Voltage clamp circuit

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Finlay_Mitchell

Joined Jan 30, 2015
1
It has been a long time since I did any electronics but I would like to do more. I need some help with a simple circuit which I'm sure you guys will be able to answer in a second.
I have an electric vehicle that uses LifePo4 batteries and I want a simple system to prevent overcharging. I was hoping to use a Tp107 Darlington Transistor and a white LED to clamp the voltage to 3.7-3.8 volts maximum during charging.
I was hoping to put the battery across C and E (C+) and a white (?) LED between B and C. Can someone help me with the calcs to allow me to select a suitable LED in terms of voltage and resistance. I am not sure whether the LED should be rated at 3.8V or 2.4V to compensate for the volt drop across the transistors. The problem is the white LED ratings tend to be a good bit higher than 2.4V.

Many Thanks
 

Bernard

Joined Aug 7, 2008
5,788
A better approach might use a stable V ref like LM336Z, 2.5 V, comparator like LM393N, & a P ch, LL FET, FD59435A, 5 A, 30 V smt, or PNP transistor rated for max charging current.
 
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