[...] The circuit I used is a standard 4 N channel MOSFETs driven from the output of a CMOS 4011 running at 12 volts, no resistors.
Nothing standard about that circuit, as it will never work correctly.
When I fired up the circuit the most voltage I can get across the motor is 8 volts from a 12 volt supply.
Be happy, that's about half a Volt more than should be expected with this configuration
The bottom MOSFETs appear to be switching completely on but the top 2, the ones where the drains go to Vcc, are not apparently.
Of course, N-ch. devices needs their gate AT LEAST 4.5V more positive than the source.
The proper device to use for the high side is a P-ch.
I just assumed the circuit would supply almost 12 v to the motor with 12 volts into the gates. This is the same circuit as in the Robot Bonnazas book. Yet they never highlighted the fact that that circuit would never supply the full 12v to the motor.
The circuit is rubbish. I am not familiar with the standards of Gordon McComb, but Myke Predko should know better!
- Using a weak "driver" like the 4011.
- No gate resistors, so the 4011 (which cannot deliver much current) will be brought to its knees (may even go into latch up) and will never be able to switch efficiently - i.e. inefficient and with high losses.
- No dead band, so same side low side and high side will be on concurrently for some time at every switch.
- Failure to realize the fact that N-ch. devices needs the gate abs. minimum 4.5V (with this device) more positive than the source.
- Use of way too slow protection diodes - but at least the MOSFETs will protect the diodes
- 100nF buffer on the power line will do absolutely no buffering at all.
- Unleashing this on beginners that will typically take the words of a "specialist book" as the Gospel on this matter, creating disappointment and lack of faith in their abilities.
An amazing list of errors in a third edition - that alone makes the book not worth the price IMO, even if you get it as a present (your time reading it is an investment as well as paying real money for it).
So now my wheels are turning alot slower. Since I used a high RPM motor than needed, this may or may not be a problem. Still need to investigate. So I assume the IRF540s are OK, that I would have the same problem with any N channel MOSFET. Maybe if I had built the H bridge which uses P and N channel I wouldn't have this problem.
Exactly (apart from the weak driver and the missing logic (to create a dead-band keeping same side L/R devices from conducting concurrently, shorting the supply).
You have two ways to get the full 12V.
1 - Use P-ch. devices on the high side (and invert the logic to keep the switching OK.
2 - Use a driver that outputs 17V..25V with the high side N-ch. devices.Remember to use ultrafast diodes
, whatever way you go.
Better still, get a complete driver...What are the metrics of your motor(s)?Voltage, current, RPM (or Kv), Locked Rotor current etc.