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symmetric dimming circuit for transformer.

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dr pepper

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I think I've done this before but cant remember how.
I want to build a pyrography woodburning pen for the mrs, one with more grunt than a little soldering iron.
And to keep things simple I've rewound the secondary of a 150va transformer to produce 3.5v at a lot of amps, and I need to regulate the ac input, this is how expensive commercial ones do it.
So I need a little circuit that will 'dim' the incomming ac, but obviously one that triggers at the same point either side of the mains waveform so little dc is produced to nuke the tranny.
Any ideas or links?
 
You could use an ordinary lamp-dimmer on the primary side of the transformer.
 
I was thinking about the crude design of an off the shelf lighting dimmer intended only for resistive lamps creating different trigger points on the + and - cycles, the resulting dc burning out the tranny.
I remember seeing a circuit that used a bridge to generate positive pulses for triggering.

I'm now thinking about a hefty current low freq pwm generator with a tl494, obviously I'd need at least unsmoothed dc to power the electrode.
 
different trigger points on the + and - cycles
I'm no expert on this but would they be very different? Won't the breakdown voltage of the customary diac in series with the triac gate swamp out any inter-quadrant gating differences?
I remember seeing a circuit that used a bridge to generate positive pulses for triggering.
I have a home-brew dimmer that does that. An SCR and its associated triggering components sit across the diagonal of a diode bridge which is in series with the load. The SCR etc get DC. The SCR triggers the same on each half cycle. The load only sees AC.
 
Hi,


"...150va transformer to produce 3.5v at a lot of amps..."

How much current?

You're right in that net DC in the primary could heat up the winding. It depends on how bad the offset is and how much resistance in the winding. There are circuits to correct this but you probably dont want to get into that.
Measure the resistance of the primary.
 
About 20 amps, maybe 50 at most around 2-3v.
DC resistance around 20r
I have lashed up a pwm circuit using a tl494 and a couple of fets robbed from a scrap pc supply, with a really low switching speed of 100hz it works well, the dc supply to it from the tranny/bridge of course being constant.
I'd still like to sort the issue of equal triggering for an ac dimmer however.
Alec I meant the triggering circuit for a triac being fed via a bridge, however what you said is also probably a good idea, if I got it right your saying use a thyristor rather than a triac, and connect up a bridge in series with the load on the ac side and connect the thyristor on the dc side, the thryristor and its trigger circuit would then see pulses of dc and wouldnt discriminate bewteen neg and pos, might try that.

Edit: My old friend wikipedia, heres a link to a circuit I mentioned above using a thyristor, interestingly the same manufacturer of dimmer shown in the article is mentioned on a woodworking site for doing exactly what I'm doing.

https://en.wikipedia.org/wiki/Dimmer
 
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What I suggested is similar to the one in the Wiki link, except that I used a diac instead of a zener.
 
Hello again,


By 20r if you mean it is 20 ohms then the offset voltage would be divided by 20 to get the offset current. So if one side was switching at 120v and the other at 119 then the offset current would be 1/20 amps which is hardly anything to worry about. If the offset was 20 volts then it would be 1 amp, but that's not likely.

With a triac, we also have the problem of turning it off. Triacs dont turn off like transistors but they must wait until the zero crossing of the CURRENT (not the voltage). And since there is inductance involved and inductance likes to keep the current flowing, you may find that some conduction angles that are possible with a resistive load are not possible with a somewhat inductive load. Adding some resistance across the primary helps but reduces efficiency.

With the full bridge solution there is the problem of suppressing the spikes that appear with somewhat inductive loads. This kind can be used with a transistor rather than an SCR, and that means you can get PWM of higher frequency than the line (with the line frequency superimposed). But some care is needed to make sure the inductive kickback does not destroy the transistor which may mean a snubber has to be introduced.

BTW diac's and things like that are from the distant past and are not needed anymore. A simple control circuit can control right down to zero volts.
 
Ok then, I've had inductive load issue problems before, and larger thyristors with capacitance make things worse.
I've thought about using a fet to pwm mains through a bridge before, and theres no shortage of high volatge fets in dead appliance psu's.
My lashup works on the secondary at low volatge, and the lashup works at 100hz, the same as the rectified i/p, I expected some weird harmonic but it works fine, the thermal mass of the wire must even things out.
I didnt know diacs were old hat, must be why theres not as big a selection on sale as there used to, can you give an example circuit of a simple dimmer that doesnt use one?
 
Hi again,

Thermal mass of the wire? What wire? Maybe a drawing of what you have already would help.

As you know, when we use a bridge rectifier with a switching device across the DC side of the bridge, any inductance of the load produces a large kickback that gets rectified with the full wave bridge and appears as a high DC voltage across the DC side of the bridge rectifier. When we do something like this in an inverter, we always have the input caps to fall back on that not only absorb the energy in the spike but also that means they are able to give that energy back when it comes time for the next pulse. But with an AC chopper circuit this is not the case so we have to provide some sort of damping, while at the same time trying to keep the efficiency up. Since the actual value of inductance is a bit of an unknown, the best bet is to do a few little experiments to see how high the spike goes with some small capacitance through a small resistance (the resistance needed so that the transistor does not have to try to dump all of the energy stored in the cap in zero time...it gets more time to do that). This also limits the minimum 'off' time otherwise the voltage in the cap would ratchet up and up and up.

