How to use variable power load as a turn on/off signal?

ivanh

Joined Nov 2, 2021
1
Hi all,

In order to have only one switch to turn on/off both headlight and taillight on an e-bike, I am trying to use the headlight as a turn on/off signal for the taillight.

The problem is that the headlight has push on - hold off electronic switch that I cannot replace with a DPST switch.

The headlight can work from 10 to 58V and at 48V draws:
2mA when off,
77mA day light,
195mA low beam,
640mA high beam.

The taillight requires 12V.

I can provide 48V for the headlight and 12V for the taillight.

My attempts to find a way to connect a relay in series with the headlight have failed. Solid-state relay LED's cannot handle 8x difference in current between day light and high beam, and I doubt a coil relay would handle that any better.

Any ideas would be appreciated.

Juhahoo

Joined Jun 3, 2019
256
Measure head light current consumption, and when it exceeds 150mA, put the rear light ON.

jayanthd

Joined Jul 4, 2015
945
Hi all,

In order to have only one switch to turn on/off both headlight and taillight on an e-bike, I am trying to use the headlight as a turn on/off signal for the taillight.

The problem is that the headlight has push on - hold off electronic switch that I cannot replace with a DPST switch.

The headlight can work from 10 to 58V and at 48V draws:
2mA when off,
77mA day light,
195mA low beam,
640mA high beam.

The taillight requires 12V.

I can provide 48V for the headlight and 12V for the taillight.

My attempts to find a way to connect a relay in series with the headlight have failed. Solid-state relay LED's cannot handle 8x difference in current between day light and high beam, and I doubt a coil relay would handle that any better.

Any ideas would be appreciated.
What is the e-bike battery voltage? 12V x 4 batteries in series = 48V to power BLDCM rated @ 48V?

The push to On and push and hold to Off switch is a tactile switch, which is used with microcontroller to control headlight or is the operation mechanical type without uC?

Headlight and taillamp are LEDs based?

You can use a 22R resistor between gate of N-Channel Logic Gate Mosfet with say VDS say 60V or more and Source to Drain current rating of 2 to 4A as your lamps max current requirement doesn't exceed 1Amps DC.

The other end of 22R gate resistor can be connected to the push to On switch if it is locking type. If it is non-locking type push button or tactile switch then it uses uC to control the headlight. That is the modern way of handling all electronics of a vehicle with microcontroller or ECU, electronic control unit.

If you give more details, then I can design a patch up or new ecu circuit for you for full e-bike electronics.

Ian0

Joined Aug 7, 2020
5,187
You can do it with a relay.
Choose a relay with a 3V coil, such as this one:
https://docs.rs-online.com/7104/0900766b80dbfca2.pdf
It has a coil resistance of 60Ω. Put the coil in series with the power to the headlight.
Put a resistor in parallel with the coil so that the combination drops 2.4V* at 195mA. So you need 2.4V/195mA = 12Ω
So you need 15Ω in parallel with the 60Ω coil to get 12Ω: R= 1/((1/12)-(1/60))
Then put a 3V 3W zener (or four 1A diodes in series) in parallel with the coil and the resistor to limit the voltage across the relay when the headlight is on high power.

*I chose 2.4V because it is the minimum pull-in voltage of the coil, in order to minimise power loss. It will also make it less likely that the relay will stay on when turning the headlight down from low-beam to day-light.

 there are some 1.5V relays about such as this one:
https://www.farnell.com/datasheets/1748112.pdf
The power loss would be even less.
You can do the maths. . . .