Simple 555 PWM - disappointing performance

[Terry Pinnell]

Mike Monett <no@spam.com> wrote:

Bob Eldred wrote:

As has been mentioned, the saturation of the 3055 isn't very good,
it needs more base drive. For three amps of collector current it
needs at least 150mA of base current with beta of 20. Use a an
emmiter follower transistor to drive the base making a darlington
connection. Definitely, as was stated, the fly off diode should be
able to handle 3 amps surge, a 1N4001 would do the job.

However, the real question is: why does the collector turn off to
a voltage of only one volt? It should go up to the supply rail,
four volts. This implies that either the transistor is not turning
off properly or, more likely, the voltage available is no where
near four volts. Maybe a bad battery?? Or, a battery that simply
can't supply the current and is dragged down during the on time
but stays down. If this is the case, it's no wonder you have low
torque. It's also possible that the 3055 has been damged by not
being protected by the 4148 diode during fly off. In this case it
might be acting like a low ohm resistor to ground with little
transistor action. These are things to check. Also, bypass the
power supply near the motor with several microfarads of
capacitance to supply the short term current needs of the circuit.

Bob

Thanks for pointing out the two voltage scales - I didn't even
see the collector has it's own scale on the right side in red.

Now that there are two separate scales, you can see that the blue
base signal goes well over 1 Volt, and has considerable ripple. This
indicates something is not quite right with the measurement. With 5
volts driving 100 ohms in the base, the base voltage should not get
much above 0.6V.

Base is driven from 12V, not 5V, as shown in screen shot. Hence plenty
of base drive across 100 or 50 ohm base resistor. (I wonder if that's
why others have also mis-identified low base drive as a problem? Maybe
I should have added '12V' to the schematic as well as showing the
waveform.)

If the measurement were true, you'd think the base current would be
many amps, which means the 3055 would be heavily saturated, and
there should be no ripple voltage on either the base or the
collector.

Next, the collector voltage in red dips below 0.4V eight times - and
even touches zero over on the right side. I don't quite know how a
power transistor can do this.

I don't follow you on that one. I'd *like* to get the collector
voltage swinging down to 0V! But please see my reply to Bob Eldred,
including my corrected 'scope shots. I set the scale wrongly on that
earlier one, adding unnecessary confusion.

Bottom line though is that Ed and Bob Eldred correctly identified the
cause: power supply. See my reply to them, and latest screen shots.

--
Terry, West Sussex, UK

As you point out, it is very strange the collector clamps at 1 Volt.
Even stranger, it is exactly 1 Volt. I suspect something is amiss in
the measurement, which needs to be sorted out before we can make any
sense of what the circuit is actually doing.

But everyone agrees it will need more base drive

Mike Monett

 

[Terry Pinnell]

Bob Monsen <rcsurname@comcast.net> wrote:

Mike Monett wrote:
Bob Eldred wrote:

As has been mentioned, the saturation of the 3055 isn't very good,
it needs more base drive. For three amps of collector current it
needs at least 150mA of base current with beta of 20. Use a an
emmiter follower transistor to drive the base making a darlington
connection. Definitely, as was stated, the fly off diode should be
able to handle 3 amps surge, a 1N4001 would do the job.

However, the real question is: why does the collector turn off to
a voltage of only one volt? It should go up to the supply rail,
four volts. This implies that either the transistor is not turning
off properly or, more likely, the voltage available is no where
near four volts. Maybe a bad battery?? Or, a battery that simply
can't supply the current and is dragged down during the on time
but stays down. If this is the case, it's no wonder you have low
torque. It's also possible that the 3055 has been damged by not
being protected by the 4148 diode during fly off. In this case it
might be acting like a low ohm resistor to ground with little
transistor action. These are things to check. Also, bypass the
power supply near the motor with several microfarads of
capacitance to supply the short term current needs of the circuit.

Bob

Thanks for pointing out the two voltage scales - I didn't even
see the collector has it's own scale on the right side in red.

Now that there are two separate scales, you can see that the blue
base signal goes well over 1 Volt, and has considerable ripple. This
indicates something is not quite right with the measurement. With 5
volts driving 100 ohms in the base, the base voltage should not get
much above 0.6V.

If the measurement were true, you'd think the base current would be
many amps, which means the 3055 would be heavily saturated, and
there should be no ripple voltage on either the base or the
collector.

