TR2 & TR3 Forum

Polyfuses come in a wide variety of amperages. The hold amperage is roughly half the trip amperage which corresponds to the sizing of most fuses used for circuit protection. I've now had 4 people burn out the switch contacts on the CDI I build by incorrect hookup. The last one burnt because the owner put a dead short across the coil with the switch in STD so the switch saw at least 75 amps. The switch contacts are paralleled and each set of contacts will withstand 8 amps before they start to heat up. So, I'm going to use a 9 amp polyfuse that trips at 16 amps. The CDI only uses 3 amps maximum (4 amps for Kettering) so the 9 amp continuous rating of the polyfuse means that it should never trip unless there is a fault even if the CDI box is at 90 degrees C. The hold current of the polyfuse goes down with increasing temperature, probably more so than a regular fuse. Fred
PS Here is a video of a guy demonstrating a polyfuse. Too bad he didn't have an ammeter in the circuit but it is a good demonstration nevertheless.

I recall a Calvin & Hobbes cartoon wherein they're driving over a bridge and Calvin asks his dad how they know what the max vehicle weight is, and his dad says "They drive heavier and heavier vehicles over it until it breaks." (His wife then screams at him "IF YOU DON'T KNOW JUST SAY SO!"

All that is to say, one method for sizing the fuse might be to just keep replacing fuses until one doesn't blow with all lights turned on...

---

Don Davis
1958 TR3A TS26313L
2018 Tesla M3 Performance
Gobs of previous BMWs, Alfas, Porsches and one NSX.

Live Fast, Play Hard, Lend a Hand