Zener circuit help please

[DanLew]

Will this work? If it will, wha are the zener and R1 values please?

If it won't work, what will?

 

[Prototyp_V1.0]

What does the leftmost [box] contains?

If it's the motor terminals then no it won't work. The current cannot go through the motor.

 

[DanLew]

It is the motor, but it shows leads coming from the motor. There are power leads going to the motor that supply 0 to about 7.8VDC, reversing polarity to run the motor in either direction. The leads shown tap off the terminals to supply power to the circuit shown. All of this is run from the same battery pack. The motor speed and direction are controlled by an electronic speed control (from radio control model application)
Here is a corrected circuit.

 

[IanP]

Assuming 0.1A current via R1, 3.5Vdc for the fan and taking into account that diode D3 drops 0.7V – the Zener diode voltage should be somewhere around 4.2V ..
4.2V Zeners are not readily available, so select 3.9V (P=3.9V*0.1A, 1W will do) and change diode D3 to Schottky, voltage drop ≈0.3-0.4V, that’s to keep the idle voltage around 3.5V ..

R1 = [7.5-3.9]/0.1 => 36Ω (at least 0.5W) is the closest ..

Rgds,
IanP
:|

 

[DanLew]

Ian,
Thank you!
I'm amazed that the circuit will work, that the help came so fast and that you provided all the specs and the calc's.
Thanks again - really helped me out .
Best regards,
Dan

 

[IanP]

I've modified voltages a little bit so the power from the battery should last longer ..

Rgds,
IanP

 

[DanLew]

Ian,
Thank you. I tried the circuit and it works perfectly. I usd a 0-500 pot to zero in on a desired fan idle speed. Ended up using a 47ohm ressitor.

Great. Thanks again.