Next, the collector voltage in red dips below 0.4V eight times - and
even touches zero over on the right side. I don't quite know how a
power transistor can do this.

As you point out, it is very strange the collector clamps at 1 Volt.
Even stranger, it is exactly 1 Volt. I suspect something is amiss in
the measurement, which needs to be sorted out before we can make any
sense of what the circuit is actually doing.

But everyone agrees it will need more base drive


If you notice, on the gif, his input voltage is 15V. Thus, with a 100
ohm resistor, he is supplying 150mA, which should be sufficient for the
motor. I'm not sure the circuit is doing this, but his gif sure implies
it (See the 'simulation' waveforms).

Correct.

I think he has the diode connected incorrectly... It's probably
backwards. When the motor starts drawing current, once it gets to be
more than 0.6V below the rail, the current flows through the diode,
causing the motor to sputter.

No, diode polarity is OK. (Now a 1N4006 BTW instead of the puny
1N4148.)

Ed and Bob Eldred correctly identified the power supply as cause. See
my reply to them, with latest screen shots.

--
Terry Pinnell
Hobbyist, West Sussex, UK

 

[Terry Pinnell]

"Bob Eldred" <nsmontassoc@yahoo.com> wrote:

As has been mentioned, the saturation of the 3055 isn't very good, it needs
more base drive. For three amps of collector current it needs at least 150mA
of base current with beta of 20. Use a an emmiter follower transistor to
drive the base making a darlington connection. Definitely, as was stated,
the fly off diode should be able to handle 3 amps surge, a 1N4001 would do
the job.

However, the real question is: why does the collector turn off to a voltage
of only one volt? It should go up to the supply rail, four volts. This
implies that either the transistor is not turning off properly or, more
likely, the voltage available is no where near four volts. Maybe a bad
battery?? Or, a battery that simply can't supply the current and is dragged
down during the on time but stays down. If this is the case, it's no wonder
you have low torque. It's also possible that the 3055 has been damged by not
being protected by the 4148 diode during fly off. In this case it might be
acting like a low ohm resistor to ground with little transistor action.
These are things to check. Also, bypass the power supply near the motor with
several microfarads of capacitance to supply the short term current needs of
the circuit.

Bob, Ed: Thanks - you're right, the cause of the problem was my bench
power supply! It was not delivering the voltage it was supposed to
under load. And the set of NiCads I used yesterday also proved to be
suspect. However, I'm pleased to say that after re-charging them
overnight they sustained 4.0 V throughout today's tests. I'm now
getting the sort of result I expected.

Nothing wrong with the base drive, BTW. I'm still puzzled why everyone
except Bob Monsen thought it inadequate?

I also carelessly set my PC 'scope's scale incorrectly, so that
earlier screenshot was a bit misleading. And adding to ambiguities,
one of my probes was ~ 100 mV adrift from zero when shorted, and I've
now nulled that out. Here is the latest result:
http://www.terrypin.dial.pipex.com/Images/PWM555-10.gif

So...apart from my screenshot carelessness, it was all down to a poor
supply. I'll make that my first check next time <g>.

The only remaining 'disappointment' is that, even with the circuit
properly powered, torque at 50% duty cycle is poor. Low enough for me
to be able to stop the motor easily by hand. That's almost impossible
with the directly driven motor. Even with the original screwdriver (2
x 1.2V C-type Nicads), it's very hard to stop it. I'd assumed that PWM
would reduce speed, but virtually maintain torque? IOW, this add-on
circuit would let my curtains close at half their present speed, but
still do so reliably, overcoming the considerable friction. Now I
don't think that will be the case. So what *is* the effect of PWM on
torque?

--
Terry Pinnell
Hobbyist, West Sussex, UK

 

[vic]

Terry Pinnell wrote:

The only remaining 'disappointment' is that, even with the circuit
properly powered, torque at 50% duty cycle is poor. Low enough for me
to be able to stop the motor easily by hand. That's almost impossible
with the directly driven motor. Even with the original screwdriver (2
x 1.2V C-type Nicads), it's very hard to stop it. I'd assumed that PWM
would reduce speed, but virtually maintain torque? IOW, this add-on
circuit would let my curtains close at half their present speed, but
still do so reliably, overcoming the considerable friction. Now I
don't think that will be the case. So what *is* the effect of PWM on
torque?

