Hi,
I’m semi new to hobby electronics, and I would like to build a variable power supply for tinkering. I found schematics and an explanation of the working principles here (http://www.qsl.net/d/dl5dbm//Projekte/N … riable.htm). It seems fairly straightforward; however I had a few questions to ask before I start playing around with high amperage electronics.
Firstly I noticed that the resistors have no power values in the schematics. Since the load can pull up to 1.5A should I use 2 Watt resistors?
Secondly, I intend to use a bridge rectifier as opposed to four diodes. I have a bridge rectifer http://www.vishay.com/docs/88608/g5sba20.pdf this one specifically. It is rated much higher than the bridge rectifier called for in the plans (100V and 3A). Can I still use this bridge rectifier?
So far for this project I have purchased the transformer, the feedback diode for the regulator, a three-position toggle switch( don’t need third position but it was on sale), and the LM317 voltage regulator.
Any suggestions?
I’m going to post this full project here when completed.
Your bridge rectifier will be fine. Use Ohm’s law to figure out dissipation for the resistors. 1/4 W will be good for all of them.
Dave is spot-on.
The major current path for the circuit is… Plug => S1 => Fuse => TR1 => BR1 => IC1 => (Your Load) => Return Path. C1 and C3 can see significant load, but the other compents don’t see anthing near the current of your external load.
Pay attention to the heatsinking.
I mounted the LM317(T) regulator on a heatsink. If you use a metal/aluminum case you can mount it right to the metal case, insulated with the mica insulator and the nylon washer around the mounting screw. Note that the metal tab of the LM317 is connected internally to the ‘Output’ pin. So it has to be insulated when mounting directly to the case. Use heat sink compound (comes in transparent or white color) on the metal tab and mica insulator to maximize proper heat transfer between LM317 and case/ or heatsink.
Thanks Dave and Dlotton, I played around with my old electronics book and ended up getting 1/2 W just to be safe.
Mee_n_Mac I bought a copper T220 heatsink for my LM317(T) so I should be good on heatsinking.
I couldn’t find a 1uF 40VDC cap, so I got a 1uF 50VDC cap instead.
I also had a question regarding the ground. I bought an enclosure that has a metal lid from radio shack. I was thinking of grounding the metal lid and then using the lid to ground any components that may accumulate unwanted charge.
Sorry if I’m asking a lot of questions, I just want to be sure I’m being safe working with main supply. I’ve got some horror stories from physics labs that justifies it haha
For power supply design, you usually want to tie all of the grounds to a single point since any voltage drop across wire leads etc., interferes with regulation. Not too much to worry about at 1 or 2 Amps, but a good habit to get into. On large supplies, usually all of the grounds are tied to the - terminal on the big (>10,000 uF) filter capacitor since it’s a screw terminal.
So just to clarify, I should tie the ground from the wall plug to the - terminal on the 2200MFD capacitor? I have all the parts and I’m just curious before I start.
The plug ground is typically tied to the chassis/enclosure. If you tie it to anything on the other side of the transformer, your output will not be isolated.
I would recommend having three banana jacks on your output, one for the OUT+, one for OUT-, and one for chassis GND. That gives you the flixibility to tie the OUT- or OUT+ to the chassis GND, or leave the output floating.
An islolated output is very handy if you want to connect supplies together and have positive and negative supplies.
The attached image is pretty standard setup for bench power supplies… chassis GND, Out-, Out+