Short circuit protection for power supply - Page 1
Forum section: electronics / beginners

author: AxleD
author_rank: Regular Contributor
timestamp: March 06, 2015, 01:01:39 pm
content: Hi,I am relatively new to electronics and i would greatly appreciate some assistance with better understanding the attached circuit which i found online.I have built a simple switchable LM7805/9/12 regulated supply and i was looking to add some short circuit protection. This is more to build on my small (but increasing) knowledge base rather than a requirement for the actual supply.I have built the attached circuit and it doesn't seem to work (I could of course have done something wrong). The explanation for the circuit is provided in the attachment. I do have reasonable knowledge on transistor operation and biasing but my question is simply:If the circuit is shorted, say to ground at the positive terminal, then wont all the current simply flow to ground with very little flowing through R1 to "properly bias" T1 and provide the necessary collector to emitter current to activate the buzzer and led?Thanks

author: JohnnyBerg
author_rank: Frequent Contributor
timestamp: March 06, 2015, 01:23:23 pm
content: Quote from: AxleD on March 06, 2015, 01:01:39 pmIf the circuit is shorted, current will flow through diode D1, and the bridge, only limited by the resistance of the wires and transformer. As there is no fuse, circuit will heat up until the transformer, the bride or the diode defects.As the transformer is rated only for 4,5 Watt it is likely to have some internal resistance and therefore limiting the current somewhat. However there still could be smokenote: the 78xx has internal short circuit protection. And thermal protection to.

author: AxleD
author_rank: Regular Contributor
timestamp: March 06, 2015, 02:40:43 pm
content: Haha, thanks. Yes the diode was on the verge of going up in smoke when i tested this last night!I am working from a standard DC wall supply (plug pack) and the supply could only provide 0.7A when momentarily shorted confirming some internal resistance on my transformer.I am aware that the 78xx has some protection, but the idea was for a buzzer to sound allowing the user to quickly disconnect the supply.The circuit attached in the original post did not seem to work and I'm still wondering whether that circuit works at all given the very, very low resistance of the diode (at say 0.5A) and the wires as you pointed out.If anyone could go through the process of calculating the current through T1's collector when the output is shorted that would be great. I calculate around 2ma which isnt enough to even light up the led but tbh my calculation may just be incorrect.Much appreciated

author: JohnnyBerg
author_rank: Frequent Contributor
timestamp: March 06, 2015, 02:47:51 pm
content: Quote from: AxleD on March 06, 2015, 02:40:43 pmThat's easyIf you shorten the output, the voltage before D1 will be approx 0,7VThat is not enough to lighten the LED or Sound the buzzer.When I was a student (long time ago), a fellow student made a alarm in his car that should go off at the moment the battery went dead

author: picandmix
author_rank: Frequent Contributor
timestamp: March 06, 2015, 05:28:05 pm
content: Regardless of any overload circuit you still want to fit simple fuses or a self resetting dc poly switch to avoid any burn outs.Different to that circuit you show is this one I also got off the web , though it does need a separate voltage supply from the one its sensing, but it is fast acting and has saved countless blown fuses when set correctly.It senses the current flow though R1 , which for the currents your regulators produce, is typically 0.1 to 0.5 ohms and 3 to 5W + depending on your load.Its current cut off point is adjustable and it isolates the circuit for a period set by C1 / R2 and activates a led / buzzer.After the delay it will restart the whole sequence like a self reseting fuse, though will cut things off again if the overload is still present.

author: AxleD
author_rank: Regular Contributor
timestamp: March 07, 2015, 09:10:54 am
content: Thank you for the attached circuit. Looks like a neat solution and i will definitely look at adding it to my project. I do have a fuse in the current design, but a "self resetting" solution sounds allot more functional!As an aside, and to whoever may be interested, i have looked at the circuit i originally posted again and redone the calculations. In theory, the circuit should be working and produce at least 16-20ma of current through the collector of T1 to light the LED. I even modeled it in spice to double check and got similar results.Of course when i build it on a breadboard it doesn't work. Not sure why, but i suspect it may be because my supply (plugpack) cannot provide more than 0.6A when shorted which means the bias current into the base of T1 is much lower than expected.Can anyone perhaps explain what is happening here then? If i assume around 1 ohm resistance on the diode (0.6V/0.6A) when shorted then my supply voltage must be collapsing to under 1V from 9V when shorted. How is that possible? or am i getting confused here?

author: picandmix
author_rank: Frequent Contributor
timestamp: March 07, 2015, 10:55:19 am
content: First off, do not believe all web circuits work ! - though that one I showed does as I built it for someone.Have a look at this link, they show a circuit similar to yours but much better documented so think its more likely to work, plus there is also a more useful switchable current protector on the same site.http://www.zen22142.zen.co.uk/ronj/add-on.html

author: JohnnyBerg
author_rank: Frequent Contributor
timestamp: March 07, 2015, 11:11:22 am
content: Quote from: AxleD on March 07, 2015, 09:10:54 am0,6V to be exactly.You're trying to redefine Ohms law. If you succeed, you win the Nobel priceTip1: instead of building it on a breadboard, try to simulate it with ltspice.Tip2: your schematic is absolute nonsens, the schema posted by pcandmix wil actually work!

author: AxleD
author_rank: Regular Contributor
timestamp: March 09, 2015, 07:23:37 pm
content: Haha.Of course, I see now what you saying.Thanks again guys, the whole current limiter idea is a far better idea.Cheers