The voltage is proportional to the speed of the motor.
The current is proportional to the torque.

vic

 

[Rich Grise]

On Wed, 25 May 2005 07:53:21 +0100, Terry Pinnell wrote:
....

upwards through at least 6V. So there should be plenty of tolerance
for some CE drop if I use say a 4 - 5V supply. All I want to do is
slow it down, maybe to 1/3 or 1/2 speed, while maintaining close to
full torque.

To make it really sexy, hide a couple of slide pots to use just
before your limit switches, lightly spring-loaded, then control
your PWM with the slide pots, from 99% (100% would be better, but I'm not
sure how much hassle it'd be to make a PWM tha'll go to 100%), tapering
down to very small just as it hits the limit. Select which pot with
another set of contacts on the up/down switch.

Cheers!
Rich

 

[Rich Grise]

On Wed, 25 May 2005 15:06:47 +0100, Terry Pinnell wrote:

Bob, Ed: Thanks - you're right, the cause of the problem was my bench
power supply! It was not delivering the voltage it was supposed to
under load. And the set of NiCads I used yesterday also proved to be
suspect. However, I'm pleased to say that after re-charging them
overnight they sustained 4.0 V throughout today's tests. I'm now
getting the sort of result I expected.

I've tried to cancel my other response, that I dashed off before
even bothering to read the rest of the thread, so if it got
through anyway, please ignore it.

(I've left the one posted where I suggest the slide pots for
soft braking.)

Thanks,
Rich

 

[James Beck]

In article <e788915cfi9qr14cj3truavse914ig6shg@4ax.com>,
terrypinDELETE@THESEdial.pipex.com says...

Joerg <notthisjoergsch@removethispacbell.net> wrote:

Hello Terry,

As Graham said, a lone 2N3055 isn't going to cut it. I don't remember
its beta but even if it was 50 at 3A the 555 would have to supply 60mA.
And the beta probably isn't even 50.

See my replies to Mike and Graham. A 555 can supply 200mA.

Use a decent size FET. Or at least use another smaller transistor in
front of the 2N3055 in a Darlington configuration. Also, don't run tens
of kHz. A 2N3055 isn't exactly a rocket.

?? As the 'scope screen shot shows, it's running at about 300 Hz.

Regards, Joerg

You can talk about the 555 sinking/sourcing 200mA all day long, BUT you
still have to deal with the fact that the 3055 isn't being driven hard
enough to work as a switch for you. You are asking a very poor
amplifier to do the job of a saturated switch.
If you want it to work you need to do the following :
Use a FET or at least an NPN darlington.
Change the 4148 out for something like a 1n4004, or better yet something
rated for 3A or more like an SB560.
Make sure you have good decoupling on your 555 circuit.

Jim

 

[James Beck]

In article <pan.2005.05.25.16.01.46.368351@example.net>,
richgrise@example.net says...

On Wed, 25 May 2005 15:06:47 +0100, Terry Pinnell wrote:

Bob, Ed: Thanks - you're right, the cause of the problem was my bench
power supply! It was not delivering the voltage it was supposed to
under load. And the set of NiCads I used yesterday also proved to be
suspect. However, I'm pleased to say that after re-charging them
overnight they sustained 4.0 V throughout today's tests. I'm now
getting the sort of result I expected.

I've tried to cancel my other response, that I dashed off before
even bothering to read the rest of the thread, so if it got
through anyway, please ignore it.

(I've left the one posted where I suggest the slide pots for
soft braking.)

Thanks,
Rich


Me too, but I still stand by my observations.

Just because it is working now does not mean I would rely on it to
continue far into the future. Parts age, and it is best to think in
worse case mode.

Jim

 

[ehsjr]

Terry Pinnell wrote:

Bob, Ed: Thanks - you're right, the cause of the problem was my bench
power supply! It was not delivering the voltage it was supposed to
under load. And the set of NiCads I used yesterday also proved to be
suspect. However, I'm pleased to say that after re-charging them
overnight they sustained 4.0 V throughout today's tests. I'm now
getting the sort of result I expected.

Nothing wrong with the base drive, BTW. I'm still puzzled why everyone
except Bob Monsen thought it inadequate?

Guilty as charged - I thought you were running the 555 from
between 4 & 5 volts. Max base I would be < 50 ma ...

