On Sat, 17 Oct 2009, Spehro Pefhany wrote:

> At 08:52 PM 17/10/2009, you wrote:
>> Hi,
>>
>> I found this schematic on the internet
>> http://farm3.static.flickr.com/2682/4020964908_49fb1cd20a.jpg
>>
>> I want to drive a 24 VDC motor with a PIC. The manufacturer
>> says the motor is a 12vdc to 36 vdc. I measured the stall
>> current at ~10 Amps. Anyway I built the cicuit above using
>> IRFU5505 for the P Channel Mosfet and a BUZ101S for the N
>> channel. Right now I am just using a 5 volt supply thru
>> 200 ohm resistors to substitute for the pic logic pins and a
>> 12volt 12 amp battery charger to power the motor. Any way when I
>> stall the motor using the above circuit it stalls at 4.5
>> amps. It seems like I am losing power in the circuit ?  If I
>> stall it hooked directly to the battery charger it stalls at 10
>> Amps. Any suggestions ? I am going to be driving a pretty heavy
>> load so I need all the power I can get.
>>
>> BUZ101S Vds 55V, Vgs 2-4V, Ciss 500pf, Id 22A, Idm 88A,
>> Rds .05 Ohms
>> http://www.datasheetcatalog.org/datasheet/siemens/BUZ101S.pdf
>>
>>
>> IRFU5505 Vds -55 Vgs -2to-4V, Ciss 650pf, Id -18A, Idm -64A,
>> Rds .11 Ohms
>> http://www.datasheetcatalog.org/datasheet/irf/irfr5505.pdf
>>
>> Thanks,
>> Kevin
>
> You are only driving the n-channel MOSFET partly on with 5V. It is
> spec'd at 10V Vgs. You will find that the BUZ101S parts are getting
> very hot very fast.
>
> You can either increase the drive voltage or use a better suited
> (eg. logic level) type of n-channel MOSFET.
>
>
>> Best regards,
>
> Spehro Pefhany --"it's the network..."            "The Journey is the reward"
> speff@interlog.com             Info for manufacturers: http://www.trexon.com
> Embedded software/hardware/analog  Info for designers:  http://www.speff.com

What's the difference between Vgs and Vgs(th) I thought the 
Vgs(th) was the voltage to turn it on fully ?

Anyway I had some IRLZ520Ns lying around and switched them 
for the BUZ101S.  I can now get a stall current of 6.5 Amps.
At a 5V drive volt, raising the drive voltage above that 
does not increase the current draw of the motor.

Regards,
Kevin

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