Electronics > Beginners
simple clamp circuit resistor calculation
khatus:
Hello guys i want to calculate the resistor R in this circuit.Can anybody help me?
The value of zener diode(VZ)=4.8 volt.
rstofer:
Before you can calculate a resistor for that final circuit, you need to know the internal resistance of the driving source. What if the source had 0 Ohms internal resistance. Then it wouldn't matter WHAT value you picked, the source would produce infinite energy if that's what it took to fry the circuit.
So, once you have an internal resistance value, you can just use Ohm's Law to figure out how you want to clamp the output. How high can the source voltage get? When the transistor turns on, how much voltage is dropped by the internal resistance based on the value of the collector resistor.
As the circuit is drawn, the transistor is working as a crowbar overvoltage device. It flat out short circuits the source when the voltage goes too high.
After you calculate the resistor values (internal and collector), you can calculate the collector current. Then you can figure out how much power is being dissipated.
As drawn, nothing really happens except for a short circuit. A better circuit would have a very fast acting fuse in the lead from the source. Once the crowbar is tossed across the + and -, the fuse blows and removes power.
What you have is not so much an overvoltage limiter but more like a overvoltage crowbar.
khatus:
well i try to simplify it
here IR = IZ = I
Rint = Internal resistance
if we take current in clockwise direction.
from loop abcd,
-Vout + I*Rint + Vz +I*R = 0
To turn on the transistor,
I*R = VBE =0.7volt
So, ( Vout - Vz - 0.7 )/Rint = I .........eqn(1)
and,
R = 0.7/I .........eqn(2)
khatus:
Is my solution is right??
fourfathom:
Forget that "simplified" circuit for now. The original circuit is not a short-circuit, it clamps at a voltage set by the (zener voltage + base-emitter voltage). The base resistor value should be low enough to drain off any zener and base leakage current, but beyond that it is not critical. You might want to set it so that the resistor voltage drop is around 0.7V when the nominal zener current is flowing, but this is not a precision circuit so you could just use a 1K Ohm resistor (giving you about 0.7mA through the zener).
You do not want to put a resistor in the collector circuit, this will limit the clamp current and negate the over-voltage protection that you added the clamp to get. Of course, if your supply / source is not appropriately current-limited (could be with a fuse) then you will likely burn out the clamp transistor.
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