Simplest way to reduce voltage from ~48v to ~36v

Thread Starter

tpw

Joined Oct 7, 2019
2
(I'm very much a beginner with electronics, so I apologise in advance)


I’m trying to use a 48v battery on my 36v ebike controller. The controller itself can deal with the higher voltage, but it has a hardcoded limit of 44v, which it reads from a wire coming from the display.

So my aim is therefore to lower this voltage somehow. The parameters are:
  • Input is 39-55v, output needs to be between 29-44v
  • Current on the wire is 80-120ma
  • It would be great it if this could be done proportionally, or with a constant voltage drop, instead of just regulating to a set output voltage
  • Single component solution would be best, since it would be easier to wateproof for riding in rain
Solutions I have researched so far:

Tvs diode:
  • Guy here used one for this purpose (52v battery but otherwise same situation) and he said it worked
  • But I’ve read that they’re not supposed to be used continuously like this, only for spikes in voltage - does that mean it could fail if used like this?
  • Also have no idea how to choose one for specific voltage drop - have read many explanations and data sheets but don’t understand difference between clamping, breakdown and working voltage. The guy in the linked post seemed to choose based on breakdown voltage = desired voltage drop, but this doesn't seem to be what breakdown voltage should mean?
Zener diode:
  • To calculate the series resistor, I would need to know the resistance of the load, correct? I’m not sure how to find this - do I measure resistance across the voltage sense wire and ground while the display
Step down converter:
  • I have one of these (appears to use a LT3800) This has a constant output and is large, so not ideal. But I would also worry that it would draw too much current? I've tried to measure the current it draws with no output, but I doubt that is particularly useful information. I tried to figure it out from the datasheet, but I don't really know what half the symbols mean
Resistor:
  • Every mention of using a resistor for this purpose (that I've seen) has said not to do it, but not said why (although i assume this is because fluctuating current would change the resistance too much?)
  • Also wasn't sure how to select the resistor value for a desired voltage drop. Is the V in V=IR meant to be the drop, or the voltage of the circuit?)
Voltage divider
  • From what I’ve read, this doesn't work once you apply a load?
Other ideas:

My main reasons for asking instead of just trying these things are:
  • Not wanting to exceed the mA level that I have witnessed on the line, because I don’t want to break anything
  • Not knowing whether my lack of understanding might lead to something breaking
  • Need something reliable, i.e. that won't fail halfway up a hill
Sorry if I've missed anything obvious - I feel like I could have researched some of these things more, but I did try. And in fairness they are bloody confusing if you have no real prior knowledge!
 

KeithWalker

Joined Jul 10, 2017
797
(I'm very much a beginner with electronics, so I apologise in advance)


I’m trying to use a 48v battery on my 36v ebike controller. The controller itself can deal with the higher voltage, but it has a hardcoded limit of 44v, which it reads from a wire coming from the display.

So my aim is therefore to lower this voltage somehow. The parameters are:
  • Input is 39-55v, output needs to be between 29-44v
  • Current on the wire is 80-120ma
  • It would be great it if this could be done proportionally, or with a constant voltage drop, instead of just regulating to a set output voltage
  • Single component solution would be best, since it would be easier to wateproof for riding in rain
Solutions I have researched so far:

Tvs diode:
  • Guy here used one for this purpose (52v battery but otherwise same situation) and he said it worked
  • But I’ve read that they’re not supposed to be used continuously like this, only for spikes in voltage - does that mean it could fail if used like this?
  • Also have no idea how to choose one for specific voltage drop - have read many explanations and data sheets but don’t understand difference between clamping, breakdown and working voltage. The guy in the linked post seemed to choose based on breakdown voltage = desired voltage drop, but this doesn't seem to be what breakdown voltage should mean?
Zener diode:
  • To calculate the series resistor, I would need to know the resistance of the load, correct? I’m not sure how to find this - do I measure resistance across the voltage sense wire and ground while the display
Step down converter:
  • I have one of these (appears to use a LT3800) This has a constant output and is large, so not ideal. But I would also worry that it would draw too much current? I've tried to measure the current it draws with no output, but I doubt that is particularly useful information. I tried to figure it out from the datasheet, but I don't really know what half the symbols mean
Resistor:
  • Every mention of using a resistor for this purpose (that I've seen) has said not to do it, but not said why (although i assume this is because fluctuating current would change the resistance too much?)
  • Also wasn't sure how to select the resistor value for a desired voltage drop. Is the V in V=IR meant to be the drop, or the voltage of the circuit?)
Voltage divider
  • From what I’ve read, this doesn't work once you apply a load?
Other ideas:

My main reasons for asking instead of just trying these things are:
  • Not wanting to exceed the mA level that I have witnessed on the line, because I don’t want to break anything
  • Not knowing whether my lack of understanding might lead to something breaking
  • Need something reliable, i.e. that won't fail halfway up a hill
Sorry if I've missed anything obvious - I feel like I could have researched some of these things more, but I did try. And in fairness they are bloody confusing if you have no real prior knowledge!
An 11 volt, 2 watt zener diode would do the job, ZY11 or equivalent.
 

Thread Starter

tpw

Joined Oct 7, 2019
2
Thanks for your answer.

Will this need a heatsink or anything? And is there a reason to use a zener over a TVS diode?
 

AnalogKid

Joined Aug 1, 2013
8,377
An 11 V zener diode in series (rather than a traditional shunt) circuit, with 0.12 A through it, will dissipate 1.32 W. A 5 W axial lead diode will work for this. It will get warm, or even hot to the touch, but not destructively so. Most of the heat is radiated from the surface of the body, but leave the leads as long as possible; they act as a heatsink. It is tempting to cover the diode and its connections with one long piece of heat-shrink tubing, but it is much better thermally to cover only the connections and leave the diode body free to radiate.

A TVS diode like a Tranzorb is a special purpose zener. It has non-standard internal construction to both lower the junction capacitance and increase the peak current capability, but it should work just fine in this application. The diode body size determines the continuous power rating.

ak
 
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