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Can a TRIAC be used on the secondary winding of an AC isolation transformer?

Say I have 230 V on the primary, but I want to use the 12 V AC secondary, will a TRIAC work on the secondary?

What I want to achieve is this: Use the TRIAC on a secondary winding (say 12-24 VAC) to use on a spot welder. The control pulse to the gate of the TRIAC will be delivered via two optocouplers (one for the positive cycle and one for the negative cycle) of the AC wave. The optocouplers will be controlled by a single-shot (mono-stable) pulse from a 555.

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  • \$\begingroup\$ @Tut - Question is, do I want random phase or zero-crossing/ \$\endgroup\$
    – fizzy drink
    Commented Dec 15, 2015 at 20:27
  • \$\begingroup\$ @Tut - Nevermind about the random phase. If I use the zero-crossing, will there be a delay between the time I hit my momentary switch, trigger the 555 pulse which triggers the opto-coupler? Will this "wait" until the zero-cross moment? \$\endgroup\$
    – fizzy drink
    Commented Dec 15, 2015 at 20:29

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A simple answer is "yes you can". The better answer is "you don't have to", because in your situation, it is better to use a triac control on the primary side. I have seen many special heating devices that use this approach, the current is smaller compared to the secondary, so you will not have trouble with short circuits where the major resistance would be your triac, thus the spot welder will heat mostly your triac and not the spot. Forget about using a zero cross detector circuit, the worst case for a transformer is to turn it on at 0V and don't use the 1.9T toroidal transformer (this is what you'll get in a shop for hallogen lamp), a use transformer designed with max. flux 1.6 Tesla or maybe less in order to keep dI/dt manageable.

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  • \$\begingroup\$ Ok, so using a regular E I core 1Tesla iron would work ok? \$\endgroup\$
    – fizzy drink
    Commented Dec 15, 2015 at 21:08
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    \$\begingroup\$ You won't get the standard EI core wonded at 1T, but 1.6T and that is OK. When you will turn on the xformer, you will get a typical in-rush current phenomena, the worst case is when secondary is left open and xformer is turned on at zero cross. With use of cheap, undersized xformer core it will saturate and your fuses will blow. With regular EI buit transformer you won't have any troubles. For driving xformer with triac it must be a random turn on type. fairchildsemi.com/application-notes/AN/AN-3003.pdf figure 3, or similar opto triac circuit for inductive load. \$\endgroup\$
    – Marko Buršič
    Commented Dec 15, 2015 at 21:27
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Yes.

Remember that there is a voltage drop across the triac (check the data sheets) and that is a more significant fraction of 12 V than it is of 230 V.

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Some details missing in your question, but I will try to answer in general.
A TRIAC operates in an AC circuit, provided that the particular conditions of a thyristor are met, that is, not exceed the maximum working voltage and ensure minimal holding-current.

You must not forget that to synchronize the switching of the TRIAC, you must take into account the zero crossing of the winding that feeds the circuit of the TRIAC.

If you can ensure this operations conditions, the TRIAC will function in the secondary of an isolation transformer.

Edit: zero-crossing & pulse

Synchronization with the zero crossings of the line, allows you to control the power applied to the load. Basically it is turning on the TRIAC after a certain time after the line voltage zero crossing. There are many methods to detect the zero crossing and to generate the shot. It would be very extensive to list them all here.

Here you can access information about phase control using a TRIAC. Here you will find information to turn a TRIAC completely in sync, without power control.

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  • \$\begingroup\$ How do I synchronize the switching? \$\endgroup\$
    – fizzy drink
    Commented Dec 15, 2015 at 19:58
  • \$\begingroup\$ I have updated my answer. Please check the links. \$\endgroup\$
    – Martin Petrei
    Commented Dec 15, 2015 at 22:19

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