# Trigger 2 triacs in parallel simultaneously

I am using a STM TPDV1240RG triac to control an AC load. This triac is rated for 40A. The current in my circuit is up to 80A so I was thinking of putting 2 triacs in parallel to share the load.

I am using a MOC3063 with DC pulse to turn on and off the triacs. The problem is that only one of the triacs is conducting (whichever is first to turn on). Is it possible to get them both to fire simultaneously?

Note: I will also be using heatsinks on each or perhaps one between the two if suitable.

Thanks.

• Any schematic do you have?
– CNA
Mar 30 '18 at 10:37
• @Dhans Things as schematics he might have, Yoda. Mar 30 '18 at 11:23

Paralleling triacs is a bad idea.

First, the one that latches faster will collapse the voltage, possibly preventing the slightly slower triac from ever turning on.

Second, even if both triacs turn on, they won't share the current well. Triacs are sandwiches of effectively bipolar devices and silicon junctions. These have lower forward voltage at higher temperature. The one that takes a bit more current will get warmer, which lowers its voltage, which causes it to take a larger fraction of the current, which makes it even warmer, etc.

Find something rated for 80 A to switch the load.

• How about adding inductors before the inputs of both traic so that the voltage does not collapse as fast and the slower triac gets some time to turn on? Mar 30 '18 at 10:55
• @ Oilin Lathrop. That's the problem, there is not much rated above 40A unless I use a solid state relay but wanted to use triacs. Mar 30 '18 at 10:56
• @ Rupesh Routray, does this ensure even turn on between both devices? Mar 30 '18 at 10:57
• @RupeshRoutray That won't help. The only solution, that is still very bad, would be to put a 10$-$100 mΩ resistor in series with each triac. This way the resistors will automatically balance everything, since they have a positive resistance coefficient. Mar 30 '18 at 11:18
• @MXG123 Don't know for sure if it can ensure even turn on, but both will turn on, with a slight delay between them and it will also ensure even current sharing between them. Mar 30 '18 at 11:20

To handle 80A with 40A triac you can try period-interleaving control of them. It will spread heat dissipation between them.

Better idea is to switch to two 80A SCR BT155W(or BT158W) in inverse parallel configuration.

Like this one:

Use one of the following circuit. Replace SCR with Triac.

First method is simple. You need two resistor capable to handle 40Amps. But with that much current, there will be too much waste heat.

For Dynamic sharing, you need two magnetically coupled inductor. i.e Both Inductor coils are wound on the same core. An isolation transformer (1:1 voltage ratio) capable to handle 40Amps through its winding will do the job.

Be careful about the polarity of the two coils though. You want them to be opposite to one another.

Let's say, when you apply the gate pulse, traic T2 turns on first. As current flows through the coil and T2, a voltage of reverse polarity is induced in the other coil connected to T1. As the voltage is of oppostie polarity, it adds up to the supply voltage. So the voltage across T1 becomes supply voltage + voltage induced in the coil, which is enough to turn it on.

• Interesting circuit configuration. I don't have an isolation transformer to hand to test. There would also be some wasted power across the resistors here to. Apr 3 '18 at 15:28
• Resistors are not necessary. Impedance of the coils are enough to limit current. Apr 3 '18 at 17:45
• Why would you want to limit the current? Its an ac switch. Apr 4 '18 at 7:11
• So that one Triac doesn't carry the entire current and burn itself. Apr 5 '18 at 15:39

I fully agree with @ Oilin Lathrop's response and comments. I can suggest the use of the product TRIAC BTA100 Series. In this case, a single piece will be able to control all the current demanded by the load.