Re: Simple 555 PWM - disappointing performance

Bob Monsen <rcsurname@xxxxxxxxxxx> 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

.

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