# High Amperage Relay

#### ttyler333

##### New Member
Does anyone know of a relay or a mosfet capable of turning over an engine? I am trying to find a product or create it myself, that is capable of switching power for the whole car on and off. This requires the starter motor to turn the engine on, which consumes quite a bit of amperage for a short peak.

I found a relay for $200 that is 500 amp with a peak of 1407A. #### MikeMl ##### Well-Known Member Most Helpful Member Both my aircraft use two different 12Vdc relays that carry the starting motor cranking current (~500A for the six cylinder 470cu. in. Continental, less for the four cylinder Lycoming). The first relay is called the Master Relay. It is mounted near the battery. It carries the cranking current, but is never operated while the starter motor is active, which means it carries the cranking current, but never has to break the circuit while the starter is operating. This means that the coil current is low enough, 1A (12W) so that the relay coil can be energized continuously without overheating. In fact, every electrical load (lighting, avionics, engine instrumentation, flap motor, pitot heat) is supplied through this relay. The second relay is called the Starter Relay. It is rated to break the circuit even while its contacts are carrying the full cranking current. This means that the spring that separates the contacts has to have a lot more force to prevent welded contacts. In turn, this requires a coil with a lot more pull-in force to overcome the stronger spring, which raises the coil's power dissipation to the point that the relay can only be operated 15sec out of a 5min period... The starter relay coil current is about 3A (36W). The Master relay is mounted right on the battery box. The Starter relay is mounted right on the starter motor on the aircraft engine. Those relays are not interchangable. Those relays look like this: #### ttyler333 ##### New Member Both my aircraft use two different 12Vdc relays that carry the starting motor cranking current (~500A for the six cylinder 470cu. in. Continental, less for the four cylinder Lycoming). The first relay is called the Master Relay. It is mounted near the battery. It carries the cranking current, but is never operated while the starter motor is active, which means it carries the cranking current, but never has to break the circuit while the starter is operating. This means that the coil current is low enough, 1A (12W) so that the relay coil can be energized continuously without overheating. In fact, every electrical load (lighting, avionics, engine instrumentation, flap motor, pitot heat) is supplied through this relay. The second relay is called the Starter Relay. It is rated to break the circuit even while its contacts are carrying the full cranking current. This means that the spring that separates the contacts has to have a lot more force to prevent welded contacts. In turn, this requires a coil with a lot more pull-in force to overcome the stronger spring, which raises the coil's power dissipation to the point that the relay can only be operated 15sec out of a 5min period... The starter relay coil current is about 3A (36W). The Master relay is mounted right on the battery box. The Starter relay is mounted right on the starter motor on the aircraft engine. Those relays are not interchangable. Those relays look like this: Woah very informative. I didn't realize breaking the current was an issue. Fortunately it shouldn't be an issue for me as the ideal setup would be a switch to turn the relay on. Then starting the vehicle. The relay would need to remain on to power the electronics of the vehicle but that current draw is minimal compared to startup. The relays i've been finding have mostly been ebay relays and I was just wanting to find alternatives. I also was partially double checking that this is the way to go. The motor it'd be starting is a v6 Vortec for a chevy blazer. Sounds like yourt airplane is a bigger motor! #### MikeMl ##### Well-Known Member Most Helpful Member You will find lots of Starting relays, but you cannot power their coil for more than a few seconds. Master (continuous duty) relays are quite rare. #### MikeMl ##### Well-Known Member Most Helpful Member #### gophert ##### Well-Known Member Most Helpful Member Hey, one airplane was designed in 1946 (Piper PA20, the one in my avatar), and the other in 1958 (Cessna 182). And my 2018 Honda still has relays. I'm just surprised MOSFETs haven't taken over the world of relays faster than they have. #### picbits ##### Well-Known Member Does anyone know of a relay or a mosfet capable of turning over an engine? I am trying to find a product or create it myself, that is capable of switching power for the whole car on and off. This requires the starter motor to turn the engine on, which consumes quite a bit of amperage for a short peak. I found a relay for$200 that is 500 amp with a peak of 1407A.
Why not just keep the starter as it is with the Motor permanently connected to the battery and use the starter solenoid for doing the starting. You can then isolate the rest of the car with a lower current relay.

I worked on a project recently where we activated the starter solenoid with a high current SSR (around 40 amp SSR).

If you are using a solenoid / relay for isolation, try to make sure the starter isn't activated when you make or break the circuit unless you absolutely have to - they won't last long switching that kind of current. Some solenoids / relays are rated to 500A but only for a few cycles.