Identifying failing controller component

Hi all of you enthusiastic people! I am trying to repair an e-bike (hub motor, some Chinese no brand controller) and I got a new controller. The bike came to life but died again shortly thereafter. I opened up both the old and the new, identical controller and found a component that looks burnt (see picture) and seems busted after checking with multimeter. The marking on the circuit board says "Rt1". Any ideas on what this is? Resistor, Thermistor, fuse or something else? Would like to repair instead of buying a new controller if possible.
Thanks a lot for any help you can provide. I'm a noob in this area

Edit: Measured the component in a very similar controller (same bike model but newer). 50 ohms resistance in a functioning one. Seems to get really hot even in this one, melting away at some nearby cables

It's the resistor that drops the voltage going to the regulator chip (thing next to it). Seems like they should have used a larger one.

The value of this part depends on the voltage you are using. What voltage is your pack?

On higher voltage controllers, I've seen them use 2 or 3 of those in series to spread out the heat. On more expensive controllers, they use a switching regulator that does not produce much heat.

Cool! Found an even smaller 51ohm resistor and soldered it as a bridge. Controller back to life! Not for to long though I suppose. I run a 36v battery on this bike. You think I could connect maybe 4x 200 ohm resistors in parallel to achieve 50 ohm and better heat dissipation?

Only one of the bikes burns this resistor. Could it be a shortage in some sensor/throttle you think?

Thanks a lot for your help

Update : As suspected the resistor pops after a short test run. Just before failing the PAS or throttle is acting strange by shortly dying and then pushing even when I'm not pedaling/using throttle.

Ifihadahifi said:

You think I could connect maybe 4x 200 ohm resistors in parallel to achieve 50 ohm and better heat dissipation?

Click to expand...

Yes, that would be much better. You need a larger surface area to dissipate the heat.

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Yes, that would be much better. You need a larger surface area to dissipate the heat.
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Great! Since I'm new at this I could really use some help calculating the Watt rating needed for this circuit (36v system). Been looking at power resistors as well (up to 50watt). Any problems using a resistor with to high rating?

Weird thing is that my 0.5W rated 51ohm resistors work just fine when I move the controller, battery and display to an identical bike. No signs of wear after 1 hour test drive. When used in the bike it is
Intended for it only lasts 10-15 min. Any ideas? I suspect throttle, PAS sensor or brake sensor malfunction but don't know how to test them.

Best regards, Andreas

Fast calculation. Worst case current is 36V/51ohm equaling .7 amp. Power is .7A x 51 ohm equaling 35 watts, If you have room for a 50W resistor, it will never burn out. Something in the circuit else likely will get smoked, though.

The power dissipation of the resistor depends on the current. Unfortunately, the only way to know the current would be to measure it. Typical controllers run around 100mA. The resistor needs to drop the voltage from battery (42v max) down to something less than 35v (the rating of the voltage regulator chip). Input to the regulator needs to be between about 15v - 35v. So at 100mA, 20v drop, the resistor would dissipate 0.2w, which is not that much.

If there is a problem with the controller so it is drawing more than normal current, it could explain the toasty resistor.

Using a resistor with a higher power rating is fine as long as you have room for it inside the case. Higher power rating will have lower operating temperature.

If you have a meter that can measure amps, you could try to measure the current going down the "key" line, or the small wire that goes to the battery positive, usually through a switch or the display unit.