Thursday, January 5, 2012

Homemade Variable Power Supply

     A while ago, I got two of these Graymark Model 803 variable power supplies. Unfortunately, they were sold as a kit and whoever assembled them before me did not do a very good job. To make matters worse, I could find no information online about this particular kit so I couldn't figure out how the circuit was supposed to be connected. It also had a mystery voltage regulator chip that I could never find any information about online. I decided that I would just go ahead and start over, using only the layout with new electronics for a new power supply.

Finished project

     I kept the original transformer after measuring it and found out it was 20.9 VRMS across each half of it's secondary windings (it had a center-tapped secondary). That translates to 29.5VPK. I used this website to help design the rectifier to get the AC line voltage to DC. I decided upon using a LM317 3-Terminal Adjustable Voltage Regulator because it was inexpensive, easy to use, and did everything I wanted with a very small component count. (Just a warning, the pin-out of this regulator is different than fixed voltage regulators, so make sure to use the correct pins for this chip.) I based my circuit on the application notes starting on page 9 of this schematic. There's also a lot of interesting applications of this regulator starting on page 16 of that data sheet. I had a ten-turn potentiometer that I used for the adjustment so I can have supreme control over the output voltage.

My schematic

The real thing

     After the electronics were installed, I noticed that the positive output was inadvertently touching the metal case that I was treating as ground. I added a foam gromment on the inside as seen in the picture above, and added a plastic piece on the outside of the case. I happened to find the plastic piece in my screw collection and it fit perfectly. You can see it on the positive (red) output on the power supply in the first picture. My next step was to design a case for it. I bent a piece of aluminum into a crude case and used J-B Weld around the edges. It is not a perfect fit, and frankly doesn't look very good, but it serves its purpose. I backed the Printrbot on KickStarter and plan to 3D print a case (and add a power switch!) when I receive it in the coming months. You can view other pictures of this project on it's album on Picasa.

My hand-bent aluminum case

     My power supply has a range of 1.3V to 28.2V. I was wondering how my power supply performed, so I took it to the lab at school to measure on an oscilloscope. Unfortunately, I was getting a lot of noise on the output of this power supply. I compared it to the lab power supply to insure that it wasn't a bad cable, and the HP E3630A power supply had barely any noise at all. You can see all the oscilloscope captures via this link. If anybody has any suggestions or improvements to get rid of the noise, please leave a comment below.

Lots of noise at 3.3V output

Edit: After looking through some of the pictures and comparing it to the schematic, I noticed I never installed the 100uF output capacitor (oops). I don't have an oscilloscope at home (I'm still on Christmas break) so it'll have to wait until I go back to school next week to see if that improved the noise issues.