Selecting electronic components.

[Mcgyver]

I fixed a Biax by replacing what I assumed must be a triac in the speed controller. everything was painted over so metal chips couldn't short things. As a result I couldn't identify the exact component.

It worked for a few hours, then the triac popped. Motor is 120V @ 3amps, and I chose a BT136 which is a 400V 4A triac (happened to have one, and I know peaks but its peak was like 25A). Just ordering some 400V 10 amp triacs to try again.

My question is, why so many triacs, transistors and FETs? Why wouldn't one always, assuming package was the same, go grossly oversized so it wasn't overtaxed and would through thick and thin. Is there some trade off - some reason why a higher rated won't work as well? Maybe price but that is insignificant for one.

Just curios and interested in advance my component selection knowledge.

Thanks

 

[Gennady]

I believe it is rhetorical question, isn't it ?
Seriously - FETs have a hundreds of parameters and, as usual, you cannot be winning one every one - voltage, amperage, capacitance of the gate, etc... To have better timing to turn it on or off you need small capacitance - small surface. That will lead to small amperage trough drain-source, It is always a compromise and trade-off.

 

[slow-poke]

Without seeing/understanding the circuit it's not possible to answer your question. In many cases (neglecting cost). Just use a bit "bigger" one is a reasonable approach for a repair. Triacs come in many flavours with different on/off characteristics relating to the quadrant of the cycle they are switching in. Some wait for zero-cross to turn on, and they all don't turn off while conducting so need to wait for zero cross to turn off. However that zero cross gets complicated with inductive loads because the current zero cross lags the voltage zero cross.

 

[djberta]

There are a lot of variables with semi conductors. Not just voltage and current. Time on off is one to think about for this kind of circuit. The lists are endless because the variables are as well.

One thing in your situation, did you ascertain why the first chip failed? If the condition that damaged the first chip is still there and has now damaged a second chip you may run in to it again. Or it may now cause worse damage if you change component size.

You mentioned that the board was painted, so likely hard to see any damage visually. Just spit balling. Maybe with the new chip install see if you can watch its heat rise when operating. Possibly add a bigger heat sink and fan. Maybe look at building more protection for the chip in circuit.

 

[Susquatch]

Agree with other comments but focussing in on other possible answers to your question.

Current sharing might have also been a design goal with them all setup with different trigger points so they cascade on smoothly as the voltage ramps up. It isn't as simple as a bigger wattage resistor to replace multiple smaller ones.