# Handling high pulse current through relay contacts

I have a circuit similar to the following:

simulate this circuit – Schematic created using CircuitLab

The peak current can get up to ~120A for about half a microsecond. The SCR will take care of the 'hot' switching -- the relay is just there to configure the load and other components not shown here.

I'm having a difficult time finding any specs on pulse current ratings through relay contacts. I've had my eye on the Omron G7L-2A-X, which is rated for 25A, but it's hard to cough up ~US\$100 for a relay that could turn to smoke with one operation. Is there a way to determine whether a relay can repeatedly survive such conditions without just buying one and trying it? Are there rules of thumb to go by?

(I've looked at previous questions here and here, but neither seem to be quite what I'm looking for.)

• I think in your diagram you meant to connect to the relay power contacts and not control terminals. Sep 28, 2017 at 15:14
• It seems like you are describing an "off-load switching" application. The contacts will open and close only when there is no power applied. The concern is closed-circuit current and open-circuit voltage capability. Those capabilities will be much higher than the switching ratings, but it is not likely that you will find them published for relays. You might find fault current ratings that would provide some indication. Neither the diagram nor your description make much sense.
– user80875
Sep 28, 2017 at 15:41
• What @CharlesCowie said... Also the relay you are showing is DC, I'm sure you can find a much cheaper AC relay that handles your currents. ALso.. check contact resistance. Sep 28, 2017 at 16:16
• @vini_i Fixed the circuit - thanks. Threw the diagram together in a hurry. Sep 28, 2017 at 17:25
• @CharlesCowie Does the updated description describe my question better? I'm basically seeking guidance on any design experience related to how much I can uprate (opposite of derate) the current capacity of relays for such a short pulse duration. Sep 28, 2017 at 17:37

Let's do the math. You have a $0.5\ \mu F$ capacitor charged at 500V.

The energy stored on that capacitor is:

$$E=\frac 1 2 C \cdot V^2 = 62.5\ mJ$$

i.e. very small.

Even if ALL that energy were discharged on the relay's contacts, the effects would be very limited on a 25-A rated contacts, which are pretty rugged. Beside that, the relay will not experience such energy dissipation, as the SCR will handle the hot switching, and there will be a $4\ \Omega$ load.

The mass of the contacts will be in the milligram range. Assuming a very conservative $0.1\ \frac {kJ}{kg \cdot K}$ heat capacity, and one milligram of contact material, you'll end up in a temperature rise of 625 °C. This is smaller than the melting point of most relay contact materials.

Considered the $0.1\ \Omega$ contact resistance, and the load, and assuming zero loss on the SCR, then the energy on the relay will be 1/40, so the temperature rise will be negligible.