H-Bridge control DC motor

[Paul F]

Hello Steve,
You are correct. The Opto-isolators are grounded to the ouptut side. Error in drawing up the schematic. I am testing using 2 bench power supplies, 5v,1A and 30V,1A at the moment but I do take the point about the caps.

Scoping the circuit shows that the spikes on the supply lines are in line with the rising and falling edge of the driving PWM signal. Seems as though the mosfets within the L6203 when switching are causing my problem.

There is a fudge I've looked at today. If I ensure that the ADC does not do a read untill the pulse has driven the motor (1ms On, 9ms OFF) then I should not read any noise on the pot. Wait for the L to H then the H to L (1mS driving pulse), pause a few millisecs and then read, having 9mS to take a reading before the next 1mS driving pulse.
I sure this will sort out the ADC reading but I'm still unhappy as to why I have not worked out how to stop the H-Bridge from spiking the supplies.
Many thanks to all for advice so far. Much appreciated.
Cheers
Paul

[mister_e]

one thing can be done... take multiple ADC reading and look at them after. if one is very different from the other, discard them/him and do an average of the others.

Maximum current driving of your 30V supply may also be the cause. If your motor ask for more or close to 1A when you apply the power on, your power supply can fall in a kind of current protection wich really drop the voltage or cause some spike. Depending on how good he is. In general motor will ask more current on the few uSec the power is apply on. The spec of the motor will tell you the average needed current without any load to drive and while he's already running for a few mSec or Usec. That has to be consider. Safe when you supply is 2-3 time higher than the current motor need.

[nomada]

Hello all
I don't know if this get in time yet, but....
Paul I hope that your workbench power supply has two different transformers, one for each power rail otherwise you maybe inducing noise right from the transformer.
In a machine I've worked with which has a vertical platform and the position was obtained from a pot I needed to take the wires away from the pot from those who controls the power (high induction through the wires) and as mister_e wrote it is a good practice to take a decent set of readings and discard the ones that goes out of the scale, in my machine the use of capacitor with the pot proved to be a bad choice so I discarded the capacitor and used a shielded cable (the pot was 2 meters away from the PIC16F877A).
Hope that you solved your problem

[Paul F]

Hello Normanda,
I think I have tried everything. Seperate bench supplies, Heavy wires, inductive filters in the supply lines, large resevoir cap 10,000uF.
The system is working now. The spikes on the supply was caused when the L6203 was being switched on and off, (pulsed driven).
I now only read the adc during the period when the L6203 is not driven and the results so far appear to be acceptable.

Many thanks for your reply especially the hint on screend cable to the pot and everyone else who offered their help and advice.

Many thanks to all
Paul