A very simple DC over voltage protector circuit is shown below. The transistor is set to monitor the input voltage applied to it from the left, in case the voltage rises above a specified limit, the transistor conducts, providing the required current to the SCR, which instantly fires, shorting the output and thus protecting the load from the hazard. It's also called a Crowbar circuit.
How it Works
The circuit shown below is very simple to understand and is quite self explanatory.The working may be understood with the following points:
The supply DC input voltage is applied from the right hand side o the circuit across the SCR.
As long as the input voltage remains under a certain predetermined value, the transistor is unable to conduct and therefore the SCr also remains shut.
The threshold voltage is effectively set by zener diode voltage.
As long as the input voltage stays below this threshold everything goes on fine.
However in case the input crosses the above threshold level, the zener diode starts conducting so that the base of the transistor starts getting biased.
At some point of time the transistor becomes fully biased and pulls the positive voltage to its collector terminal.
The voltage at the collector instantly passes through the gate of the SCR.
The SCR immediately conducts and shorts the input to ground.
This may look a bit dangerous because the situation indicates that the SCR might get damaged as it shorts the voltage directly through it.
But the SCR remains absolutely safe because the moment the input voltage drops below the set threshold the transistor stops conducting and inhibits the SCR from going into damaging extents.
The situation is sustained and keeps the voltage under control and prevents it from reaching above the threshold, in this way the circuit is able to accomplish the DC over protection function.
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Regarding a crowbar for a negative supply…
Use the same circuit.
Just connect it the right way round across the supply.
Anode of the SCR to the positive and cathode to negative. The circuit does not care which is ‘ground’.
Hello. The principle of operation of the circuit would be the same as for positive voltage. Only with negative tension. It would have to be a circuit with TRIAC (I think, because the SCR doesn’t work with negative voltage. Right?).
In other words: Close short (and blow fuse) if the voltage exceeds the pre-established limit.
With this protection you will know before burning the components of the circuit that is powered if the problem is in the circuit itself or in the source that supplies it.
(for example in a symmetrical source where you have + VCC and -VCC.
+ VDC- Voltage 16v = OK. voltage 16.5v = Burn out fuse.
-VCC – voltage-16v = OK. voltage-16.5v = Burn out fuse.)
Independent protections.
This circuit is for + VCC.
And for -VCC? How to make?
I don’t know if I made myself understood …
Searching on Google I found nothing regarding the CROWBAR protection with negative voltage.
You could help me?
If possible with the formulas for calculating the stresses.
I thank you for your attention.
Sorry for the translation (Google)
Luiz Pavão
You mean to say crowbar for AC?
You can try the following design:
Olá . Gostaria de saber como fazer esse tipo de circuito para tensão negativa.
Obrigado.
Pavão
Hi, sorry I did not understand how the negative voltage crowbar would work?
Hi, I have a tiny wind generator ( a toy ) and would like to use it as the basis for a strong wind alarm..is it possible to modify the crowbar circuit to incorporate a buzzer?
It will generate about 5 v with enough output to light an LED in normal circumstances..I would like to connect to a modified version of the crowbar to a relay board that I have so that if the wind speed rises and the unit generates 6 v or more it can trigger the alarm relay circuit.
Regards
David B
Hi, you can connect the buzzer in series with the 4k7 resistor. If the transistor does not conduct satisfactorily, you may try reducing the 4k7 to 1k.
Input is 18v
try 18 V zener diode, and also replace 100 ohm with 1K
I want the set limit to be 15v, how will I select appropriate components value
What is the input volts and from where it is acquired?