TECH: Using a second MOSFET to protect your fire modes

ok…after my last how to on attaching a second MOSFET to handle the motor current and protect your primary MOSFET with all the sexy fire modes there were a bunch of questions about how to do this for other blaster models…

My original working solution was done whilst I was building an active brake which worked a treat but might be a bit confusing for the non tech savvy to get with the program.

The previous how to was done with a store bought active brake which changed a couple of things and…well…that screwed the … on me…muppet moment…

things you NEED to be good with

  1. Blaster MOSFETS are usually negative switching…that is they turn the negative motor wire on and off
  2. To test this, use your multimeter and check that the positive battery terminal and the positive motor terminal have a direct connection - this will indicate you are indeed negatively switched.
  3. Store bought active brake MOSFETS are usually positive triggered

I have opted to use a small reed relay which will make it easy for anyone to do this on any blaster.

Questions regarding steps should probably go with the progression images for the benefit of everyone and organisation of the brain dump.

wanna see it run…

an as you see it wiring diagram (by popular and rather persistent demand)


my bits

  • Kriss vector replacement MOSFET board JF428A
  • T238 active braking MOSFET
  • Bestar BR-1050 reed relay from Jaycar

other things

  • Heatshrink
  • Cable ties
  • Flux
  • Solder
  • Other tooling you’d expect to need…or should expect to need.

NOTE: I am a religious heatshrink user and love cable ties to ensure nothing can just get yanked accidentally. You will do as you will do but if your work isn’t electrically insulated and you get a mid use short your LiPO may swallow your blaster into a small puddle of woe and smoke…food for thought…

The logic to the solution

1: you pull the trigger and your primary MOSFET fires, the motor lines come alive
2: your live motor lines engage the relay
3: the relay triggers the secondary MOSFET by connecting the positive battery pad on the second MOSFET to the trigger input
4: well…thats it really…motor goes whirrrrr……

Your process will vary a little depending on your bits and location on but loosely it goes like this…

    • Attach the motor negative wire to the motor negative output in your secondary relay
    • Wire the relay with plenty of wire on the inputs so you can run them over to the primary MOSFET motor output pads
    • I’ve chosen to attach the reed relay and the second MOSFET together…glued them
    • Wire one terminal of switched relay output to the battery positive pad on the second MOSFET
    • Wire the other switched relay output to the second MOSFET trigger input
    • Solder battery negative and positive pads on second MOSFET giving yourself some extra lead
    • Connect the motor positive terminal with some lead also

Now you will have 2 negative leads from the battery to the two MOSFETs and 3 positive leads to the two MOSFETs and one directly to the motor

At this point TEST…

Group the two battery negative and 2 battery positive leads so you can power your beast - add power
Pull the trigger and you should whirrrr….if you let the smoke out you’ve been a muppet somewhere, order parts and try again in 2 months…

But if you’ve gotten this far without fragging your hardware you’re ready to tidy up and install it into your beast.


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You sir are a legend

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Just an every day muppet m8

Easy as

What are your thoughts on using a solid state relay?

Was my go-to last time around but what I found was the non latching as relay got all latchy on me

The reed relay selection was more about size than anything else.

It’s sooo small!

I’m redoing my harness on the one too…going to stuff bits in other places put of the way and make it all purty and hidden like*

perfection is achievable…it’s a matter of iteration

Round two…electrically field ready but not quiet aesthetically perfect

Thanks for this, starting to get my head around this now lol.

Q: can you skip the relay and go straight to the T238? If you get time perhaps explain the relay part a bit more would be great.

Thanks for spending time on this. I enjoy learning new things, keeps the mind active.

no. you need to relay to convert the switching and to separate the current

the relay is <5$ from jaycar

but you definitely need it to handle the signal switching

Cool thanks, I’ll order a couple.

I do like the DIL package relays.

preferred product?

always open to improving the list at my end

Why can’t you use a solid state relay? That’s what I don’t understand.