PWM DC motor circuit?

T

Terry Pinnell

Jan 1, 1970
0
Could someone kindly save me a bit of time in finding a suitable
circuit for driving a DC motor with PWM please? My first hour of
googling has found many threads, but so far no specific relevant
schematics.

The DC motor is from a cordless screwdriver, used originally with two
NiCad C cells, and duly marked as '2.4V'. I ran it briefly at 5V and
assume it would tolerate that, giving me higher torque potential. It's
geared, and (unloaded with 2.4V applied) gives 250 rpm. That's too
fast for my curtain control application so I want to be able to reduce
it. Hence PWM.

I'll probably use limit switches and a C/O relay for the
forward/reverse/off control, at least while getting the challenging
mechanicals sorted (as per separate thread). So I don't need to
consider the complications of combining PWM with an H-Bridge.

My first thoughts are to use a 5V supply (derived from a convenient
fairly heavy duty 12V supply I have nearby) with a variable duty cycle
555 (or maybe a 4001/4011 equivalent), directly driving a medium power
MOSFET, with the motor between its drain and 5V. I'll breadboard that
shortly, but meanwhile:
- will that simple approach be reliable?
- any snags or protective measures to watch out for?
- is there an 'optimum' frequency range?
- if I designed a *high* duty cycle from 555, which I recall makes it
easier to achieve a wide d/c range, what is most efficient way to
configure the output MOSFET stage?
- I happen to have a 2N3055 (or similar) already neatly mounted on a
h/sink, from a previous project; any major reason why I shouldn't use

As mentioned in the curtain thread, I'd like to build this asap, as
the full-speed motor action is adding to my design and testing
problems.

TIA.

P

Paul Burridge

Jan 1, 1970
0
Could someone kindly save me a bit of time in finding a suitable
circuit for driving a DC motor with PWM please? My first hour of
googling has found many threads, but so far no specific relevant
schematics.

This is a really clever design that works very well indeed in
practice:

http://www.solorb.com/elect/solarcirc/pwm1/

P

Paul Burridge

Jan 1, 1970
0
finished printing out that very circuit! One of two I'd found at
following further googling this afternoon.

I'm also going to try my own 'simple 555' design mentioned in my OP.

The main problem with using a 555 based design is going to be the
1/3-2/3 range of available supply volts. That may not be a problem in
your particular case but it's nice to have the full rail-rail o/put
this quad op-amp gives you, IMHO.

J

John Jardine

Jan 1, 1970
0
[clp]
Terry Pinnell
Hobbyist, West Sussex, UK
My cooker extracter uses a pot', setting a 555 pwm, driving a TIP110
darlington, directly feeding an ex Alfa-Romeo radiator fan. Frequency is
arbitrary at a couple of kHz. No heatsinking, no protection or
jiggery-pokery parts, other than a diode across the motor.
There must be all of 50p's worth of bits and it's run fine for years.
regards
john

T

Terry Pinnell

Jan 1, 1970
0
John Jardine said:
[clp]
Terry Pinnell
Hobbyist, West Sussex, UK
My cooker extracter uses a pot', setting a 555 pwm, driving a TIP110
darlington, directly feeding an ex Alfa-Romeo radiator fan. Frequency is
arbitrary at a couple of kHz. No heatsinking, no protection or
jiggery-pokery parts, other than a diode across the motor.
There must be all of 50p's worth of bits and it's run fine for years.
regards
john

Thanks. I'm probably going to have to get a bit more complex in this
case, due to a combination of the reversal requirement plus automatic
stopping at each extreme. In considering that very simple '555 driving
MOSFET' appoach, (or the epanorama site version), I'd vaguely thought
I could implement these additional facilities with a relay and
microswitches. But of course that's not possible; the motor will be
permanently connected with one specific polarity, one end to Vcc and
the other via the MOSFET (or NPN BJT) to ground.

However, I might *still* build such a simple PWM circuit, purely to
get the speed down while I experiment. At present, I barely get time
to apply power before the cord flies off the pulley or fouls up in
some way!

But for my final circuit, it now looks as if I'll need an H-Bridge
approach. That looks more easily done with two NPN and two PNP BJTs,
but I'd welcome others' views please.

How's the Alfa getting on without its fan?

P

Paul Burridge

Jan 1, 1970
0
How's the Alfa getting on without its fan?

Alfas don't have fans anymore. They've all gone over to Ferrari.

