Testing PiBorg Reverse power control without motors

Submitted by lars_the_bear on Tue, 02/09/2016 - 17:33

Forums:

Hi

This is a follow-on to a question I asked last week. Because I'm writing all the code myself in C, I want to ensure I can make the Reverse behave the way I want it to, before I bite the bullet and order a bunch of motors.

Can I simulate the behaviour of the motor in its PWM environment with, for example, a simple R/C low-pass filter? Of course it doesn't need to be an exact model -- I just need to see that the motor voltage can be made to go up and down and not exceed the rated voltage.

I would guess that something like a 100R resistor in series with a 100uF capacitor would probably do, but I'm not sure what the PWM frequency is, and I'm not sure whether the Reverse will only work with a low-resistance load like a real motor.

Any suggestions much appreciated.

Best wishes
Kevin

piborg's picture

piborg

Wed, 02/10/2016 - 11:18

It should be fine to attach an RC circuit to the outputs from PicoBorg Reverse for testing purposes.
We would recommend you keep the current through the resistor low to ensure it does not get too hot.
Ideally this should be a small number of milliamps if you are using a typical through-hole resistor.

100Ω may actually be a bit too low, it depends on the voltage.
If you are running with 12V for example:
12V ÷ 100Ω = 0.12A or 120mA
This would mean the power through the resistor is:
100Ω × 0.12A² = 1.44W, which is a fair amount.

If you cannot tell what the wattage of the resistor is I would suggest assuming it is 0.1W or 100mW to be safe.
This would give us a minimum resistance of:
12V² ÷ 0.1W = 1,440Ω or 1.44KΩ
Higher voltages will give larger values, but I expect either way a value in the KΩ range will be wanted.

The PWM output should be at 7.8125 KHz approximately for both outputs.

lars_the_bear

Thu, 02/11/2016 - 19:00

For the record, a 2k2 resistor and 10uF capacitor seem to work OK as a test load. I guess the values are not critical. I was worried that the output stage would not work at all unless the current were of a realistic level, but it seems fine.

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