Triac in parallel with a Relay problem

parallel triac

Hi all,

I made a device to measure current in used by a 220V device.
It uses a Relay to switch on the device and a hall effect sensor in the same circuit measures the current. It works fine.
I added a Triac in parallel with the relay to implement zero cross on / off switching, so I turn the triac on for 200 msec before the relay and switch it off 200 msec after I set the relay on. As I could measure the waveform showed that it switched on at zero cross.

After a couple of weeks of use with ~500mA load the Relay failed. It stuck in ON state.
On another pcb I disassembled the relay in it and turned on the Triac all the time and toggled the Relay 2sec ON / 2sec OFF.

Suprisingly (for me) when it switches on and off it sparks! WHY?
I could measure just 2-3 volts difference between the relay's 2 pole while all the current flow through the triac and the relay was open.

Why is this?

thanks,
Zedman

paralleling triacs

I figured out that if I turn on the triac first it's ok, turns on @zero cross, but when the relay hits ON state it may get all the current from the triac and the triac will not have enough current to stay open, altough I feed it's gate constantly.

The triac is a MAC8SNG.

Is it true?

problem triac

Using a relay parallel with Triac will off the triac, this is obvious because for turning the triac off we used the the force commutation in which we short the triac terminals. your circuit is doing the same, mean you are using the force commutation.

triac not opening

I think I don't understand clearly, we do not short the triac's terminals in any way.
It's ok there will be no voltage difference between the terminals of it when the relay is on.

Added after 6 minutes:

Is there any way to keep the Triac ON to prevent the relay sparks?

zero cross switching triac

The way you've drawn it, the relay does short the triac's A1/A2 and therefore it can not fire anymore even if you drive the gate constantly. As I understand triac-output optocouplers, Rt1 should be superfluous, BTW.

If you want to add a zero cross start to the circuit you'd need to have the triac in series with the relay contact/load (and switch the relay first, then the triac).

The sparks you noticed in your experiment without switching the relay are probably due to your load not being resistive. A 250V (or 140V if you use 110V mains) VDR in parallel with your load should suppress those.

sample problem for triac

Yes you all right on shorting the terminals, I was wrong.

I'll try to put on a VDR.

zero cross detector opto triac

Sparks will occur natuarlly when the conatcts are smaller compared to the supplied load in proportion to the voltage. Thatz why the last relay stuck in ON position as the load was over its capacity of the contact ratings. You may try snubbers to reduce sparks after you have used a better contact relay with more capacity.

is it possible to parallel triacs

You're apparently using a opto triac with zero-crossing detector. I doubt, if it can work correctly in combination with a bouncing relay.
You may want to check the behaviour with a non zero-crossing detector opto triac.

triac paralel

I always use anopto diac for triacs. i have controlled 4600 lamps i a display. and are working properly. having no problem.
but i dont understand why you are using the relay.

difference beteween relay and triac

Huh,

forum engine failed so my last posts are gone.

So:

Pranam77: It's a Tyco PE014A05 Relay with 5A contacts, and it switched only 500mA.

FvM:

I put in the triac just for the zero cross switching feature. Why it doesn't work with a relay in parallel?

mfarooq1:

The relay is used because it's bistable -> no power used to hold it on. Even the triac need some mA. Another thing is the heat the Triac generates @>1A.

Added after 2 minutes:

I put on a 275V / 4500A MOV and an RC from a 0.1 uF X1 cap and a 47ohm / 2W resistor. Now it does not spark even when switching @3A.

Does the resistor need to be > 1W?

thanks
Zedman

triac terminals

I put in the triac just for the zero cross switching feature. Why it doesn't work with a relay in parallel?

Click to expand...

I guess, that you get fast rising voltage during relay bounce. The zero crossing detector possibly prevents triac triggering in this situation.

Does the resistor need to be > 1W?

Click to expand...

Continuous power rating isn't an issue, but the resistor should also withstand the pulse load when switching at voltage maximum.