power supply  - Page 1
Forum section: electronics / beginners

author: christos
author_rank: Regular Contributor
timestamp: December 18, 2013, 07:40:35 pm
content: i have a power supply..very big and i want to make a protective circuit..when i touch the (+) and (-)..can any one find a circuit that its cheap and easy to build?

author: con-f-use
author_rank: Unknown
timestamp: December 18, 2013, 09:19:29 pm
content: AC? DC? What voltage? What frequency? Protect what from what?If you just touch one output (either plus or minus) and your power-supply is not grounded, you should be fine. Else you might need anisolation transformeror anRCD.

author: christos
author_rank: Regular Contributor
timestamp: December 19, 2013, 01:57:29 pm
content: dc ..its a short circuit protection

author: tszaboo
author_rank: Super Contributor
timestamp: December 19, 2013, 04:19:17 pm
content: It is called "fuse".

author: christos
author_rank: Regular Contributor
timestamp: December 19, 2013, 04:23:38 pm
content: not fuse lol..every knows that..never mind..it uses transistor and some resistors so when the output makes a short circuit all the current flows to the transistor and the resistor..i think the transistors work like a switch in this case

author: madires
author_rank: Super Contributor
timestamp: December 19, 2013, 04:45:33 pm
content: A current limiter or constant current mode?

author: Skimask
author_rank: Super Contributor
timestamp: December 19, 2013, 05:27:30 pm
content: Maybe a crowbar?

author: christos
author_rank: Regular Contributor
timestamp: December 19, 2013, 07:55:54 pm
content: https://www.google.gr/search?safe=off&espv=210&es_sm=122&biw=1280&bih=922&tbm=isch&sa=1&q=short+circuit+protection+circuit+diagram&oq=short+circuit+protection&gs_l=img.1.1.0i19l10.2074.2074.0.3759.1.1.0.0.0.0.266.266.2-1.1.0....0...1c.1.32.img..0.1.266.WcTpG-SbSogif you know this..i want something like this but without transistor if its possibly

author: Mandelbrot
author_rank: Regular Contributor
timestamp: December 20, 2013, 05:57:59 am
content: Is there any particular reason you don't want to use transistors? What voltages/currents are expected across the output terminals? We really just need more information.

author: Rerouter
author_rank: Super Contributor
timestamp: December 20, 2013, 06:35:27 am
content: buy a resettable circuit breaker, you wont be able to make a DC current limit without some kind of transistor

author: christos
author_rank: Regular Contributor
timestamp: December 20, 2013, 10:36:10 pm
content: i have a fried in germany and he made a circuit only with resistors..he lived here in greece and i cant fiind his number now..thats why..he made one power supply for me from a pc and made that circuit in it and when i build my own with lcd display i could not find the power supply he made for me..if you dont know anything .thats ok..i just thought someone would know here

author: Dave
author_rank: Super Contributor
timestamp: December 20, 2013, 10:59:38 pm
content: Quote from: Skimask on December 19, 2013, 05:27:30 pmAcrowbarprotects from overvoltage, not overcurrent.OP, you are going to have to cough up more info if you want any useful help. Engineers don't like guessing.

author: Rerouter
author_rank: Super Contributor
timestamp: December 20, 2013, 11:38:00 pm
content: you still havent elaborated on just what the hell power supply it is you have? as for a possible current limit method such as your freinds,if its a regulated power supply then you find a normal incandescent globe with a cold resistance low enough to drop about 2-3V at your ideal current, and a voltage rating higher than the supplies maximum output, you hook this globe in series with the output and move the feedback network of the supply after the globe so it still outputs the correct voltage for small loads,when you exceed the current and the globe will start to get hot making its resistance increase limiting the current furthur, as an added bonus you have a nice bright indicator that your drawing excess current

author: christos
author_rank: Regular Contributor
timestamp: December 21, 2013, 08:33:41 pm
content: its just a normal powe supply..never mind..ill just put some fuse and its donehe made that when the outputs touch each other..the current flow throu the resistors somehow..he used the resistor as a load for the current to flow..he had a LED as a warning so he could see if there was a problem in the project he made....if you dont havee anything in mind how this works ..no problem..

author: Rerouter
author_rank: Super Contributor
timestamp: December 21, 2013, 09:10:23 pm
content: ok next best guess for his, a lm317 regulator in constant current mode, plenty of info online about how to set up one of those, likely with a comparator to turn on the led, (these are active devices like transistors, so when you said no transistors i was working out a truly passive method)

author: Simon123
author_rank: Regular Contributor
timestamp: December 21, 2013, 09:20:50 pm
content: You can use transistor and current sensing resistor:Calcuate what resistor you need with ohms law and connect output of transistor to rs latch and to relays, which disconnect output.

author: christos
author_rank: Regular Contributor
timestamp: December 21, 2013, 11:54:01 pm
content: There was no transistor or anything..olny 2 or 3resistors..if you dont know.anything about it..i just make one from the internet..simply

author: Zero999
author_rank: Super Contributor
timestamp: December 22, 2013, 12:50:29 am
content: A simple resistor will limit the short circuit current but unfortunately, the voltage drop may be too high.You can buy special PTC (Positive Temperature Coefficient) resistors which have a low resistance when cold and high resistance when hot. Another name for these is polyswitch/fuse. The PTC resistor is connected in series with the load. Under normal conditions the voltage drop across the resistor is negligible but when it's short circuited, it heats up and the resistance increases so a very small current flows.Here's a link to a typical datasheet:http://www.vishay.com/docs/29085/29085.pdf

author: TheBay
author_rank: Unknown
timestamp: December 22, 2013, 02:34:29 am
content: Seems like this guy doesn't want any help or listen to the suggestions given

author: Bored@Work
author_rank: Super Contributor
timestamp: December 22, 2013, 06:44:46 am
content: Quote from: TheBay on December 22, 2013, 02:34:29 amWasn't that obvious from the beginning? Probably a bullshit PC power supply that should now magically become a lab power supply.

author: christos
author_rank: Regular Contributor
timestamp: December 25, 2013, 02:24:31 pm
content: well.. i used some fuses..if i find my friedns number ..ill ask him how he made the circuit with only resistors and ill post it here thx

author: christos
author_rank: Regular Contributor
timestamp: December 26, 2013, 06:32:14 pm
content: Quote from: TheBay on December 22, 2013, 02:34:29 amsorry about that..its ok.. i used fuses

author: techm
author_rank: Newbie
timestamp: January 10, 2014, 07:17:37 pm
content: For those still interested, I had been thinking about this also for my build and ran across a neat circuit using optocoupler, a transistor and a relay. Basically, the optocoupler internal light is powered from the output, when the + and - are shorted, the voltage across the optocoupler light is effectively zero (or close to it) so it goes out. On the other side of the coupler (transistor side) this removes the base current to the NPN transistor which completes the circuit to the relay, opening it and shutting the power off. Place the relay before the optocoupler and it latches off. A simple momentary switch to restore base current to the transistor latches it back on. Here's where I found it:http://www.learningelectronics.net/circuits/short-circuit-protection-for-balanced_14.htmlI guess the potential drawbacks are having a relay on the output (and its mechanical bounce) and the fact you have to manually reset it, though I'm working on modifying that.