Electronics > Beginners
Protection circuitry for opto-triacs
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Zero999:

--- Quote from: Circlotron on June 07, 2018, 11:51:46 am ---
--- Quote from: vector on June 06, 2018, 07:50:33 pm ---The MOV is supposed to protect the opto-triac from the voltage spikes due to the relay coil,

--- End quote ---
When the LED drive stops, the triac will continue conducting until the relay coil current pases through zero. That being the case, there will be no energy left in the relay coil and consequently no voltage spike. A small RC snubber across the triac to slow down the sudden jump in voltage as the triac turns off might be nice though. Others can advise of the value of R and C.

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One of the problems is it's important to know the inductance and resistance of coil being switched, which is often unknown. If you know the inductance and resistance, then all that's needed is a capacitor and series resistor to make the RLC circuit critically damped, so there's no overshoot in voltage.
https://en.wikipedia.org/wiki/RLC_circuit
vector:

--- Quote from: Hero999 on June 07, 2018, 08:39:58 am ---Yes, the opto-coupler is not designed to switch the load directly. Interestingly the MOC3063 data sheet doesn't specify a continuous current rating. It only gives a peak rating of 1A for 100μs, at a repeat rate of 120pps (120Hz). Presumably this is just long enough to trigger a TRIAC.  The output stage is only rated to dissipate 150mW and the peak on state voltage is 3V at 100mA, which implies that the continuous current rating will be much lower.
http://www.mouser.com/ds/2/348/rb168l150-774152.pdf

The Z0103 appears to be suitable for driving the relays.
http://www.st.com/content/ccc/resource/technical/document/datasheet/af/ee/c1/7b/df/8a/46/d7/CD00002268.pdf/files/CD00002268.pdf/jcr:content/translations/en.CD00002268.pdf

A PTC resistor, i.e. a resettable fuse which, should be designed not to fail short, but open circuit, if it conforms to the relevant safety standards, which in my experience is what happens when the component reaches the end of its lifespan.

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Ok, thanks. I have to be careful when reading the datasheets :D



--- Quote from: T3sl4co1l on June 07, 2018, 09:28:39 am ---It's not like you're going to protect anything, even though it's a TRIAC, I doubt it's anywhere near capable of fusing even a fast-blow type.  Let alone a PTC that takes an eternity to go open (~100ms).

On the upside, AFAIK, high voltage PTCs are ceramic based types, much higher reliability than polymer types.  Downside: R25 and resistance ratio suck.

Tim

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--- Quote from: Hero999 on June 07, 2018, 10:10:50 am ---Yes, a PTC resistor is no good at protecting semiconductors such as TRIACs. To be honest, I didn't even consider that the original power was intending to to that.

A PTC resistor is only there to protect the cable from melting down, causing an even bigger short circuit and fire.

An alternative to using a really small fuse, is to use a much larger TRIAC, with a much higher fusing than a fast blow cartridge fuse. The downsides are more cost and board space.

Another thing to watch out for is the breaking capacity of the fuse, which is the maximum current it can safely interrupt, without blowing up, overheating or arcing over. It needs to be higher than the maximum short circuit current in the circuit, which is only limited by the impedance of the wiring. A small, possibly fusible resistor could be used to limit the short circuit current, as long as it's a much lower value, than the load impedance.

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Well, the point of the PTC fuse would have been more a protection in case of shorts to avoid fires... even if it blows the optocoupler, I can always change it :D (probably I was not clear in the beginning about why I put the PTC fuses)



--- Quote from: Circlotron on June 07, 2018, 11:51:46 am ---When the LED drive stops, the triac will continue conducting until the relay coil current pases through zero. That being the case, there will be no energy left in the relay coil and consequently no voltage spike. A small RC snubber across the triac to slow down the sudden jump in voltage as the triac turns off might be nice though. Others can advise of the value of R and C.

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--- Quote from: Hero999 on June 07, 2018, 12:36:48 pm ---One of the problems is it's important to know the inductance and resistance of coil being switched, which is often unknown. If you know the inductance and resistance, then all that's needed is a capacitor and series resistor to make the RLC circuit critically damped, so there's no overshoot in voltage.
https://en.wikipedia.org/wiki/RLC_circuit

--- End quote ---

Ok, goot to know. I will look at the specifications of the relay to see if I find anything useful.
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