Jump to content
Electronics-Lab.com Community

DC Potentiometer in AC circuit to control DC output.


abador

Recommended Posts

I finally finished a power supply last night after finally finding a working step down transformer. I put the 5K audio DC potentiometer in one of the AC legs from the transformer (series) and was amazed to find that I could get anywhere from about 18.6V to 3.6V. More recently I tried to charge a battery with the rectified and filtered DC output but I couldn't seem to get it above 3V and it was jumping around a lot. Could this unpredictable nature be because I have the potentiometer outside of the DC output? Could the AC ruin the DC potentiometer? I also have the fuse on the other leg of the transformer, would this protect from over current in the load that the power supply is connected to?

Link to comment
Share on other sites


Please post a schematic.

You can't use a potentiometer to vary the voltage to a large load because the output impedance of the potentiometer is too high so the output voltage will drop. Furthermore, the potentiometer is probably only rated to something like 0.25W and will burn out if the current is too high.

Link to comment
Share on other sites

Ok, I got it. Sorry it's a little messy. I tried cleaning it up but the simulation software I was using wouldn't let me take off some labels, the blued box at the bottom was only a command for the program. Basically the fuse is on one side of the circuit and the potentiometer is on the other side. The load obviously is not part of the power supply. I hope this helps give a better view of it.

post-62473-14279144323338_thumb.jpg

Link to comment
Share on other sites

The resistor in series with the LED is 1k and the load is 1k, giving a total load impedance of 500R, ignoring the LED's forward voltage drop.

The output voltage can be calculated using the potential divider formula which can be found on Wikipedia.
http://en.wikipedia.org/wiki/Potential_divider

Link to comment
Share on other sites

I actually have a rectifier that is in one unit so I guess I drew the rectifier wrong, sorry about that. I ended up removing the potentiometer from the AC part of the circuit and put it in the DC in series with the output. I have still had trouble with the power supply though. I changed out the Capacitor to a 3300 uf just because that was the only other high capacitance cap that was within the voltage range. (more capacitance than I really need) I then checked the resistance on ether side of the DC circuit (+ and -) to check for shorts and the resistance was pretty high so I doubt that is the problem. I even checked the potentiometer which came out clean. I get a voltage reading every so often where it is supposed to be but this part is unpredictable and when ever I hooked it up to a 6V battery to charge, it would go back down to less than 1 volt. I am hoping if something is bad that it is anything other than the transformer just because it is so rare to find a working transformer when salvaging them. Could it maybe the 1K resistor and LED?

Link to comment
Share on other sites

I probably should have labeled the out terminals in the schematic instead of labeling the resistor as load. Basically I'm just trying to use the LED as an indicator that it is working right/is on. The "load" is actually just supposed to simulate a load that could have been used on the output. I  think that made it a little more confusing.

Link to comment
Share on other sites

The load can be connected in parallel with C1 which can be increased in size, if needs be.

The circuit you posted is a bad idea because the potentiometer also increases the impedance of the power supply so the current and voltage to the load will also fall. The brightness of the LED will vary depending on the load connected to it.

Link to comment
Share on other sites

  • 2 weeks later...

Thanks for all the replies everyone. I ended up putting the potentiometer in the DC part of the circuit and in series with the load. I still don't know why the readings I kept getting were so unpredictable, all I can think of is that maybe it's the fact that my multimeter only cost me $12. I put a 4.7K resistor in series with the potentiometer to bring the voltage to a more workable level but I will probably increase that to a 10K. I was able to charge a battery up to a certain point but then found out it was bad after the voltage kept dropping fast when a lead was removed. I kept trying to measure the voltage across the output and was surprised that the potentiometer was not changing the voltage at all, then I remembered that the voltage will always measure the the total across an open and put a 10K resistor across the output to get a better reading. I am now getting a stable output. Thanks for all the input.

Link to comment
Share on other sites

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.

Guest
Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.

Loading...
×
  • Create New...