[Solved] Input voltage of Regulated Power Supply DIY Kit - Page 1
Forum section: electronics / projects

author: kalel
author_rank: Frequent Contributor
timestamp: October 25, 2017, 08:52:17 pm
content: These things are available from multiple Chinese vendors:0-30V 2mA-3A Regulated Power Supply DIY Kit"Input Voltage: 24V AC"I'm wondering if the input voltage has to be AC, or could be DC. Also, does it have to be 24v, or can it work as a step down converter with say 15-18.5 volt laptop power supply bricks? This is just one option I'm considering, but I don't want to work with AC transformers as I feel that enclosed Laptop DC supplies might be safer to work with.Also interesting:"Provide current limiting, LED display, over load and short circuit protection."I don't see an LED display, could be some SMD LEDs or a description error.There's an LED below the big capacitor.

author: jaycee
author_rank: Regular Contributor
timestamp: October 25, 2017, 09:41:10 pm
content: Any schematic ?From a look at the board, it looks as if it's the same sort of circuit used in the likes of the Mastech supplies. Id imagine the input has to be AC so that it can create the extra rails needed for the opamps to run on.

author: kalel
author_rank: Frequent Contributor
timestamp: October 25, 2017, 09:55:43 pm
content: Quote from: jaycee on October 25, 2017, 09:41:10 pmMany sellers don't provide one but Banggood does provide this document for a very similar (I think the same) one:https://img.banggood.com/file/products/201505080459530-30Vinstall.pdfThere are so many modules out there, I'm sure I will be able to find a DC input equivalent if not compatible.QuoteThat answers my own question.

author: VEGETA
author_rank: Super Contributor
timestamp: October 26, 2017, 08:21:54 am
content: I guess you can use 24v DC power since the input is a full-bridge so when using DC voltage only 2 diodes will be on. Maybe you will lose 1.4v as diode drop voltage but overall nothing wrong will happen.They made it accept 24v AC is maybe to allow people using it within their own PSUs.

author: glarsson
author_rank: Frequent Contributor
timestamp: October 26, 2017, 08:27:40 am
content: No. Can't use DC. It doesn't use a simple bridge rectifier; look at R2 and following components. They provide a negative supply for the op amp.

author: VEGETA
author_rank: Super Contributor
timestamp: October 26, 2017, 08:43:05 am
content: Quote from: glarsson on October 26, 2017, 08:27:40 amOh, I didn't notice that. Thanks for pointing it out. If so, then this kit is not really practical because not everyone have a transformer to use at bench. It is not safe too.

author: jaycee
author_rank: Regular Contributor
timestamp: October 26, 2017, 03:23:25 pm
content: It's a fairly old hobby design that's been around for quite a while. It does work but the opamps are powered directly from the input supply, which means an upper voltage limit. I also disagree with it being 0-30V @ 3A with just a 24VAC input - there will be too much sag to maintain the 30VDC output at that level of current.Also because it is a linear supply, all of the dissipation is in Q4. If you set for example 5VDC and pulled 3A from it, you would quickly burn out Q4 as it would be dissipating a LOT of power.I'd probably use it for 0-20V @ 1A with a good heatsink on Q4, or maybe more current by replacing Q4 with a bank of power transistors in parallel (with 0.22 ohm emitter resistors) on a BIG heatsink.As for using a transformer being unsafe.. well.. only unsafe if you are not confident about doing mains wiring safely. I personally think if you can wire a 3-pin plug, you can wire a transformer safely too.. if you want to prevent the possibility of touching anything live, just put the transformer in a plastic box.

author: Kleinstein
author_rank: Super Contributor
timestamp: October 26, 2017, 04:06:19 pm
content: These kits are often sold with TL081 or similar. So there supply should not be higher than about 36 V. With a negative supply of around -4 to -6 V depending on the kit. This leaves 30 to 32 V for the positive supply. So be save that transformer should be no more than about 20 V, maybe only 18 v with a small transformer (as the no load voltage will be higher).With a 20 V transformer you may not reach 20 V under all conditions (e.g. low line voltage and higher current). At the lower voltage and with really good heat sink one might be safe up to 2 A or so, 3 A more like only with an even lower transformer voltage.

author: not1xor1
author_rank: Frequent Contributor
timestamp: October 27, 2017, 07:12:41 am
content: Quote from: Kleinstein on October 26, 2017, 04:06:19 pmand with just 3300µF as a filter capacitor when you have 3A of load you get around 9Vpp of ripple (calculating ripple as I / (f * C) )when designing a PSU, one has to take into account main voltage variations (+/-10% according to specifics, at least in Europe), transformer derating (RMS secondary current is about 60-70% higher than DC load current), ripple voltage and power regulator dropout, max supply voltage, input limits and output limits of the opamps, SOA of the power devices, etc...those people didn't take into account any of thatthat kit is just a bad joke

author: jaycee
author_rank: Regular Contributor
timestamp: October 28, 2017, 05:46:51 pm
content: Yes. Shove a decent 6800uF capacitor in it, and use it with 18VAC input, and you'll get a nice 0-15V at max 1.5A supply.. but certainly not what they claim.