Scr firing

Hi, R. The easiest, most straightforward way to go is to purchase a solid state relay (SSR) that will handle your problem. SSRs have the optocoupler and thyristor built in. Heating lamps generally provide a more or less resistive power factor, so I'm not sure why you seem to be focused on inverse-parallel SCRs. For most loads, a triac-based SSR should do well.

A couple of caveats here. First, to be safe, think 2 watts power dissipation in the SSR per amp of load current. Purchase a heat sink accordingly, use thermal heat sink compound to mate the SSR to the surface of the heat sink, and use forced air where appropriate. Make sure to purchase a SSR which is "random turn-on". *DO NOT* get one which is "zero crossing", unless your control algorithm takes that into account.

Apart from that, you might get better advice if you described your problem a little better. For instance, what kind of heating lamps do you have, are you shooting for phase control, integral half cycle control, or proportioning on/off control of your load, and of course, what's the load power, current, and voltage?

Note also that, if this is a school project, a SSR probably won't cut it -- you'll have to "show your work" rather than just buying a module.

Feel free to post again.

Cheers Chris

Why do you want to do this? What would define a successful project? Why not an off the shelf solution?

For SCR controlled AC, you need to sense zero crossing, and add a delay for phase modulation. For slow response like a heater, you can use PWM techniques, but only if the PWM frequency is much lower than the AC line. In other words, you just turn on the power for a few seconds, and then turn it off for a few seconds. You probably don't want to use the PWM module for this.

Induction heating is something else altogether, using RF coils to heat a metal object. Heat lamps are more properly called radiant heating, using infrared to heat the object.

For antiparallel SCRs, you need two isolated gate drive circuits. Pulse transformers are the simplest and cheapest, but may not be as reliable as a well defined gate pulse circuit, which uses a narrow high current pulse followed by a "back porch" of gate current just over the minimum turn-on spec. You also should be careful to avoid gate current during the quadrant where the SCR has reverse voltage applied to it.

You can also use a single SCR connected to the + and - terminals of a full wave bridge, so you can use a single gate circuit. However, the diodes waste power and the SCR works twice as hard.

For any SCR gate drive, be sure the isolation is sufficient. An optoisolator like 4N25 has isolation voltage of over 4000 volts, which is just enough for a 480 VAC circuit. If you build gate drive power supplies, make sure the transformers are rated for the mains voltage you are controlling. If you use DC/DC converters, isolation voltages are often only

500V, which is just barely enough for 120 VAC. The isolation barrier must be designed to withstand high voltage transients on the power line without degradation. A breakdown will allow mains power to connect to your logic circuitry, with dangerous and catastrophic results.

These concerns are another reason to use a commercially available solid state relay.

Paul

Your two main choices,m ere are phase controlled power (fast smooth and electrically noisy) or burst fire integral half cycles (slower, simpler, lumpier and electrically quieter)

For quantity one, you best bet is an appropriate solid state relay. The line voltage isolation is included, both for the control signal and for the heat sink.

There are instantaneous turn on types, if you want to use a small transformer to provide a line frequency zero crossing reference for the micro, so you can delay the firing pulse with software, for phase control. There are zero crossing turn on types for burst pulse control, where you need only a slow PWM digital output to control the average power.

There are also units available that convert a variable DC control voltage or current into phase or burst pulse power control, if you have a DC control output available. The last kind are most expensive, but you will definitely spend more designing, building and debugging your own, if you are going to need only 1 or a few.

I have done PID loads of times over 25 years for motors, but its a similar principle for controlling heating. In all cases I used the mains as a reference and the output from the PID was scaled to provide a pulse of between 0 and 9ms. A triac was used in every project. I have to say that PID is not always to convert into a real project especially for a beginner. I found that often a bit of fine tuinin gwas required to get the critical damping just right. If its not right you will get hunting of the speed or the motor will grumble badly (not a problem with heat lamps)

Try also looking at:

Might give some ideas.

Jon