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Minky

Boring Circuit Diagram Post

I know this isn't a service offered, but if anyone is particularly bored please could you check my circuit?



.. And if you're really, really bored please critique my LED circuits..



EDIT: I'm going to calculate the resistors later..  Then have a horrible session of trying to read resistor codes and sort the pile I've bought
OldNoob

For 2-3s and LEDs with a 3v forward voltage a 460 ohm resistor works fine. Save you calculating. I just buy 50 of those a year. You can run 2 LEDs per resistor if you are being lazy.
Minky

I like lazy. Thanks ON
Boff

Are you deliberately using a P Channel FET in the first diagram? If so, it's going to need to move. P Channels go on the high side of the load rather than the low side as N Channel MOSFETs do. My brain is struggling to tell you where it should move but move it must. Smile
SSGT

If you want to maintain the same voltage drop, and therefore brightness, when using a single resistor to power multiple LEDs in parallel you need to use a lower resistance than you would powering a single LED. Two LEDs in parallel means double the current draw so, if you needed a 460Ω resistor for one LED, you'd need a 230Ω resistor for a pair of LEDs (Voltage drop across a resistor = Current through resistor * Resistance of resistor - i.e. V=IR).

Boff, that is an N-FET (arrow pointing towards gate pin) it's just upside down (probably because of where the battery is located - I thought it was P-FET when I first looked at it). It probably would work although it's not ideal using an N-FET as a kill switch if you're also using an N-FET as the rev switch as the rev FET's source pin will effectively be floating when the kill switch FET isn't "on" (or, for some reason isn't fully "on"). It's not so much of an issue in this sort of setup but, as a general rule, you should keep the source pin permanently connected to source (be that negative or positive depending on whether you use an N or P-FET) since the source is effectively the reference pin that the FET compares the gate voltage to (as is, if you switch any of the ancillary lights on whilst the kill switch FET is "off" the gate and source of the rev FET will be pulled up to supply voltage minus the forward voltage drop of one of the LEDs). One alternative is using a P-FET moved to the +ve side of the battery with the source pin connected to, and gate pulled up via a resistor to, V+ (the opposite of an N-FET) but, to be honest, unless you absolutely need to due to space (or you are running something like Wolverines), I'd personally just use a full-size 21A microswitch for the rev switch and, if you can find one, preferably a decently rated switch for the kill switch aswell (a FET that has been switched "off" will still conduct some current albeit a very small amount - that said, you shouldn't be leaving a pack in there for any significant period of time anyway). If you must use two FETs, maybe put the kill switch in series before the rev switch and use it to keep the rev FET off and use the other N-FET to control just the ancillaries as per below. Basically let the FET you're already using to switch the motors also act as the kill switch for the motors, that way you can get away with using a cheaper/smaller FET as a kill switch for just the ancillary lighting - if nothing else it's one less FET for the current in the motor circuit to flow through (depending on the FET you use that could be the equivalent of shortening a 16AWG wire run by a few metres).


(Ancillary lighting removed for convenience)

You will also need resistors (or a resistor) in series with the rev LEDs. EDIT: Nevermind, if they are "12V LEDs" they'll already have resistors built-in.
Minky

Thanks all!  I'm fairly sure people pay for this kind of advice in the real world..

Well I've just found out my free circuit app has corrupted the file I'd been working on, or maybe it couldn't handle the amount of upside down Mosfets.

Does anyone know whether 12V LEDs will run on 3s okay?  I hadn't realised Lipos drop down to 11.1V as they do.

I'm going to mull over the kill switch thing.  I like SSGTs solution but maybe (as Franksie had previously told me) I'm over engineering it but I've bought a spare Mosfet and a rubbish switch that I need to compensate for or throw away I'll play with some physical circuits and see what makes sense.
SSGT

A "12V LED" is usually just a normal LED with an appropriately specced resistor built into it. As long as that's the case then, if you run it at 11.1V (or 9.9V which is as low as you should ever let a 3S LiPo go), it'll just be less bright than if you ran it with a 12V supply. If it's meant for automotive applications then it'll be a little on the conservative side since the electrical system can run at anything up to 14.8V.
Minky

Damn,  I thought it would be 4 times as bright.  Oh well,  thanks anyway

Edit:
You mention that Mosfets carry a lot of resistance, if I were to include one on the LED circuit should I account for this when choosing resistors for the LEDs?
Also, I'm unused to reading the data sheets for these, what value indicates resistance? (If it's 'Resistance' sorry for wasting your time.)
SSGT

The "on" resistance should be listed at RDS(on) (resistance drain-source "on"). It's not a huge amount of resistance but it can be significant at higher current draws especially relative to a simple piece of wire.

It'll depend on the resistors you use but if it's under 20mΩ (certainly if it's under 10mΩ) then it probably isn't worth including in the resistance calculation since it'll likely fall within the tolerance band of most resistors you'd be using anyway (a 230Ω resistor with a gold tolerance band, which is considered pretty good, will have a 5% tolerance i.e. it may actually have a resistance anywhere between 218.5Ω and 241.5Ω - a 10% tolerance "silver band" 460Ω resistor can be anywhere between 414Ω and 506Ω).

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