Lower rated wattage for current spike?

Hi folks - if I have a circuit that will pull a max of 2A for 1-2 seconds at 8 Volts (so Power = 16W for 2 seconds), is it ok to use a lower wattage resistor for that?

I realize it would be better to use a resistor with wattage capability higher than that, but those resistors are much bigger and won't fit on my PCB.

How can I make the determination if I'll run into temperature problems for just those few seconds?

This is an example resistor I'm thinking about (datasheet link there): https://www.digikey.com/product-detail/en/vishay-dale/ALSR014R000FE12/ALSR1F-4.0-ND/256964

You might get away with it but 2 seconds is a rather long time in electronics. For millisecond pulses you could probably get away with handling the average power.

Typically you'd want to derate resistors, so use a 20W resistor if you need to dissipate 16W. In your case, maybe you could get-by with a 10W resistor, maybe less.

Are you sure a failure/error won't hold that power for a long period of time?

You can try it... How bad are you going to feel if the resistor burns-up and your PC board turns black?

XY problem for a start but one approach is to use two 10 W resistors. Three 7 W.

How can I make the determination if I'll run into temperature problems for just those few seconds?

If the resistor explodes or emits brown smoke, wrong choice.

But your calculation of 16W cannot be correct, unless the resistor IS the circuit. What is the circuit and what is that resistor supposed to do?

Sure - happy to give more detail, thanks all.

I’m building a circuit to wirelessly trigger e-fuses for fireworks. Actually, I already have one built and working but I used a second power source and am reworking it to all run off of one power source.

Each board can handle 8 fuses; each of the 8 fuses has a current spike when it fires, so I need a resistor to limit the current it can pull so it doesn’t starve the controller that’s also running off the same 9V battery. (the whole battery only needs to last for about a 20 minute show).

I could be wrong about the 1-2 seconds…it could be that the e-fuse only really pulls high current for microseconds; I’m not sure how to know, and the datasheets on these things are non existent. They are like this (although not this exact brand): https://www.aahfireworks.com/e-fuse-3-meter.html

Schematic attached.

How do you know the fuse will even work if you limit the current like that?

Why depend on a small battery? A 9V PP3 block battery would be an extremely poor choice for a high current application (if that is what you mean by “9V”). Just use a 12V motorcycle battery, or a 6xAA battery pack.

In any case, you need to do some experiments.

I don't know it will work if I limit current like that; I'm waiting to get them in the mail so I can test; but I believe the firing amperage for them is between 1 and 1.5A.

Depending on a small battery because I have about 15 of these boards placed over a large area and more/bigger batteries adds up and is a pain.

The previous 2 power source iteration used 4 AA for the controller and 1 9V for the firing circuits and that was fine....am trying to get rid of the 4 AAs....

I have about 15 of these boards placed over a large area

And one 9V battery is supposed to power all of them, over long wires?

Seems a bit early to be posting about issues that you don't know exist, or designing PCBs without extensive field testing.

Sorry if I wasn't clear - there is 1 9V battery per board.

The 9V battery on each board powers the 8 ports for the board; so the wires are not that long - max is about 4 ft down to about 1 ft. I have that all already working. The only difference is that the previous board iteration powered the controller separately because firing a fuse was causing the controller to starve briefly and reset.

So in theory current limiting the firing circuit should fix that.

Which leads me to needing to know how big a deal the brief wattage above resistor capability is....

I'm happy to do more field testing when the fuses get to me...but I'll just be testing to see if the resistor explodes. Right? Seems like a valid question to ask here?

Apologies I'm not trying to waste folks' time - if there are other tests that would be material I'm happy to do them...

NodeMCU (3.3volt logic) + IRF540?
Did you test that @ 2Amp, and did that work?
Dropping 9volt to 3.3volt with a 100mA LP2950 for the whole board, including NodeMCU that needs 100mA with ~400mA peaks? Did the LP2950 survive that?
Which resistor. The 6ohm one in the sources?
Leo..

'm happy to do more field testing when the fuses get to me...but I'll just be testing to see if the resistor explodes. Right? Seems like a valid question to ask here?

The idea is unworkable to begin with. The energy from the battery either goes into heating the fuse, the resistor, or both. Energy that heats the resistor is completely wasted.

Come back when you have a genuine, rather than an imagined problem to solve.

My apologies for not having enough background to be worth your time.

I appreciate the attempted help anyway.

I think I will still be cautious - it sounds like you're saying I should make the problem genuine by just trying a lower watt rating and then coming back here if it explodes. I prefer a safer approach to build my understanding as I learn (I'm still a beginner).

I'll look elsewhere or try to devise safer ways to learn and test. I'm sorry if I'm misunderstanding what you're saying but I'm doing my best.

Thanks anyway for your efforts.

I confess to not knowing anything about firing fireworks. When my dad used commercial explosives the device he used to fire them used a capacitor charged to a high voltage. The capacitor was dumped across the detonator. High current for a short time. I guess if were doing what you are doing I'd try that approach.

Before going too far, I would take an oscilloscope and see what the voltage drop on the 9v battery itself was when pulling 2 amps for 2 seconds. Those batteries are not built for high-current applications and the internal resistance may give you problems.

Do you know how much current your control circuitry draws? Another approach might be to use a large value capacitor to hold up the voltage for that, fed from the battery through a diode (to prevent backflow) and resistor (to limit current spikes when first connected).

it sounds like you're saying I should make the problem genuine by just trying a lower watt rating

No, drop the resistor idea. Use an appropriate battery instead. Model rocket enthusiasts use an AA battery pack to ignite fuses.

It may be that a FRESH 9V block battery (with no resistor) would work for you in that particular situation and for the duration of the show, but you won't know until you try the experiment.

So it was a complete XY problem after all! :astonished:

One of the better examples thereof!

Hi,
Why not store the extra energy required for the instantaneous current for the e-fuse in a capacitor on the e-fuse side of the current limit resistor?

Have you got any circuit testing facilities on you firework control system?
That is before you fire, you test the continuity of the circuit?

Tom... :slight_smile: