Hall Effect Chopper

[legomanww]

Would it be possible to make a current-limiting chopper circuit using a hall effect sensor, instead of a sensing resistor?

I was thinking of using a hall effect sensor, a voltage comparator, MOSFET, and either a d-type flip flop and a clock, or a small PIC. The current sensor would detect the current, the voltage comparator would compare the output voltage to a reference voltage, and the PIC would keep the switching frequency to a reasonable level.

Since the current range is so high, it could be used for large motors. Also, I would imagine amplifying and offsetting the output with an opamp would provide finer control around the value of interest.

It seems to me like this would be a better (maybe just different?) solution than a sensing resistor because it would not require a big resistor or extra resistance in the circuit.

I tried this a while ago, but didn't properly think it through so it didn't work as expected, but I was thinking I would give it another go.

Any comments?

Here are the sensors I was going to try it with:
http://www.allegromicro.com/en/Produ...rentsensor.asp

[neilw20]

Negligble loss if you use a current sense transformer.
Up to 10A a resistor is quite practical if the sense voltage is low.
You need a current mode controller.
Various ones are upto 50% duty cycle.
Others do the full 100%. Tha'ts what you need.
Look at the data on UC3842 family. They are cheap too.
It will do drive the MOSFET, has current sense, voltage reference, oscilator, comparitor, etc. Very few components needed.
That's one of the easiest way to do it.
If you use hall sensors, staying within the linear range becomes a problem, but you can complicate it by by driving a cancelling coil driven by feedback from the hall sensor. Then the cancelling current is proportional to the measured current.
You can measure right down to DC with response times less than 5uS if it is done correctly. This is really good way to go above 100 Amps.
IMHO it is only worth this trouble above 100 amps.

[legomanww]

The whole point of the hall effect current sensors I linked to is that they are linear and output a voltage directly proportional to the current. DC currents also work fine from my experiments.

I like that family of UC3842 chips, those are neat.

[neilw20]

Originally Posted by legomanww The whole point of the hall effect current sensors I linked to is that they are linear and output a voltage directly proportional to the current. DC currents also work fine from my experiments. I like that family of UC3842 chips, those are neat.

Legomanww.
Hall sensors are only linear until you saturate them.
Extra care is need when running current mode, as a missed response can be destructive.
If you have transient spikes from something saturating they could be missed. Check it out with a really good CRO to make sure.

[legomanww]

No offense, but I don't think you actually looked at the chips I linked about.
For one of them, the specs claim a 0.8% nonlinearity over the full sensing range (200A).
The output responses also show the output remains constant when a (constant) DC 200A load is measured.

[Madclicker]

The answer to your question is yes, it's probably possible to chop the current using hall effect sensors as the current detectors. I wouldn't do it that way for a motor drive. For one thing resistors are much cheaper. Another is that using sensing resistors is a well documented method...a no brainer.