Glad you got it working. I think you may get better torque
with a higher frequency.

Here's another possible approach: run it direct (no PWM) and
move the limit switch actuator to shut the motor off sooner.
That will give you full speed. Alternatively, you could go
with two limit switches like Rich (?) mentioned - the first
one slows it down (via the PWM circuit or just a resistor)
and the second one stops it.

Ed

I also carelessly set my PC 'scope's scale incorrectly, so that
earlier screenshot was a bit misleading. And adding to ambiguities,
one of my probes was ~ 100 mV adrift from zero when shorted, and I've
now nulled that out. Here is the latest result:
http://www.terrypin.dial.pipex.com/Images/PWM555-10.gif

So...apart from my screenshot carelessness, it was all down to a poor
supply. I'll make that my first check next time <g>.

The only remaining 'disappointment' is that, even with the circuit
properly powered, torque at 50% duty cycle is poor. Low enough for me
to be able to stop the motor easily by hand. That's almost impossible
with the directly driven motor. Even with the original screwdriver (2
x 1.2V C-type Nicads), it's very hard to stop it. I'd assumed that PWM
would reduce speed, but virtually maintain torque? IOW, this add-on
circuit would let my curtains close at half their present speed, but
still do so reliably, overcoming the considerable friction. Now I
don't think that will be the case. So what *is* the effect of PWM on
torque?

 

[Pooh Bear]

Terry Pinnell wrote:

Mike Monett <no@spam.com> wrote:

Terry Pinnell wrote:

[...]

What is the main reason for its poor performance please? I didn't get
around to trying it, but would a cap (100nf?) across the motor help?
What other changes would get me closer to the idealised result? (BTW,
I do get that when I swap the motor for a purely resistive load.)
Terry Pinnell
Hobbyist, West Sussex, UK

Needs lots more base drive to saturate the 3055.

Thanks, but I'm not sure why I need more than the 200 mA max output
current capability of the 555? With 3A collector current, the 2N3055's
gain only has to be 15 or more to drive the motor fully from that,
yes?

From Motorola's data.

Saturation is measured at Ic/Ib = 10

@ 3 A Vce sat = 0.5V ; Vbe = 1.25V

That may help explain the 'high' Vbe commented on by others.

Graham

 

[Terry Pinnell]

Fred Bloggs <nospam@nospam.com> wrote:

What is the main reason for its poor performance please?

The output torque must be closer to ~ (duty)^2 and not ~(duty), so that
at duty=50% you will get somewhat less than torque equivalent to <1V
across motor. Increase your duty to 70% and see what happens- and as
stated replace the 4148 with a 4001.

Ah, that explains it, thanks. Intuitively I'd have expected a linear
relationship, with torque proportional to (average) current. And
therefore I'd hoped that I could use a supply of say 4.8V @ 50% duty
to get same torque as 2.4V direct, but at half speed. From that ^2
relationship, I'd need to go to 10V, which might be a tad *too* high
for a motor originally using two Nicads!

Is that ^2 proportionality easily derived?

--
Terry Pinnell
Hobbyist, West Sussex, UK

 

[Terry Pinnell]

vic <vic@paris> wrote:

Terry Pinnell wrote:
The only remaining 'disappointment' is that, even with the circuit
properly powered, torque at 50% duty cycle is poor. Low enough for me
to be able to stop the motor easily by hand. That's almost impossible
with the directly driven motor. Even with the original screwdriver (2
x 1.2V C-type Nicads), it's very hard to stop it. I'd assumed that PWM
would reduce speed, but virtually maintain torque? IOW, this add-on
circuit would let my curtains close at half their present speed, but
still do so reliably, overcoming the considerable friction. Now I
don't think that will be the case. So what *is* the effect of PWM on
torque?

The voltage is proportional to the speed of the motor.
The current is proportional to the torque.

vic

Thanks, but see Fred's message and my reply.

--
Terry Pinnell
Hobbyist, West Sussex, UK

 

[Terry Pinnell]

ehsjr <ehsjr@bellatlantic.net> wrote:

Terry Pinnell wrote:



Bob, Ed: Thanks - you're right, the cause of the problem was my bench
power supply! It was not delivering the voltage it was supposed to
under load. And the set of NiCads I used yesterday also proved to be
suspect. However, I'm pleased to say that after re-charging them
overnight they sustained 4.0 V throughout today's tests. I'm now
getting the sort of result I expected.