If you are on the secondary side the situation is a little better. Since the output is low to start with a high spike like 100 volts will only mean we need a transistor of say 150 volts. But of course some experimentation and measurement is in order for sure. See how high it goes after adding some small capacitance and series resistance and go from there.

"Dimmer" circuits have come a long way since the diac. But amazingly even a decent one isnt that complicated. After all, all that we are trying to do for any dimmer is *time* the pulse to the triac, to get it there at the time that correlates to the desired conduction angle. To see how simple this really is, say we want to 'fire' at 1/2 of the total sine wave half-period. And say we are at 50 Hz here. 50 Hz has a period of 20 milliseconds, so half period is 10 ms. That means of course that the 90 degree point of the sine (and that is half way of half the total period) is at 5ms and at 15ms (the negative and positive excursions). So all we have to do is:
1. Detect the zero crossing of the power line sine, and
2. Start a timer that times out 5ms.

So if we look at the first cycle, it starts at t=0.000 seconds, and that is when we start a one shot. The one shot is adjusted for 5ms, so after 5ms have passed (t=0.005 seconds) the one shot fires and that is used to drive the gate of the triac. So the triac turns on at 5ms and we see a sharp rise in voltage at the load, then it starts to come down as a sine does until it crosses through zero again and that turns the triac off, but also starts the one shot all over again. Then after another 5ms the one shot fires again and that turns the triac on again (t=15ms) and that's where we get the negative part of the output.

There are a lot of ways to do this, all of which involve an adjustable one shot and possible another one shot to fire the triac with s short pulse. The adjustable one shot is made adjustable via a potentiometer that the user can adjust to get smaller or larger phase angles. The beauty here is that if the one shot can adjust down to 100us say, then we have almost full conduction, and since it can certainly adjust up to 9.9ms, we have adjustment all the way to zero output...something that a diac circuit might have trouble doing. That's why many of the older diac circuits could not dim all the way down to near zero light output with a standard bulb.

Sound simple enough?

Oh yeah, dont be afraid to switch at 1kHz or even 10kHz as the MOSFET's have no trouble with this and you'll get a more well behaved output.
 
For pyrography purposes even 100Hz is a lot faster switching than is strictly needed. You could get away with whole cycle omission to control the heating element.
 
For pyrography purposes even 100Hz is a lot faster switching than is strictly needed. You could get away with whole cycle omission to control the heating element.

Hi there alec,

Yeah you could, but you'd also have to put up with hearing the banging of the secondary being turned on and off :)
Would be worse with the primary however.
 
Good point.
 
Pretty much all the tips will be nichrome wire, however pyrographers tend to use anything from 30swg to 12swg, hence the need for variable amps, to make things worse you can get branding irons to fit the pen which use a coil of a few turns of nich wire to heat the brand, not a massive inductance but probably much more than the leads, the brands are usually brass which isnt going to be an issue, the cheeper ones are cast which will bump up the inductance, I'm sure this is why they say they screw up psu's.

I'll look into measuring the inductive kick back, thats an interesting idea, the fet I tried is rated at 1200v so doesnt struggle, but I could use a lower voltage fet with a lower rdson/gate charge and dissipate even less heat.
Efficiency doesnt worry me much, so long as the 'sink isnt as big as the kitchen sink, I was thinking of using a mov accross the fet just in case the mrs gets some of the cheep iron brands and jumps up the kick back.
Interesting about the kick back with the circuit controlling the primary, that would put lots of stress on the transformers insulation.

Allthough I have written software for phase angle control I dont have that much experience with simple diac circuits.

I like 100hz as it doesnt make annoying squeels, just a bit of a rattle, I dare say 20kc would still be ok.
 
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Hi again,

Well maybe when you get something together you can post a few pics.
 
OK were going to lash up a trial to make the father in law's anniversary present, I'll take some pics.
 
I have the pics but my camera's having a bad day.
I gave up on the transformer idea, I had a couple of pc power supplies so I adapted one and used it in conjunction with a simple circuit using a tl494 and a irf540 as a switching reg.
Works well.
One thing I'm surprised at, when burning wood using nichrome wire you can hear the switching frequency (now 1kc) from the tip when in contact with the wood, I didnt think 26swg nichrome would respond temperature wise anything like as quick as that, the wire runs off 5v so I dont think the moisture in the wood is responding to the applied voltage, bit strange that.
 
The sound may be due to vibration caused by a magnetic effect: either mutual attraction/repulsion between the adjacent wire sections near the tip or (don't quote me on this) perhaps magnetostriction.
 
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Hi,

The linear expansion could be causing the noise as the wire would get longer and shorter by a significant amount along the total length. This would mean the wire would actually knock against the surface of the wood and even if it was a very small movement it would still make a noise. So even if the wire didnt actually change temperature by a super big amount it could be enough to cause vibration. It could be along the width, but the action along the length would be more significant.
You could look up the linear coefficient of expansion for the type of metal used and see what it is.

BTW how long is the wire?
 
Well theres a thing magnetostriction, that used to be an issue with olde lopty's on tv's.
The nichrome wire is only around 1 1/2", I'm relying on the resistance of the cable to the wand to prevent the psu from going into overcurrent and shutting down.
Heres the completed burner as promised, the choc block on the end of the wand is porcelain, so it doesnt melt from the heat of the tip, the pot is 10 turn, pyrographers are very fussy:

Photo0401.jpg

Photo0402.jpg
 
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