My first power supply rebuild - Page 1
Forum section: electronics / projects

author: n3vti
author_rank: Contributor
timestamp: March 20, 2016, 01:43:47 am
content: Hello everyone,I apologize in advance for the long post, but here goes:About 16 years ago, I wanted to get serious about electronics and soldering stuff. But I had never built anything of my own. I found a book on power supplies with some schematics, so I wanted to build one. I had just started working at RadioShack (and, for the record, I'm still there (sigh)) so I was able to obtain all the parts I needed. I built it and it has served me well for all of this time. I used an LM317 variable regulator, and it outputs about 1.5 - 16 volts DC, at about 1.5 Amps.Fast forward to now: I built a more compact power supply with similar specs and I use it too.(see attachments below.)I decided to open up my first supply, and I noticed a lot of things wrong. How in the world did this not kill me is beyond me. I guess I was eager to get it done and I rushed it at the time.(Please don't get mad, or bang head against wall, etc. This was 16 years ago when I was young and stupid!)Clearly, I need to rebuild this to make it proper. All I've done so far is heat-shrunk the exposed mains wires and replaced the main on/off switch. Here's my question: I can use an LM317 again if I want (i can probably salvage most of the parts on the board), however, my transformer is rated 25.2 VAC, at up to 2 Amps. Is there a way I can take full advantage of this? I can't use the LM338 since I have a plastic case, and I'm not looking to spend a lot on extra/replacement parts. I'm also not looking to use the full range of the regulator. The most I'll need is maybe 15 volts, max. I had a schematic on my computer, but I think I forgot to scan it in. Will do so in the next post.Thanks, Chuck

author: Kleinstein
author_rank: Super Contributor
timestamp: March 20, 2016, 08:44:34 am
content: With only 110 V and a hopefully working GFI circuit electricity is not that dangerous - you very likely survive even touching a bare live wire. Still wired need to be fixed - soldered joint may brake. So even a loose wire should not touch anything critical.The second danger is fire: if things get to hot they might cause a fire. This is why the fuse is a good thing. With the closed case it may be a good idea to have a thermal fuse or protection circuit as well, to shut down, when the inside temperature rises above about 60 C.A linear regulator like the LM317 will cause quite waste heat - so they are problematic inside a closed plastic case. No matter what type of linear regulator you use the heat somehow was to get outside. With a 25 V transformer you may end up at something like 35 V after the filter cap and thus a worst case power of about 35 W for a 1 A short circuit current. The relatively small heat sink at the LM317 may actually be a good thing, as the chip will go into thermal shutdown before it gets to hot inside. The LM317 also has SOA limiting that might reduce the maximum current at large differences.So my guess is that you can't get the full 1 A current rating at low voltages (e.g. 5 V) for an extended time.If you really need more current something like a metal top of the case or ventilation holes and a fan would be needed.When you don't need the full voltage, it can be a good idea to have a resistor before the filter cap - this reduces the voltage a little. As peak currents are reduced the total heat production is actually reduced a little (up to about 20%) and also the RMS current seen by the transformer is reduced.

author: n3vti
author_rank: Contributor
timestamp: March 20, 2016, 05:46:51 pm
content: Thanks! I made a schematic of the rebuild. Attachment below. I went with a full wave rectifier design. I calculated the voltage at C1 to be about 18 volts going into the regulator. It should be enough to give me the full sweep of 1.5 - 15 volts before it drops out of regulation. Also, with less voltage going in to the regulator, the less it has to drop for the lower voltages, and less heat dissipation.Does anyone have a better idea of a schematic layout? Or is this good as is?

author: rdl
author_rank: Super Contributor
timestamp: March 20, 2016, 06:09:40 pm
content: I always use this as a basic starting point. Note that D1 also protects against potential discharge of load capacitance back through the regulator. You can often get away with a value for R1 higher than 120 ohms using better quality LM317s or provide the minimum load in another way, such as an LED or small current sink on the output.

author: n3vti
author_rank: Contributor
timestamp: March 20, 2016, 07:39:13 pm
content: Thanks! I remember the caps from an earlier blog video, so I will incorporate them in. I will also add the "bleeder" diode in (D1 in your diagram.) I have R1 set at 437 Ohms (390 + 47 in series), and R2 at a 5K variable. This will give me up to 15.552 volts max output.I'll post an updated schematic when I get home from work.

author: Kleinstein
author_rank: Super Contributor
timestamp: March 20, 2016, 08:33:09 pm
content: There should also be the small (e.g. 100 nF) capacitor at the input side (c3 in the circuit with the diodes), close to the regulator. The large cap it not really effective for high frequencies.

author: n3vti
author_rank: Contributor
timestamp: March 21, 2016, 01:14:13 am
content: Okay, I have a new design in mind. See attachment below. I also "reverse engineered" the current supply I have, before I started to get rid of my major mistakes. What was I thinking when I built that supply? I think I had wanted to do a battery backup or something like that... but I don't need it. I did drill some holes on the back of the enclosure for the heat.Sorry about the chicken scratch

author: ZeTeX
author_rank: Frequent Contributor
timestamp: March 22, 2016, 05:46:50 pm
content: Quote from: n3vti on March 21, 2016, 01:14:13 am1. R1 is too low, your led will die. (50mA is too much for small led) try 2.2k instead.2. ~100n cap at the adj pin might reduce some noise that gets amplified by the regulator.3. you should also put electrolytic capacitor at the output.. so if your load just need a peak current the capacitor will supply it.

author: n3vti
author_rank: Contributor
timestamp: March 22, 2016, 06:32:56 pm
content: Quote from: ZeTeX on March 22, 2016, 05:46:50 pm1. Where did you get 50 mA from? The output from C1 is about 18 volts. (It would have been 36 volts had I used a bridge rectifier and not used the center tap. Even then, if I was doing that configuration, I would have used 1.5K or 1.8K.) The LED I'm using is a 2.1V, ~20mA. I can bump up R1 to 820 which would limit current to 19 mA.2. I thought about that; I might put it in. I don't need this to be perfect.3. I'm on the fence on this one. Some say yes, others say no. I'm not using it for high frequency applications.My next design I'll be using the LM338. I'm just trying to make the most out of this supply using the existing parts.

author: ZeTeX
author_rank: Frequent Contributor
timestamp: March 22, 2016, 07:56:07 pm
content: Quote from: n3vti on March 22, 2016, 06:32:56 pmyep you are right about the first one, you should still change R1 to even higher then 820, less risk of the led dying.