PWM constant current source ... help

If you move the sense resistor to above the motor, things get a lot easier. The voltage stays below the positive supply and does not have the fast, large swings riding on it.

There are also differential amplifier based chips available that tie to such a resistor and produce currents that are proportional to the shunt current that can drive negative rail circuits, such as Zetex ZXCT1008:

This series is limited to 20 volt supplies, but it may give you some ideas.

Hi folks,

I'm trying to think of a good way to implement a PWM cc source for an inductive load (a small DC motor). For resistive loads like battery chargers it's obviously enough to just toss a current-sensing resistor into the source leg of the low-side driver. But for an inductive load I need to take the current trough the freewheel diode into account. Currently my basic current-sensing circuit looks like this:

,---+--+27V | | Motor | V prop | | / to I /+|-----+ D1 / / | | | --+-R1---

robert, there is not so much inductance as you think, and it will vary a bit depending on the commutator , but there is an opposing voltage source that models the back EMF of the motor and is depending on the rpm. Very important is the DC resistance, which can be easily measured and is the foundation of your design. That lightbulb is IMHO an appropriate test object for the stalled motor, but when moving the voltage source might even revers the current, what cannot happen with the light-bulb. You can also easily measure the inductance with a L-meter.

Is it really? In the design i'm currently pursuing it isn't, but do you mean I can replace the differential I measurement (including the I through the flyback diode) by a more simple measurement with a source resisitor if I know more about the motor charcteristics?

I should have clarified: I didn't use the bulb as a "dummy motor", but as a PTC current source in series with the motor. I'm seeking to replace the bulb with an electronic circuit that is more accurate and gets less hot.

robert

I know, but I can't, as I have mentioned in my post. The motors are internally tied to a common +24V supply rail.

robert

Yes, measure first the DC-resistance and inductance. Then measure the rpm and I with different voltages applied, 6/12/18/24V.. in a no_load condition or if that isn't possible in the normal load condition. You can now calculate the back EMF vs. rpm(should be quadratic). Then there will be some friction, a more constant term. You can easily measure these things by applying a regulated voltage with a Lab supply which can also sink current.

o |+ / \\ (_M_) \\_/ |- | | .--------. | |LAB +| o -' |Power | |Spply | | | | -| o -. '--------' | | | === GND (created by AACircuit v1.28 beta 10/06/04

hope this helps

Surely the back emf is proportional to rpm, not quadratic?

In article , Robert Latest wrote: [....]

You may want to put a small inductor in series with the motor and a small capacitor right at its leads. The bruch noise from the motor and be trouble for circuits.

How accurate do you need things to be? A rail-to-rail op-amp and 4 resistors could make a diff-amp. Even the good old LM324 would work like this:

(-) ---/\\/\\--+---/\\/\\----+----/\\/\\------- ! ! ! ---C1 --!-\\ ! --- ! >---------+--- ! --!+/ GND ! ! (+) ---/\\/\\--+---/\\/\\----+ ! ! ---C2 / --- \\ ! / GND ! GND

If C1 and C2 are large enough to filter the switching frequency signal down to about 0.1Vp-p at the input of the LM324, the LM324 will be happy enough. If you make them small enough that the time constant they create is well above the gain cross over of the loop, they will not effect loop stability.

This could all be a switching regulator chip. A "current mode" booster chip could take the current sense input as its feedback. The advanage the "current mode" circuits have is that they will react very quickly to a shorted motor etc and yet allow a slowish loop involving the LM324.

There is a (limited) relationship between Average motor current and the Source current, depending on the ratio of the clock frequency of the PWM, to the L/R time constant of the motor.

With a slow clock, (PWM period large compared to L/R), the conduction time of the flyback diode is small compared to the OFF time. In this case the Average motor current is approximately equal to the Average of the Source current.

With a fast clock, (PWM period small compared to L/R), the diode conducts for the whole of the OFF time. A Sample-Hold of the Source current in the centre of the ON period is approximately equal to the Average motor current.

If the clock is fast enough, the motor ripple current is small and a simpler Track-Hold of Source current during the whole of the ON period might be ok. You can judge whether it is a fast/slow clock by scoping the Source voltage during the OFF period. If it is up at (Vsupply + Vdiode) during the whole of the OFF period then the diode is conducting continuously.

But that's a bog-standard linear cc source, and in my application it would burn ~24W with the motor stalled (27V supply voltage with the motor dropping 3V at 1A). I'd rather not dissipate that much heat because I don't want to add a fan to my enclosure.

Thanks, robert

Thanks, Tony, for your good advice. Of course I haven't gotten round to taking any measurements but I'll soon do that. I'm curious how the rpm of the motor influences the flyback time, if at all.

robert