Using Bafang 6V Wire on a Mosfet

[furcifer]

I have a 60V LED headlight I'd like to switch using the display. I think I should be able to use the 6V wire connected to the gate (positive) and drain (negative) legs on an n mosfet to switch the light.

So if I'm not mistaken 52V positive to the light, 52V negative to source leg, 6V positive to the gate leg, 6V negative to the drain leg and then run the negative to the light from the drain (the hole in the body of the mosfet)

Kinda like this?

 

[amberwolf]

It's almost right.

First, you'll need to make sure you use a MOSFET with a 6v gate voltage, and use a resistor between the gate and the 6v light output from the Bafang with a value that allows enough current to charge the gate quickly but not damage the light output. If the 6v is not enough to turn on the gate fully, then the FET will dissipate more power as heat than if it were just a switch, and you'll need to be sure it's not so much you need a heatsink (or add one if you do).

Next, connect the FET's source to system ground. The 6v's ground has to be the same ground point as well, or else you will need to use a driver chip that isolates the 6v system from the battery voltage system.

Next, connect the headlight's positive to battery positive.

Next, connect the headlight's negative to FET drain.

There's a number of webpages out there with more info if you need it, like here:
https://electronic-products-design.com/geek-area/electronics/mosfets/using-mosfets-as-general-switches

 

[furcifer]

Thanks for that link. I've read a few similar pages but found they jump from p to n mosfets without being explicit and it gets confusing. I don't know much about mosfets to begin with.

If I've read you right, I need to bump up the 6V (+) signal voltage to the the mosfet so it switches on faster and doesn't dissipate too much current as heat in the process?

I'm still not sure I understand mosfets. I'll have to do some more reading,

 

[docw009]

Choose an N-channel MOSFET with a gate voltage around 3 volts and you should be fine. Usually, the display uses the same same ground as the battery and the one wire should suffice. If it doesn't work, it means the display is on a separate ground from the battery and then you do not want to use the MOSFET method. How do you know? Measure your "6V" signal using the battery negative as the reference.

The resistor is mainly there to protect the output on your display so it won't see a short circuit if your wiring has a fault. There's little or no current flow, so size of the resistor is not important.

Probably easier to solder the wire to the drain terminal of the MOSFET rather than the tab. They are the same.



Edit: And it would be prudent to put a fuse between the lamp's positive terminal and the battery positive.

 

[fechter]

Most MOSFETs will turn on fine with 6v. The headlight doesn't draw much current. If you look for a part with "logic level gate", it's made for 5v gate drive so should be perfect. docw009's drawing looks correct.

 

[furcifer]

Most MOSFETs will turn on fine with 6v. The headlight doesn't draw much current. If you look for a part with "logic level gate", it's made for 5v gate drive so should be perfect. docw009's drawing looks correct.

With the daughterboard being part of the internal controller I can't see it not having a common ground to the display. So that's good.

It's a 16 T6 LED, I don't have an amp meter but I'd be surprised if it pulled 8A.

The only other problem I can think of is with the driver. They used the stupid 5 mode driver (who needs SOS and tactical strobe in their headlights???)

I don't use them, but if the light were to go into SOS mode, I think there needs to be some capacitance in the circuit so the source voltage doesn't fluctuate and turn off the fet? I just thinking between the dots, if the voltage in the circuit drops below 3 or 6V it's going to switch the gate.

I think you can turn the modes off by removing a resistor on the driver board but I can't find a schematic. I would prefer not to have to deal with the SOS and strobe modes but until then I may have to.

 

[fechter]

The schematic above assumes the light runs off pack voltage. They make drivers that can do this. If you're using a 12v or 6v light, you will need a separate dc-dc converter to go from pack voltage to light voltage. The 6v from the bike controller can only support a very low current.

You might have to play with the light. Most of the multi-mode ones I've messed with will default to a certain setting when you first apply power. If it stays off long enough, it will go back to the default. If the default mode is not what you want, then it sort of sucks since you have to toggle the power to switch modes.

I had one that defaulted to "off". One press on the button switched to the desired on mode, so I added a small capacitor from the switch contact to the power line which fooled it into just turning on when power was applied.

 

[furcifer]

The schematic above assumes the light runs off pack voltage. They make drivers that can do this. If you're using a 12v or 6v light, you will need a separate dc-dc converter to go from pack voltage to light voltage. The 6v from the bike controller can only support a very low current.

You might have to play with the light. Most of the multi-mode ones I've messed with will default to a certain setting when you first apply power. If it stays off long enough, it will go back to the default. If the default mode is not what you want, then it sort of sucks since you have to toggle the power to switch modes.

I had one that defaulted to "off". One press on the button switched to the desired on mode, so I added a small capacitor from the switch contact to the power line which fooled it into just turning on when power was applied.

It's running off the charge line on the battery since it's live and just sitting there. I was worried about the amps but it seems fine and I can't get a definitive answer on how many amps it can deliver (the charger is 5A so I know that's safe, coming or going shouldn't matter). Converting it to the pack wouldn't be hard to do if necessary.

It does default to HI, but there must be some capacitance or it wouldn't recognize the plug-unplug-plug method of changing the modes. How long in between plugs and unplugs before it resets is a bit of a mystery. It seems like it holds memory for 5-10 seconds.

eta: it's a "12-80V" light. So there is a DC-DC converter on the driver. But the board probably converts to 4V, I believe that's the nominal voltage for LED's. It could run the LED's in series but looking at the product line they use from 4 LED's to 16 LED's in the same housing. I wanted to open it up to take a look but don't have the tools. The unit is sealed from the front with notched ring holding in the lens.