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Old 06-18-2011, 08:44 PM   #1
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Default 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.


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Old 06-19-2011, 11:59 AM   #2
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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...


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Old 06-19-2011, 02:25 PM   #3
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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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Old 06-19-2011, 03:37 PM   #4
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Quote:
Originally Posted by rod734 View Post
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.
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-b...ht_2903wt_1139
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Old 06-19-2011, 08:33 PM   #5
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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...
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Old 06-21-2011, 05:13 PM   #6
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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...
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Old 06-21-2011, 07:33 PM   #7
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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%)?
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Old 06-21-2011, 07:39 PM   #8
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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.
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Old 06-21-2011, 07:46 PM   #9
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Quote:
Originally Posted by Countrysquire View Post
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?
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.

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How often should it fire when turned all the way up?
It will just stay on all the time. Constant voltage on the output.

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

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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.
No problem. Let's see if we can get it working....
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Old 06-21-2011, 07:57 PM   #10
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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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