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 Operation
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:
Power Supply Short Circuit Protection using SCR and Relay
The next circuit is another over current protected power supply circuit which employs an SCR and a relay for the required protection and cut-off.
It's basic yet efficient in managing up to 24 VDC. The maximum voltage output may depend on the secondary voltage of the transformer. With the help of the 500-ohm potentiometer, the output voltage could be changed.
As soon as the protecting circuit detects an upsurge in current, the SCR will activate the relay coil, shutting the load's power and placing the circuit in idle mode. S1 should be pressed to deactivate the SCR once the over current issue has been fixed.
The 2N3055 transistor works like the voltage regulator pass transistor. However, this transistor section could be entirely replaced with any other standard voltage regulator IC such as LM317.
The 1-megohm variable resistor can be used to control the sensitivity of this SCR protection circuit.
Short Circuit Protection for Variable Power Supply Circuit
If you are having a variable power supply and want the short circuit operation to work even at voltages below 3 V or 1.5 V, then you can try adding the following short circuit protection circuit to your power supply circuit.
The idea was designed and submitted by one of the avid readers of this blog.
The above was tested and found to be working satisfactorily. The images show how the circuit was practically implemented into a variable power supply unit.
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