FET how-to
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Posts: 46,084
Hi gurus!
Does anyone on this group know how to use FETs?
MOSFET
N-Ch, Enhancement Mode
High Speed Switch
Positive to Drain, Motor between Source and Ground.
Gate connected to pos, motor runs fast, FET's get hot.
Gate to ground, Motor runs very slow, FETs get very hot very fast.
These FET's have the following specifications:
Drain-Source Voltage, VDSS
60V
Drain-Gate Voltage (RGS = 20k Ohm), VDGR
60V
Continuous Gate-Source Voltage, VGS
±30V
Drain Current, ID
Continuous
Pulsed
35A
152A
Maximum Power Dissipation, PD
125W
Operating Junction Temperature, TJ
+175°C
Storage Temperature Range, Tstg
-55° to +175°C
Maximum Thermal Resistance, Junction-to-Case, RthJC
1.2°C/W
Typical Thermal Resistance, Junction-to-Ambient, RthJA
60°C/W
If I'm correct, these should be able to handle 35A continuous and I
have three(3) in parallel and they still get hot. The motor is an
electric wheelchair motor, and those will carry someone all day on a
charge so they shouldn't pull that much current.
Any ideas?
Thanks.
Doug
Does anyone on this group know how to use FETs?
MOSFET
N-Ch, Enhancement Mode
High Speed Switch
Positive to Drain, Motor between Source and Ground.
Gate connected to pos, motor runs fast, FET's get hot.
Gate to ground, Motor runs very slow, FETs get very hot very fast.
These FET's have the following specifications:
Drain-Source Voltage, VDSS
60V
Drain-Gate Voltage (RGS = 20k Ohm), VDGR
60V
Continuous Gate-Source Voltage, VGS
±30V
Drain Current, ID
Continuous
Pulsed
35A
152A
Maximum Power Dissipation, PD
125W
Operating Junction Temperature, TJ
+175°C
Storage Temperature Range, Tstg
-55° to +175°C
Maximum Thermal Resistance, Junction-to-Case, RthJC
1.2°C/W
Typical Thermal Resistance, Junction-to-Ambient, RthJA
60°C/W
If I'm correct, these should be able to handle 35A continuous and I
have three(3) in parallel and they still get hot. The motor is an
electric wheelchair motor, and those will carry someone all day on a
charge so they shouldn't pull that much current.
Any ideas?
Thanks.
Doug
Comments
Motor does stop, though with gate to ground, anf runs full speed (or
close enough) with the gate to pos. FETs still get burn-ya hot in
just a few seconds. Do I need to go with some other devices? These
were salvaged. I have 12 of them all alike and I was trying to make a
h-bridge motor controller. Had these on hand so thought I'd try
them.
Thanks for your help.
Doug
> To: <basicstamps@yahoogroups.com>
> From: "Ray McArthur" <rjmca@u...>
> Date: Fri, 6 Apr 2001 22:46:36 -0400
> Reply-to: basicstamps@yahoogroups.com
> Subject: Re: [noparse][[/noparse]basicstamps] FET how-to
> You are running the FET in "source-follower" mode, so there is lots of power
> dissipation in the FETS. Connect the source to gnd, the drain to motor, and
> other motor lead to positive. The gate goes to a positive ON/OFF control
> voltage. When the gate is at zero, or gnd, the motor should be off. With
> the gate at positive control voltage, the motor should run.
>
> Ray McArthur
>
>
Original Message
> From: Doug Simpson <veewee77@a...>
> To: <basicstamps@yahoogroups.com>
> Sent: Friday, April 06, 2001 5:11 PM
> Subject: [noparse][[/noparse]basicstamps] FET how-to
>
>
> Hi gurus!
>
> Does anyone on this group know how to use FETs?
>
> MOSFET
> N-Ch, Enhancement Mode
> High Speed Switch
>
> Positive to Drain, Motor between Source and Ground.
>
> Gate connected to pos, motor runs fast, FET's get hot.
>
> Gate to ground, Motor runs very slow, FETs get very hot very fast.
>
>
>
>
>
>
> Your use of Yahoo! Groups is subject to http://docs.yahoo.com/info/terms/
>
>
>
per sink.
These came out of an APC smart-ups 400. There are 4 sets of 3 that
drive the power output transformer to produce AC. Do the math. They
are at 24v in the UPS, two sets of three per side. The unit will
drive 250W continuous load on the AC output, and if you do the math
that's 250W/24V, that's 10.41A that the FETs are switching. And they
would get pretty warm if you pull full load on the UPS on batteries.
Probably more info than you need, but I'm needing to get this
resolved if possible before school is out because we may be going to
do a summer enrichment thing at school and we are thinking about
doing a section with robotics and I want to spark the student's
interest before then.
Thanks, again!
Doug
> To: <basicstamps@yahoogroups.com>
> From: "Tim and Jo" <dema@t...>
> Date: Fri, 6 Apr 2001 20:37:42 -0700
> Reply-to: basicstamps@yahoogroups.com
> Subject: Re: [noparse][[/noparse]basicstamps] FET how-to
> Hi Doug,
>
> What heatsinking do your transistors have?
