Please could someone tell me how to connect this TVS diode?

[DarrenB]

I am trying to copy someone else's idea and drop the voltage to my ebike display by using a TVS diode (P6KE24CA). The displays max voltage is 60V and I want to connect a 20S li-ion battery to the system which has a fully charged voltage of 84V. I know that I need to add the TVS diode in series with the displays power cable but I am unsure which way around to put the diode or even if I have selected the right diode. I think the P6KE24CA is a bi directional TVS, is that ok to drop 24V? The person who did this originally used the same part but with a 18V drop. The display only uses a few mA when powered on. I would appreciate it if someone could explain how this works and why a bi directional TVS diode is used over a uni directional (I don't know what either are)?

https://www.mouser.co.uk/datasheet/2/240/Littelfuse_TVS_Diode_P6KE_Datasheet_pdf-741008.pdf

 

[DarrenB]

I'm still not clear where the relay is connected.

Here is the schematic for the voltage reducer. The output voltage is Vbatt * R2 / (R1 + R2) - 0.6 V.

The current taken by the resistors will be Vbatt / (R1 + R2). With the ratios that you are wanting, R2 will dissipate more power than R1. R2 will dissipate R2 * (Vbatt / (R1 + R2)^2

The output voltage will reduce slightly depending on the current taken by the display. If G is the gain of the transistor, and the display takes a current of I, the voltage will drop by:- I / G * R1 * R2 / ( R1 + R2 )

You can try various values for R1 and R2. To reduce the voltage by 25%, R2 will be three times the value of R1.

If R1 and R2 are too low, they will take too much current and get too hot. If R1 and R2 are too large, the voltage will reduce too much from the current taken by the display.

Thanks. So I can keep the 10k and change the 22k for 30k and maybe use 3x91k or 2x62k in parallel?

The relay connects to a specific output from the controller which was designed for a low powered front and rear light. Because this output is from the controller it can be switched via the display. It hasn't got enough capability to power the light directly with only 6V 500mA so it will be used to power a 6V relay instead. I plan to use a 24V step down powered from the battery which the relay can switch to give 24V 750mA to the light. I think the display cable is in series with the controller so all current it needs goes via the display first and therefore the voltage reducer you rigged up for me. I am not sure if this is wrong or not but I think this is right. I remember you said that the NPN would need to dissipate this as heat but I was wondering if there was anything else that seemed wrong?