Some problems with my dump load controller

Question

The requirement is an automatic dump load controller; for balancing lead acid or lifo batteries. While the input voltage is above 1.75V the circuit should dump 10A into the load continuously.

^{simulate this circuit – Schematic created using CircuitLab}

So in theory (I think). R1, R2, Q1 and Z1 form a variable zener trigger; switching on Q2 while the input voltage is above that set by R2. Z2 should then sense the voltage across the load resistor; keeping current at 10 amps, by pulling Q2 low.

The problem is that I need to balance the batteries down to 1.75V, and the smallest zener I can find is 2.4 Volts.

Any other problems I'm not seeing?

Answer 1

Just a couple of obvious things wrong with the circuit as it stands now:

1. You are designing at the TIP's absolute maximum 10A rating, the transistor will not lead a long and prosperous life.
2. Collector–Emitter Saturation Voltage at 10A can be up to 3V, the circuit won't won't work as expected at 1.75V.
3. Similar 3V value for base-to emitter voltage.
4. Expected base current is 10A / h(FE) = 20mA, across the 1k R4 this adds up to 20V, not to mention R1 and R2.
5. Z2 will clamp the base voltage to approximately 0.7 Volt, the transistor Q2 will never conduct.
6. The feedback intended with Z2 won't work. The base voltage will vary only very slightly, not in the order magnitude the zener diode will attempt to stabilize it.
7. If you want to use Q1 -emitter voltage reference, drop R3 as that makes the zener voltage dependent on the emitter current. Better to use a collector side current source for that.
8. Q1 base will only marginally influence the rest of the circuit due to R3 and the fact you are only tapping into the base current.

Check the datasheet.

I'm convinced I didn't catch all issues with the design, but enough issues to reconsider the architecture.