Problem with thyristor circuit control

Ok so I brought a thyristor circuit motor control governor to use in a heating mantle with a nichrome element and upon turning it on I found that even with the potentiometer on the thyristor turned all the way off down to zero it still let's enough current thru to
Have the element get to 190'c and as I want a finer control this is unacceptable.now the lucky part is I have two of these so I was wondering if I can turn it down to zero and then put the reduced output from this into the input of the second thyristor circuit and use the second ones potentiometer to try and get a fine control of the temperature.is there anything wrong with this idea? below is a pic of the $3 thyristor circuit I'm talking about.there all over eBay for a few bucks and the two I have are from different sellers and are similar.im not sure if the problem is bcoz of the thyristor circuit allowing that much current thru even when turned down to zero or whether that's meant to happen.what I want to know is am I going to damage the circuits putting them back to back or am I going to be pleased that it works fine?

You have provided almost no information. On the assumption that this is a phase angle control device feeding the output of one of these units does not make sense. (That reasoning would also apply if they are PWM devices.).
What is the supply voltage ?
What voltage reading do you get at the heating element ? (This will not be accurate due to the waveform but it will give us some idea.)
What temperature does the heating element run at connected directly to the power supply ?

Les.

If the heating element would normally be in water and you're running it in air then it'll get hot.

Mike.

What happens if you adjust the preset resistor on the board? Looking at photos of the PCB in ebay listings, it's connected directly across the external pot so sets the minimum power level.

You could even remove that completely to give the main pot a wider control range; or if you can get it low enough on the preset with the main pot unplugged, changing to a higher value main pot may give the range you need.

You cannot cascade two units, or not with any functional effect; it may damage the second one.

Les Jones said:

You have provided almost no information. On the assumption that this is a phase angle control device feeding the output of one of these units does not make sense. (That reasoning would also apply if they are PWM devices.).
What is the supply voltage ?
What voltage reading do you get at the heating element ? (This will not be accurate due to the waveform but it will give us some idea.)
What temperature does the heating element run at connected directly to the power supply ?

Les.

Click to expand...

Ok here's the type of circuit I'm talking about.https://www.ebay.com.au/itm/2000W-AC-50-220V-SCR-Electric-Voltage-Regulator-Motor-Speed-Control-Controller-/282514749543
Supply voltage is 240v.
The whole unit is said to be 400w.
It's a scientific heating mantle so water is not involved.the heating element needs to be controlled somehow as if it was connected straight to the mains then it would melt the nichrome and stuff everything else as well.i believe that it originally was controlled by a triac circuit of some sort(don't have more info than that).
The problem with using the thyristor circuit which I was told would be ideal for this is even with the potentiometer turned down to zero it still heats the mantle to 190'c so there's no way to get a lower temperature than that and without that it's useless.the original board got fried from a power surge or something.the copper track was peeling off the circiutboard.i don't have a multimeter so can't measure voltages.was wondering whether putting the output of the thyristor turned down to almost zero into the input of another one will give a much finer control over the temperature the element gets to.
If anymore information is needed let me know.if this will stuff up the second thyristor circuit let me know

rjenkinsgb said:

What happens if you adjust the preset resistor on the board? Looking at photos of the PCB in ebay listings, it's connected directly across the external pot so sets the minimum power level.

You could even remove that completely to give the main pot a wider control range; or if you can get it low enough on the preset with the main pot unplugged, changing to a higher value main pot may give the range you need.

You cannot cascade two units, or not with any functional effect; it may damage the second one.

Click to expand...

I wouldn't know which way to adjust it or whether I was raising or lowering the output.would one need a multimeter to do this?your talking about the blue Square thing with the tiny brass knob on top right?how much would it need to be adjusted?
A quarter of a turn, a full turn, 5 turns?

Yes, the blue one.
Try it just a couple of turns and see if that makes the minimum setting hotter or cooler.

Then take it all the way in whichever direction is cooler - 15 turns or so to be sure, or until you hear it faintly clicking as you turn it.
That will be the minimum you can get without changing parts.

