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Old 10-04-2012, 07:00 PM   #1
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Default PWM trouble

After reading the forum on using a PWM to control a boil kettle i decided to attempt to make my own. I ordered the parts Walker suggested and built the board following his diagram. After testing the circuit out however I find that the PWM is not operating. After hooking it up to a flashlight i expected to see it turn on and off as the pot turned up and down but nothing happened. The light stayed on regardless of where the pot was turned to except for when I turn the pot off then the light turn off. Not knowing anything about circuit boards I am not sure if I did something wrong or broke something so I was hoping someone could help me trouble shoot this. The only difference between the board I made and the one Walker used is that mine has two rows on the outside where as his had only one. (I used the inner row on each side)

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Old 10-04-2012, 08:24 PM   #2
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I'd review the entire thread. Somewhere in it someone spotted a problem with the way it was written up. Walker acknowledge the problem, but I don't know if the doc got updated. Perhaps that's the issue.

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Old 10-04-2012, 08:30 PM   #3
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Can you add a link to the project? If it's designed for an electric kettle it's possible that the modulation frequency is higher that what is visibly detectable, however if that were the case you would notice an intensity change in the light. What tools do you have? DMM? Logic probe? O-Scope?

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Old 10-05-2012, 05:53 AM   #4
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Here is the link to the thread.

http://www.homebrewtalk.com/f170/pwm...us-how-221301/

I did notice that he made an edit and I built my pwm according to what I believe was a good diagram (one made after he recognized the problem). I have rechecked my circuit board a number of times and everything seems to be in place.

The image for the diagram is on forum post 180. The only difference I know of is that this is the circuit board I used...http://www.mouser.com/ProductDetail/...SlqRz1Eg%3d%3d.. which is not a bread board like his.

Maybe I fried something and that's why it is not working properly? That is my only other guess.

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Old 10-05-2012, 01:41 PM   #5
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A couple of things to check. 1) Make sure you're using a 12v power supply and 2) make sure the capacitor you replaced is in place with the correct polarity (if the capacitor you bought has that).

If you could post a picture of it that would be helpful. When I built mine I had a similar issue. It wasn't until I powered it with a 12v source that it worked correctly. I was able to hack an on PC power supply to get my 12v.

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Old 10-05-2012, 01:56 PM   #6
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This is my first attempt at loading a picture so hopefully it works. Also I am currently using a 5V wall wart, I thought i read somewhere that the voltage of the power supply wouldn't matter. I'll see if I can find a 12V laying around and try that

181.jpg  
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Old 10-05-2012, 02:13 PM   #7
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Just checked around the house, no 12V adapters. I do have 3 computer adapters but they are all like 18-19V. I also have a 6V and a 9V, would any of these work better? If not I may need to go get a 12v?

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Old 10-05-2012, 02:47 PM   #8
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The voltage that it works at will depend on the SSR that you are connecting it to. For your flashlight check it should work fine at 5v. It's hard to tell from the photo, but it looks like you have everything in the right spots. There are a million things that could cause your failure mode, but I'll take a stab at it. The most common cause of failure with circuit cards is workmanship. My guess is a solder bridge. Just a little whisker of solder can cause failure to function. If you have an ohm meter buzz out the adjacent connections. From your description of the mode of failure it could be caused by the capacitor only discharging when it is shorted to ground. This could be caused by having a low resistive path to Vcc via some other means than through the potentiometer. This would happen if there was a bridge between pins 2 and 3 on the 555 IC. (or the same nodes near the diode or pot connection)

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Old 10-05-2012, 02:56 PM   #9
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Is there a reason you built your PWM from scratch? I bought mine for my stir plate off of Amazon for less than $10 including shipping. Comes in a small enclosure with a screw terminal block for inputs/outputs: http://www.amazon.com/gp/aw/d/B007V1...9448887&sr=8-4

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Old 10-05-2012, 05:39 PM   #10
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normally for LEDs, the PWM frequency is very high, in the kHz range, in order to avoid flickering. the problem with driving a SSR at high frequency is that it both creates additional heat, but also more importantly- the fact that most common SSRs are zero-crossing means that there are only 120 times per second, or once every 8.3 miliseconds, when the SSR is allowed to turn off (when the AC wave crosses zero volts).

if you are driving the SSR at 100s or kHz frequency, the extra delay caused by waiting for zero crossing can heavilly effect the proportion of on/off time (if you set 50%, it might be actually 80%, or if the PWM frequency is high enough, it will be on nearly 100% all the time).

post #3 in that "PWM show us how" thread talks about needing to lower the frequency. if you had a non zero-crossing SSR, it wouldnt matter.

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