1. can a SMAJ26 last long enough at 36volts for a PPTC 1.5amp resettable fuse to trip?

the fuse has a 33volt maximum but i can't find one higher with the package size and trip current i need.

2. think the extra 3 volts will burn the fuse? someone said they build in a 10% safeguard.
thanks

 

They don't like a lot of overvoltage - they catch fire.

 

1. can a SMAJ26 last long enough at 36volts for a PPTC 1.5amp resettable fuse to trip?

It should. My recollection is that you allow a maximum voltage of 24V. Why are you using a 26V TVS?

2. think the extra 3 volts will burn the fuse? someone said they build in a 10% safeguard.

As I recall, you're working on a commercial product. I'd operate components well within their maximum ratings to avoid any financial liability down the road.

That said, I have used a PTC fuse at a voltage higher than its maximum rating because it was what I had on hand. But that was for personal use, and I accept the risks from doing that.

 

It should. My recollection is that you allow a maximum voltage of 24V. Why are you using a 26V TVS?

Some may want to power with 2 12 volt batteries in series. I thought 26 volt would give a little room. After looking at breakdown voltage for the 24volt diode which is 26.7volt, I may try the 24volt diode. I have to stay below 36 volts because of 40volt max driver chips so I decided to only go slightly above 24volts. Some users have 3 12volt batteries in series and could accidentally hook up to 36volts.

What happens when the diode starts to breakdown but stays below clamp voltage?

That said, I have used a PTC fuse at a voltage higher than its maximum rating because it was what I had on hand. But that was for personal use, and I accept the risks from doing that.

These are completely contained inside a heavy metal housing and supply voltage is supposed to be fused by the user. I really only need proof that my dumb buddies hooked it up to 36volts and fried it. I thought about just hooking up the diode to a very thin trace that would burn up instead of using a resettable fuse. More parts equal more failures.

 

Some may want to power with 2 12 volt batteries in series. I thought 26 volt would give a little room. After looking at breakdown voltage for the 24volt diode which is 26.7volt, I may try the 24volt diode. I have to stay below 36 volts because of 40volt max driver chips so I decided to only go slightly above 24volts. Some users have 3 12volt batteries in series and could accidentally hook up to 36volts.

24V TVS diodes are used to protect 24V circuits. Regarding 3 battery systems, you can't protect from stupid. But, a TVS diode with a series fuse should be sufficient for even stupid users.

These are completely contained inside a heavy metal housing and supply voltage is supposed to be fused by the user. I really only need proof that my dumb buddies hooked it up to 36volts and fried it. I thought about just hooking up the diode to a very thin trace that would burn up instead of using a resettable fuse. More parts equal more failures.

Can't really protect from stupid; it would cost too much. Stupid people are your potential repeat customers.

Burning up traces seems like an odd way to prevent damage from overcurrent. The whole point of using PTC fuses is that they're self-resetting. A burnt trace eventually needs to be repaired. Even a good old glass fuse is easier to replace.

 

Can't really protect from stupid; it would cost too much. Stupid people are your potential repeat customers.

Stupid people will blame you for the problems they cause. They might not always be repeat customers. They may move on and blow up someone else's product.

 

24V TVS diodes are used to protect 24V circuits. Regarding 3 battery systems, you can't protect from stupid. But, a TVS diode with a series fuse should be sufficient for even stupid users.

I'm adding a reset fuse with the TVS. The 1.5amp fuse can go up to 3amp before tripping so I had to widen my traces to carry the current. I'm unsure what happens when the fuse starts opening and the TVS starts conducting. Will the LEDs continue to work until the fuse opens completely? How long before the fuse opens? I have parts ordered to test.
Thanks

 

I'm unsure what happens when the fuse starts opening and the TVS starts conducting.

PTC fuses never "open". Their resistance increases when their current rating is exceeded, but voltage is never actually removed from the protected circuit.

Will the LEDs continue to work until the fuse opens completely?

Like glass fuses, PTC fuses don't operate instantaneously. Resistance will start increasing when the fuse heats up and will continue to increase until it reaches maximum. So there won't be an abrupt removal of power from your device. Resistance will decrease as the fuse cools.

How long before the fuse opens?

It depends on the PTC fuse you select. As with datasheets for glass fuses, the datasheet for the PTC will give you it's specifications.

 

PTC fuses never "open". Their resistance increases when their current rating is exceeded, but voltage is never actually removed from the protected circuit.
Like glass fuses, PTC fuses don't operate instantaneously. Resistance will start increasing when the fuse heats up and will continue to increase until it reaches maximum. So there won't be an abrupt removal of power from your device. Resistance will decrease as the fuse cools.

got the board built with smaj26 tvs and fuse.
i went up to 42volts(clamp voltage) with the supply voltage but the tvs burnt up before the fuse tripped.
and the tvs failed to short which kicked off my power supply.
what would happen if i was powering from a battery? tvs catch fire?
would a smbj do any better?
could the fuse be bad or is this idea not going to work?

on a positive note, i went to 42 volts pretty quick and it didn't damage the driver chips.
thanks

 

FYI
this technique will NOT work with resettable fuses.
they trip too slow before the tvs burns up.
i tried a bigger tvs (smbj) with same result.
i'm considering using a quick blow fuse instead.
https://www.vishay.com/docs/28747/mfuserie.pdf

1. do you think they will blow before the tvs burns up?
2. i need to still use a tvs for voltage spikes. Will a spike from a high current load dump like an engine starter trip a quick blow fuse?

please respond.
thanks