Cheap bench power supply - Page 1
Forum section: electronics / general

author: salamander
author_rank: Contributor
timestamp: June 10, 2014, 01:24:57 pm
content: Hi,I am looking for some "cheap and cheerful" bench power supply. I am just a hobbyist, doing some work with microcontrollers, mainly 3,3/5V stuff.Can someone tell me if the UNI-T UTP3701 (http://www.uni-trend.com/UTP3701.html) is not a complete sh**?Make note that I am from Czech Republic, so getting something from eBay is quite difficult due to shipping costs, the UNI-T is sold in a local store for around 103 USD, so is it good enough?Thanks for answers

author: Richard Crowley
author_rank: Super Contributor
timestamp: June 10, 2014, 01:31:08 pm
content: In other discussions in these forums on meters, the "Uni-T" products were deemed to be decent quality and good value for the price. If I were in your situation, I would buy it.

author: saturation
author_rank: Super Contributor
timestamp: June 10, 2014, 01:54:49 pm
content: $103 for a 3 output PSU 30V adjustable, 5V fixed? That's a good price.If you can return it to the store if it turns out to be bad then test it at home for ripple, regulation, and know how to tear it down for a quality inspection, that is the best price I've heard for a 3ple power supply. In the US such would go for about $200.https://www.eevblog.com/forum/testgear/looking-for-goodinexpensive-bench-power-supply/255/

author: salamander
author_rank: Contributor
timestamp: June 10, 2014, 02:55:23 pm
content: That's the problem ... the price seems to be too good.Anyway, I'll order it, it should be more than enough for my "projects".I would like to test it, but my I'm quite short on lab equipment, no dummy load or anything like that .... I'll try to think of something.Thanks for your advice

author: Richard Crowley
author_rank: Super Contributor
timestamp: June 10, 2014, 02:58:51 pm
content: A "cheap 'n cheerful" test load could be a variety of incandescent bulbs from an auto-parts store. A wide variety of loads from very small "clearance" and "dome" bulbs up to high-current headlamps.

author: dannyf
author_rank: Super Contributor
timestamp: June 10, 2014, 03:15:11 pm
content: QuoteComputer power supplies, ATX or laptop, are cheap and powerful.I have used simple USB plugs to power some of my boards.

author: rob77
author_rank: Super Contributor
timestamp: June 10, 2014, 03:33:23 pm
content: Quote from: dannyf on June 10, 2014, 03:15:11 pmATX is a NO GO - if you leave the main output voltage unloaded, then the regulation doesn't work correctly.the USB is a good idea for small 5V projects.the laptop supply with a LM317 on a decent heat-sink would be a usable poor man's regulated supply

author: salamander
author_rank: Contributor
timestamp: June 10, 2014, 05:05:29 pm
content: By cheap I mean I can afford the UNI-T if it is worth the money

author: dannyf
author_rank: Super Contributor
timestamp: June 10, 2014, 05:07:06 pm
content: QuoteWhat's worth is highly subjective and I am afraid that no one can answer that question for you: what's worth the money for me may not be worth it for you, and vice versa.

author: suicidaleggroll
author_rank: Super Contributor
timestamp: June 10, 2014, 05:40:49 pm
content: Quote from: rob77 on June 10, 2014, 03:33:23 pmThen put a load on it...Power resistors are cheap, that's hardly a deal-breaker.

author: rob77
author_rank: Super Contributor
timestamp: June 10, 2014, 07:26:28 pm
content: Quote from: salamander on June 10, 2014, 05:05:29 pmi think it's definitely worth of the money, but anyways sooner or later you'll need more power supplies. so the laptop charger with the simple LM317 circuit is not a bad idea as an additional supplybtw.. i've seen that supply (or a very similar one) at GM electronic - probably it's worth of buying there - having the 2 years warranty. (i bought my smps lab supply there )

author: dannyf
author_rank: Super Contributor
timestamp: June 10, 2014, 09:52:11 pm
content: QuoteIf you are a newbie, I would suggest that you get a "wimpy" power supply - so that if you did make a wiring mistake, sparks wouldn't fly. That's where those little USB chargers are incredibly handy - they cannot do much damage.If you do get a powerful power supply, form a good habit by first powering up your circuit through resistors (ie, making it wimpy).

