Hey guys, ive been following rESCue lighting control project.
I wanted to install a simple mosfet switch to control some leds.
I’ve followed the schematics posted on the rESCue site, i bench tested it at 36v and it worked great, using 5v rail from the vesc to turn it off/on.
But when you throw 15s at it, the mosfet just turns on with no 5v signal from the vesc.
Wondering if there’s any combo of resistors or diodes that i could use to get this thing working?
After doing more research, it appears i can use (2) 100ohm resistors to make a voltage divider.
That will get me down to 31v which is in working range for the mosfet switch to operate at.
Has anyone used a voltage divider on a build before? What kind of heat should i be expecting
I tried a similar setup with a higher voltage MOSFET module, one that could handle the pack voltage (FR120N). The issue I ended up having (or so I think), is that although the module was rated to handle the current draw of the lights I was looking to power, it couldn’t handle the inrush current to the capacitors on the buck converter. It’d work 5-7 times, then fail on permanently.
Still looking for a solution to this, so let me know if you manage to find a working solution. Someone mentioned a NTC thermistor as a nice simple solution (I don’t need to switch lights on/off quickly), but I sort of moved on to other problems.
@JoeyZ5 supposedly has some revised high side switches that’s he is going to be offering soon that should solve our issues. Fingers crossed, I’m on the home stretch of this build.
Do you have any links or info on the ntc thermistor method?
I have a bunch of them laying around.
Nothing a google search wouldn’t show up, but here’s a good intro.
Seem to remember getting stuck at trying to spec a suitable thermistor. At the voltage (12v) and current (about 0.5a) I was going to use, the thermistors I seemed to find still maintained enough resistance once warmed up to generate a concerning amount of heat. And if you did for whatever reason quickly turn the circuit on and off quickly, then you could still blow the MOSFET as the thermistor would still be heated up (low resistance).
I think a proper switch module with pre-charge is the way to go.
You need to use a buck converter. There are many problems with your plan to use a voltage divider. Also, you need to make sure whatever you do it is cut off from the battery when off.
If you just hook 2x100 ohm resistors up to your battery they will burn. Power is V^2/R. For 15s that means at full battery you’d be putting 20 W into 2 resistors or 10 W a piece. Normal resistors you just have lying around can handle anywhere from 0.25 W up to at most 2 W.
You could raise the resistance of the resistors to reduce the passive draw and not burn out your resistors instantly, but then any current used by the lights will be going through a larger resistor will cause large losses and a lot of heat again. Also if your draw is large compared to the current passively through the resistors your voltage will dip.
In general, voltage dividers are fine if you’re drawing extremely little current. Then you can use large resistances to have little passive draw and you won’t run into big losses and a drop in voltage due to pulling current. They are not what you want for this.
You need to look for a 60 to 30 volt buck (stepdown) converter or something similar.
Hard to help without a properly drawnout schematic,
Is it a NFET? PFET?
Are you trying to switch on low or highside?
Look in the datasheet for the Vgs(th) parameter, thats the potential you need over gate and source in order to toggle the FET. Adding a diode as a transient absorbtion could help the FET deal with some lethal transients.
I agree with @chr1spe if the divider is meant to carry current. Divider works ok to adjust gate voltage tho since its not dependent on current but on voltage. But not a very sexy solution.
I was not wanting to use a voltage divider in the first place.
The mosfet switch was recommended by the guy that makes the rESCue.
I was just following his schematic.
Also i am using a buck converter to power the leds, the mosfet is what was supposed to turn on/off the buck converter.
I believe it’s an (nfet) low side switch, as it uses 5v from vesc to open/close the ground gate.
If i supply less than 40v to the mosfet it will turn the buck on/off normally like it should.
Once supply voltage gets up over 40v, the mosfet constantly sends power to the buck, bypassing the mosfet switch.
Looking for sexy solution!!! @linsus
Can you just link the mosfet you’re using? It sounds like you’ve bought a mosfet that isn’t spec’ed for what you’re doing. The solution is buying a properly spec’ed mosfet. Using a voltage divider will still not work for your situation for multiple reasons. Also, almost anything you add between the mostfet and battery will have the entire problem you’re trying to avoid by adding a mosfet, which is current draw while off.
