I have "repaired" a cnc power nad speed control board from yoocnc (china) and it worked fine for a day. the next day the spindle just started running as power was applied. the mosfet was shorted, the pwm signal is fine.
Schematic here: https://maher-ramblings.blogspot.no/2014/08/cnc-engraver-schematics-yoocnc-nt65-3x.html
On this board I have shorted R3 and R4 and removed R5. R4+3 was suprisingly in the 8Kohm range and must have blown R5 and the optocoupler, but they are not in use so I just dissabled it.
Initially the ne555 also didnt work and was replaced, along with the Q1(IRF740), I didnt have a IRF740 but i put in a IRZL44 as thats what i had and it should tolerate 12vg-s.
I also cannot see the purpose for D3 diode, so any enlightenment there would also be appreciated.
Best Answer
R3 and R4 are from a current threshold warning. When current trough the shunt (R3+R4) rises above the threshold of U3 the controller gets a STOP signal.
For example, when the spindle is jammed.
With this circuit bypassed, you can dump the full transformer current into the spindle and FET risking damage to both.
Here are some ways you can destroy this FET:
- Slow gate. if the 555 gate drive is too slow, or not high enough, it results in the FET being in the linear region too long until it overheats.
- Back-EMF from the spindle should go through D4 and be dissipated. If D4 is open, high transients will occur and the FET will be damaged.
D3 prevents the gate voltage from going above 18Vac rectified + Vforward, if that supply is available.
The IRF740 is a 400 Vds n-channel. Yours is an IRFZ44, with only 60V. For a circuit running at rectified 36Vac (50Vdc) that is very close.
Even with D4, you'll still be looking at transients. Since diodes take a small amount of time to conduct.
This would mean you most likely have burnt it by back-EMF or gate voltage.
Vgs is not allowed to be higher or lower than 20V or 12V from ground. (in this circuit) D3 clamps the gate voltage at rectified 18V with diode drop. This will be around 25V. Beyond maximum ratings.
My suggestion would be to replace it for a >200 Vds fet with Vgs equal or better than +-20V, and protect the gate with a 15V zener diode.