Add this Short Circuit Protection to Your Power Supply

A rather cheap yet reasonably effective short circuit protection circuit is explained below which can be used for safeguarding power supply circuit

Introduction

A power supply unit is an indispensable unit for every electronic enthusiast and engineers working in the relevant field. Though today all of us use hi-tech power supply units having built-in protection, there are folks who still rely on ordinary types of power supply units with no protection facility.

The biggest enemy of all power supply units is a possible short circuit that might occur at its output terminals due to an accidental connection or due to the fault with the connected load.

There are various electronic circuits that might be employed with a power supply unit for checking this problem, however these circuits sometimes themselves are at risk of getting damaged due to the limitations with many electrical parameters.

A very innovative way of rectifying this problem has been shown in this article. A single relay is used for sensing as well as tripping the output from the involved malfunction.

Circuit Description

Referring the circuit diagram, we see that a relay is connected directly to the output of the power supply DC output, however the connection is made through the N/O contacts of the relay. These contacts are also terminated as the output of the unit.

N/O means normally open, that means the contacts are open initially, which in turn keeps the output disconnected from the positive of the power supply.

Now when the shown push button is momentarily pushed, the N/O contacts are bypassed allowing the current to flow across the relay coil.

The relay coil energizes, closing the N/O contacts, which in turn latches and sticks to the position even after the push button is released.

The relay latch maintains this latched position as long as the output is used under normal conditions, but in an event of a short circuit across the output terminals, there may be a sharp drop in voltage, the instant this voltage drops below the coil voltage of the relay, it loses its holding strength and immediately releases the contacts, and trips, cutting OFF the supply to the output and in the course switches OFF the latch preventing the short hazard conditions.

This brings the relay to its initial condition and needs resetting in order to restore power at the output.

The circuit diagram for the power supply short circuit protection is shown below:

 



80 thoughts on “Add this Short Circuit Protection to Your Power Supply

  1. Have questions? Please feel free to post them through comments! Comments will be moderated and solved ASAP.
  2. sir,
    Thanks for your innovative circuits….i would like to convert 220vd.c to 5vd.c am using a step up 220v transformer that is not from the mains a.c directly….i would be grateful if you can give me a circuit that can convert 220d.c from a step up transformer to 5v d.c.
    Regards.

  3. sir,i dont know if you can help me with a transistor that can withstand 220v d.c ,i tried 2n5551 but it could only with stand 160v d.c…
    Thanks and regards

  4. hello sir Swagatam want to know if you have a power supply circuit variable from 0 to 30 volt with fine adjustment of tension or rolling out source this to a variable … from already thank you very much.
    ATTE: Manuel Argentina

  5. Hi Swagatam,
    I am a senior design student looking to create a biomedical sensor. This circuit could be useful in the protection of our power source. With your permission may I make a copy of this diagram for our documentation?

    Regards,
    Chris McCord

  6. oomg i already bought 12 v relays e.e.

    aaannd dont u have a simple circuit like this for overcurrent protection ¿

    once again thanks!.

    • thanks for answering , but that is for an ac cicuit right ¿, what im doing is a power supply, using the 337, and i mandatory need to protect my dc circuit from overload !

      i have already implemented the shortcircuit protection and it works great!

      ATT: Steven.

    • The DC comes from the AC after all. So the AC control will be directly proportional to the DC.

      Anyway with a 337 IC you can do the following:

      use a BC557 transistor.
      connect collector to ADJ, emitter to ground-relative to (-).

      connect base to ground via a 100 ohms and put another resistor between the emitter ground connection and base ground connection…meaning the emitter/ground joint and base/ground joint must be bridged with a resistor.

      This resistor decides the current limit.

      find it by using R = 0.6/limit current = Ohms.

    • you can put it anywhere you like, before or after the regulator….it will do its job as specified.

      by the way if you are using a 78XX regulator then you probably wouldn't need the above arrangement because all modern regulator have built-in short circuit protection and are litrally blow-out proof

  7. I have added a led at the output to know when the protection is on , but how can i add another led to know when the shorcircuit occurres ¿, sorry for asking so many question this is my first try with electronic circuits

  8. Hi Swagatam! I've built the circuit and it works great! thanks!
    I just have two questions:
    1 ) is the diode really needed? what is it for? having it means that I will always have a 0.7 voltage drop and I would like to avoid it because i'm puting the circuit after a previously regulated power supply.
    2 ) Is it safe to use this circuit as a short circuit "detector" to repeatedly check solderings, etc. (as I usually do with the buzzer in my multimeter)

    • Thanks Daniel,

      The diode can be removed.

      this circuit is a crude design and will work only for high current short circuit scenarios….it's not suitable for detecting low current soldering faults.

  9. hi again,
    Thanks for your responses 🙂
    As I said before, I'm placing this circuit after a voltage regulator. The problem now is that when I regulate the voltage to lower than 3.5 volts, the relay opens, so I cant use lower voltages than that. The reason why this appens is quite obvious, I just wanted to ask if you have any suggestion on how to solve it.

    By the way.. have you thought of this circuit as being one bit of digital memory?

