Featherweight Current limiter

[ewan]

I was wondering if anybody knew of any resonably priced components that would be needed to create a current limiter which ran in line with a relay. I, to be honest do not have a clue about anything on the lines of current limiting... so help would be helpful

I am looking at current limiting at a rating of about 30-40amps on 18v to a motor. It would not be working with a speed controller, It would only be working in line with a relay.

Cheers, Ewan

[mattan]

why dont just ordet a speed controller whit built in current limit, lock at some 4QD,s

Does anyone know if the 4QD NCC 24V 35A from technobots current limited, and if it will run happily at 18v? I know the responce, even in the robot version 4QD controller is not excellent, but all I need is an on/off function.

Does the 4QD NCC 24V 35A run off a pot (servo) or will it connect directly to most standard RC (futaba being standard) sets?

Cheers, Ewan

[ewan]

ok, thats probably my best bet...

Ill earn the money somehow...

[mark_m]

In responce to Ewans original post, it seams that Noel Poncelet of Hassocks Hog has found a component that does everything that Ewan was asking for:http://www.hassockshog.co.uk/build_diary_2.htmhttp://www.hassockshog.co.uk/build_diary_2.htm
go to the entry on may 28th.
It talks about a fully auto-protected power MOSFET (VNP49N04).
This sounds really interesting to me if it works. Ive been thinking for a while now of designing a bolt on current limiter that modifies the signal going to an ESC or somehow switches the output off when the current reaches a preset level, but it seems a much simpler solution to just stick this mosfet in series with the ESC outputs.
would this work though?
Problems I can see:
1) dissipating the heat it produces
2) it would waste some power
3) its another component that could fail
assuming these problems could be overcome would this work to protect motors from over heating or help batteries last a bit longer?

Mark

[ewan]

Wow, thanks Mark, Ill do some testing, and get back with the results.

Cheers, Ewan

[talulah]

I dont think you could use the VNP49N04. The problem in particular is the section that states When the current limiter is active, the device operates in the linear region, so power dissipation may exceed the capability of the heatsink.

This means that rather than turning off altogether, the gate-source voltage (the signal that turns the MOSFET on and off) is reduced so the MOSFET is operating in the linear region. The linear region means that the MOSFET is neither fully on nor fully off. Only when it is fully on or off is it dissipating reasonable amounts of power. When it is half on and half off, the power dissipation can be hundreds of times larger. Then the temperature will rise rapidly, and the MOSFET temperature protection will shut it down. Im not sure this is a nice way to operate the device, and will certainbly reduce its reliability.

I have designed a current limiting circuit, but its built into a speed controller design. You can find it in section 10 here: http://homepages.which.net/~paul.hills/SpeedControl/SpeedControllers.htmlhttp://homepages.which.net/~paul.hil...ntrollers.html
This could be redesigned to reduce the PWM ratio based on the current. I havent time to do this at the moment Im afraid, but Ill bear it in mind for the future.

Have a go at using the 40N04 by all means, and report your results. Good luck!

[big_nipper]

Ive used these in Little Spinner. I started with the 20A versions and worked fine till I connected the disc to the motor. The Motor drew 23A starting current (connected directly to the battery) and with this MOSFET it overheated and shut down in less than half a second. It then pulsed as it cooled down and heated up so qucikly it was audiable. I replaced it with the 50A version and it worked fine from then on.

If you are wondering the heat sink was a solid ally block approx 10cm x 2cm x 2cm and it still overheated the MOSFET. The motor stall is not over 50A so in effect it should never hit this. I would not recommend using it in a PWM application unless its very low freq as its got a very long turn on/off time. I only use it in case of a short circuit in a on/off application.