>
> Tim
>
>
Original Message
> From: Doug Simpson <veewee77@a...>
> To: basicstamps@yahoogroups.com <basicstamps@yahoogroups.com>
> Date: Friday, April 06, 2001 7:55 PM
> Subject: Re: [noparse][[/noparse]basicstamps] FET how-to
>
>
> >Tried it that way, too. Still gets very hot. No changes actually.
> >
> >Motor does stop, though with gate to ground, anf runs full speed (or
> >close enough) with the gate to pos. FETs still get burn-ya hot in
> >just a few seconds. Do I need to go with some other devices? These
> >were salvaged. I have 12 of them all alike and I was trying to make a
> >h-bridge motor controller. Had these on hand so thought I'd try
> >them.
> >
> >Thanks for your help.
> >
> >Doug
> >
> >
> >> To: <basicstamps@yahoogroups.com>
> >> From: "Ray McArthur" <rjmca@u...>
> >> Date: Fri, 6 Apr 2001 22:46:36 -0400
> >> Reply-to: basicstamps@yahoogroups.com
> >> Subject: Re: [noparse][[/noparse]basicstamps] FET how-to
> >
> >> You are running the FET in "source-follower" mode, so there is lots of
> power
> >> dissipation in the FETS. Connect the source to gnd, the drain to motor,
> and
> >> other motor lead to positive. The gate goes to a positive ON/OFF control
> >> voltage. When the gate is at zero, or gnd, the motor should be off.
> With
> >> the gate at positive control voltage, the motor should run.
> >>
> >> Ray McArthur
> >>
> >>
Original Message
> >> From: Doug Simpson <veewee77@a...>
> >> To: <basicstamps@yahoogroups.com>
> >> Sent: Friday, April 06, 2001 5:11 PM
> >> Subject: [noparse][[/noparse]basicstamps] FET how-to
> >>
> >>
> >> Hi gurus!
> >>
> >> Does anyone on this group know how to use FETs?
> >>
> >> MOSFET
> >> N-Ch, Enhancement Mode
> >> High Speed Switch
> >>
> >> Positive to Drain, Motor between Source and Ground.
> >>
> >> Gate connected to pos, motor runs fast, FET's get hot.
> >>
> >> Gate to ground, Motor runs very slow, FETs get very hot very fast.
> >>
> >>
> >>
> >>
> >>
> >>
> >> Your use of Yahoo! Groups is subject to http://docs.yahoo.com/info/terms/
> >>
> >>
> >>
> >
> >
> >
> >
> >Your use of Yahoo! Groups is subject to http://docs.yahoo.com/info/terms/
> >
> >
> >
>
>
>
>
> Your use of Yahoo! Groups is subject to http://docs.yahoo.com/info/terms/
>
>
>
dissipation in the FETS. Connect the source to gnd, the drain to motor, and
other motor lead to positive. The gate goes to a positive ON/OFF control
voltage. When the gate is at zero, or gnd, the motor should be off. With
the gate at positive control voltage, the motor should run.
Ray McArthur
Original Message
From: Doug Simpson <veewee77@a...>
To: <basicstamps@yahoogroups.com>
Sent: Friday, April 06, 2001 5:11 PM
Subject: [noparse][[/noparse]basicstamps] FET how-to
Hi gurus!
Does anyone on this group know how to use FETs?
MOSFET
N-Ch, Enhancement Mode
High Speed Switch
Positive to Drain, Motor between Source and Ground.
Gate connected to pos, motor runs fast, FET's get hot.
Gate to ground, Motor runs very slow, FETs get very hot very fast.
When you fix the circuit:
If your motor runs at all with the gate grounded, the FETS were damaged from
overheating.
Ray McArthur
Original Message
From: Doug Simpson <veewee77@a...>
To: <basicstamps@yahoogroups.com>
Sent: Friday, April 06, 2001 5:11 PM
Subject: [noparse][[/noparse]basicstamps] FET how-to
How much voltage are you applying gate-source? Normally, the FETS are
operated as a switch, full ON or OFF. To turn the FETS on fully, you must
apply 12 volts to the gate. If you use logic-level FETS, 5 volts will turn
the FETS on fairly well. To control motor speed, you pulse the FETS on and
off, (PWM), so the duty cycle controls motor speed.
How much current does the motor draw with full voltage applied? Is there a
part number on the FETS?
Ray Mc
Original Message
From: Doug Simpson <veewee77@a...>
To: <basicstamps@yahoogroups.com>
Sent: Friday, April 06, 2001 5:57 PM
Subject: Re: [noparse][[/noparse]basicstamps] FET how-to
> Tried it that way, too. Still gets very hot. No changes actually.
>
> Motor does stop, though with gate to ground, anf runs full speed (or
> close enough) with the gate to pos. FETs still get burn-ya hot in
> just a few seconds. Do I need to go with some other devices? These
> were salvaged. I have 12 of them all alike and I was trying to make a
> h-bridge motor controller. Had these on hand so thought I'd try
> them.
What heatsinking do your transistors have?
