Triac in parallel with a relay

[zedman]

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 or 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.

Why is this?

thanks,
Zedman

 

[crutschow]

It may be that the relay closed-contacts keeps the triac from turning on (all the current is going through the relay contacts so there is not voltage across the triac or its gate). Then when the relay opens, the triac can not turn on fast enough to absorb the current and the contacts have to break all the current.

You might measure the voltage across the relay contacts when it opens, if you can, to see what is looks like.

 

[zedman]

Yeah, it's possible that the triac wont turn on while the Relay is on. That's sad.
But why does it spark when turns on? The triac IS on, I tried it with a hairdryer (>1A) and it was on but when the relay switched on I've seen sparking.

Any idea how to eliminate this?

thanks,
Zedman