Re: Simple 555 PWM - disappointing performance
- From: Terry Pinnell <terrypinDELETE@xxxxxxxxxxxxxxxxxxx>
- Date: Wed, 25 May 2005 14:01:25 +0100
Mike Monett <no@xxxxxxxx> 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
.
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- Simple 555 PWM - disappointing performance
- From: Terry Pinnell
- Re: Simple 555 PWM - disappointing performance
- From: Bob Eldred
- Re: Simple 555 PWM - disappointing performance
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