Using zener diode for crowbar circuit

Thread Starter

KevinHowJones

Joined Jun 19, 2022
30
Hello,

I have looked online for crowbar circuits and most of what I find are circuits which use either an op-amp comparator or a zener diode which switches on a MOSFET at a threshold voltage, thus providing a short circuit across the input of the circuit to be protected, and quickly blows the fuse cutting of voltage altogether.

I was wondering about simply using a zener across the input of the circuit to be protected, and when the voltage reaches the zener voltage, begins conducting and never allowing the input voltage to exceed the zener voltage, and if the overvoltage is high enough, will blow fuse. I have also found circuits online like this mentioned, but the MOSFET design seems much more ubiquitous.

The circuit I want to protect draws less than a half an amp, so I would only be using a 1 or 2 amp fuse. I want the threshold to be at 14V. It would seem that if I use a 2W or 5W zener, with no current limiting resistor from the power supply which has very low internal impedance, the fuse would blow before the zener ever heats up enough to fail.

Is there are reason the MOSFET design seems favored? Would the zener-only design not react as fast? Or are there other considerations?

Zener circuit:
 _________________                             ____________________________
|                 |--(+)-----fuse---|---------|                            |
|     P.S.        |                 |         |  circuit to be protected   |
|                 |                (Z)        |                            |
|                 |                 |         |                            |
|                 |--(-)------------|---------|                            |
 -----------------                             ----------------------------
 
Last edited:

crutschow

Joined Mar 14, 2008
34,455
What is the power source?
How much current can it supply?
How much overvoltage would you expect to be applied?
 
Last edited:

dl324

Joined Mar 30, 2015
16,930
A zener diode isn't likely to work. What you want is a TVS diode; essentially zener designed for a high surge current. They usually short when they fail, which is what if you have a fuse before it.

There's a difference between a crowbar circuit and overvoltage protection. A crowbar puts a short on the supply, overvoltage protection doesn't have to.
 

Sensacell

Joined Jun 19, 2012
3,449
The Zener would dissipate a massive amount of power before the fuse blows.

This is why "crowbar" protection usually means an SCR- when it goes into conduction, it basically shorts the circuit to ground, it's able to pass massive current without too much power dissipation.
 

Ian0

Joined Aug 7, 2020
9,826
A TVS diode and a PPTC "polyfuse" or a PTC thermistor works well, provided that you get them sized correctly. The TVS has to dissipated quite a lot of power in the time it takes the PTC to heat up and change to its high-resistance state.
 

BobTPH

Joined Jun 5, 2013
8,989
What are you using a power supply? What makes you think it will suddenly increase its voltage? I suppose it is possible, but the more likely failure is a short somewhere, which causes increased current, not voltage.
 

MisterBill2

Joined Jan 23, 2018
18,538
A shunt-zener diode with a series fuse scheme will not provide adequate over voltage protection for sensitive electronics because a fuse will not pop instantly. Of course that also depends on the source of the power. The only place I have seen that arrangement sort of work is on some British motorcycles, where the alternator output was shunted by a HIGH CURRENT RATED Zener diode mounted on a rather serious heat sink. Some of those have been modified to use a series transistor regulator to avoid the serious problems of the zener diode failing and allowing the lights to burn out from over voltage. ( That failure usually happened when riding fast on dark roads.)
And like others,I have never seen a mosfet or other transistor type used in a "crowbar" protection scheme. Aside from other reasons, it would not work because as soon as the voltage started to fall the bias voltage would drop and the device would stop conducting. So wherever you saw that suggestion is an UNRELIABLE source, and not to be trusted.
 

Thread Starter

KevinHowJones

Joined Jun 19, 2022
30
Excuse me, I miswrote. Yes, the majority of the circuits I've seen use an SCR, not a MOSFET.

The power supply is rated at 2A, 13V output. It is a "wall-wart" type.

The load is a surveillance camera. The wall-wart mentioned above was provided with the camera. Measurements show me that the camera draws less than 1A at any given time (600 mA is the highest I've observed), so the supply should be able to blow a 1A to 2A fuse.

The possible surge in voltage can come from line voltage power surges we have in my area from time to time. The power goes out frequently, and when it comes back online that is when the problems occur. Line voltage surge protectors apparently have not been 100% successful in protecting the load as it has created havoc with the camera, sometimes repairable with a hardware reset, but in a couple of cases has caused permanent damage to the camera.