Nothing wrong with the base drive, BTW. I'm still puzzled why everyone
except Bob Monsen thought it inadequate?

Guilty as charged - I thought you were running the 555 from
between 4 & 5 volts. Max base I would be < 50 ma ...

Glad you got it working. I think you may get better torque
with a higher frequency.

Here's another possible approach: run it direct (no PWM) and
move the limit switch actuator to shut the motor off sooner.
That will give you full speed. Alternatively, you could go
with two limit switches like Rich (?) mentioned - the first
one slows it down (via the PWM circuit or just a resistor)
and the second one stops it.

Ed

Thanks for all the follow-ups. Appreciate the advice.

Ed: Potential snag with 'earlier switch off' is that curtains are then
unlikely to reach their limits. In particular, when opening, the last
few inches requires the greatest torque, because of 'bunching'.

James: May well try a MOSFET to see if there's any noticeable
improvement.

--
Terry Pinnell
Hobbyist, West Sussex, UK

 

[Pooh Bear]

Terry Pinnell wrote:

< snip >

Nothing wrong with the base drive, BTW. I'm still puzzled why everyone
except Bob Monsen thought it inadequate?

Didn't realise you were running the 555 off 12 or 15 V.

Also haven't used a 555 in ages - decades actually ( well actually I just did
recently - but just driving a tiny load and wasn't interested in the load current
specs ).

< snip >

The only remaining 'disappointment' is that, even with the circuit
properly powered, torque at 50% duty cycle is poor. Low enough for me
to be able to stop the motor easily by hand. That's almost impossible
with the directly driven motor. Even with the original screwdriver (2
x 1.2V C-type Nicads), it's very hard to stop it. I'd assumed that PWM
would reduce speed, but virtually maintain torque? IOW, this add-on
circuit would let my curtains close at half their present speed, but
still do so reliably, overcoming the considerable friction. Now I
don't think that will be the case. So what *is* the effect of PWM on
torque?

Have you tried stopping the motor when driven with a DC voltage of 1.2V ? Is it
comparable to the 50% duty cycle ?

Note that when the 2N3055 is off - the motor continues to rotate but acts as a
motor-generator and isn't it loaded by the 1N4004 ( as it is now ) . That is
likely to reduce both torque and power.

Half bridge would eliminate this effect.

Graham

 

[john jardine]

"Terry Pinnell" <terrypinDELETE@THESEdial.pipex.com> wrote in message
news:1fm991dbm2bi04d3a1oql2j4ml9sng4dql@4ax.com...

vic <vic@paris> wrote:

Terry Pinnell wrote:
The only remaining 'disappointment' is that, even with the circuit
properly powered, torque at 50% duty cycle is poor. Low enough for me
to be able to stop the motor easily by hand. That's almost impossible
with the directly driven motor. Even with the original screwdriver (2
x 1.2V C-type Nicads), it's very hard to stop it. I'd assumed that PWM
would reduce speed, but virtually maintain torque? IOW, this add-on
circuit would let my curtains close at half their present speed, but
still do so reliably, overcoming the considerable friction. Now I
don't think that will be the case. So what *is* the effect of PWM on
torque?

The voltage is proportional to the speed of the motor.
The current is proportional to the torque.

vic

Thanks, but see Fred's message and my reply.

--
Terry Pinnell
Hobbyist, West Sussex, UK



I just measured a small screwdriver type motor. It came out at 6mH. If yours

is similar (or worse) then at 300Hz the inductive current will never rise to
it's optimum value. Maybe only gets to 60% of where you think it should be,
which makes the apparent torque even worse than square law.
Chop the motor at a much lower 50-100Hz.
regards
john

 

[Fred Bloggs]

Terry Pinnell wrote:

Fred Bloggs <nospam@nospam.com> wrote:




What is the main reason for its poor performance please?

The output torque must be closer to ~ (duty)^2 and not ~(duty), so that
at duty=50% you will get somewhat less than torque equivalent to <1V
across motor. Increase your duty to 70% and see what happens- and as
stated replace the 4148 with a 4001.


