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[on:]

Hi

I have a couple of these motors
http://banebots.com/pc/MS-25XXX-180/MS-25010-180

Nominal v   : 4.5v
No Load RPM   : 1280
No Load A   : 0.2A
Stall Current   : 6.7A

But i really cant seem to find any motor driver which can handle the low nominal voltage and the relative high stall current.

I imagine i need a driver somewhere about 7 - 8A

Am i right? Any ideas where i can such driver, or just MOSFET for it?

[Reply #1 on:]

The other approach is to use a supply that limits the current to less than the stall current, so that the controller doesn't have to carry such a high current.

If you need the full torque from the motor from standstill then this approach wouldn't be appropriate, but for lighter duty use it could be useful.

Note that overloading a supply like this causes its voltage to drop substantially and you cannot really share it with other loads.

The low voltage is an issue as most large current MOSFET bridges require a higher voltage to drive the gates.  Consider running from a somewhat higher voltage but limit the PWM to some percentage that is equivalent to the 4.5V.

[Reply #2 on:]

That worries me... I would have said Logic level MOSFET.. Though 5v is best, as I understand logic level MOSFETS are even capable at 3.3v, with gate voltages as low as 2v in spec.  A Logic MOSFET would seem the thing I would now go with.. Am I missing something?

[Reply #3 on:]

That worries me... I would have said Logic level MOSFET.. Though 5v is best, as I understand logic level MOSFETS are even capable at 3.3v, with gate voltages as low as 2v in spec.  A Logic MOSFET would seem the thing I would now go with.. Am I missing something?

No, logic level usually means 4.5V Vgs test voltage.  High power MOSFETs rarely are driven direct from low voltages because of the large coupling from drain to gate via drain-gate capacitance(*).  Usually the drive would be via a MOSFET driver chip that has its own 12V supply.  For instance the MIC4422 that I've used and takes 5V to 18V supply and will run from 3V3 logic and drive practically the largest MOSFETs.  Most low-gate-voltage MOSFETs are surface mount and of modest Vds.

(*) When power switching the rapid change of voltage on the drain couples significant current back through the gate via the gate-drain capacitance and would easily overpower and destroy low voltage logic without good protective circuitry.

[Reply #4 on:]

http://parts.digikey.com/1/parts/1024707-mosfet-n-ch-60v-12a-ipak-rfd3055.html

I just purchased these, they're rated for 12A @ 60v.. Logic level, n channel mosfet.  Got twenty of them from another vendor, seven bucks shipped. I will be using to switch power LEDs, but unless I am totally confused, this should switch perfectly up to their rating with no external FET driver.. Am I wrong.. And if so, what should be an appropriate driver?

I don't mean to be hijacking the thread, hehe.. It really is the same question, though..

[Reply #5 on:]

You could use an H-bridge made from logic level mosfets, similar to the one in the attached schematic, but drive it from a 5v supply instead of 12v, use logic level mosfets throughout, and reduce the 1K resistors to around 330 or 470 ohms. Take care not to drive both Arduino outputs high at the same time (although the diodes provide some protection if you do that).

[Reply #6 on:]

Just an FYI... The logic MOSFETs I purchased seem to work perfectly with a 330 ohm resistor on the gate.. Right now switching a bit over three amps at twelve volts, cool as a cucumber...

[Reply #7 on:]

I am back.

Sorry for the late answer, i have been kind of busy... 

Looks like ill go for the Logic Level MOSFETs. Am i right when i say that what defines a Logic Level MOSFET is the low gate threshold voltage (Max 5v) ?

I just purchased these, they're rated for 12A @ 60v.. Logic level, n channel mosfet.  Got twenty of them from another vendor, seven bucks shipped.

Thanks focalist, i appreciate your offer. Is it seven bucks for all twenty?

The other approach is to use a supply that limits the current to less than the stall current, so that the controller doesn't have to carry such a high current.

Good approach! But i do really need a good torque.


Are there any good MOSFET selection guides out there anyone could recommend?


 

[Reply #8 on:]

An L298 H-bridge will drive as low as 2.5 V. You can use the current sense pin to monitor the stall condition via an adc input on the arduino.

[Reply #9 on:]

Looks like ill go for the Logic Level MOSFETs. Am i right when i say that what defines a Logic Level MOSFET is the low gate threshold voltage (Max 5v) ?

The gate threshold voltage is the voltage at which it *starts* to conduct, not the voltage at which it is fully on A logic level mosfet is one that has a defined Rds(on) at logic level voltages. Typically, Rds(on) is quoted at 4.5v for logic level mosfets. The gate threshold voltage for logic level mosfets is normally quoted as 2V or less.

I use the Farnell site to search for logic level mosfets. Don't filter by Rds(on) test voltage, that way you will miss most logic level mosfets because Rds(on) is typically tested at both 4.5v and 10v for these mosfets, and it is the 10v test that gets used if you filter by test voltage. Instead, filter on Vgs(threshold) <= 2v (or >= -2v for a P-channel mosfet), then check the datasheets to see if Rds(on) is quoted at Vgs=4.5v.