J

John Jardine

Jan 1, 1970
0
Terry Pinnell said:
John Jardine said:
[clp]
Terry Pinnell
Hobbyist, West Sussex, UK
My cooker extracter uses a pot', setting a 555 pwm, driving a TIP110
darlington, directly feeding an ex Alfa-Romeo radiator fan. Frequency is
arbitrary at a couple of kHz. No heatsinking, no protection or
jiggery-pokery parts, other than a diode across the motor.
There must be all of 50p's worth of bits and it's run fine for years.
regards
john

Thanks. I'm probably going to have to get a bit more complex in this
case, due to a combination of the reversal requirement plus automatic
stopping at each extreme. In considering that very simple '555 driving
MOSFET' appoach, (or the epanorama site version), I'd vaguely thought
I could implement these additional facilities with a relay and
microswitches. But of course that's not possible; the motor will be
permanently connected with one specific polarity, one end to Vcc and
the other via the MOSFET (or NPN BJT) to ground.

However, I might *still* build such a simple PWM circuit, purely to
get the speed down while I experiment. At present, I barely get time
to apply power before the cord flies off the pulley or fouls up in
some way!

But for my final circuit, it now looks as if I'll need an H-Bridge
approach. That looks more easily done with two NPN and two PNP BJTs,
but I'd welcome others' views please.

How's the Alfa getting on without its fan?

I really missed that Alpha after they sent it to the
great-scrapyard-in-the-sky. At the time it had been a useful source of 12V
project motors and that nice multi-strand cabling that only seems to turn up
in car wiring looms.

For the project I wouldn't dream of using H bridges. Damned things are
always messy, ugly and require effort to make 'em work reliably .
Why the motor can't be reversed?. Surely just regard the Vcc and fet wires
like what they basically are, i.e. A pos and neg supply for the motor. Run
'em through the reversing relays and limit switches as normal. The wiring
runs will add a bit of resistance and inductance but at (say) audio chopping
frequencies, the motor won't remotely notice any difference.
regards
john

B

Bob Wilson

Jan 1, 1970
0
that, or a cheap single-chip microcontroller + MOSFET + capacitor/diode
snubber? =)

Al

Using a microcontroller to replace a simple quad-comparator-based PWM circuit
(such as the one mentioned) would seem to be a little silly. Not only is the uC
more expensive than the comparator, it also requires time spent on programming
(and debugging). The comparator-based PWM is so simple, that it would be up and
running before the code for a uP version wer even half finished.

Bob.

T

Terry Pinnell

Jan 1, 1970
0
Another potentially interesting link I found on the epanorama site was

Could someone give it a try and confirm that it's inaccessible please?

Googling on some of the key words from the epanorama outline also so
far unsuccessful.

"Motor controller uses fleapower - A simple, permanent-magnet dc motor
is an essential element in a variety of products, such as toys, servo
mechanisms, valve actuators, robots, and automotive electronics. In
many of these applications, the motor must rotate in a given direction
until the mechanism reaches the end of travel, at which point the
motor must automatically stop. This circuit implements The design is
optimized for a supply voltage of 3 to 9V, making it well-suited to
battery-powered applications."

J

John Woodgate

Jan 1, 1970
0
I read in sci.electronics.design that Terry Pinnell <[email protected]
Edial.pipex.com> wrote (in said:
Another potentially interesting link I found on the epanorama site was

Could someone give it a try and confirm that it's inaccessible please?

Remove '#dc' from the URL and instead scroll down the page you then get
'Circuits'.

A

Al Borowski

Jan 1, 1970
0
Using a microcontroller to replace a simple quad-comparator-based PWM circuit
(such as the one mentioned) would seem to be a little silly. Not only is the uC
more expensive than the comparator, it also requires time spent on programming
(and debugging). The comparator-based PWM is so simple, that it would be up and
running before the code for a uP version wer even half finished.

Want to bet on that?

I already have code for simple things like PWM - I'd use a cheap 8 pin
PIC with ADC ($2 AUD in single quantites). The only parts you'd need would be an 8 pin uC, a pot for PWM adjustment, and the FET and snubber. I reakon, if I had the parts on hand, I could get it going in 10 minutes on breadboard. No having to hunt down a quad comparitor or the needed resistor values - I just have a heap of cheap micro's for jellybeans when I do things like this. cheers, Al S Steve Taylor Jan 1, 1970 0 Al said: Want to bet on that? I already have code for simple things like PWM - I'd use a cheap 8 pin PIC with ADC ($2 AUD in single quantites). The only parts you'd need
would be an 8 pin uC, a pot for PWM adjustment, and the FET and snubber.

