PWM Controller

[RiccardoR]

Hi
I'm new to circuits and this site. I have currently started building a pwm controller . I currently have two issues I'm struggling to fix
1) I cant seem to get a square wave out of pin 3 555 timer
2) The motor runs and I have a 10k potentiometer which is working as a switch rather than the varying the speed of the motor

Can anyone think of any way to resolve these issues, any help would be great!
Thanks

 

[(*steve*)]

Can you post a diagram of the circuit you're using.

The are several possibilities, that the circuit is wrong, that you've built it wrong, that noise from the motor is disrupting the 555, out that you've damaged the 555 out other components (possibly from spikes from the motor).

We may have to go through a few things in order to figure out which it is.

I'm thinking of looking at the options in the order listed above (unless something points me in another direction), so the next thing I'm likely to ask for is a picture of your construction. Providing both now might help

 

[RiccardoR]

Can you post a diagram of the circuit you're using.

The are several possibilities, that the circuit is wrong, that you've built it wrong, that noise from the motor is disrupting the 555, out that you've damaged the 555 out other components (possibly from spikes from the motor).

We may have to go through a few things in order to figure out which it is.

I'm thinking of looking at the options in the order listed above (unless something points me in another direction), so the next thing I'm likely to ask for is a picture of your construction. Providing both now might help

 

[Arouse1973]

Hello Riccardo. Just a question, why are you using two 555 timers for PWM? With the addition of a couple of diodes you can just use one.
Thanks
Adam

 

[(*steve*)]

One major problem is that your 10k pot can cause the discharge pin to be connected directly to Vcc. When the transistor in the 555 turns on to discharge the capacitor it immediately gets destroyed.

The 555 datasheet (or at least some of them) will suggest a minimum resistance between Vcc and the discharge pin (the problem can also occur when your cap is connected directly to the discharge pin, but in your case the capacitor is small). I recommend you place a resistor in series with your 10k pot (say 1kΩ) and replace the 555 (U1)

 

[RiccardoR]

One major problem is that your 10k pot can cause the discharge pin to be connected directly to Vcc. When the transistor in the 555 turns on to discharge the capacitor it immediately gets destroyed.

The 555 datasheet (or at least some of them) will suggest a minimum resistance between Vcc and the discharge pin (the problem can also occur when your cap is connected directly to the discharge pin, but in your case the capacitor is small). I recommend you place a resistor in series with your 10k pot (say 1kΩ) and replace the 555 (U1)

I have a break in the track next to pot so I can't place the resistor next it , also do I have to replace 555 timer and holder it sits in or just the 555

 

[(*steve*)]

Just the 555, but wait until I have another look... (Just in case there's something else)

 

[(*steve*)]

The 1N4148 used to protect the transistor is a small signal diode and is probably not the best option. I'd use a 1N4001.

The diode would probably fail short circuit which would then take out the transistor, followed by the 555 (U1). You've said the motor still works in some respects, so the diode likely hasn't failed, and the fact that it's not on all the time probably means the transistor is OK (If the motor only turns off when the pot is at one extreme end, this is because the motor is being shorted out by the pot and the damaged 555, however I don't think that's what you observed.

Just to be sure,remove the motor and the power from the circuit and use your multimeter on the ohms range to read the resistance at the following points:

1. Across D1 (both ways). The resistance should be significantly higher one way than the other. If your meter has a diode test range, it will typically read 0.6 one way and OL the other way.
2. Across the transistor T1, again both ways. The resistance should be high (tens of kΩ or higher)
3. From pin 3 of 555 U1 to the positive and negative supply rails (or pins 1 and 8). The resistance should be in excess of 1kΩ.
4. From pin 7 to pin 1 of the 555 U1. It should be a high resistance (greater than 10kΩ) but it is likely it is low (under 1kΩ) due to the lack of a resistor in series with your pot having damaged it.

I've tried to estimate resistance values that you'll get in-circuit regardless of most likely other failures. If you're uncertain, post a list of all the resistances you've read and I'll try to interpret them

It might also be useful to see the other side of your board so I can check for any obvious mis-wiring.

 

[RiccardoR]

The 1N4148 used to protect the transistor is a small signal diode and is probably not the best option. I'd use a 1N4001.

The diode would probably fail short circuit which would then take out the transistor, followed by the 555 (U1). You've said the motor still works in some respects, so the diode likely hasn't failed, and the fact that it's not on all the time probably means the transistor is OK (If the motor only turns off when the pot is at one extreme end, this is because the motor is being shorted out by the pot and the damaged 555, however I don't think that's what you observed.

Just to be sure,remove the motor and the power from the circuit and use your multimeter on the ohms range to read the resistance at the following points:

1. Across D1 (both ways). The resistance should be significantly higher one way than the other. If your meter has a diode test range, it will typically read 0.6 one way and OL the other way.
2. Across the transistor T1, again both ways. The resistance should be high (tens of kΩ or higher)
3. From pin 3 of 555 U1 to the positive and negative supply rails (or pins 1 and 8). The resistance should be in excess of 1kΩ.
4. From pin 7 to pin 1 of the 555 U1. It should be a high resistance (greater than 10kΩ) but it is likely it is low (under 1kΩ) due to the lack of a resistor in series with your pot having damaged it.

I've tried to estimate resistance values that you'll get in-circuit regardless of most likely other failures. If you're uncertain, post a list of all the resistances you've read and I'll try to interpret them

It might also be useful to see the other side of your board so I can check for any obvious mis-wiring.

When you said about the motor only stopping once it's at one extreme end I do experience this. The motor works fine I can't turn it on and off with the pot I just can't seem to get varied speed from it
I'll start going through the test you've given me to do
Thanks

 

[RiccardoR]

I re-draw out my schematic today does the components look better placed.
(I am still carrying on with my old design) just wanted a second opinion on this recent drawing

 

[RiccardoR]