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'[EE] High side motor driver'
2004\11\29@034838 by Jinx

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I've got a cordless drill motor (~2A) that I need a high side
driver for. At this time of day, unable to run out and buy a P-ch
FET, I've tried some PNP transistors (eg TIP42, TIP120).
Base drive is PWM from a 555, driving the transistor directly
or through an NPN switcher on the PNP base. So far the
results have not been encouraging. The waveform looks terrible
(not the nice square wave with an N-ch FET low side I tried),
the transistors get very hot and the motor's got no grunt

The reason I would like a high side driver is because the state
of the motor is measured by a circuit that needs the motor on the
low side. That's not set in stone but......

Q. Is it worth persisting with this or just get a P-ch FET ?

TIA

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2004\11\29@043522 by Wouter van Ooijen

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> FET, I've tried some PNP transistors (eg TIP42, TIP120).

I realise TIP120 is an NPN darlington:
http://www.fairchildsemi.com/pf/TI/TIP120.html?

Wouter van Ooijen

-- -------------------------------------------
Van Ooijen Technische Informatica: http://www.voti.nl
consultancy, development, PICmicro products
docent Hogeschool van Utrecht: http://www.voti.nl/hvu


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2004\11\29@044821 by Jinx

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> I realise TIP120 is an NPN darlington:

Sorry, my mistake. TIP126

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2004\11\29@060259 by Russell McMahon

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> I've got a cordless drill motor (~2A) that I need a high side
> driver for. At this time of day, unable to run out and buy a P-ch
> FET, I've tried some PNP transistors (eg TIP42, TIP120).
> Base drive is PWM from a 555, driving the transistor directly
> or through an NPN switcher on the PNP base. So far the
> results have not been encouraging. The waveform looks terrible
> (not the nice square wave with an N-ch FET low side I tried),
> the transistors get very hot and the motor's got no grunt

You can use an N Channel FET if (and only if) you provide a gate drive
voltage for it that is suitably above the high side rail. This may be able
to be done with the usual capacitor pump type circuit but it may cause
problems here if there is no low side driver to provide a cleanish drive
square wave. You may need to make a separate drive inverter (capacitive
would do). For testing the easy solution is to use a 9 volt battery for the
gate drive. With a little care you should get hundreds of hours from a  std
9v "transistor battery" (PP3 or whatever). A slightly complexified Joule
thief type inverter or similar one transistor self oscillating type would
also work. The input PWM drive  may provide all you need for a capacitor
pump or this could drive a transistor with inductor in its collector to
"ring" to the required voltage.


       RM

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2004\11\29@065216 by Jinx

face picon face
> You can use an N Channel FET if.......

But is it worth the hassle ? Maybe it is. Getting a P-ch FET
won't be a problem later on. I know the range is limited but
something like the 12A MTP2955V from RS is fairly cheap,
although the Ron (0.23), is quite a lot higher than the original
N-ch in the drills, either STP60NF06 (0.014) or IRFZ46 (0.02)

The implied question in my OP was whether FETs are generally
superior to normal transistors in an application like this. Looking
around the web the vast majority of high side drivers seem to be
P-ch FETs

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2004\11\29@081737 by Russell McMahon

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>> You can use an N Channel FET if.......

> But is it worth the hassle ?

Only you can say.
But, probably not.

> The implied question in my OP was whether FETs are generally
> superior to normal transistors in an application like this. Looking
> around the web the vast majority of high side drivers seem to be
> P-ch FETs

P FET is easy.
Transistor is harder to drive, higher drive circuit dissipation.
FET hasn't got bipolar's second breakdown problems.
FET often as cheap. A more "pleasant" solution if it suits your needs.
I'd probably use a PFET.


       RM


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2004\11\29@094904 by Martin K

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Jinx wrote:
> I've got a cordless drill motor (~2A) that I need a high side
> driver for. At this time of day, unable to run out and buy a P-ch
> FET, I've tried some PNP transistors (eg TIP42, TIP120).
> Base drive is PWM from a 555, driving the transistor directly
> or through an NPN switcher on the PNP base. So far the
> results have not been encouraging. The waveform looks terrible
> (not the nice square wave with an N-ch FET low side I tried),
> the transistors get very hot and the motor's got no grunt
>
> The reason I would like a high side driver is because the state
> of the motor is measured by a circuit that needs the motor on the
> low side. That's not set in stone but......
>
> Q. Is it worth persisting with this or just get a P-ch FET ?
>
> TIA

Use a MOSFET. At 2A, the drop of a transistor dissipates ~2W minimum.
Add in the slow switching [you didn't mention what frequency?] and
pretty soon you're burning up a lot of power.
An N-channel fet with an R_DS of .015 would dissipate 60mW and you could
switch it above audible frequency with ease, directly from the 555 with
a small gate resistor. Driving a motor low-side is much easier than
high-side.

What is it that your sensor circuit measures? current?
If you put the N-MOSFET on the low side and measure the voltage drop
across D->S, you would get a pretty good idea of the current to the
motor. (the MOSFET wouldn't heat up much to change the R_DS)
--
--
Martin Klingensmith
http://infoarchive.net/
http://nnytech.net/

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2004\11\29@112043 by olin_piclist

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Martin K wrote:
> At 2A, the drop of a transistor dissipates ~2W minimum.

