PSU design question - Page 1
Forum section: electronics / beginners

author: dcbrown73
author_rank: Regular Contributor
timestamp: July 04, 2020, 06:00:21 pm
content: I just had a quick question about my PSU design.While the PSU has a main power switch, I want to be able to turn off the DC power without having to turn off the entire device. I want to avoid repetitive inrush current during normal use, as I'm only using a thermistor for inrush protection.Anyway, I'm using an LM338 for the voltage regulation, the question I have. If I want to add a power switch, should this be done in front of the LM338, or behind it? I just wanted to ensure that if I put it behind the LM338, there was no risk to the LM338 from the voltage still applied to the input8. I don't expect there would, but I figured it doesn't hurt to ask before I go through this and end up damaging the regulator.Thanks,Dave

author: TimFox
author_rank: Super Contributor
timestamp: July 04, 2020, 07:45:31 pm
content: If there is a substantial capacitance at the output of the regulator, you risk reversing the input-to-output polarity, which can be destructive, with the switch ahead of the regulator. To protect against that, add a diode from input to output of the regulator, normally reverse-biased.

author: Gyro
author_rank: Super Contributor
timestamp: July 04, 2020, 09:14:14 pm
content: You might want to look into schemes where you pull the adjust pin of the LM338 to approx -1.2V to effectively shut it down instead of an in-line switch. Some protection diodes would probably still be a good idea (check the datasheet).

author: eblc1388
author_rank: Frequent Contributor
timestamp: July 05, 2020, 12:12:24 am
content: Does your PSU has an output voltage meter?If it has, it is logical to place the output switch as the last element before the output terminal, so that the actual output voltage can be adjusted or verified before one turns on the output.

author: MosherIV
author_rank: Super Contributor
timestamp: July 05, 2020, 11:42:46 am
content: Have a look at transistor emitter follower circuits.Since bipolar transistors are base current controlled, if you use a emitter follower as the final output device, you can turn off the output by cutting off the base current. The base current is an order of magnitud smaller than the output current, so you can use cheap low current switch instead of expensive ligh currentswitch.Make sure youuse a high power (current) transistor like the classic 2n3055.

author: rdl
author_rank: Super Contributor
timestamp: July 05, 2020, 10:59:02 pm
content: I'd probably use a DPDT switch directly on the outputs, but watch out for contact resistance. Using a relay is another option.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: July 06, 2020, 03:25:13 am
content: Just use reasonable values of capacitors, and prefer bank-wound style transformers over shell or toroid styles.(The bank-wound style has poorer regulation, i.e. output voltage variation with load current; but that's fine, because you're regulating it anyway!)This is in contrast to a lot of audioph{ool|ile} projects that use a huge toroid and the most ungodly amount of capacitance...Tim

author: dcbrown73
author_rank: Regular Contributor
timestamp: July 07, 2020, 01:27:57 pm
content: Sorry about the delay in my response. Life had a few twists and turns recently.Quote from: TimFox on July 04, 2020, 07:45:31 pmYes, I'm adding some diodes for protection. I will post the current work in progress schematic below.Quote from: Gyro on July 04, 2020, 09:14:14 pmSo pulling the adj pin to -1.2v will stop it completely? I know if I ground it I get around 1.23v. I didn't realize pulling it negative would continue to lower the voltage. I will test this, though I'm not exactly sure if that is how I want to proceed. I currently have not designed a negative voltage rail for it.Quote from: MosherIV on July 05, 2020, 11:42:46 amI've thought about this as I've seen them in the past, but I don't have any experience with emitter follower circuits. I will have to spend some time getting a better grasp on these types of circuits.Quote from: rdl on July 05, 2020, 10:59:02 pmI only have single output so far. So I suppose a SPST would be enough. It's what I initial had planned to use, but I was just worried about any possible damage it could cause.I do have another question. On power supplies, I should use lineir pots and not audio (logarithmic) pots right? I was testing last night and I noticed that the entire voltage control was primarily controlled with only half (50%) of the pot on my test circuit. The other half did nothing. I don't suppose someone can tell me why this is vs how to resolve it so the entire pot is used when controlling the range of voltage.Finally, if I wanted to add current control / limiter support as my commercial PSUs. I suppose I need to learn and understand how to create that type of circuit. Right now, it's only voltage controlled. Are there preferred ways to do this?Here is a current status of the schematic.

author: MosherIV
author_rank: Super Contributor
timestamp: July 10, 2020, 11:45:31 pm
content: Adding current limit means moving away from LM388 or other variable voltage regulators.Trying to add current limit to them often means voltage regulation side fighting the current limit side leading to instability and oscilkation.Best to start again from scratch and design your own voltage and current regulation.Voltage regulation is as simple as using a power transistor in emitter follower mode, voltage on base is output on emitter. Use opamp to feed voltage into base and feedback to control voltage.Current limit is basically measuring current through current shunt, measuring the difference between that and current limit set by voltage and forcing the output voltage down to limit the current. Again done by using the difference between both inputs of opamp. The clever part is ORing the output of the voltage control opamp and the current limit control opamp.Have a look at the schematic from this sitehttp://www.microsyl.com/index.php/2010/03/31/bench-power-supply-0-25v-0-5amp/comment-page-2/I found it the easiest to study and understand.

author: eblc1388
author_rank: Frequent Contributor
timestamp: July 10, 2020, 11:56:32 pm
content: It is probably not a good idea to use LED as fuse in the transformer primary.How does it get there in the first place?

author: dcbrown73
author_rank: Regular Contributor
timestamp: July 11, 2020, 01:07:59 am
content: Quote from: MosherIV on July 10, 2020, 11:45:31 pmThank you. I will look closer to this tomorrow when I have more time.

author: dcbrown73
author_rank: Regular Contributor
timestamp: July 11, 2020, 01:12:22 am
content: Quote from: eblc1388 on July 10, 2020, 11:56:32 pmActually, the LED is not the fuse and I have that LED placed incorrectly. The LED is a 120v AC LED just to show the power is on though I believe it should just go from + to GND (and probably a current limiting resister for it) and not inline as it shows in the schematic.. The actual fuse is a slow blow fuse above the LED in the circuit. Then I have a thermister for inrush current protection. (on that side of the transformer anyhow)I will probably rework the entire circuit based on MosherIV's feedback. I'm still learning. I also want to add a voltage and current meter and if I can add limiter for the current that would be awesome.

author: ferdieCX
author_rank: Regular Contributor
timestamp: July 11, 2020, 02:05:16 am
content: Hi, the way you connected R3 could be dangerous if the pot wiper fails.We had a tread about that some days agohttps://www.eevblog.com/forum/projects/question-about-potentiometer-wiper-failure-in-bench-power-supplies/msg3122980/#msg3122980

author: rdl
author_rank: Super Contributor
timestamp: July 11, 2020, 03:57:53 am
content: From that thread ferdieCX mentions check this link:http://www.imajeenyus.com/electronics/20160517_potentiometer_feedback/index.shtml