The following article explains how to build a power amplifier short circuit or overload protection circuit for safeguarding loudspeakers from burning.
Why we need a Short Circuit Protection
While working with high power amplifier designs, two things become crucial, the protection of the amplifier and the protection of the speakers from an accidental over current influx.
Especially when the amplifier design involves costly mosfets, the design becomes specifically vulnerable to short circuits at the outputs.A short circuit at the output may be caused due to mishandling or ignorance from the part of the user.
Whatever might be the reason, the end results in the destruction of the precious mosfets inside the amplifier box.
The above mishap can be prevented by adding a small circuit for detecting a short circuit conditions at the outputs of an amplifier.
Circuit Description
The given amplifier short/overload protection circuit diagram, shows an inexpensive design using just a single transistor for implementing the intended feature.
Normally a low value resistor is usually employed at the output of mosfet amplifiers, the current developed across this resistor can be well exploited for tripping a relay in case it exceeds the safe maximum current value.
The current threshold across the above resistor is sensed by an LED inside an optocoupler, which lights up the moment a short or overload conditions is sensed.
This instantly triggers the opto transistor which in turn switches ON the transistor driver and the associated relay mechanism.
Since the relay coils support the amplifier connection with the speaker output, disconnects the amplifier from the output connection, preventing the amplifier devices from a possible damage.
The capacitor at the base of the transistor keeps the transistor switched for a few seconds so that the relay does not oscillate randomly.
Fernando Cavallero says
Hi! Nice circuit. How do you decide to use 1.5 or 2 in the above formulas? That number means the voltage drop in the resistor to trigger the diode bridge. Thanks
Swag says
Hi, thanks, I am glad you liked it! for the opto coupler to conduct optimally the current limiter resistor must develop a voltage equal to the sum of the forward voltages of two series diodes from the bridge and the LED inside the opto….the total may reach upto 2V minimum, that’s why the value 2 needs to be used in the formula.
Dondon says
Hi sir …IAM planning to build your design what is the computation for my power amp are 1000 watts at 80 volt ….t.y.
Swag says
Dondon, I have explained the formula in the article, please do it according to it…
Dondon says
Thank you sir …I figure it out and calculate the exact value of the resistor…..t.y…
limar says
only detects DC at speaker out but not detects SHORT at speaker out…. optocoupler needs DC supply in order to work… how…in this circuit provides a SHORT protection, let’s say a faulty (shorted)speaker wire to the amplifier?
Swag says
did you check how much voltage is generated across the sensing resistor during the intended overload? did you check whether that voltage was actually sufficient or not for triggering the opto LED?
the bridge is already present to convert the pulsating DC into rectified DC?
check all the parameters above, and set up the resistor accordingly as per your overload threshold, the circuit will cut off the relay at the set point
limar says
circuit NOT WORKING . . .
Swag says
find the fault in your design, it will work
RAHUL.R says
Sir,
This circuit also prevent DC from amplifiers O/P? means, this circuit act as a speaker protection circuit?..
How can calculate The resistor value in +75v -0- -75v 250W amplifier?
Swagatam says
Rahul, yes you are correct, it will protect speaker from burning.
for 75V 250 watt, the current limiter resistor can be calculated as
250/75 = 3.33 amps
R = 2/3.33 = 0.6 ohms
watt = 2 x 3.33 = 6.66 or 10 watts
tsalz says
Does anyone have a COMPLETE working circuit (including missing +12V to relay and opto) with all parts IDENTIFIED? Has anyone tested or built this circuit?
saravanan ck says
Thank g
saravanan ck says
What is number of diodes
Swagatam says
use 1N4007
saravanan ck says
This circuit is new it OK can we use in4148 diode
saravanan ck says
Thank you sir so I want to ready 6 circuit for 5.1 amplifier
Swagatam says
you are welcome, yes that's right!
saravanan ck says
Sorry I can't understand which voltage
Input to transformer is 220 volt and the output of the transformer is 24 volt the transformer current rateing is 5 amps
Mean while I get only 1 ohmes in India
Thank-you
Swagatam says
the 0.5 ohm shown in the diagram will need to be calculated as explained using the formula in the previous comment.
If your amplifier operating volatge is 24V then
100/24 = 4.16
R = 1.5/4.16 = 0.36 ohms
wattage of resistor will be = 1.5 x 4.16 = 6.24 or a 10 watt will do.
if you are getting 1 ohm then use two in parallel, in place of the indicated 0.5 ohm
Swagatam says
…..two 1 ohm in parallel will do in place of 0.36 ohm
saravanan ck says
100 Watts 5.1 amplifier system it is designed by MOSFET IC I have changed my speaker for more than 8 times in 2 years
Thankyou
Swagatam says
for 100 watt you can use the following formula
100/V = current
therefore R = 1.5/current
here V is your amp supply voltage
saravanan ck says
Can I use MCT2E instead of 4N35
Swagatam says
yes you can!
saravanan ck says
Sir thank you for reply but in India only 1 omhs is available can I use that one
Thanks again
Swagatam says
saravanan, please tell what is the max wattage limit do you prefer to implement? the resistor value will depend on this
saravanan ck says
Say the value of resistance 0.5 ohm iam not able to get in india
Swagatam says
use 5nos of 0.1 in parallel
saravanan ck says
OK it is useful circuit I want to know the number of opto couple
Thank-you
Swagatam says
use 4n35 optocoupler
saravanan ck says
Useful but I want the number of opto couple
mohammad yusuf says
thank,,, its usefull
Anonymous says
have you built and tested this project/the project is impressive does this protection also include d.c currents against the speakers?
Swagatam says
not tested, but i am sure about it.
yes it would also handle DC shorts across the speaker
Anonymous says
HI
this diagram seems not complete how does the relay power up?where is the source of its power?
Swagatam says
Hi,
yes by mistake the positive connection is not shown in the diagram, a separate +12V should be connected to the link that connects the relay coil and the opto transistor.
sanusi emmanuel says
see, talk of ingenuity this is one of them, it obviously agreed with my slogan "simple and functional" keep it on.
Swagatam says
thanks!