Triac control of a power tranformer

[cowboybob]

I know it doesn't work. (Sorry for misspelling transformer)

I've already (mistakenly, and without enough study) tried a triac (which instantly shorted). So I tried another triac with a snubber circuit (in several different configurations). That too failed to protect the triac against the off phase current.

I've been at it for about 2 months now. Googled myself blind. And I've gotten nowhere.

I've seen that there are some very experienced and bright members on ETO and I consider you all THE source.

So, I'm asking (begging, more like it) for a solid-state solution to controlling the input (or the output) of a 110VAC to 6VAC (high current, 10-20 amps) transformer with a very low level (1 - 5 volt) rapidly changing AC or DC signal. This control might span as long as 5 to 15 minutes per session.

A opto-isolated mechanical relay I don't think will work due to the resultant arcing damage to the relay points.

And if not a solution, then maybe some thoughts on the problem.

 

[KeepItSimpleStupid]

We used to do that all the time until I put a stop to it and used a DC power supply. Our heaters were about 40 V, 10 Amps or so. We had analog voltmeter, ammeters and a variac. the is would fit on a 4RU panel. I said, enough is enough and used a 1RU DC power supply. These took up less rack space and had integral voltage and current meters. Another positive side effect of this change was that our in house wound tantalum heaters lasted much longer.

You need four things for it to work: 1) Phase angle firing and 2) (Very important) Current limiting and 3) An overrated back to back SCR's or Triac and 4) slowly ramp up the voltage.

Back to back SCR's are a better configuration than a triac. I forget the reason, but there is a good reason to fire both at the same time.

We used triacs rated for 25 Amps, and I^2t fuse rated to to protect the triac and a 3AG fuse (typically 10 Amps) so that we would not blow the 25 A fuse. This worked VERY well. The I^2t fuse was $25 each. This drove a 120 Variac and was stepped down to 40 V or so.

See: https://www.electro-tech-online.com/custompdfs/2012/02/1008.pdf

You have to keep firing the SCR/Triac once you turn it on until the voltage is zero. The SCR will turn off when the current is zero or the voltage is zero, I believe. This is the major difference between a light dimmer and a motor "dimmer". e.e. Fan controller.

Something like: https://www.nuwaveproducts.com/nwupcm-1p.html We used Eurotherm controllers. You can't beat them.

How is that?

PS: i should also note that we had controllers that would control a 3V, 200 A secondary from a 208 V primary transformer for another application.

 

[cowboybob]

Thanks, KISS for the response.

From the info I saw, this is also considered a Triac? If you search Jameco for "SCR" they have it listed in the Triac section. And the symbol is that of a triac, even in my SIM. Although the datasheet (Motorola, in this case) clearly states that they can be used for motor control, I took that to mean inductive control, i.e., a transformer as well. Am I right?

I guess my question is, can I, by simply applying a sufficiently high DC signal to the gate (they only list a peak gate voltage), can I turn on and off the transformer. And can I do so at frequencies of, say, 100 to 10KHz?

I know that at those frequencies the output voltages of the transformer will never reach 100%, but that's fine. I'm actually looking for a variable pulsed effect.

lastly, I understood the difference between "zero" and "phase" firing (I'm pretty sure, anyway), but I was majorly unsure how to sync up the phase firing correctly.

Incidentally, I'm controlling a string of parallel wired 6V lights that are in a system I can't convert to a DC source.

 

[KeepItSimpleStupid]

Traics and Bandaids. An SCR is a unidirectional triac and a triac is a bidirectional SCR. The same class, only different.

SCR's and Triacs are current controlled, but watch the quadrant issues. It will tell you the polarities required. There is also sometimes a difference in the parameters as well.

Back to basics for the time being:
1. Suppose you had a DC source connected to a load and you 1) interrupt the load ad 2) you short the SCR. In BOTH cases the SCR will turn off, right?

So you now have this Ac thing which has voltage and current out of phase by something. So you turn it on at zero voltage and a few ms later it turns off because the voltage is 50, but the current is zero. You can't have that.

So, you detect a zero cross and start providing continous pulses to the gate. The off time should be at least the turn off time or more. The on time at least the turn-on time or more.

Now another voltage zero cross comes around, and then you turn off your pulses.

You need some sort of table to convert phase angle to voltage UNLESS your doing power control.

Start-up is a little different, because you have to start gradually applying power. Let's say start at 0.5% of full voltage.
At every half cycle you can increment the voltage paying attention to the current. If it is exceeded, you have to turn off the drive.

You maybe able to immediately turn off the drive if some peak current is exceeded.

The other complicated issue is to determine what is the polarity of the gate current and the quadrant your in. That's probably whay back to back SCR's are better. You CAN PROBABLY trigger both SCR's simultaneously, but only one will respond.

You are really only concerned about each half cycle, not full cycle. Does taht make any sense?

Your not going to get a variable pulsed efect at the high frequencies, only the low ones. You will still turn on the device, but you cannot wait for it to turn off by itself because it turns off at two places (0 voltage and 0 current).

You can easily see why a pulse less than the turn-on time is pointless and an off time > than the turn-off time is pointless. I'd start with 2x those values. Then try to figure out what a minimum increment should be.

Lamps make things tricky, because the cold resistance is 15x less than the hot resistance. So, not you can see the benefits for overrating the triac by bunches. If you current limit, you don't have to over rate by much.

Triacs can turn on spontaneously if dv/dt is exceeded, hence the need for a snubber.

Hopefullly, you can see all of the things getting in the way.
1. Inductive load (continuously pulse as work-around)
2. Cold resistances (slow start, current limit as a work around)
3. Polarity of trigger signal (trigger transformers, back to back SCR's as a work around)

Work with half-cycles, not full cycles.
Detect zero voltage crossing cycles.
You need a table of rms voltage vs phase angle.
You need to measure current.

Further complications if needed.
1) Voltage compensation
2) Actual power control.

This device in industry might be a 0-10 V, 0-20 mA or 4-20 mA controlled device with 0-20 mA being the most common.

The Eurotherm power modules implemented current limiting in the modules.

The earlier power modules were two transformer triggered SCR units. The latter versions used industry standard signals of 0-10 and 0-20 mA.

Take a look here: https://en.wikipedia.org/wiki/TRIAC and pay particular attention to "triggering Modes"

They don't do as good of a job here: https://en.wikipedia.org/wiki/Silicon-controlled_rectifier, but it's in the details.

You can easily reverse the polarity of the gate by reversing the leads of the trigger transformer.

 

[cowboybob]

Thank you again, KISS.

It'll take me awhile to absorb this.

I'll get back to ya.

CBB