marcophys
I can get the speed down to 29 rpm @ 5.3V 4.7A (using the slow charge switch on the charger).
35 rpm @ 6V 5.1A (using fast charge switch)

(It looks like that 5A fuse is too light)
Difficult to be certain, but 5A may be 'just right', or perhaps even too high. Problem is, the motor was designed for low duty cycle, intermittent operation from a DC battery source, and has been re-purposed to serve in a high duty cycle design fed from a rectified power source. Ultimately, both fuse elements and motor windings fail due to excessive heating, and for sustained overload protection the trick is to select a fuse that will clear before motor windings are damaged. Off-the-shelf, general purpose, continuous duty motors are typically rated for a 40°C temperature rise. You'll probably be OK by closely monitoring motor heating until you are comfortable it isn't about to become a flambé.

I haven't looked up the clearing curve for a BS 1362, 5A fuse, but came across this interesting article comparing 3 amp and 13 amp fuses, and learned a 3A fuse will hold on indefinitely at 4.5 amps. My guess is for a 5A fuse it'll be on the order of 7.5 amps.

I vaguely recall IEC standards having something similar, but NEMA DC motor ratings include a "power code". Power code A is battery powered (PWM output is between A and C, although close to A than C), code C (nearly all the motors I worked with were code C) is DC derived from a full wave bridge, and code D is rectified DC from a half wave bridge. One way to overheat a DC motor is to feed it voltage with an 'uglier' waveform that what it was designed for. For example, I've seen FHP (fractional horsepower) motors cook when their drives lost an SCR, and effectively morphed from code C to code D.

Given this, my guess is part of your motor heating is a consequence of the supply waveform. If you have a largish (in the 100 to 330 uF range) electrolytic capacitor handy, temporarily clip it across the battery charger output, and see what happens. The motor ought to run cooler, and provide smoother low speed operation.

It looks like your project is coming along nicely. Kudos.

marcophys
Tom.G
Hey guys, after sleeping on it I realized the added resistive load must at least equal, preferably exceed, the inductive load; and will probably be more effective when connected to the dimmer. What say @jim hardy , how much must the resistive load exceed the inductive load to keep the TRIAC on? Taking account of both phase shift (power factor) and leakage inductance of a very low cost transformer

If put on the xformer secondary there is still the leakage inductance etc. on the primary side. If put across the motor (after the rectifier), there is the additional problem of not being connected around zero crossing. Those may be enough to upset the dimmer.

Yea, the above is weasel-worded. That's because I've never tried the resistive load on the secondary and would be interested in the result. On the primary we know it works. If you care to teach all of us, try it on the secondary and enlighten us. (Some oscilloscope traces would be nice if you have access.) If you just want to get the project done, keep the resistance on the primary.

Asymptotic and marcophys
jim hardy
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how much must the resistive load exceed the inductive load to keep the TRIAC on? Taking account of both phase shift (power factor) and leakage inductance of a very low cost transformer

If put on the xformer secondary there is still the leakage inductance etc. on the primary side. If put across the motor (after the rectifier), there is the additional problem of not being connected around zero crossing. Those may be enough to upset the dimmer.

All above is true.
Resistive load need only pass holding current of triac at end of trigger pulse
and since we don't know what is value of that holding current or duration of trigger it'll have to be found by experiment what size lamp does the job.

He has an advantage in not needing full AC voltage, that's easier on the transformer (remember volt-seconds and saturation) .

Trial and error is the approach for this one. See if a 40 watt 12 V lamp on secondary behave noticeably different from a 40watt 240 volt lamp on primary ?
To avoid rectifier drop he could connect it here

old jim

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marcophys
Pauses in development are useful.
They are usually caused by external forces, but in fact they are anyway required.

I must clear up something else tomorrow, and then I'll get back on this.
I'll implement the fuse as suggested, and then look again at resistive loads.
I have the typical 240V lamps and 12V auto lamps.

Thanks for the considered thoughts.