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use a Power Mosfet instead of a 30amp auto relay

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mazur50

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I was wondering if someone can point me in the right direction. First I will give you a brief history of what I need. I race slot cars and on our controls we use 30amp automotive relay as a fully power relay to get the max power out of the cars in the straights. But the biggest problem is it makes the car twitchy because the relay is slow to react. So I would like to replace the relay with a Power MOSFET because they react faster and I won’t have to turn off the relay on the tight twisty courses.

Let me know if you can point me to a circuit that will work to replace a 30amp relay

Thanks
Mike
 
30Amp mosfets have large internal geometries. So they tend to have alot of capacitance as a result. In order to switch them "fast" you will also nee d a suitable gate drive circuit. First things first, you need to pick a power mosfet to use that will be a suitable replacement. Consider also paralleling several smaller ones.

We can get a better idea of how to replace the 30A relay if you can provide a schematic of how & where the relay is currently being used.

In the most basic application, your tie your 30A load to the drain(s) of the Power FETS and drive the gate w.r.t. the source with something like 10-15V but if all you have is 5V in your system, there are ways to drive the fets with that.. Again, we need some more detail & schematic if you want "detailed" help.
 
Take a look at this link it talks about how to use a 30amp auto realy for a full power realy

**broken link removed**

and here is another.

**broken link removed**

the track voltage is from 11.5 to 16 volts
 
how slow is this relay? what is your control voltage (coil voltage) on the relay primary? I take it you are trying to switch 11.5 to 16V range by your description?

this sounds like an easy one fwiw. just need more info.
 
most of the time the track voltage is 14.5 but some tracks run no less then 12.5 and they do Qualifying at 16 volts. the voltage the coil will see will be the same as the track power. the realy is slow enough that it makes the car glitchy when comeing in and out of corners. becasue it is not release the mill sec i let off the triger. that is why i figured a Mosfet might do a better job.

thanks
Mike
 
How much current will your signal to turn on the mosfet be able to provide? It sounds like you could do this with a single inexpensive FET real easy.
 
Yep, that'd do. I might go up to as much as a 50V FET just because I don't know how the AC component of these DC power rails look, just to protect against any spikes, but that 2.5mohm on resistance is awesome on that part. No heatsink necessary with that guy.
 
If you can privide a diagram of how you were intending to connect the relay then we might be able to provide you with a diagram of how you should connect the MOSFET.
 
The relay is of 30 AMP rating, but how much current is actually being used? The MOSFET would have to be selected for that wattage. By firing the gate by a small transistor works well. I have used it to drive 10 AMP electromagnet to vibrate it upto 200 Hz without any problem.
 
the motor draw 4-8 amps at 5 volts i have never put a meter on the track to see the draw when the car is running i am sure that when the car goes in the turns and stops and goes the amps go up over 10amp
 
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