Bosch: powering light from 36 V?

[Alavin]

Hi all,

Would it be possible to use Bosch main 36 V that goes to motor to power front light?

I have a Performance Line motor from 2016 that provides only 6 V and 1.5 A (if I remember correctly) for lights.

I was thinking about connecting main power to light via a MOSFET so that original light switch would work.

Regards.

 

[amberwolf]

YOu can't use the 36v to/from the motor to do it, because it will brighten and dim as applied motor power changes.

But you can use the 36v from the battery directly to do it, as long as there is a place you can tap into that preferably is turned on and off with the bike (so you don't have to remember to turn the light on/off separately).

You can either use one of the many headlights that works on voltage within the 36v range (around 29v to around 42v) or you can use a DC-DC that converts from that voltage range down or up to whatever voltage the light you want to use requires.


If you prefer to have the original light switch connect power, you can use a light or DC-DC that has an enable input that when held "low" or grounded turns it on, and use a simple transistor (2n2222 or similar) driven by the original light output to ground the enable line when the light is activated. No need to use a MOSFET to actually handle the power itself.

 

[Alavin]

Thank you for your comments!

I was being unclear: by "motor" I meant the drive unit and by the 36 V cable the one that enters drive unit.

I wonder if in Bosch system that cable is powered all the time when there is a battery attached or only when system is powered on. In case the latter (and if I didn't care about light on/off switch) I could just splice the cable and route cable to light via a fuse. I could use Busch & Müller light that can take 42 V.

Do you happen to have any links to suitable DC/DC modules? I'll have to familiarize myself with basic transistor circuits to be able to implement that DC-DC converter plus 2n2222 circuit.

 

[Alavin]

Just realized that LM2596 step-down regulator has enable pin. Modules with that chip are commonly available. It seems enable pin is grounded in those modules but it should not be difficult to cut it and solder input from 2N2222 circuit.

 

[amberwolf]

Yes, that should work.

A transistor circuit using 2n2222, 2n3904, BC547, etc (any generic NPN transistor) would just use something like a 1kohm to 10kohm 1/4watt resistor on the base pin to the control input (lightswitch output from bosch), and the emitter goes to the battery ground (negative), and the collector goes to the enable line. A small voltage on the resistor input makes a current thru the base and and emitter whch turns on the collecotr to ground the enable line. Not much heat is generated so it can be stuffed in wherever is convenient, with just a bit of heatshrink over it all to keep from shorting to anything. (heatshrink the individual leads/connections, too)

If the voltage on the input is going to be higher, and the signal it is switching doesn't have to go completely to ground to turn it on, you can add a 1kohm resistor on the base to ground, too, but for your usage this shouldn't be needed (and might cause problems).

I have a bag of a few hundred such transistors I've had around for a few decades that I have about half-emptied at this point, using them for little things like this. (they're so old they all come in little metal cans instead of plastic!) I keep a few higher-power similar transistors for switching actual loads on with basically the same circuit. When i am going to toss out some old dead electronics, I first open them up to mass-desolder bigger useful parts like these, and resistors; I don't typically sort them out but just dump them in a bag or box for later "hmmm....do I have one of these?" before I order stuff (cuz shipping is ridiculous on small parts like this). Lots of stuff uses these little transistors, or ones that are close enough to work, and often has the right resistors in the same part of the circuit, so if you have old unused stuff around you're likely to toss out....

You can use FETs for the purpose too, but depending on the FET they can be harder to correctly drive the gate, and static electricity tends to damage them easier than it does plain old small transistors.