If that's not low enough you could remove the blue preset completely... Desolder it or just twist it back & forth until the connections break.

The basic circuit is capable of being set to zero output, if the appropriate value pot and/or preset are fitted; what it's capable of as it is depends on the parts the manufacturer chose.

Be very careful as the whole circuit is "live" when power is connected, including the control pot wiring. Don't touch it unless it is totally isolated.

No way I'd touch it while it's plugged in.

I dont think daisy chaining them would work.
It is possible to build a phase angle control circuit that regulates down to a low level, if it is phase angle control.
I can think of 2 reasons why they did that, 1 to reduce interference, and 2 to make the control pot have a better control range, instead of getting all the power at the last little bit of rotation.
You'd probably need to redesign the e;ectronics, or use a different controller.

dr pepper said:

You'd probably need to redesign the e;ectronics, or use a different controller.

Click to expand...

They are completely normal, minimal triac + diac type "light dimmer" controllers, the circuit is obvious from various photos on ebay.

The only limiting factor is how high the resistance of the control pot can be set, for the lowest power output level.

It only uses really two wires from the pot and the multiturn preset is connected straight across those, providing an adjustable low-point setting.
Not knowing how the preset is set up when they are sent out, it's a bit of trial and error to see if if has sufficient range to adjust to near zero, or if a different pot is needed.

Depends on how low the op wants the o/p, a 20v diac is going to set the min level, multiplied by any division ratio in the pot divider when its at min.

I've turned the preset variable resistor down as far as it will (until it started clicking)and while this reuced its output with the pot turned down to zero from heating the element to 190'c down to 125'c it definitely can't be set to zero output at all.but now it's useable where it wasn't before.im now trying out a 2nd thyristor circuit from a different company and it seems to have better control than the first one.will test this one out and see how it goes.

The two thyristor circuits I have are the same component wise but have different potentiometers as one is sitting on the PCB and one is a corded one so the pot can be installed away from the PCB.theyre both 500k.pics of the two below.im just wondering and I don't think it should but does this make any difference to how fine the temperature control is?

What is the resistance of the heating element and what is the supply voltage ? What power is required for the hearing application ?
Les.

It won't make any difference if they are the same values, or not more than a few percent due to tolerances; it's just whichever mounting style is most convenient.

As I said previously, removing the blue preset pot completely would allow it to go lower still, if that is needed.

Is the preset pot a variable resistor of some sort? So removing the preset pot entirely and not putting anything in its place will somehow make it be able to have less output? That doesn't sound right to me but im no expert.

Don't know the resistance of the element is,supply voltage is 240v
The whole element at its maximum is rated at 400w.the second thyristor circuit I tried seems to have better control and when turned down to zero the element sits at about 80'c.only problem is it's pot isn't corded so it can't be installed inside the unit

draculic acid69 said:

Is the preset pot a variable resistor of some sort? So removing the preset pot entirely and not putting anything in its place will somehow make it be able to have less output? That doesn't sound right to me but im no expert.

Click to expand...

Yes, exactly.

In principle, the lower the pot resistance, the more current through it to charge the capacitor used for timing and the sooner in each half cycle the triac fires.

The blue preset pot is connected directly across the main control pot, so there is always some current through that, limiting how low the "minimum" point can be.

This is the basic "light dimmer" circuit they have copied, it's been a common design for decades. The component labels will be different, but that's it. These PCBs have the extra blue pot in parallel with the main pot.

I've just found a photo of the PCB underside again on ebay - you can see the preset pot terminals connect straight across the main pot connections. Removing the preset means the main pot is the only current path in the control circuit.

I've just put the corded potentiometer on the 2nd thyristor circuit that I got to sit at 80'c and with the corded potentiometer its new minimum temp when set to zero is 140'c so it seems to me that the source of variance is the potentiometers either due to one being cheap and inferior or to save money it's not 500k but is labelled as such

Could it be just a quality issue? The one with the better resistance has a logo on the back and the crappier one doesn't have any brand markings.is this a possibility?