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Sunday, February 6, 2011

Free Energy and Bad Pot

I have more or less finished moving and unpacking everything and I decided to rest a bit and do something relaxing like fix a broken power supply! I decided to fix this not because I need the power supply right now, or because I found it in the garbage 2 days ago but because there is nothing better to motivate you to unpack as having the need to find something. For example: my soldering iron and digital volt meter. Both of which I had to use to fix the power supply and in the process of finding them I found other useful stuff too - like toenail clippers!
The power supply that I found is an Extech 382200 that has a 0-30 volt output and up to one amp. Both the voltage and the current are adjustable so this is a handy thing to have. When I plugged this thing in and turned it on the voltage display read 51.1 volts! That is a bit higher than the 30 volt rating and the voltage adjustment knob didn't do anything.
EXTECH Power Supply (broken)
At first I thought that the output transistors might be shorted so they were the first things that I checked. If the transistors short then the output usually goes higher than what it's supposed to be and it's not adjustable - of the fuse blows. The transistors are mounted to a big heatsink on the back of the power supply but to get at them I had to take the cover off. Check out the picture below!

Inside The Power Supply
There is a big toroid transformer at the bottom of the power supply because it's a linear power supply and you can see the black heatsinks on the back. I checked the transistors with an ohm meter and they looked OK. I probed everything on the board as well, the diodes, the transistors but everything looked OK.
At this point I was starting to wonder if this was going to be an easy fix or not. After checking the solid state devices I decided to check the potentiometers on the front panel (sometimes referred to as pot's). I didn't think that they were the problem because typically they are reliable but of course the voltage pot was open instead of 7K ohms as it was marked! What? Is that all that's wrong?
A potentiometer is a device that changes it's electrical resistance when you turn it (Check out the work potentiometer above it's a link to a Wikipedia page about potentiometers if you want to know more). There are many ways to do it but usually there is a band of some resistive material and a little 'wiper' that runs across that band as you turn the knob. I took the broken potentiometer apart and probed this band with the meter and found a crack in the resistive material. Check out the exciting picture below!
Closeup Picture of Bad Pot
You can see in the above picture the white plastic cap that turns with the knob and holds the wiper. Kinda blurry in the upper right is the metal cover that I pried off with a screw driver to get at the insides. between the black and red meter probes is where the crack in the resistive band material is. I went to the trouble of taking this apart to be sure that the problem was in the pot itself and not in the external solder joints or a broken solder post.
Now that I knew what the problem is I had to replace the potentiometer. Unfortunately I don't have a 7k ohm pot laying around because that isn't a common value, but I do have a few 10K's. There is a way to make a potentiometer of one value become a smaller value by adding a resistor in parallel with it. Click HERE for a Wikipedia link of what I am talking about if you are curious. One disadvantage of putting a resistor in parallel with a potentiometer is you change the 'resistance curve' of the potentiometer. In other words if the pot is linear putting a resistor will make it non linear. HERE is a great link to a pdf explaining what I am talking about and also an in depth discussion about how to change pot values with fixed resistors.
Having said all that here is what I did. I took the 10K ohm pot that I had and added several resistors in parallel with it to get 23.5K ohms. That value in parallel with 10K ohms gives you 7K ohms. Check out the Wikipedia link to see why if you are curious. The side effect of adding that resistor is that the 10K ohm linear pot was no longer linear. To see how much I changed it's linearity I made the cool graph below!
Linear Potentiometer with a Parallel Fixed resistor
You should be able to click on the picture for a better view if you want to see it. The interesting thing is the green line is the linear value of the pot and the curved red line is the actual resistance values I got after adding the resistor in parallel. It's not linear anymore but for what I am doing that isn't going to matter that much. The potentiometer that I am using is a '10-turn' pot which means that you can turn the knob ten times to get the entire range so fine adjustment is easier than a pot that only turns 270 degrees for full value.
OK enough about all that, I drilled out the hole where the original pot was mounted to fit the new pot that I had and I put the entire thing back together, turned it on and it worked!
Repaired Extech Power Supply
In the above picture you can see the repaired power supply and how ugly it looks with the new pot installed. The knob off the old broken pot doesn't fit but I don't really care. I could have bought a replacement 7k ohm pot and made it look nice but then this post wouldn't have been about Free Energy and Bad Pot.

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