Electrical – Failing to understand and to fix a PWM spindle controller circuit

component-failurepowerpwm

My CNC controller worked for many hours without trouble. But recently the resistor R5 (red circle) started to melt each time I switch my CNC spindle on, and even at low RPMs. It is 100 ohms, compared to the large 2 ohms R3 and R4, so why does it drive so much current? And without blowing the optocoupler?

The schematics below was reverse engineered, so it may have some little errors, but I checked that it matches the circuit I am trying to fix.

Also, I do not need the STOP signal at the bottom (any clue what is it for btw? PWM feedback?). So removed R5 completely with the idea I would "remove" the loop in the bottom right of the schematics, but it then prevents the spindle from turning at all. How can it be?

Other hints:

  • The spindle power supply line reads about 60VDC instead of stated 48V.
  • the spindle resistance is about 2 ohms, the fuse never melts
  • Initially, I realized that R4 was not soldered properly but I fixed it before changing R5 again. No luck, R5 burns again after a few seconds.
  • I checked the PWM signal with a scope (OK), the MOSFET gate-source (Q1), the flyback diode (D4), the AL817 opto diode (U3), and they all seem OK.
  • The C9 capacitor is larger than stated on the schematics (1000uF in place of 100). It is also OK, but the polarity leaves me puzzled.

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Best Answer

Well you have what looks like 48 V DC being produced by rectifying the AC36V input supply. This connects to your spindle motor(?) and it might take a big current at start up thus several tens of volts may be forced to the top of R5 (100 ohms). R5 feeds an opto diode that is grounded and so you might see 20 or 30 volts for several seconds across R5. This will dissipate a power of 4 watts to 9 watts for some short period of time while the spindle device is running up to speed.

If you are using a surface mount resistor, it will likely melt. It seems to me that 100 ohms is too low as it will also force up to 300 mA through the EL817 opto and this is rated for 60 mA absolute maximum.

These numbers I have used might be out by a factor of two or three but I definitely smell a problem with R5 being underpowered and too low in value. 1 kohm might be more appropriate.

Also, I do not need the STOP signal at the bottom (any clue what is it for btw? PWM feedback?)

No, because I have no idea what you have reverse engineered.

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