Tim
Original Message
From: Doug Simpson <veewee77@a...>
To: basicstamps@yahoogroups.com <basicstamps@yahoogroups.com>
Date: Friday, April 06, 2001 7:55 PM
Subject: Re: [noparse][[/noparse]basicstamps] FET how-to
>Tried it that way, too. Still gets very hot. No changes actually.
>
>Motor does stop, though with gate to ground, anf runs full speed (or
>close enough) with the gate to pos. FETs still get burn-ya hot in
>just a few seconds. Do I need to go with some other devices? These
>were salvaged. I have 12 of them all alike and I was trying to make a
>h-bridge motor controller. Had these on hand so thought I'd try
>them.
>
>Thanks for your help.
>
>Doug
>
>
>> To: <basicstamps@yahoogroups.com>
>> From: "Ray McArthur" <rjmca@u...>
>> Date: Fri, 6 Apr 2001 22:46:36 -0400
>> Reply-to: basicstamps@yahoogroups.com
>> Subject: Re: [noparse][[/noparse]basicstamps] FET how-to
>
>> You are running the FET in "source-follower" mode, so there is lots of
power
>> dissipation in the FETS. Connect the source to gnd, the drain to motor,
and
>> other motor lead to positive. The gate goes to a positive ON/OFF control
>> voltage. When the gate is at zero, or gnd, the motor should be off.
With
>> the gate at positive control voltage, the motor should run.
>>
>> Ray McArthur
>>
>>
Original Message
>> From: Doug Simpson <veewee77@a...>
>> To: <basicstamps@yahoogroups.com>
>> Sent: Friday, April 06, 2001 5:11 PM
>> Subject: [noparse][[/noparse]basicstamps] FET how-to
>>
>>
>> Hi gurus!
>>
>> Does anyone on this group know how to use FETs?
>>
>> MOSFET
>> N-Ch, Enhancement Mode
>> High Speed Switch
>>
>> Positive to Drain, Motor between Source and Ground.
>>
>> Gate connected to pos, motor runs fast, FET's get hot.
>>
>> Gate to ground, Motor runs very slow, FETs get very hot very fast.
>>
>>
>>
>>
>>
>>
>> Your use of Yahoo! Groups is subject to http://docs.yahoo.com/info/terms/
>>
>>
>>
>
>
>
>
>Your use of Yahoo! Groups is subject to http://docs.yahoo.com/info/terms/
>
>
>
A floating gate can lead to overheating and may damage the chip.
A pull down resistor about abput 100K should be used.
ACJacques
Doug Simpson wrote:
>
> Tried it that way, too. Still gets very hot. No changes actually.
>
> Motor does stop, though with gate to ground, anf runs full speed (or
> close enough) with the gate to pos. FETs still get burn-ya hot in
> just a few seconds. Do I need to go with some other devices? These
> were salvaged. I have 12 of them all alike and I was trying to make a
> h-bridge motor controller. Had these on hand so thought I'd try
> them.
>
> Thanks for your help.
>
> Doug
>
> > To: <basicstamps@yahoogroups.com>
> > From: "Ray McArthur" <rjmca@u...>
> > Date: Fri, 6 Apr 2001 22:46:36 -0400
> > Reply-to: basicstamps@yahoogroups.com
> > Subject: Re: [noparse][[/noparse]basicstamps] FET how-to
>
> > You are running the FET in "source-follower" mode, so there is lots of power
> > dissipation in the FETS. Connect the source to gnd, the drain to motor, and
> > other motor lead to positive. The gate goes to a positive ON/OFF control
> > voltage. When the gate is at zero, or gnd, the motor should be off. With
> > the gate at positive control voltage, the motor should run.
> >
> > Ray McArthur
> >
> >
Original Message
> > From: Doug Simpson <veewee77@a...>
> > To: <basicstamps@yahoogroups.com>
> > Sent: Friday, April 06, 2001 5:11 PM
> > Subject: [noparse][[/noparse]basicstamps] FET how-to
> >
> >
> > Hi gurus!
> >
> > Does anyone on this group know how to use FETs?
> >
> > MOSFET
> > N-Ch, Enhancement Mode
> > High Speed Switch
> >
> > Positive to Drain, Motor between Source and Ground.
> >
> > Gate connected to pos, motor runs fast, FET's get hot.
> >
> > Gate to ground, Motor runs very slow, FETs get very hot very fast.
> >
> >
> >
> >
> >
> >
> > Your use of Yahoo! Groups is subject to http://docs.yahoo.com/info/terms/
> >
> >
> >
>
>
>
> Your use of Yahoo! Groups is subject to http://docs.yahoo.com/info/terms/
I had the same problem when I was constructing my
H-bridge motor controller. The problem I had were the
fets weren't "logic level" so what I had to do was
supply them with a higher voltage than the 5 volts
coming from the stamp. I used a circuit which came in
the "Microcontroller Application Cookbook" (great book
well worth having) It put's the signal through a 7407
(hex buffer) which upped the voltage and it works
marvelously.
P.S.If you'd like the scematic I could probably e-mail
a scan of it to you personally but I'm pretty sure the
whole copy-write thing means I shouldn't post it.
Best regards
Aaron
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