simple clamp circuit resistor calculation - Page 1
Forum section: electronics / beginners

author: khatus
author_rank: Regular Contributor
timestamp: August 26, 2019, 03:35:14 pm
content: 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.

author: rstofer
author_rank: Super Contributor
timestamp: August 26, 2019, 03:45:30 pm
content: 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.

author: khatus
author_rank: Regular Contributor
timestamp: August 26, 2019, 04:10:31 pm
content: well i try to simplify ithere IR= IZ= IRint= Internal resistanceif we take current in clockwise direction.from loop abcd,-Vout+ I*Rint+ Vz+I*R = 0To turn on the transistor,I*R = VBE=0.7voltSo, ( Vout- Vz- 0.7 )/Rint= I .........eqn(1)and,R = 0.7/I .........eqn(2)

author: khatus
author_rank: Regular Contributor
timestamp: August 26, 2019, 04:14:59 pm
content: Is my solution is right??

author: fourfathom
author_rank: Super Contributor
timestamp: August 26, 2019, 04:55:11 pm
content: 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.

author: khatus
author_rank: Regular Contributor
timestamp: August 26, 2019, 11:17:59 pm
content: Quote from: fourfathom on August 26, 2019, 04:55:11 pmok So what will be the value of the resistor R?

author: fourfathom
author_rank: Super Contributor
timestamp: August 26, 2019, 11:46:57 pm
content: Quote from: khatus on August 26, 2019, 11:17:59 pmAs I said, 1K Ohm will work just fine.If you want to calculate it, first chose how much current you want to flow through the zener. Anything from 0.1 to 10mA is probably OK for a small-signal zener diode, but you should check the device specs. Let's say you want 5mA through the zener (this seems to be a typical test current for a 1/2W zener).Then look at the base-emitter voltage for your transistor. Let's assume it's 0.7V when conducting.Then R1 would be 5mA / 0.7V = 140 Ohms. That will work, but so would 1K. The zener voltage vs current curve is quite steep, the transistor base-emitter voltage changes with collector (clamp) current, so the value of R1 will have little effect on the performance of the clamp. If you make R1 too small, then the zener current will be excessive.What is the clamp voltage you are trying to get, and what is the current that this clamp needs to shunt? These are far more critical factors than the value of R1.