Ah, that explains it, thanks. Intuitively I'd have expected a linear
relationship, with torque proportional to (average) current. And
therefore I'd hoped that I could use a supply of say 4.8V @ 50% duty
to get same torque as 2.4V direct, but at half speed. From that ^2
relationship, I'd need to go to 10V, which might be a tad *too* high
for a motor originally using two Nicads!

Is that ^2 proportionality easily derived?

No as this means the power supply delivers current to both rotor and
stator- but it is the only other explanation given that your
measurements and circuit are shown correctly.

 

[Rich Grise]

On Wed, 25 May 2005 17:01:05 +0000, ehsjr wrote:

Terry Pinnell wrote:

Bob, Ed: Thanks - you're right, the cause of the problem was my bench
power supply! It was not delivering the voltage it was supposed to
under load. And the set of NiCads I used yesterday also proved to be
suspect. However, I'm pleased to say that after re-charging them
overnight they sustained 4.0 V throughout today's tests. I'm now
getting the sort of result I expected.

Nothing wrong with the base drive, BTW. I'm still puzzled why everyone
except Bob Monsen thought it inadequate?

I think that's also been adequately explained by now, so not to worry.

Guilty as charged - I thought you were running the 555 from between 4 &
5 volts. Max base I would be < 50 ma ...

Glad you got it working. I think you may get better torque with a higher
frequency.

This is interesting - I'm nexting through the thread, and the last
(most recent) post said TP should use a lower frequency. This could be a
whole topic of its own. My last successful (in the smoke test sense - no
commercial potential, because the "customer" turned out to be a swindler)
PWM ran at about 100 Hz. It drove a 12V, 1.5A PM motor, which is probably
comparable to a screwdriver motor.

Here's another possible approach: run it direct (no PWM) and move the
limit switch actuator to shut the motor off sooner. That will give you
full speed. Alternatively, you could go with two limit switches like
Rich (?) mentioned - the first one slows it down (via the PWM circuit or
just a resistor) and the second one stops it.

Well, It was I who suggested something like this, but actually,
since I like doing stuff with physical hardware, I had suggested
an arrangement of a couple of slidepots, which would give a very
dramatic (theatrically) slowing effect, such that it comes to a
complete stop just as it actuates the limit switch. There are
those who would call it a klooge, but "those people" are probably
of the same ilk as those who misspell "klooge", to make it look
like it rhymes with "sludge." ;-)

Cheers!
Rich

 

[Rich Grise]

On Wed, 25 May 2005 21:03:47 +0100, Terry Pinnell wrote:

ehsjr <ehsjr@bellatlantic.net> wrote:

Here's another possible approach: run it direct (no PWM) and
move the limit switch actuator to shut the motor off sooner.
That will give you full speed. Alternatively, you could go
with two limit switches like Rich (?) mentioned - the first
one slows it down (via the PWM circuit or just a resistor)
and the second one stops it.

Thanks for all the follow-ups. Appreciate the advice.

Ed: Potential snag with 'earlier switch off' is that curtains are then
unlikely to reach their limits. In particular, when opening, the last
few inches requires the greatest torque, because of 'bunching'.

James: May well try a MOSFET to see if there's any noticeable
improvement.

Yes, screwdrivers aren't noted for coasting a lot!

Of course, I like my own, which the determining factor is a constant-
current driver - really quite simple, just the standard emitter
resistor and voltage-regulated base drive, but it gave full torque
down to zero RPM.

It ran at about 100 Hz. The guy whose design I was one-upping at the
time had evidently used a freq. somewhere in the Khz range, because
one of the complaints from the customer was that it "whined".

I've since misplaced the schematic, but it's trivial. A two-
transistor astable, where the two base resistors are replaced
by a pot with about 1K at each end, and the wiper to +V. Standard
emitter follower, a couple of 1N4148s, and a TIP33C current sink.
I picked the TIP33C for its robustness, and MPSA06's for the
little trannies, for the same reason.

But I had 20-40V of headroom, and needed the robustness because it
was supposed to be in a MIG welder spool gun.

