Help with DPC-18 display testing

[matth53]

Hello, I recently had my BBS02B controller seemingly fry so I bought a new one. However, it won't turn on. I want to check if the DPC-18 display also fried so I got documentation of the 5-pin green Higo cable plug for the display. There is a PL pin and an P+ pin, and apparently pushing the power button "shorts" these two cables, thus turning the controller on.

So I took a multimeter and inserted staples into these two pinholes, and measured the resistance with the ohmmeter. When power button is not pushed, I get OL (open loop). When I push the power button, I get about 2M ohms. I would think it should be <=10 ohms, but the power button cable also goes through the display and there may be other circuitry inside affecting this. (The cable with the 5-pin Higo comes out of the display and is not directly connected to the power button.)

Does anyone know anything as technical as this? I just want to confirm if the display is dead before I throw in another $100 for a new display (in case the problem is actually that the new controller is just dead from the box).

 

[amberwolf]

You can't directly measure the resistance when it is powered, because there is potentially battery voltage present across it when the switch is off (potentially damaging your meter), and when it is on battery voltage is fed into the switch (really a transistor controlled by the power button via an MCU), and because the switch doesn't have zero resistance there should be voltage across that resistance.

A meter reads resistance by placing a voltage across it, then measuring the resulting current and calculating the resistance. (or it wil push a known current thru the resistance and meausre hte voltage drop across it). So your meter wouldn't read the resistance correctly because of the existing voltage already present.

What you want to measure is when you turn it on if battery positive shows up on the switched output wire. If it does, you can then use something like a 10kohm to 100kohm resistor from that pin to ground (battery negative) and then measure at that pin to see if battery voltage is still present. if it isn't, it means the transistor either isnt' being turned on, or it has failed and can't pass enough current to operate the controller.


Alternately, if you are very careful not to touch any pins except Battery positive output pin on hte controller, and the "ignition" line input pin on the controller, you can connect those together (which is what that transistor in the display does), and see if the controller powers on. If it can't talk to the display it will probably shut down quickly, but it might respond to control inputs for a moment. If it doesn't, you can at least check for 5v at the throttle connector if you have one.

Note that if you touch battery voltage to *any* wire except the one intended for it, it will probably destroy the controller, as the other two wires are low voltage data lines, and the last is ground, which would make a direct short circuit of the batteyr thru the controller's wiring and circuit board.

 

[matth53]

Thanks! I was trying to test the display without any power - meaning without the display plugged into anything. I thought, based off what I read, that the P+ and PL pinholes would be "shorted" by pushing the power button. But there's probably some internal resistors maybe, or something inside the display screen that prevents this (the power button is routed via the display screen, then to the green Higo plug. Presumably the power button does work because as I mentioned, the ohmmeter drops from OL to 2M ohms, I just wasn't sure if it is supposed to drop to <=10 ohms or not. What I will do is probably test this display on another e-bike (just need to find an associate who also uses DPC-18). But if anyone knows specifically if this 2M ohm reading is actually correct or not, would be good to confirm it.

 

[amberwolf]

Thanks! I was trying to test the display without any power - meaning without the display plugged into anything. I thought, based off what I read, that the P+ and PL pinholes would be "shorted" by pushing the power button.

Measuring like that won't work because the "switch" between those two lines is a transistor that has to be turned on by an MCU that detects the power button is being pushed. The MCU requires battery positive and ground to be connected to teh display to provide power to run it.

Presumably the power button does work because as I mentioned, the ohmmeter drops from OL to 2M ohms, I just wasn't sure if it is supposed to drop to <=10 ohms or not.

Doesn't matter if the power button itself works, unless you can test the actual function of the display providing battery positive back to the controller, which is not done by the button (only triggered by it).

What I will do is probably test this display on another e-bike (just need to find an associate who also uses DPC-18). But if anyone knows specifically if this 2M ohm reading is actually correct or not, would be good to confirm it.

In this case, it doesn't matter what that reading is; it doesn't tell you whether the system is able to connect battery positive to the controller's "ignition" input and provide that to the controller's internal LVPS that powers it's MCU, etc, which is all that matters. You'd have to test it with power on, via voltage and/or current, not resistance, to find out if it is actually working.

(the only thing the resistance will tell you is whether your transistor has the same off resistance as other people's transistor; it doesn't tell you if the transistor works, or if the MCU that turns the transistor on works, or if the power button is capable of turning on the MCU, etc, all of which are required for the display to turn the controller on).


A better way to test just the transistor (which you don't need to do, and which I do not recommend) is up the display, find the transistor that does the switching, and either remove it and test it in a transistor tester, or test it in-circuit by applying the correct voltages (whatever those are) at the resistors that feed it, and measuring the voltage that results at the switching pin.

But this only tests the actual transistor, and not the power-on function--for that, you have to power the display normally and measure voltages at the "switch" pins (PL P+, etc) to see whether they are correct for the state chosen with the power button.

 

[matth53]

Thanks. That does help to clarify. I will go ahead with testing the display on another bike.