Archive for BRIT NERF Home of the British NERF Community
 

The free forums are now under new ownership, a full announcement will be made shortly

       BRIT NERF Forum Index -> Q&A and New members
Broz

Long question about wiring and mosfets for higher currents

Ok so following advice from blastersmiths I'm looking to rewire my hellcat Stryfe to take into account the increased current (I also mean to do this with my Peasant 180 RS, but maybe that should warrant a separate topic.)  Therefore I have a couple of general areas of inquiry...

My understanding is that Hellcat's can draw up to 22a stall, thus outstripping 18awg wire and 10a rated switches. So my first question is can I address this simply by going up to 16awg and a 22a or 25a rated switch, given that I can fit the larger switches in all my semis and don't have space as an
issue?  This would have the benefit of being simpler and likely cheaper than introducing mosfets etc,
assuming it is a viable option, is it? or am I missing something through inexperience?

If the above is not viable or advisable, then I would like to run past my understanding of how one wires in a Mosfet and the attendant gubbins. Bearing in mind that I am working on an already rewired blaster done along the MTB video guide lines where the entire original wiring and switches have been removed. I've no idea how the community feels about MTB in general, but I personally find their videos and wiring guides very easy to follow.

After watching ON's Demolisher video and looking at the eldritch mandala that is the circuit diagram, would this be correct in terms of what needs sticking to what?


Switch Common to Mosfet Gate Pin No1 (would this be negative or a feed?)

Mosfet Source Pin No3 to battery negative.

Bridge the two with a 10K resistor.


Switch Normally Open to motor negative.

Mosfet Drain Pin No 2 to the motor negative  (negative for wire colour I take it, or is that a feed?)

Motor positive to battery positive

Bridge the two with a diode, silver end on the positive side.


Would this be correct, or am I going to start a fire?
Boff

If you're going for a Stryfe with Hellcat 180s then, yes a switch rated for 44A (you've got 2 motors, remember that 22A is for ONE motor) and 16AWG would be fine. Obviously, getting one to fit and spending the time grinding out the trigger cluster is the challenge.  We recommend MOSFETs these days since once you get round to it, they're not that difficult to manage.

If you're trying to retrofit from an already rewired mechanical system then I'd suggest you stick with that and find a decent rated switch and do a straight swap. If that's not an option (and it wasn't when I did the research). retrofitting will let you keep your current 10A micro switch and you can then use either recovered wire from elsewhere to do the gate pin wiring or buy some 22AWG or 24AWG from eBay to cover that low current side of things. Of course, you could wire the whole thing in 16AWG but that's overkill!

Your wiring list is indeed correct. Remember to test with alkaline batteries (you want a minimum of 6V) before you try with anything high powered. If the FET or the cells get hot then you've wired something wrong.
SSGT

As Boff mentioned, two Hellcats is 44A stall total. The problem with switch ratings is that they're usually AC resistive ratings - a 21A Omron switch is actually only rated to 21A continuous for a resistive load at 250VAC. If it can handle 21A at 250V you'd expect it can handle much more at only 8-12V but because DC is more damaging to switch contacts that isn't the case. That said, at low voltages switches aren't normally de-rated from the AC spec meaning that at 8VDC that same switch should still be capable of 21A continuous for a resistive load. The next problem is that motors aren't resistive loads - most decent switches rated to 21A @ 250VAC are only rated to 7A continuous for an 8VDC motor load (as per spec sheet linked above).

The reason why we can still use these switches is because that 22A current per motor is the stall current so the switch only has to handle it for a fraction of a second. The switches themselves are specced accordingly - the switch above, for instance, should still be capable of handling a pair of Hellcats since it's max. inrush current for a motor load (Note 4 from beneath the ratings table) is specced as 7x the continuous current i.e. 49A burst @ 8VDC (at 3S you might want to de-rate it a little bit but it should still be capable of at least 46A burst @ 12VDC - it's capable of 35A burst @ 30VDC). The only potential issue is in the event of a stoppage although, in that case, if you don't release the trigger and let the motors sit at stall you'll likely damage the motors anyway.

Your connections look right other than the switch normally open (NO) which should be connected to the battery +ve or motor +ve (effectively the same thing in this case) rather than motor -ve - it won't cause a fire it just wouldn't switch on (well, it might but if it does the motors won't be running at full speed).

Switch Common -> FET Gate (Pin 1) is technically a "feed" from a switch rather than a pure negative so I'd go blue.

Motor -ve -> FET Drain (Pin 2) is debatable. Ideally you'd use a separate colour to avoid confusion but, if we're going by "blue = feed from switch", I'd probably stick to black.