Wanna buy one? You've seen the pic of part^H^H^H^Hmost of one of the
prototypes:
http://www.abiengr.com/~sysop/images/PWM-prototype-bottom.jpg
http://www.abiengr.com/~sysop/images/PWM-prototype-top.jpg

The TIP33C mounts to the heatsink, of the same footprint, but with
about 3/4" fins - if anybody asks, I'll go dig out a whole one.
<time passes>
Ah, heck. Before hitting "send", I always like to take a little
break, such that if I make a complete spectacle of myself, I at
least know I'm being foolish. Well, I went out to the RV to refresh
my tipple, and they just jumped out at me. There's a couple more
pix there, which I've gone ahead and uploaded just because they're
there. You should be able to figure out which is which:
http://www.abiengr.com/~sysop/images/

Yeah, they all exhibit crappy workmanship. Sorry, but I was underpaid,
and I've always been terribly, terribly lazy. If I got paid accordingly,
I do have the capacity to really dress stuff up. (of course, they
always say, "dress up first!" Screw'em. I'm too old for that crap.

And I don't know if there's anybody with significant money who'd
want to pay me enough to cut off my gorgeous hair. ;-P

BTW, the pot isn't shown - it mounts off-board, and solders to the
three turret terminals on the right-hand side. The DC input is on the
left. The motor negative lead goes to the central turret terminal where
the collector of the TIP33C is. I don't remember what the other turret
terminal is for. :-Z

Cheers!
Rich

 

[Terry Given]

Terry Pinnell wrote:

"Herbert Blenner" <a1eah71@aol.com> wrote:


Replace Q1 with a PNP transistor in common collection configuration.
Now this circuit can take advantage of the sizable current sink
capability of the 555 timer. Further since you are now switching
voltage across the motor, D1 is not needed.


Thanks, Herbert, but according to the LM555 datasheet, a 555 "output
can source or sink 200 mA."

--
Terry Pinnell
Hobbyist, West Sussex, UK

and besides, he is completely wrong about the "dont need the diode",
which of course you still do. The diode is placed to ensure the motor
current has somewhere to commutate to when the switch turns off. for a
pnp switch, the pnp collector goes to the motor and the cathode of the
diode; the other end of the motor and the diode anode are at zero volts.

advice like that you can live without.

BTW, the point Ban made is quite correct. The motor has some inductance,
Lmotor. This is what "forces" the current to keep flowing when the
2n3055 switches off. When the switch is on, current ramps up, current
slope = dI/dt = Vbattery/Lmotor. Eventually this current gets so high
the voltage drop across the winding resistance gets in the way (eg in a
relay, Rwinding sets Irelay). When the transistor switches off, the
voltage on the winding reverses (Lenz' law) forward biasing the diode
to, say, 1V. The inductor current now ramps down, dI/dt = Vdiode/Lmotor

Because Vdiode is small (1V cf 15V) the current ramps down slowly. This
will limit the performance you get from the machine. A simple fix for a
low power single-ended circuit is to add a zener in series with the
diode (or a few more diodes), but beware the losses!

Cheers
Terry

 

[Terry Given]

Terry Given wrote:

Terry Pinnell wrote:

"Herbert Blenner" <a1eah71@aol.com> wrote:


Replace Q1 with a PNP transistor in common collection configuration.
Now this circuit can take advantage of the sizable current sink
capability of the 555 timer. Further since you are now switching
voltage across the motor, D1 is not needed.



Thanks, Herbert, but according to the LM555 datasheet, a 555 "output
can source or sink 200 mA."

--
Terry Pinnell
Hobbyist, West Sussex, UK


and besides, he is completely wrong about the "dont need the diode",
which of course you still do. The diode is placed to ensure the motor
current has somewhere to commutate to when the switch turns off. for a
pnp switch, the pnp collector goes to the motor and the cathode of the
diode; the other end of the motor and the diode anode are at zero volts.

advice like that you can live without.

BTW, the point Ban made is quite correct. The motor has some inductance,
Lmotor. This is what "forces" the current to keep flowing when the
2n3055 switches off. When the switch is on, current ramps up, current
slope = dI/dt = Vbattery/Lmotor. Eventually this current gets so high
the voltage drop across the winding resistance gets in the way (eg in a
relay, Rwinding sets Irelay). When the transistor switches off, the
voltage on the winding reverses (Lenz' law) forward biasing the diode
to, say, 1V. The inductor current now ramps down, dI/dt = Vdiode/Lmotor

Because Vdiode is small (1V cf 15V) the current ramps down slowly. This
^^^

4V. RTFP

will limit the performance you get from the machine. A simple fix for a
low power single-ended circuit is to add a zener in series with the
diode (or a few more diodes), but beware the losses!

Cheers
Terry