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Re: Triac Power Control

You wrote:

If I use only resistive ballast (heater elements) to limit my 5KW pig,
 couldn't I also use a big Triac for power control? 
 SNIP


Hi Greg,

Iīm sure you could control your pole pig via SCRīS. I built myself an AC/DC
TIG welder setup using 4 SCRīs. The AC part is of interest to you. I use two
SCRīs connected antiparallel. The SCRīs are triggered via two small
transformers (+ a diode and a cap). These trigger transfomers are hooked up to
a phase angle cut in controller. You can vary the output current very nicely.
The SCRīs have a nice high dv/dt time so the pulses are really hard. Two
antiparallel SCRīs will give you a nice sine shaped wave, unlike TRIAC stuff.
My SCRīs are small hockey puck type with a continues current rating of 340 A
(enough for any TC pig) and can stand a 7200A pulse -at- 10ms with ease. Oven
elements would help to dissapate the power, but arenīt really necessary as
long as you limit the current via the phase angle control. (Read: add a
resistor to the control potentiometer, so that you can only run the max
current your house fuse will hold)
The TO3 type you mention wonīt last long in a pig circ running at high power,
because the TO3 case wonīt be able to dissapate the heat fast enough even on a
big sink. If you use these you would have to connect several in parallel and
if one goes they all go. My welding SCRīs are mounted on a stainless steel
heat sink and are water cooled.

There are two things you WILL have to watch out for:

1.) You must decouple the SCRīs with a cap & res array or they might trigger
at the wrong time.

2.) You must be absolutely certain that no RF can wander back to the SCR or it
will die shortly. My welder circ uses one MOV per SCR to prevent this and has
a power RFchoke between the HF ignition circ and the SCR bridge.
 
I have included a pic of the SCR and safety board and RF choke so you can have
a look.

coiler greets from Germany,
Reinhard 


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