Ideally you'd have a second, relatively low resistance (100Ω or so), resistor at the entrance to the gate to prevent damage to the FET from a sudden inrush of current (and to prevent "ringing" if you decided to apply a PWM signal to the gate in the future) but it isn't essential. You could actually remove the need for the 10kΩ resistor entirely by connecting the switch normally closed (NC) to battery -ve but then you have battery +ve and battery -ve connected to the same switch - probably not a good idea unless you know exactly what you're doing.
Franksie

There's a diagram in my Rapid pistol blog if it helps to see it in a picture.  Once I can get to a desktop I can post the pic I have of the stryfe circuit too
Broz

Thank you very much gents, all very awesome.

Failing to consider the doubling of the current for two motors is exactly the sort of thing I was expecting to have missed, having a lack of basic mathematical sense...

I don't mind ripping out the trigger cluster, I like a very short, clicky pull on the rev trigger anyway and years of modeling and converting has left me quite deft with a pair of cutters and bits of plasticard.

While there are plenty of 22/25a switches thanks to SSGT's very illuminating advice, as you say  Boff, if I can get round to it I might as well go the MOSFET route and I did ask the questions because I was looking to step up my electronics skill a little anyway.

SSGT, Thank you for explaining the differences in resistance, makes sense to me now when looking at switches on Farnell and RS.

So just to clarify for my understanding on the wiring, I'd run  Switch NO to motor + then on to battery + ?  I ask because this was the specific bit that confused me initially when trying to work things out.  

That would be great Franksie, RS was my next step and the more pictures the better.
SSGT

Broz wrote:
So just to clarify for my understanding on the wiring, I'd run Switch NO to motor + then on to battery + ?  I ask because this was the specific bit that confused me initially when trying to work things out.


Yes, you want the switch NO to connect to the positive supply either at the +ve connection to the motors or the +ve connection from the battery. I know you don't like diagrams but it might help explain why it can be either.



As far as we're concerned that whole top rail (coloured in red) is +ve. You can connect the NO of the switch to any point along that rail, be that the +ve terminal of the battery or the +ve side of the motors, it really doesn't matter - connect to whichever is closest/most convenient (pack connector is probably closest but you might not want an additional wire coming off it). A FET is a voltage controlled device, pull it's gate up to a "high" voltage and you switch the FET "on", pull it down to a "low" voltage (i.e. 0V/GND) and you switch the FET "off" (at least for N-channel FETs but that'll be what you'd use anyway). All that switch does is pull the voltage of the gate up to supply voltage when you switch it over (the 10kΩ resistor pulls the voltage of the gate down to ground/battery -ve when the switch isn't switched over - ideally you wouldn't need a resistor there but if you just used a plain wire you would cause a short circuit as soon as you flicked the switch). Likewise one side of the 10kΩ resistor could be connected either to the FET gate pin or the switch common, and the other side could be connected either to the FET source pin or the battery -ve (in this case it makes more sense to keep the resistor as close to the FET as possible to reduce the number/length of wires i.e. bridge the gate and source pins with it).

This is what you're original connection list would have made (i.e. switch NO to motor -ve):



In this case the switch will only pull the FET gate up to the voltage at the -ve side of the motors. Ideally we want this to be 0V otherwise the motors won't be experiencing the full voltage of the pack. If the FET does switch on this means that the gate has been pulled up above it's threshold voltage and everything between the gate and the -ve side of the motors will be at around 3-5V. This means the motors effectively have only 6-8V across them - the additional 3-5V would be wasted as heat through the FET.
Broz

Wow, thanks for going to this degree of trouble, massively appreciated.

I think it's more the Blastersmiths diagram, I don't understand some of the notation and I suspect the  colours and their background contrast throw off my eye because yours is completely clear to me.

My mistake was not having an understanding and trying to just complete a circuit without an appreciation of how the FET interacted with it. If I correctly grasp your explanation I was essentially just isolating it and half the circuit.

Just one more point, for the sake of wire gauge, which are the low voltage parts? Is it just the Com to Gate?
SSGT

Broz wrote:
Just one more point, for the sake of wire gauge, which are the low voltage parts? Is it just the Com to Gate?

There is no "low voltage" section per-se, everything will either be at 0V or 11.1V. That said, the connection from from the battery/motor +ve to the switch NO and the switch common to the gate (technically the connections to/from the resistor aswell but if you're wiring it directly to the FET pins then there aren't any additional wires there anyway) can be thinner gauge than the rest since they won't be exposed to very much current. As long as there is a nice thick "pipe" for the current to flow to the motors, through the drain/source of the FET and back to the battery that's all that matters.
Broz

And that would me posting late, forgetting what little I had learned about electronics when I was twelve and using "voltage" and "current" interchangeably like a noob...

       BRIT NERF Forum Index -> Q&A and New members
Page 1 of 1
Create your own free forum | Buy a domain to use with your forum