Building Walker's PWM, need help

Attempted to build a PWM unit based on Walker's schematic today. Have everything soldered up, but I don't think that I got it right. Whether using a 9 volt power supply or a 14v, I only see about 2.7 volts on the output and not enough to spin a 12v computer fan. To make it worse, it has behaved differently at different times, sometimes keeping the fan spinning after starting it by hand, sometimes not. I've rechecked all the terminations and checked to make sure that there was no continuity across any of the traces from sloppy soldering.

Questions:

Should I have such a large voltage drop?

What is the best way to test one of these?

Thanks.

Well it appears that I found the problem AFTER I unsoldered the parts from the board. One of the pins on the chip was bent underneath and not seating in the socket. Back to the soldering board...

I was going to put one of these together for my controll panel, but I found one on ebay assembled and tested for about $12 shipped so I decided to save myself the time and eyestrain.

rod734 said:

I was going to put one of these together for my controll panel, but I found one on ebay assembled and tested for about $12 shipped so I decided to save myself the time and eyestrain.

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Sure, that's the logical thing, but I never let logic stand in my way. If this second attempt doesn't work, I'll be ordering one as well. I don't want to screw around and have as much in this as a PID would cost...

If attempt number two doesn't work, thinking about one of these for $5 shipped. http://cgi.ebay.com/New-LED-Dimmer-brightness-adjustable-controller-DC12V-/170657259504?pt=LH_DefaultDomain_0&hash=item27bbf71ff0#ht_2903wt_1139

Well, the second attempt to build this did not lead to success. Thinking the chip might be bad as everything else seems to check out. I had continuity between two of the pins before anything else was connected, so perhaps I fried it on my first attempt...

Installed a new chip, still no love. Zero voltage when measuring across the two outlet leads (reset and discharge). Not sure what I'm missing, but it's getting to the point that I don't want to let this thing kick my butt...

wow. sorry to hear you're having problems. I've built a few of these and have not had problems.

One comment about checking between the two leads. You are only going to see a voltage drop there when the PWM is "firing". Do you have the pot knob cranked all the way up (meaning, set to 100%)?

I have tried it all the way up, down, and in the middle. A few questions:

The circuit shows a ground, but I don't have it grounded to anything other than the - lead from the power supply. Do I need something else here?

How often should it fire when turned all the way up?

Does it matter which of the two outer posts on the pot that the Shottky diodes are on?

Thanks for your help Walker. I've done plenty of bigger electrical work, but this is the first time that I've tried to build an electronic circuit on a board.

Countrysquire said:

I have tried it all the way up, down, and in the middle. A few questions:

The circuit shows a ground, but I don't have it grounded to anything other than the - lead from the power supply. Do I need something else here?

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No... I don't know why I have it shown with a ground in the diagram. That symbol shouldn't be there and it should just be connected to the "-" side of your DC source.

Countrysquire said:

How often should it fire when turned all the way up?

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It will just stay on all the time. Constant voltage on the output.

Countrysquire said:

Does it matter which of the two outer posts on the pot that the Shottky diodes are on?

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What kind of POT do you have? I posted a link to the one I actually used one time and it is a pot with 5 connections on it because it has a power switch built into it. People have mis-wired that pot more than once because of the 5 connections.

Countrysquire said:

Thanks for your help Walker. I've done plenty of bigger electrical work, but this is the first time that I've tried to build an electronic circuit on a board.

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No problem. Let's see if we can get it working....

It's a pot with three posts, no switch. The diodes are on the outer two posts, the output on the center post.

Countrysquire said:

It's a pot with three posts, no switch. The diodes are on the outer two posts, the output on the center post.

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OK.... that sounds correct then.

And your diodes are pointing in opposite directions, right?

I am assuming you have a multi-meter? It might be worthwhile checking a few points in the circuit.

Yes, diodes are opposite direction and I do have a Fluke meter. I'll be home in about a half hour and can do some checking.

I would check that you are getting 14V (or whatever it is) on your power supply lines first.

Then move the "+" probe to the power pin on the chip (upper right) as well as the reset pin on the chip (lower left) and make sure you are still reading 14V at those locations.

Countrysquire said:

I'll be home in about a half hour and can do some checking.

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OK. I will be leaving my office in about 45 minutes, so we might not cross paths again for a while. I am tinkering in my garage with another project later.

Anyway, here's what I would check.

14V seen between + and - lines of power supply
14V seen between VDC pin of the 555 and - line of power supply
14V seen between reset pin of the 555 and - line of power supply
14V seen between ground pin of the 555 and the VCD pin of the 555.
14V seen between the grounded pin of each capacitor and the + line of the power supply.

Then you can go crazy checking for continuity between all of your components. Basically check for 0 Ohm resistance between each pin of each component that you believe should be wired together to make sure that they are indeed wired together. Don't put the leads on the wire you soldered in. Actually touch the pins of the diode or capacitor, or chip or whatever.

The fact that you measured 2.4V in your first attempt says to me that there is a short in there somewhere, so visually inspect the connections to make sure that no solder blobs made any bridges or frayed strand of wire is touching something it shouldn't be touching.

GGRRRRR.... It looks like some of the traces have delaminated from the board. The voltage jumps around as the board is flexed. Looks like it's time to start over.

Thanks again for your help Walker.

Looks like we have success! Full voltage with the pot turned up, pulses as it's turned down. Now I wait for the SSR to make its way across the Pacific.

Thanks again for your help Walker.

Awesome. Glad it worked.