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

dcstep-downvoltagevoltage-drop

(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!

Best Answer

You are able to use a fixed voltage drop of about 11 Vdc at about 120 mA. This is fairly easy.

schematic

simulate this circuit – Schematic created using CircuitLab

The transistor is a Darlington device in a TO-220 package and has a reasonable gain of greater than 1000. The Vbe drop is about 1.2V. Choose the appropriate Zener diode for the desired voltage drop.

The total power dissipation is about (11V) * (0.12A) or about 1.3 Watts. You will need s small heatsink to keep the temperature rise reasonable. A small heatsink from an old computer motherboard or CPU is a good choice. Note that the transistor tab is connected directly to the incoming supply voltage. Don't let it touch Ground.

Note that there is NO current limit and no other protection. It's up to you to keep bad things from happening. Do NOT allow the output to short to Ground.

[Edit]

From the comments:

1) First thought was to simply use a 5W Zener diode. But those can be hard to come by these days. Next easiest is to use a smaller Zener diode coupled with a buffer transistor - the Zener handles only a few milliwatts and the bulk of the heat comes from the transistor - it's easy to get increased surface area (small heatsink) and thus keep the transistor cool.

2) I'm simply mentioning that there isn't any current limit in this solution. If there isn't any chance that you will "Oops" and touch the output to ground, then don't worry about it.

We can also substitute a LM317HV regulator for the transistor - this does have significant over-current and thermal protection. But the transistor is less expensive and may be easier to get. Mention in your comments if you want to explore using a LM317HV.

Do note that any of that transistor family will work: TIP120, TIP121, TIP122. The transistor is dropping only about 11 Vdc.

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