author: jpb
author_rank: Super Contributor
timestamp: June 10, 2014, 10:12:41 pm
content: Quote from: dannyf on June 10, 2014, 09:52:11 pmWith that Uni-T supply it should be possible to set a low current limit which will have the same effect - though this can lead to odd "bugs". When I first tried out my gps module I thought it wasn't working because it needed 30mA and I'd set the current supply limit to around 10mA and hadn't noticed.

author: rob77
author_rank: Super Contributor
timestamp: June 10, 2014, 10:13:56 pm
content: Quote from: dannyf on June 10, 2014, 09:52:11 pmand what about setting the C.C. to a current corresponding to the expected draw of the circuit ? that's why we're using a lab supply - adjustable constant voltage with adjustable constant current capability. if the CC doesn't kick in - all good. if CC kicks in, then it's time for troubleshootingi have a relatively powerful supply (1-36V , 0-10A) and yet the cc can be set to 100mA - just perfect for first power on (smokeless smoke testing)

author: Jeroen3
author_rank: Super Contributor
timestamp: June 11, 2014, 02:24:07 pm
content: Quote from: rob77 on June 10, 2014, 10:13:56 pmDon't be fooled, 100 mA through the wrong components is still enough to create smoke. If you can find a fused/overcurrent feature on a bench supply, get it.And an atx pc supply isnota bench supply. But as a power supply for afinished productit is excellent.There is no protection against continuous or repetitive short circuit or overload condition. They are designed not to ignite in such case, so some do have some protection. Others just have a mains fuse or ptc.So, yes you can can use them, but do not leave them unattended with your device under test.

author: janoc
author_rank: Super Contributor
timestamp: June 11, 2014, 08:10:05 pm
content: Quote from: rob77 on June 10, 2014, 03:33:23 pmMost recent ATX PSUs have a load resistor inside for exactly this reason, so that the PSU will actually power up correctly even when not loaded otherwise. However, a 20A+ PSU with no adjustable current limiting is a big bang/lot of smoke waiting to happen. They do have a protection circuit that shuts the PSU off when there is a short on the output, but that will not protect your circuit from being blown to bits - your micro will be a red glowing lava already, with 10 amps passing through it and the PSU will not even flinch at that ...I have one for when I need to power something requiring a lot of juice but not for general bench work.Quote from: rob77 on June 10, 2014, 03:33:23 pmIt is a good way to fry the USB ports of your computer. At least use a cheap powered USB hub.

author: rob77
author_rank: Super Contributor
timestamp: June 11, 2014, 08:28:06 pm
content: Quote from: janoc on June 11, 2014, 08:10:05 pmthe atx is a no- go because of the incorrect regulation if the main output is not loaded (and don't try to tell me that there is resistor drawing few amps inside) which is usually 12V, so the 5V output might be out of the +-5% spec.have you ever heard about the small power supplys/ adapter (or call them whatever you want) with USB connector output ? because it was about using those in your project - not your computer's USB ports.

author: janoc
author_rank: Super Contributor
timestamp: June 13, 2014, 12:55:59 pm
content: Quote from: rob77 on June 11, 2014, 08:28:06 pmYes, there is a fairly beefy resistor inside and no, you don't really need to have several amps going through it for the regulation to work. At least not in recent supplies. That used to be the case for the old PSUs, but not anymore. I haven't measured the resistor in mine, but it looks like a 2W one in there.Um, most of the ATX supplies don't really have separate regulators for 12V and 5V rails. It is all typically done by something like TL494 PWM controller. It is enough to load one rail and the other one will be in spec too once the feedback loop is closed. Granted, the regulation is not great, the ATX specs are fairly loose when judged by the bench supply standards! That's one reason why most PC hardware has local regulators too.Quote from: rob77 on June 11, 2014, 08:28:06 pmWell, that's not what most people will think about when speaking about USB, IMO. But yeah, these little adaptors can be OK if you get a decent one, not some cheap one transistor deathtrap which are unfortunately so common.

author: AmmoJammo
author_rank: Frequent Contributor
timestamp: June 13, 2014, 10:01:11 pm
content: Quote from: janoc on June 13, 2014, 12:55:59 pmPlease stop replying.