PWM circuit from 20vdc

jpanhalt

Joined Jan 18, 2008
11,087
Are the grounds of the 14V and 20 volt supplies connected to each other? In other words, everything must have the same ground. John
 

jpanhalt

Joined Jan 18, 2008
11,087
I would step back and look at it as separate problems. Problem #1 is getting the controller to work. Problem #2 is getting the controller and motor to work from the 20VAC line.

Will your motor run on 12-14 VDC? If so, power everything from the same 14 V DC supply. If you can get that to work, then address the separate supply problem.

Something is drawing too much current. I assume you do not have an oscilloscope. Does your voltmeter have a Amp function? If so, put it in series with the 14 V supply and see what it is drawing.

We are having a big snow storm right now. Nothing is moving in the city. So, I will check back frequently.

John
 

Thread Starter

nitrochicken

Joined Mar 7, 2008
24
Hi John,
Sorry for the delay,
I have just checked the current through the 14v supply and its only drawing between 8-16mA from end to end on the pot.
The motor will run at 14v, I just tried it and the transistor stayed very cool, it was drawing 1.3A at this voltage. I need it to run at 20v though as it isn't fast enough.
I'm still not getting the full range of speed control, so I think something else must be wrong as this was working the first time round. Could the 2200uf cap and the 10nf cap to pin 5 have affected this?
Thanks
Dale
 

jpanhalt

Joined Jan 18, 2008
11,087
I would handle the problem with the speed range last, as almost anything you do not could affect those settings.

Your most recent schematic shows a 47K pot. Your setting must be pretty close to the low end, as the 1K resistor on top would only draw 14 mA when shorted. That is a bit of an aside, though. The current measurements I meant to get were the total from the supply and at Pin 8 .

Re. cap at Pin 5: Pin 5 is to an input to one of the comparators in the 555. It can be used to set the threshold voltage and sometimes can be left open (floating) without a problem. However, it is generally not a good practice to leave the pin floating, thus the use of the capacitor. I don't see how it could be creating a problem. I have built a controller with a 555 and had it connected to the cap.

Motor current: Your value sounds good. Remember it is PWM'd, so you are seeing only some estimate of the average, not the peak to peak with your meter. The meter is not fast enough for that. Does your meter have a duty cycle and frequency setting? If so, see what the duty cycle reads. Caution: The readings can be confusing at the extremes. That is, as you get above 80 to 90%, the meter may suddenly start showing 1-duty cycle.

The good news is that it is working at 14V, which I assume is from some sort of supply or battery and is reasonably go DC. So, the question seems to be why the mosfet is apparently not turning on completely with the 20V supply. You have already confirmed the grounds, but double check that the 20V supply ground (negative DC) is connected to the same ground as the 14V supply and the Source pin of the mosfet. If that checks out, do you have a small value, high wattage resistor? I am thinking that noise from the supply could be affecting your results. If you put it in series with that supply before the motor, you will set a maximum current. A 4 to 10 ohm 25W resistor will probably get pretty hot, but will give you enough time to see what the mosfet is doing. The big capacitor across the supply also acts as a filter, so I would leave it in place.

It sounds like you have made good progress. Only one big step -getting it to work with the separate supply - is left. The range of speeds should be relatively easy. You may not get a full zero to 100%, but you can get pretty close.

John
 

Thread Starter

nitrochicken

Joined Mar 7, 2008
24
Hi John,
I've just been checking over everything again and I've sorted the problem with the speed range. One leg of the diode from pin 7 was not soldered, I hadn't even pushed it through the board!
With that sorted the circuit works really well on the 14v supply(the battery from my cordless drill!) and the transistor stays stone cold.
When I tried the 20v supply it started to get very hot again. (I think the bridge rectifier diodes are ok now though)

I have checked again and the ground from the 20v supply is definatley connected, and linked in with the 14v ground.

My meter does only ac/dc V, A, Ohms and diode check unfortunatley.

I don't have any large resistors, only thing I have is a 5ohm 240v AC heating element(just a coil of resistive wire), would that work?
Also, I'm not sure what that would allow me to check?

What do you mean by noise from the supply?