  10. hi swaggatam, how do you make a circuit like above to short circuit protect a lithium bank of 200v?
    Im working on a project and i noted that an accidental increase in current draw would lead to fires,

  11. hi swagatam
    can you please guide me i would like to make a variable resistor, which component should i replace and value? i already have transformer which is 24v 6amp. thanks advance

  12. hi swagatam, can you please guide me what component and value should replace in diagram i want to make an variable resistor which is 30v is the max output by using an 24v 6amp transformer

  13. Hi Swagatham
    Can you publish an LM317 based 1.25V to 24V, 5A variable powersupply circuit. I know 317 can only handle 1.5A, so a power transistor(s) can be used to handle current more than 1.5A.
    But I don't want that bulky power transistor. I prefer an N-Channel power mosfet for enhansing LM317's current capacity. Is it possible….?
    I searched google for such a circuit, not yet found. If you can add output SHORT CIRCUIT PROTECTION, with latching/auto reset after removing short circuit. That will be an added advantage.

    • Hi, Anil,

      although a BJT looks more appropriate as the outboard current booster device since it requires just 0.7V to trigger, let's try this with a mosfet and see how it responds…I'll design and post it soon.

  14. hello sir as u said above "could be used in variable power supply unite. ." how does relay going to react in case of low voltage is being set eg: 12max out put relay set to 12vminmum if turn down voltage to 2v then I hope relay going to be Delatch .? right sir ?
    Thank you in advance

  15. Hi sanjay bharvad,
    I believe it is possible if you have a step down transformer if it is 50-100HZ ac or high frequency circuit which will step down the 9kv to easy to manage power like 12v. you can then utilize the relay powering circuit from 12v external source and use zener diode to cut out form a cut-off point when the 12v which is directly influenced by the 9kv fluctuates to the level you may want a shut down.
    I have utilized similar circuits in lithium ion short circuit controls for electric cars.

  16. Sir
    I made your short circuit protection circuit it works fine for transformer power supply , if I connect to battery 24v 10Ah ,circuit not working ( when shorting output voitage across relay not fall to zero ,instead huge spark and melting wires) please give any modification or new circuit.sorry for my bad english

    • Deepak, it’s because a transformer has limited current which is able to drop, whereas a battery may have relatively huge current which may not drop quickly causing melting of wire.

      add a current limiter in series with the battery, and you will find the system working perfectly. You can do this by adding a low value resistor such as a 2 ohm 5 watt etc or you can calculate it using Ohm’s law

      alternatively you can try the following design which is designed for battery use

      http://www.homemade-circuits.com/2012/04/how-to-make-automotive-electronic-fuse.html

      • Thanks Bro, I made your new electronic automotive fuse it works fine,one again thank you for your fast response, I made another circuit 24v battery to 12v 200mA circuit using lm317 adjustable but lc get heated ( some power loss approx 3w as heat) please give any efficent simple circuit for 12v 200mA, thank you.

          • Bro another help please, I want to add cheap multimeter ( 2$) as voltmeter for my variable power supply unit , multimeter is 9v battery powered ,I replace it by 7809 ic but while measuring (i.e self powered multimeter) improper reading , please give any simple isolated power source circuit 9v or any suggestion/ideas thank you…

          • sure bro, the 7809 should have worked correctly according to me, first confirm whether or not it is giving an accurate 9V without the meter connected…if it is fine, then you can try connecting a high value capacitor across the output and ground of the 7809 and see if that correts the issue….you can try a 1000uF/16V or even higher. alternatively you an simply use your cellphone charger output and use it for powering the meter, it worked great in my meter.

  17. Thanks for your suggestion bro, multimeter power supply 9v by adding capacitor in output is not working on my cheap meter . cell phone charger idea help little bit that is while touching the negative by hand gives proper reading , finally I decided to leave on battery power supply with on/off using relay (one COB multimeter gets defective while measuring voltage more than 3v) . thank you …

      • Hai Bro, your help please .
        three dc source with one voltmeter, I need a circuit to measuring voltage of dc source ( common ground) by selecting with one push switch ( 3 way sequential selector switch ) using relay , thank you

        • Hai bro sorry for disturbing you again and again , I thank you once again for your quick response, I tried sequential push selector switch it works perfect but some delay in selection ,then i changed the capacitor between pin 13 &14 with 0.5uf instead of 1uf it works charmingly thank you ..
          I made variable PSU using lm338t plastic package t220 , when output get shorted ic gets heated instantly with in 4 sec ic gets damaged in min/max output ( but using lm317t long term short circuit and work absolutely perfect without ic gets damage ) even though using proper huge heat sink and fan ,please provide any short circuit protection circuit for variable psu ( but while using lm338k its works fine for some limits time approx 1 to 4 mins , I tried your electronic fuse works perfectly for constant voltage 5 to 33v thank you…

          • Bro, LM338 is the 5 amp version of the LM317…and both these ICs are thoroughly protected from overheat and overload situations…..if your LM338 is burning due to overheat that means it is a duplicate IC not original make.

            short circuit protection is absolutely not required for these ICs…you can check the details in the datasheet of the IC.

  18. Hai sir….*
    I tried this circuit it’s working well…
    I have one adapter type smps…
    O/p 12v 1.5A …

    Its working good…This smps contains short circuit protection…
    If i short it…smps power led goes OFF…if i un short it…smps power led ON…

    I can’t able to find the circuit diagram…fully molded …
    So pls guide me sir…
    Is there any circuit same like…

    Whenever short happen….smps power led off and in normal condition smps power led ON….

    Pls guide me sir….*



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