Not really.  A suitable bipolar transistor should be able to do much better
than 1V C-E drop when saturated.  2A is not that much for a PNP power
transistor.  You should be able to get 300-500mV drop without expensive
components.


*****************************************************************
Embed Inc, embedded system specialists in Littleton Massachusetts
(978) 742-9014, http://www.embedinc.com
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2004\11\29@113207 by Bob Blick

face picon face
> I've got a cordless drill motor (~2A) that I need a high side
> driver for. At this time of day, unable to run out and buy a P-ch
> FET, I've tried some PNP transistors (eg TIP42, TIP120).
> Base drive is PWM from a 555, driving the transistor directly
> or through an NPN switcher on the PNP base. So far the
> results have not been encouraging. The waveform looks terrible
> (not the nice square wave with an N-ch FET low side I tried),
> the transistors get very hot and the motor's got no grunt

You say "the waveforms look terrible" did you perhaps forget to include a
freewheel diode? You always need one in a PWM circuit. Across the motor.
As it is, the stored EMF in the motor is being wasted in the form of heat.
I'm surprised your transistors survived the voltage.

Cheerful regards,

Bob


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2004\11\29@131035 by Jinx

face picon face
> You say "the waveforms look terrible" did you perhaps forget to
> include a freewheel diode?

I've used 5 1N5819s. b-0V / b-12V on the BC337 NPN base
driver,  b-0V / C-E on the PNP and one across the motor plus
100n filter caps to 0V and case plus a pair of reverse-connected
15V zeners across the terminals

> I'm surprised your transistors survived the voltage.

The first BC337 didn't without its diodes ;-(  That's why I
wondered if it was better to stick with FETs

Think I can stop wondering and start ordering

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2004\11\29@133100 by Jinx

face picon face
> What is it that your sensor circuit measures? current?
> If you put the N-MOSFET on the low side and measure the voltage
> drop across D->S, you would get a pretty good idea of the current
> to the motor. (the MOSFET wouldn't heat up much to change the
> R_DS)

Presently there's voltage across a sense resistor which goes to a
comparator. In series with the motor are a pair of cross-wired
end-stop microswitches and the motor itself goes through a polarity
reversing relay. The person I'm doing this for originally had (what
he thought was) a good deal on 24V gearboxed motors. I added
an MC34063 booster and everything was sweet. Now he's found
a very cheap source of similar 12V motors but they're way too fast,
hence the PWM to slow them down. Unfortunately installations are
not all the same so he needs to set the optimum PWM in situ.else I
might have suggested a series resistor. There is a simple solution with
a FET in here somewhere, I think I just lucked out trying bipolars
and got side-tracked. As you do


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2004\11\29@141334 by Dwayne Reid

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At 11:30 AM 11/29/2004, Jinx wrote:

>Presently there's voltage across a sense resistor which goes to a
>comparator. In series with the motor are a pair of cross-wired
>end-stop microswitches and the motor itself goes through a polarity
>reversing relay. The person I'm doing this for originally had (what
>he thought was) a good deal on 24V gearboxed motors. I added
>an MC34063 booster and everything was sweet. Now he's found
>a very cheap source of similar 12V motors but they're way too fast,
>hence the PWM to slow them down.

Why can't you just put the source lead from the N-ch FET in series with the
sense resistor?  Make sure that the flyback diode ties across the motor.

In other words, what about the N-ch FET version doesn't work?

dwayne


--
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Trinity Electronics Systems Ltd    Edmonton, AB, CANADA
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____________________________________________

2004\11\29@145408 by Martin K

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Olin Lathrop wrote:
> Martin K wrote:
>
>>At 2A, the drop of a transistor dissipates ~2W minimum.
>
>
> Not really.  A suitable bipolar transistor should be able to do much better
> than 1V C-E drop when saturated.  2A is not that much for a PNP power
> transistor.  You should be able to get 300-500mV drop without expensive
> components.

Fine, but I'd still find it easier and more efficient with an N-MOSFET.
--
--
Martin Klingensmith
http://infoarchive.net/
http://nnytech.net/

____________________________________________

2004\11\29@155719 by Jinx

face picon face
> Why can't you just put the source lead from the N-ch FET in
> series with the sense resistor?  Make sure that the flyback diode
> ties across the motor.
>
> In other words, what about the N-ch FET version doesn't work?
>
> dwayne

I'll be trying to find out today whether an N-ch driver will in fact
work. I had thought of doing what you suggest. There's still the
comparator to sort out. A rectifier/filter on the chopped DC should
do it

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2004\11\29@172301 by Bert Koerts

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>I'll be trying to find out today whether an N-ch driver will in fact
>work. I had thought of doing what you suggest. There's still the
>comparator to sort out. A rectifier/filter on the chopped DC should
>do it

I cannot find your previous email but if I remember it right you use a low
side current sensing resistor. It is best to put this resistor inside the
flyback circuit so you will also include the recirculation current from the
motor in your measurement. There is some good info at
http://www.analog.com/analogdialogue. (Current measurement in solenoids for
automotive control systems)

Bert

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2004\11\29@180429 by Russell McMahon

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> I  might have suggested a series resistor.

A FET plus opamp (LM358 which costs ~~= $0) can simulate a variable resistor
of desired value. Using the cheapish TO247 IRFD250 (if I got the prefix
right) you can handle 60 watts continuous on an adequate heatsink and 100
watts plus burst.


       RM

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