Simple 555 PWM - disappointing performance

Needs lots more base drive to saturate the 3055.

Mike Monett

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.

Herbert

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).

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.

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.

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.

Regards, Joerg

Hello Graham

But maybe not at three amps where Terry wants to use it.

Regards, Joerg

A couple of immediate comments.

Might also work better if you used a half bridge configuration ( Class D PWM ).

Graham

fT is 800 kHz IIRC.

If you treat it right though...... Some surprisingly nice audio amps used them. We're spoilt with 20-40 MHz output devices now !

Graham

But fT is designed to look good.

It's where Ic/Ib falls to 1 in the emitter follower configuration IIRC.

Btw - RCA made rugged single diffused 2N3055s with huge SOA and low fT whereas Motorola made faster ones ( 2 MHz ? ) with rather less durability. Different processes.

Took JEDEC ages to make up their mind what to do. The RCA device was renamed 2N3055H IIRC.

Similar happened with the 2N3442. Was very popular over here in the 70s ' Linsley Hood ' kit amplifier. If you got the 'wrong' one though it made a nice oscillator !

Graham

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:)

Mike Monett

Make sure that the supply voltage is rock solid. If it is routed through the breadboard to the emitter and motor, re-wire with direct leads from the power supply. My bet is that the supply (as "seen" by the motor/xsistor circuit) is not capable of the current that the motor draws. I'd also bypass at at the breadboard. After doing that, you can address stronger base drive as necessary. I agree with others - you may not have enough.

Ed

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

But you do not need any relays in this case, the H-bridge can do all what you need. Also the supply voltage should be increased, otherwise you might not be able to put enough current into the motor. Do not only monitor the voltages, but far more important is the current.

Because when the switch is off, the motor keeps turning and creating the back-EMF into basically an open circuit. This may even cause problems with the bearings, additional heating and vibration.

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?

I reckoned so, but both you and Mike have raised the question so far, so I'm beginning to wonder. But 200 mA should surely be enough? (Min gain 15, collector max 3A.) FWIW, reducing that 100 ohm base R to 50 made no difference.

OK, thanks, I'll change them.

Understood. But for this undemanding application I'd like to use a very basic single-ended approach if possible. When driven directly from my bench power supply, the motor operates reliably from under 2V 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. There's also the issue of integrating this 'add-on' with my existing circuit, which uses two relays, respectively to apply power and reverse current direction. I suspect a half bridge configuration might add complications in that respect.

Practicalities apart, I'm also curious why it doesn't give a cleaner output.

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

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

Thanks. I'll take another look at the supply, but I did try both my home made bench power supply and a 4.8V battery set. The former should be well able to supply 3A. The latter was 4 full-charged D-type NiCads. Both gave similar result. But I'm wondering if my earth returns were properly wired up; I'll check later this morning. I'll also try another set of batteries.

See my earlier replies on this.

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

Well, the switch is the 3055, isn't it? With 300Hz the motor will keep turning when the PWM is switching the transistor on and off. The motor generates a voltage when turned, I suppose it is a DC-motor with magnets on the stator.

The 2N3055 isn't saturating. Those guys have a miseraby low beta, and I'm not sure how much current a 555 can source either. I'd either reverse the sense of the PWM and add a PNP switch, or use an NPN and PNP, to up the base drive. It wouldn't hurt to pull the base down for off-times, either.

Not until you get the 3055 out of its linear area.

How much current does the motor draw?

Good Luck! Rich

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

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