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555 speed controller... help using transistor on output

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5ky

New Member
Hi. I made a simple speed controller with a 555 timer. (just using variable resistor to control duty cycle, a ghetto-PWM)

Though it's simple and works fine, I know I won't be able to run anything with high current on this setup since the 555 timer has it's limitations so I decided I would use the output to drive a high current transistor. There's a problem though, I'm using the discharge output (pin7) which shorts the caps to the ground, which means it's going to be the negative output. The NPN and PNP transistors I have both require positive voltage for the base. :( (tip31a, tip42g are the two I have on hand)

I'm fairly new to electronics but I know most of the basics. Any help or anything pointing me in the right direction would be greatly appreciated. I know there are some simple speed controllers out there using mosfets but I would really like this one to work out mostly because I don't like to give up if there's a possible solution.

Thanks for your time!

Oh and here's a link to the schematic I used:
pwm.png (image)
 

smanches

New Member
Use the output pin, not the discharge pin. You might also need a smaller transistor to drive the larger one, since the 555 can only output so much current. Might not be enough to turn on large BJTs.

EDIT: Guess I should have looked at the schematic first. I've not used that configuration before. Now I'm going to test it. :)
 
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BrownOut

Banned
PNP transistors "turn on" when the base is negative WRT the emmitter. If connected and biasted correctly, it will turn on when the output of the 555 is 'high.' You can also use a NPN transistor, properly biased, to turn on when the output is low, and pin 7 is not turned on.

It will work, I've done it both ways.
 

MikeMl

Well-Known Member
Most Helpful Member
That is poor circuit. Here is a better one
 

5ky

New Member
Use the output pin, not the discharge pin. You might also need a smaller transistor to drive the larger one, since the 555 can only output so much current. Might not be enough to turn on large BJTs.

EDIT: Guess I should have looked at the schematic first. I've not used that configuration before. Now I'm going to test it. :)
thanks! i googled and found a couple other schematics for the same thing that used the output pin like you mentioned which works perfectly with my tip31a.

so... you mentioned the 555 possibly not being able to put out enough current to turn the tip31a on... so you're suggesting i get a small transistor (like a 2n2222) and use it power the tip31a... so would this be like a darlington pair?

thanks for the responses, i really appreciate it.
 

Chippie

Member
so... you mentioned the 555 possibly not being able to put out enough current to turn the tip31a on... so you're suggesting i get a small transistor (like a 2n2222) and use it power the tip31a... so would this be like a darlington pair?
Thats right, of course you could just replace the TIP31A with a drop in Darlington power device....Select one that has an appropriate current handling capacity to match your load..
 

5ky

New Member
Thats right, of course you could just replace the TIP31A with a drop in Darlington power device....Select one that has an appropriate current handling capacity to match your load..
amazingly it was able to switch the tip31a just fine without a smaller transistor to drive it.

so now that it works i have some simple questions:

i tried switching the .1 uf caps for .01 uf caps and i don't notice any difference in power delivery except for the motor makes alot of high pitch noises with the higher frequency setup. is there any difference? should i just go with the .1 uf caps to keep this thing quiet?

thanks =)
 

Chippie

Member
You can go either way with the cap value...raise its value to lower the frequency and vice versa...Change it for a value that you are comfortable with..
 

smanches

New Member
amazingly it was able to switch the tip31a just fine without a smaller transistor to drive it.
Were you driving it at your full load? More drive current is required as the collector-emitter current goes up.

i tried switching the .1 uf caps for .01 uf caps and i don't notice any difference in power delivery except for the motor makes alot of high pitch noises with the higher frequency setup. is there any difference? should i just go with the .1 uf caps to keep this thing quiet?
You could also go much higher, out of hearing range, which is where most speed controls work. Over 20Khz or so.
 

5ky

New Member
Were you driving it at your full load? More drive current is required as the collector-emitter current goes up.
no I've only applied 120mA so far because I don't have anything layin around that will pull more current but I see what you're saying. would a small 2n2222 npn transistor feeding into the tip31a as a darlington pair suffice?

You could also go much higher, out of hearing range, which is where most speed controls work. Over 20Khz or so.
I guess I need to find some smaller capacitance caps then because with the 0.01 uf it was still audible and it was pretty annoying. i don't think i have any caps smaller than that, so i guess i need to start throwing them into series? (i think?)
 

smanches

New Member
I think the general ratio of base to collector current is around 1:10, but I'm sure someone can give much more accurate information. A 2n2222 would drive a 500ma base current into the Tip31a, which would be able to push a 5A load? Seems like it should handle more than that, so I must be wrong.

I actually have no idea how fast you can get a 555 going. Maybe you can't even get above hearing range. Would have to do some tests.

You should put a small cap, .01F or so, between the collector and emitter of the tip31a. Place it as close to the transistor as possible. It will reduce some high voltage ringing that could cause issues, as well as some noise. Might also try a 1uF cap across the motor which would reduce the noise as well. If this becomes bi-directional speed control, you'll need to make sure it's not a polarized cap.

You will also need a freewheel diode across the motor, in reverse of normal current flow. This prevents the inductive kick of the motor windings shutting off from kicking the crap out of your main power transistor.
 
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5ky

New Member
ok well i went out and bought an assorted bag of small ceramic caps and found some just small enough to hit a frequency outside of my hearing range (don't have an oscilloscope). the only problem i found is that the higher the frequency, the lower the torque is to the motor and takes quite a bit of duty cycle before the motor begins turning. once it starts turning though i can turn it back down. the max rpm power seems alot higher now though.

with the frequency alot lower (using .1 uf caps) there were lots of vibrations and audible humming noises but the lower rpm torque was alot stronger.

which makes me wonder if there's a way to have variable frequency, whether it be a low and high setting that switches at the halfway mark or to have it fully variable without straying away from the 555 timer setup?

i'm not sure if i made that very clear but i'm basically building an underwater ROV with my buddy and the speed controls will control the thrusters and i want really good low speed control for accurate and precise maneuvering. with the higher frequency setup i crank the duty cycle to nearly 30% before the motor starts spinning and when it does it's too much speed.

hope that made sense.

UPDATE: heck a really quick burst of current to get the motor spinning would work with the high frequency setup. but even that i'm not sure how to go about doing.
 
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Chippie

Member
So you learnt that the higher the pwm frequency the lower the motor's torque due to the inefficienies of the motor...

With a little experimentation, you should be able to find a 'sweet spot' where the pwm frequency isnt too bothersome and the motor's torque is acceptable..
 

colin55

Well-Known Member
These's another way to do it. It detects the feedback from the motor (spikes) to create a constant speed over a wide range of load.
It also provides high start-up torque:


I have used it in thousands of kits and found it to be very successful.
 
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