# Capacitor calculation

I am new to the concept of pyro-fuses, if the firing current is 1.2A/2ms, this means that I need to provide at least 1.2A for 2ms for the pyro to be triggered. Now if I want to use a capacitor to provide this firing pulse for a 2-3 Ohm load, how to calculate how much capacitance needed? The calculations I am getting don't make sense, and I am not sure if I understood the concept correctly.

• What starting voltage on the capacitor are you willing to consider? Commented Jan 11 at 18:56
• I think similar question was just asked. Show how you did the calculations, they may be correct, and thus the result will be that the capacitor values required are not sensible, which will also means, capacitor is not a sensible solution to the problem. Commented Jan 11 at 19:03
• @404 No, I wouldn't say that. I would say I don't know enough about your application. For example, is this to be delivered through some very long wire? Commented Jan 11 at 20:13
• @SamGibson thank you for clarifying, I wasn't aware of that, during the editing thought it was because of the rule compliance, I will be more careful then
– 404
Commented Jan 11 at 20:58
• Thank you very much @periblepsis
– 404
Commented Jan 12 at 10:18

C=I*t/U. So you would need 2.4mF per Volt or 0.66mF at 3.6V.

• Just to clarify - you are using SI units and "mF" means milli-Farads and not micro-Farads aren't you? Commented Jan 11 at 20:28
• Yes, all SI style. Commented Jan 11 at 20:28

I also new to the concept of Pyro fuse, but based on information,I found the relate document from here.

Rémy Ouaida, Jean Francois de Palma, Gilles Gonthier. Hybrid Protection based on Pyroswitch and Fuse Technologies for DC Applications. Symposium de Genie Electrique, Jun 2016, Grenoble, France. ffhal-01361696

That gave the deffinition of Pyro-fuse as the device that combine Pyro-switch and fuse in parallel.

The Pyro-switch is the switch that use Pyro-technique chemical in cut through the busrbar, result in high voltage, high current fast response cut-off switch. to trigger it, it require some energy to ignite the chemical.

The paralleled fuse is under rated and be there to keep low voltage across pyro-switch then it will opened afterward.

In sumarized, the pyro-fuse is safty cut-off switch that work on external trigger which required amount of current in some specific duration to start the action.

For application and designs, I found this application note document from TI called "L9680 Cut-off Battery and Pyro Fuse" and the L9680 Datasheet and this application circuit is quit interesting.

This figure show that "Energy reserve capacitor" which get charge by using 5V or 3V3 power with boost converter to 25V or 33V (Programmable) the BOM of application circuit from datasheet required min 2.2mF to 4.7mF maximum. This design for deploy with 1.25A or 1.75A current to pyroswitch.

However, both of the application note and datasheet doesn't shown the calculation or rules to select the value of capacitor.

The correct answer will depend on the circuit and scinario of usage. For example le assumed that I just use the switch to discharge the capacitor to pyroswitch from fully charge cap. The pyroswitch required 1.2A for 2ms with resistance of 2.5Ohms the minimum voltage to get 1.2A is IxR = 3V

Then for RC discharge voltage equation

Vc = Vsexp(-t/(RC))

then solve this equation and we got

C = 1/(1250*log(Vs/3))

As it depend on fully-charge of capacitor then let assumed it tobe 5V We got C = 0.00156609 or about 1.56609 mF, don't forget to consider safety factor, ESR and wire resistance.