I wanted to add something on the DC side to maybe give a little more protection. I don't know for sure if that would work, but just going on the "couldn't hurt" principle.

I suppose my thinking with the zener is that, along with the fuse, it would provide a kind of hybrid over-voltage/crowbar protection. As someone pointed out, the fuse will not blow instantly, so in the meanwhile the zener is providing overvoltage protection until the fuse blows, so the voltage to the camera will not rise above Vz. Then, since the supply has very low internal impedance, a lot of current will start flowing if the voltage spikes above the zener voltage, and the fuse will blow quickly enough that not a lot of heat will be dissipated from the zener before it blows.

Anyway, I thought it would be great if I could do this with just one device and a fuse. Just my bright idea anyway.
 
Last edited:

MisterBill2

Joined Jan 23, 2018
18,538
An option that will work very well depends on the no-load output of that supply, or some other supply, which is to add an in-line 12 volt regulator. That would totally solve the problem . The challenge is that most regulators need to drop a few volts to be able to function. You could build your own regulated supply with a transistor, zener, and a rectifier-bridge and a filter capacitor, and if the capacitance was high enough it could even keep the camera working during power drops.
 

LowQCab

Joined Nov 6, 2012
4,075
If your Camera actually pulls less than ~1-Amp, ( average Current ),
You can probably get away with not having to provide a Heat-Sink for this Regulator.

This Regulator can withstand an Input-Voltage as high as ~60-Volts,
which would equate to a Mains-Surge of roughly ~600=Volts, ( 1,200-Volts if your Mains are 240V ),
and still maintain stable Voltage-Regulation.
.
.
.
Resistor Calculations 1 .PNG
.
 

MisterBill2

Joined Jan 23, 2018
18,538
Consider that all linear voltage regulators require a higher input voltage than they deliver, usually at least two volts, often a bit more than that. So you might need a different module.
 

Thread Starter

KevinHowJones

Joined Jun 19, 2022
30
Consider that all linear voltage regulators require a higher input voltage than they deliver, usually at least two volts, often a bit more than that. So you might need a different module.
The one LowQCab suggested has a drop out of 350mV to 425mV. There are several other LDO modules available from different manufacturers as well.
 

LowQCab

Joined Nov 6, 2012
4,075
Using a few more discreet Components,
You can easily build an almost "Zero"-Drop-Out-Regulator, rated for much higher Surge-Voltages,
but it will take-up more space, and probably cost twice as much.

How high is "up" ?
.
.
.
 

MisterBill2

Joined Jan 23, 2018
18,538
Prior to action, discovering the minimum voltage that the video camera requires for satisfactory operation will reveal what the options are. If the 12 volt supply feeds an internal regulator that drops the voltage down to 5 volts, then an ordinary cheap regulator delivering ten volts would be totally adequate.
 

Thread Starter

KevinHowJones

Joined Jun 19, 2022
30
Prior to action, discovering the minimum voltage that the video camera requires for satisfactory operation will reveal what the options are. If the 12 volt supply feeds an internal regulator that drops the voltage down to 5 volts, then an ordinary cheap regulator delivering ten volts would be totally adequate.
The camera will work fine on 12V. I'm guessing their technology is designed around 12V nominal, and they use a 13V supply to compensate for any loss over using a long cable for power. I wouldn't want to go below 12V as power is used for operating the motors for PTZ.

With a 13V supply, I think a 12V LDO regulator like the one discussed should do just fine.

On another note, is there a way to determine if the supply is using old-school step-down transformer/bridge/filter or if it uses switching technology, without using a saw?

If it is a simple transformer type, then it seems reasonable that the line voltage-to-output would scale (roughly) proportionately as LowQCab mentioned. However, if it is a switcher, who knows what might happen during a surge (possibly the line voltage appearing at the output maybe?)
 

Thread Starter

KevinHowJones

Joined Jun 19, 2022
30
Using a few more discreet Components,
You can easily build an almost "Zero"-Drop-Out-Regulator, rated for much higher Surge-Voltages,
but it will take-up more space, and probably cost twice as much.
I'd like to see an example of such a circuit, got any lying around?
 
Top