I reakon, if I had the parts on hand,

....and all the software written and debugged.....
;-)
Steve

A

Al Borowski

Jan 1, 1970
0
Steve said:
...and all the software written and debugged.....
;-)

/me points to where I wrote "I already have code for simple things like
PWM"

cheers,

Al

T

Terry Pinnell

Jan 1, 1970
0
John Woodgate said:
I read in sci.electronics.design that Terry Pinnell <[email protected]

Remove '#dc' from the URL and instead scroll down the page you then get
'Circuits'.

Thanks John (where've you been?), but that gives same result I got.
IOW, I end up on page
http://www.reed-electronics.com/ednmag/index.asp?layout=siteInfo&doc_id=30982

which says: "We are sorry, but the article you've requested cannot be
found. You may try searching for the title in our Article Search
above."

T

Terry Pinnell

Jan 1, 1970
0
John Jardine said:
I really missed that Alpha after they sent it to the
great-scrapyard-in-the-sky. At the time it had been a useful source of 12V
project motors and that nice multi-strand cabling that only seems to turn up
in car wiring looms.

For the project I wouldn't dream of using H bridges. Damned things are
always messy, ugly and require effort to make 'em work reliably .
Why the motor can't be reversed?. Surely just regard the Vcc and fet wires
like what they basically are, i.e. A pos and neg supply for the motor. Run
'em through the reversing relays and limit switches as normal. The wiring
runs will add a bit of resistance and inductance but at (say) audio chopping
frequencies, the motor won't remotely notice any difference.
regards
john
I used to *add* stuff to my cars, rather than take bits away <g>.
Started more than a couple of decades ago on my own cars, when even
basics like windscreen wiper speed control were not always provided.
Later, whenever I got a new company car, I'd have to strip out any
increasingly generous, there was less need to add my own projects
anyway. What car now doesn't have a burglar alarm, ice alert,
sidelights-on warning, etc? In fact, it was only as recently as last
week that I got around to adding my very first gadget to my '96 BMW
328SE. It's one of the few things apparently not covered in all modern
cars: what I call a 'Pedestrian Horn'. A footswitch just above my left
foot (the car's auto) initiates a rapid and *relatively* quiet
bip-bip-bip of the horn/siren, to warn little old ladies or horse
riders etc just around the next bend, or that apparently oblivious
driver or passenger emerging from a car ahead. Also has advantage that
I don't need to take hands from steering wheel. The challenge was
access to the horn wiring and getting it inside the car. (Just a low
duty cycle astable driving the existing horn relay.)

Back to curtain motor control. (Can't postpone the admission any
longer!) I'm embarrassed to say that you're quite right, of course.
Don't know what I was thinking. Clearly I should be able to use a
simple single MOSFET or BJT circuit for the PWM speed control, and my
relays/microswitches for F/Off/R control, as you say.

Your post couldn't have been more timely, as I was going to get stuck
in today and build the H-Bridge design I found yesterday in this
month's edition of EPE mag. That's actually a neat design (NPN & PNP
pairs), and incorporates all the facilities I want. But it would have
been time-consuming to biild and test. Thanks for the heads up.

C

cpemma

Jan 1, 1970
0
Paul said:
The main problem with using a 555 based design is going to be the
1/3-2/3 range of available supply volts. That may not be a problem in
your particular case but it's nice to have the full rail-rail o/put
this quad op-amp gives you, IMHO.
At lowish PWM frequencies (50-150Hz) a simple 555 + switcher transistor
circuit can give about 5%-95% duty cycle at almost full supply volts, but a
2-3 opamps will give 100% for little more in parts count or cost.

Some more circuits at http://www.cpemma.co.uk/pwm.html

J

John Woodgate

Jan 1, 1970
0
I read in sci.electronics.design that Terry Pinnell <[email protected]
Thanks John (where've you been?),

Very busy, and travelling abroad a lot.
but that gives same result I got.
IOW, I end up on page
http://www.reed-electronics.com/ednmag/index.asp?layout=siteInfo&doc_id=30982

which says: "We are sorry, but the article you've requested cannot be
found. You may try searching for the title in our Article Search
above."

I think you may be picking up a bad proxy. Even the URL with the #dc in-
page reference works for me. I've copied the text on DC motors and I can
send it to you as a Word document if you want it.

T

Terry Pinnell

Jan 1, 1970
0
John Woodgate said:
I read in sci.electronics.design that Terry Pinnell <[email protected]

Very busy, and travelling abroad a lot.

I think you may be picking up a bad proxy. Even the URL with the #dc in-
page reference works for me. I've copied the text on DC motors and I can
send it to you as a Word document if you want it.

http://www.reed-electronics.com/ednmag/index.asp?layout=siteInfo&doc_id=30982
yet you can. Must admit I don't really understand what 'picking up a
bad proxy' means. My personal hygiene is pretty good ...

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