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Dan_jackson1985

Mosfet for MTB Rhinos in Stryfe

Hi all,

I need some advice on what mosfet to use with mtb rhinos. I have be using an IRFZ44N with intermittent success(mainly failure!).

before building any circuit i prototype it on a board first

I tried following the BSUK circuit diagram, and could not get it to work. (think i fried the Fet)

I then used the FDS video, after watching it a few times i worked out what was going on and managed to get my circuit working.

i knocked up my own circuit diagram and soldered up the circuit for real. Success!

i then installed it in the blaster, closed it up connected my battery....... reved it up and smelt the smell of the magic smoke escaping!! Crying or Very sad

Are Mosfets sensitive to too much heat during soldering?

Do i need a different Mosfets for my motors?

thanks in advance!
FDT OldNoob

If it worked on the bench you shorted something when you installed. I have only smoked the FET when I wired it wrong.
This diagram by Franksie is correct.



Did you insulate all your connections PROPERLY WITH HEAT SHRINK? Because if you didn't that would be my first suspect.
Boff

An IRFZ44N is rated to 3 times the combined current draw of a pair of Rhinos so if you're frying it, you're doing something wrong. A IRFZ44N is rated to 49A, a pair of Rhinos only pulls 16A at stall.

Common candidates are not having the diode installed correctly, having wire short through the motor casing from poor soldering to your terminals and shorting the source/drain again by poor soldering. I've done the first two at least once in the last week so don't worry, they're common enough errors. Smile
Dan_jackson1985

No shorts, tried it with 6v test pack. When i pull the rev trigger my battery voltage drops but no spinney spinney.

iv also checked all wires for breaks etc.

how forgiving is the fet? Is one mistake then dead?

I might go back to a good ole' micro switch for this project!
FDT OldNoob

That FET is dead, you will need another one. That's now why it's not working.
Dan_jackson1985

My desk is turning into a Fet grave yard!! I'm gonna scrap this loom and start from scratch.

Is there a way of testing Fets prior to install?
FDT OldNoob

Yes, you wire them right then they work. You won't get a "bad" one, it's highly unlikely die to their extreme simplicity. Before you abandon it I suggest taking some decent pictures and posting them so we can see what you are doing wrong.
I still blow the odd FET. Yours should be on the +ve battery feed, just to check.
Boff

Agreed. In depth photos are a good idea. If FETs go pop, they don't always explosively disassemble themselves, sadly. I had one go in a Desolator the other day that just scorched the inside of the case rather than fragmenting everywhere. Smile
Dan_jackson1985

OldNoob wrote:
Yours should be on the +ve battery feed, just to check.


This is interesting, as my instinct is to agree but all the wiring diagrams I'v looked at it appears to be on the negative side unless I am miss interpreting them.

I would post pictures but i have disassembled my circuit (and am still to learn how to post pic on a forum Rolling Eyes )
FDT OldNoob

There is a sticky on picture posting. I use Imgur as the host but it requires you to view in desktop to set the picture size to the forum correct 400x600.
I could be wrong about that FET, if not the wiring diagram I posted is wrong. I just follow my own video. That's why I made it. For rhinos a FET is overkill anyway.
SSGT

That reminds me, that post probably needs updating (for example, you don't need an imgur account anymore and you can paste straight from the clipboard).

The image is definitely correct though. The FET gate is pulled up to V+ by the rev switch (and held down otherwise by the resistor), the drain is connected to the motors and the source is connected to GND/V-.

A FET is definitely overkill for Rhinos. A decent 21A switch can easily handle 40+A motor inrush currents at 11.1V.

If the voltage is sagging but the motors aren't spinning that suggests a short - either bypassing the motor (which will mean a lot of current through the FET) or passing through the gate of the FET to source. Either way the FET will likely be dead or damaged but you'd need to check for other issues elsewhere.
Franksie

When I sent the diagram to Mike at BSUK the reply I got was:

Your MOSFET is in the correct place with it being on the negative side of the inductive load. Smile
Dan_jackson1985

I will check out the sticky thanks.

I will probably stick to switches for my rhino builds then as I'v already got 3 successes under my belt.

Saying that I will not be giving up on the mosfet method for other projects as it does seem like a really nice way of doing things. I would like to mount all the components on a circuit board.
FDT OldNoob

The problem with that us the 40-60A stall currents in the flywheel motor circuit where the MOSFET is required. Also a circuit board limits where you can put the components. In a fully battery tray expanded Stryfe for example there is very limited dead space.

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