Hi Forum,
Refer to trailing edge dimmer schematic: https://sound-au.com/project157a.htm , I have a question: How can I do the short-circuit protection for back-to-back N Mosfet?
Thanks.

 

I may be getting confused with the term back-to-back. Is that in reference to the two MOSFETs (Q10 and Q20) in this circuit below?

Short circuit protection, the simple answer is to use a fuse, which is already there. A better option might be to sense the peak current in Neutral using a resistor or maybe better, a Hall sensor and when the peak current gets larger than a predetermined value, pull down the reset pin (pin 4) of the NE555.

 

That's not easy to do.
A fast, instrument-type fuse may work but, as experience has shown, the transistor is often faster than the fuse.

Otherwise you would need a circuit to monitor the current (typically using a small shunt resistor in series with the load) to turn off the MOSFETs when the current becomes excessive.
Not sure that can be easily done with a circuit controlling AC.

 

Hello,

@DickCappels , did you read the text as the Q1 and Q2 have a remark sign, that calls to refer to the text.
Also the back to back mosfets are also used in solid state relays as the AQV21:

Bertus

 

Thank you. I did not see the sign. Is the schematic in post #2 the correct one?

 

Hello Dick,

Is the schematic in post #2 the correct one?

It is upto the TS wich of the two schematics on the page is used.
He did not mention wich is used.

Bertus

 

Sorry about my late reply. @DickCappels, Yes it is Q1, Q2 in your picture.
I also agree that MOSFET will be faster than fuse, so that I will do the simulate using the R-shunt, and comparator solution

 

@crutschow,
I did the simulation and see that's good.
Because of the bidirectional current of R-shunt, I used LM324 to amplify the voltage of R-shunt and add to 1/2*VCC. Then using window comparator with latch output to active the 2N3904 to drive the gate of MOSFETs to Ground, if the voltage of R-shunt is outside of window limit.
Here is the picture and the LTspice file. Do you think is it ok?
Thanks