My ebike was failing when going up steep hills. I would switch it off for 20 seconds and then the motor would start again. This failure slowly got more regular and worse. Yose Power said the motor was faulty and, as it was still under warranty, they sent a replacement. I finally fitted the new motor and set off, and after 100 metres it failed. And this time it wouldn't re-start.

This got me looking at the Controller ( KT36ZWSRT-01). The two capacitors on the circuit board looked suspect and my friend, who is a deft hand with the soldering iron, replaced them. My ebike is now fully working again. Cost of components less than a fiver.

I am now looking for a wheel so I can utilise my spare motor

I doubt if the caps had blown because when they do the evidence will be clear to see and smell.

When the bike fails on hiils it is usaually due to slow speed and too much current on steep hil, this causes a lot of heat and the mosfets will eventually reach their olsp temp and thermally cut out bt temporary shorting. Once cooled they will switch again until they get hot again, better mosfets are needed or more mosfets.

The two capacitors on the circuit board looked suspect:)

 

Suspect in what way ?

Suspect in what way ?

 

They were visibly damaged

It looks like the heat from the adjacent resistor (which looks like a wire link), which was excessively darkenened by excess current, melted the plastic covering of the capacitor and caused the top of the capacitor to expand. There was no visible damage to the mosphets.

Electrolytic capacitors that are failing, do expand.

Electrolytic capacitors that are failing, do expand.

 

What do you want me to say? Visibly damaged capacitor was replaced. Ebike working fine now.

But what caused the damage, is it likely that both capacitors were heated and melted at the same time ? If it were my bike I would be curious.

 

Electrolytics do fail, due to overvoltage, or not having been used in a while. The start gassing internally and to stop them injuring people they normally have weak spots, triangular lines, in the stop of the alluminium can.

 

Did the capacitors have a suitable voltage rating ?

I burnt out the previous motor after climbing a long and very steep hill. The heat generated by this probably contibuted to the eventual failure of capacitor.

 

Ive no idea if the capacitors had a suitable voltage rating. I guess they did as the controller came as part of the kit.

Photos showing the blackened resistor and the two capacitors, one with a split plastic cover. The assumption is that the resistor overheated and transferred the heat to the capacitor causing the plastic to split in the process.

Well, its possible the current measuring resistor got hot enough, cant say I have touched one when the controller is in use. I have a similar KT controller and it did get failry hot the other day going up a long steep hill an max assist.

 

The capacitors in the picture are 50V so OK for a 36V battery.

The 470uf 50v capacitor showing the melted plastic covering

The 470uf 50v capacitor showing the melted plastic covering

 

 

Yep, that capacitor has blown. Have a look at the top, it is slightly bulged. There was a huge problem with dodgy electrolytic caps 10 to 15 years ago, causing all sorts of electronic kit to die prematurely. I used to be given loads of "dead" motherboards and get them running again by replacing the caps. You'd see bulging at the top initially, then soon after they would often short and fail with a bang. There are probably still some bad caps out there on the market in China.

 

Good job getting it sorted

There was a huge problem with dodgy electrolytic caps 10 to 15 years ago, causing all sorts of electronic kit to die prematurely.

 

I used to be given loads of "dead" motherboards and get them running again by replacing the caps.

 

BTDTGTTS.

 

If you had a controller apart and found one capacitor bulging, would be an idea to replace the lot, they are not expensive.

BTDTGTTS.

 

If you had a controller apart and found one capacitor bulging, would be an idea to replace the lot, they are not expensive.

 

Soooo true

 

Another thing to check I find with a lot of modern lead free soldered PCBs is the quality of the joints on electrolytics, they are often loose and could do with a drop of fresh solder.

Another thing to check I find with a lot of modern lead free soldered PCBs is the quality of the joints on electrolytics, they are often loose and could do with a drop of fresh solder.

 

Especially the surface mount ones.

 

It does not take much of a knock to break the (tiny) soldered joints.

 

Lead free - orrible stuff.

Hi All, am new to the forum, just joined. I have a kt controller that is not working. The board looks exactly like this one in the pics. The shunt resistor is a bit charred and the big resistor next to it also had a bit of black on the legs. I removed it out of the board and it measured 198k ohms. What is the value of this resistor? I cant exactly tell what colours the bands on the resistor are.

cheers

The one in Caps2.jpg could be 1K 5% 1W. But both boards look darkened around the shunts, suggesting they're overheating enough that problems will recur.

 

You could reduce the controller's maximum current to 80% so that the shunt makes 2/3 of the heat, or try replacing it with a string of physically larger shunts, but that only helps if the problem's the board design. I'd have guessed at a shorted winding if OP hadn't already swapped the motor.

Thx for the speedy response. Attached is a pic of the resistor on my board. Like i said i pulled it out and measured it and it shows 198k ohms. I dont know its value, so i dont know if its busted and whether i should replace it or not. Attached is a pic of my controller model.

Edited June 6, 2025Jun 6 by Balmy

That too _could_ read 1K and it's failed open circuit: if it were more than 7K a smaller and cheaper one could have been fitted. The cause of failure still needs finding and fixing.

I think that large carbon resistor is part of the regulated lower voltage and 5volt circuit. One end is connected to the 48/36v and the other end goes to the voltage regulators. Its shown in the bottom of the circuit diagram I have attached below as R62 and 82ohms/3watt but this resistor number and ohms value varies with different versions of the KT controllers as far as I know.

If you google 'KT controller circuit board' and select 'images' you will see examples of this resistor with different values on different versions of the controllers as photo below.

The fact that the resistor has 'blown' would lead me to suspect that there may have been a fault on the 5 volt line or that the voltage regulators may be faulty, so there is no guarantee that replacing the resistor will work. Also I suppose it would be wise to check the external wiring as a short on the 5 volt line could possibly cause this resistor to blow.

I have attached a pdf of KT controller general circuit diagram below if it helps.

 

.

S06S-Controller (2).PDF

Edited June 6, 2025Jun 6 by Sturmey

That too _could_ read 1K and it's failed open circuit: if it were more than 7K a smaller and cheaper one could have been fitted. The cause of failure still needs finding and fixing.

thx

I think that large carbon resistor is part of the regulated lower voltage and 5volt circuit. One end is connected to the 48/36v and the other end goes to the voltage regulators. Its shown in the bottom of the circuit diagram I have attached below as R62 and 82ohms/3watt but this resistor number and ohms value varies with different versions of the KT controllers as far as I know.

If you google 'KT controller circuit board' and select 'images' you will see examples of this resistor with different values on different versions of the controllers as photo below.

The fact that the resistor has 'blown' would lead me to suspect that there may have been a fault on the 5 volt line or that the voltage regulators may be faulty, so there is no guarantee that replacing the resistor will work. Also I suppose it would be wise to check the external wiring as a short on the 5 volt line could possibly cause this resistor to blow.

I have attached a pdf of KT controller general circuit diagram below if it helps.

 

[ATTACH type=full" alt="63438]63438[/ATTACH].

Thx for the info. Are you suggesting i replace it with a 82 ohms/3 watts resistor?

It needs to drop the voltage by around 20v at a current of between 100 and 200mA, so it'll be 100 Ohms to 200 ohms. Colour look like brown black brown, so 100 ohms. It's value isn't that important as long as itin that sort of ball park. The shunt would only overheat if you connected the battery with the wrong polarity, which would also soon blow the capacitors, or if a MOSFET has blown, so you should check them before going any further.

You are right. I did indeed short the controllers positive with the negative, but the battery was not attached. I tested the resistor out of the circuit and it showed 198 ohms. I checked the mosfets, they are all ok. Visually the capacitors seem ok too.