Just to check, I'll let you know my transformer wiring. Its a 75VA, has primary 240v ac side connected to mains, and 2 secondary coils with 4 outputs marked 0, 20vac, 0, 20vac from left to right. I have parallelled the 2 secondary coils up and have them going into my rectifier. They output 21v

Thanks

Dale
 

Thread Starter

nitrochicken

Joined Mar 7, 2008
24
Not sure its relevent but just to mention that the motor has 3 small unmarked caps on, 1 between legs and 1 from each leg to case.

Thanks
 

jpanhalt

Joined Jan 18, 2008
11,087
Not sure its relevent but just to mention that the motor has 3 small unmarked caps on, 1 between legs and 1 from each leg to case.

Thanks
Not a problem. Those are some of the caps I mentioned in my earlier post. They are for electrical noise suppression. This doesn't happen to be a model airplane motor, does it? John
 

jpanhalt

Joined Jan 18, 2008
11,087
I've just been checking over everything again and I've sorted the problem with the speed range. One leg of the diode from pin 7 was not soldered, I hadn't even pushed it through the board!
With that sorted the circuit works really well on the 14v supply(the battery from my cordless drill!) and the transistor stays stone cold.
Great! You are almost there.

I don't have any large resistors, only thing I have is a 5ohm 240v AC heating element(just a coil of resistive wire), would that work?
Yes, probably.

Also, I'm not sure what that would allow me to check?
It's a current limiter, current indicator (heat), and known load. Simple power supplies with a rectifier and capacitor (essentially what you have) are helped with a load.

What do you mean by noise from the supply?
Simple rectified AC is not "pure" like from a battery. There will be ripple from the half-waves. The capacitor and load help smooth it. Your operating PWM frequency is about 4X line frequency, so there could be some coupling effects.

... to mains, and 2 secondary coils with 4 outputs marked 0, 20vac, 0, 20vac from left to right. I have parallelled the 2 secondary coils up and have them going into my rectifier. They output 21v
Check the AC voltage from the transformer. If it is 20 V RMS, the rectified voltage should be about 28 V.

John
 

rwmoekoe

Joined Mar 1, 2007
172
nitrochicken,

are you using the low
please check the output level of the 555 (pin 3). to get it done maybe it'll be easier to hold the output at each level for awhile while you're measuring. (short pin 2+3 to ground to get high level output, short pin 2+3 to vcc to get low level output).

please make sure that the low level output is well below 2 volts, and the high above 8 volts.

are you using 7555 or other low power 555 type?

anyway, if things checked out right and you still got the heat sink too hot to touch, you should try adding a driver circuit between the 555 and the mosfet.
 

Thread Starter

nitrochicken

Joined Mar 7, 2008
24
Eureka!

It works, it works!

I checked over everything again and found I had made another mistake. I'd put the 2200uf cap in parallel with the motor and not over the supply as in the diagram. Basically, should have gone in the next hole over! I'm trying to make it small so its very fiddly!
I guess that cap is doing a good job after all in smoothing the rectified supply.
It now runs perfectly with no detectable heat from the mosfet.
The rectifier diodes do get a little warm but nothing too bad, I can touch them.

Interestingly, from what you said about the rectified voltage being higher its actually lower. I checked and the ac is coming out at 22.4v and the rectified dc is 21.5v.

I suppose you are wondering what it is I'm making, you were miles away John! Its actually a coffee bean roaster I'm attempting to make. It started life as a hot air popcorn popper I got for £7 from argos. The motor was powered from rectified AC with the voltage being dropped by a second 5ohm heating element that I mentioned earlier.
I also have a PID controller that I'm going to use with a SSR to control the heating element. I haven't started that part yet but I think it shouldn't be too hard.

Its kinda overkill really, and who knows how well it will work. It would probably have been cheaper to buy a home roasing machine, but there's more fun if I'm making it!

Thanks to everyone for your help,

I might be back with PID problems at some point.

Regards

Dale
 

jpanhalt

Joined Jan 18, 2008
11,087
That is truly great to hear. Be happy you are not in Cleveland, Ohio. I just finished shoveling a foot of snow. Freshly roasted coffee would be really good right now. John
 
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