# Excess voltage protection?

#### joewales44

Joined Oct 8, 2017
218
is there a simple way to protect my circuit from too high voltage?
i need to limit to max of 26 volts.
is there a part that temporarily goes open like a reset fuse but for voltage instead of current?
or a way to clamp excess voltage without a bunch of extra parts?
if not, is there a definite way to indicate excess voltage has been used?
thanks

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#### crutschow

Joined Mar 14, 2008
33,369
i need to limit to max of 26 volts.
What's the normal voltage?
a way to clamp excess voltage without a bunch of extra parts?
You will need some extra parts.
Define how many is a "bunch"?

#### dl324

Joined Mar 30, 2015
16,197
is there a part that temporarily goes open like a reset fuse but for voltage instead of current?
or a way to clamp excess voltage without a bunch of extra parts?
if not, is there a definite way to indicate excess voltage has been used?
A TVS diode would be one possible solution. Most will become a short when they fail. To provide the best protection, they should be used with an upstream fuse that will open before the diode fails.

#### MisterBill2

Joined Jan 23, 2018
16,637
There is a common scheme that uses a zener diode to trigger an SCR to short out power when the zener is biased into conduction. It is quite effective and operates fairly rapidly. But it is old-school and a bit primitive.

#### KeithWalker

Joined Jul 10, 2017
2,930
There is no explanation of what the circuit is used for, and there is no indication on your circuit diagram of what the input(?) P1 is. so my answer to your question is "Probably".

#### Ian0

Joined Aug 7, 2020
8,968
i need to limit to max of 26 volts.
Why? The IC will withstand 40V

A TVS is the normal method, as @dl324 says, but it already has one of those, so what more protection does it need?

#### joewales44

Joined Oct 8, 2017
218
supply voltage is 12 volts.
i've used the zener cutoff before and it worked great until there was a voltage spike that blew the zener.
i'm still repairing boards from that fiasco 18 years ago.

will the tvs diode blow open or short?
the supply voltage is fused.

seems there isn't an easy solution?
thanks

#### Ian0

Joined Aug 7, 2020
8,968
I use PTCC fuse and TVS diode.

#### MisterBill2

Joined Jan 23, 2018
16,637
What IC?? or is that the "2861" little block connected across the current sense resistor??
My experience with TVS elements is that usually they fail in a permanent slow resistance mode and pop the fuse.
Given that we have no hint as to the current capability of the power source, and we see no circuit protection element, we have no way to know what sort of shunt device to suggest for clamping the supply spikes.

#### Ian0

Joined Aug 7, 2020
8,968
What IC?? or is that the "2861" little block connected across the current sense resistor??
My experience with TVS elements is that usually they fail in a permanent slow resistance mode and pop the fuse.
Given that we have no hint as to the current capability of the power source, and we see no circuit protection element, we have no way to know what sort of shunt device to suggest for clamping the supply spikes.
PAM2861 LED driver. Not much of a clue in the schematic but Mr Google says that PAM2861 is the only LED driver IC with 2861 in the part number, and the TS's circuit matches the application circuit in the datasheet. Really - the amount of trouble we have to go to on these people's behalf!

#### dl324

Joined Mar 30, 2015
16,197
will the tvs diode blow open or short?
the supply voltage is fused.
The predominant failure mode is short, but they can fail open. That's why you use a fuse upstream of the diode that opens when before the diode current is large enough to cause failure.
seems there isn't an easy solution?
A TVS diode seems like an easy solution to me. Especially when the circuit is already fused.

If you tried to use a regular zener diode, you didn't choose the right part. TVS diodes can handle much higher currents than zener diodes because they're constructed from many match zener diodes in parallel.

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#### MisterBill2

Joined Jan 23, 2018
16,637
The benefit of using the SCR and Zener diode scheme is that it is simple to test, substituting a lightbulb for the fuse, and repeated checking does not deteriorate any of the components.

#### joewales44

Joined Oct 8, 2017
218
Pam2861 has 1 amp limit running at 0.5amp.
power source is 12volt marine battery with load dumps from starter creating spikes. TVS is for catching these spikes.
D4 is reverse polarity protection.
customers accidentally connect to 38+ volts and blow the chip and TVS diode.

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#### MisterBill2

Joined Jan 23, 2018
16,637
Pam2861 has 1 amp limit running at 0.5amp.
power source is 12volt marine battery with load dumps from starter creating spikes. TVS is for catching these spikes.
D4 is reverse polarity protection.
customers accidentally connect to 38+ volts and blow the chip and TVS diode.
Another option is an actual 1 volt regulator module. More complex and more costly, but the right module can also protect against really stupid customers.
AND, the US Army has implemented a module I came across that only switches on for the correct power voltage and polarity. Not sure who makes it, but it will only switch on for 24 volts DC (actual range 20 volts up to 29 volts DC). Much better than letting some fool burn up a radar set or a bombsight computer or a new encrypted radio.

#### joewales44

Joined Oct 8, 2017
218
The predominant failure mode is short, but they can fail open. That's why you use a fuse upstream of the diode that opens when before the diode current is large enough to cause failure.
A TVS diode seems like an easy solution to me. Especially when the circuit is already fused.
total current for light will be around 1 amp. Even though i recommend a 3amp fuse, my buddys may use much larger fuses than the TVS can withstand.
So... guess I'll add a reset fuse to the circuit board.
1. what hold and trip current would you recommend for the fuse? and it must be surface mount.
2. can the SMAJ26 withstand up to the fuse trip current before it burns up? Should I use a different TVS?
3. I'm unsure where to put the fuse since i must have reverse polarity protection and there are 2 inputs after the switch. i can't put it before the switch. seems like it should go right before or after D4 ?
4. are the fuses very reliable? i don't want to introduce another failure point.
5. how long does it take for the fuse to reset?
thank you very much.

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#### MisterBill2

Joined Jan 23, 2018
16,637
OK, so now we know that this is a question in the development of a commercial product, not an individual's personal project.
Commercial consulting rates apply..

#### joewales44

Joined Oct 8, 2017
218
is this location ok for fuse placement?
fuse is rated for 60V, holds 1.1A, trips at 2.2A
SMAJ26 maximum clamping voltage is 42v. should i go to SMAJ22 which clamps at 39V ?
i would like to let them use 24 volt input.
will the fuse open before excess voltage damages the driver chips?
thanks

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