# How to calculate the resistor value or wattage value in the given circuit

#### nisha p

Joined Dec 26, 2017
20
can someone tell me formulas or as to how to select the resistor R2(value and wattage) in the given circuit.C4 charges via R2 from supply and apply to load.
Thankyou

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#### ericgibbs

Joined Jan 29, 2010
11,111
hi nisha,
Is this a homework assignment, if so, I will move it to the Homework forum.
You should also show your attempt at solving the question.
E

#### nisha p

Joined Dec 26, 2017
20
its not homework question .its something am working on.

MOD: thanks for the update. E

Last edited by a moderator:

#### Threeneurons

Joined Jul 12, 2016
30
"Worst case" senario, is if the FET is ON continously (either intended, or broken). Then its 1000V / 1200 ohms = 0.833A (833mA). 0.833A * 1000V = 833W.

You can also do the algebra first, with the two fundamental equations: V=IR & P=IV, and solve for P (Power in watts), given V & R: then I=V/R. substitute that in the power equation: P=(V/R)*V = V^2/R = (1000V*1000V)/1200 = 833W.

Nominally, I suspect the load is a very small duty cycle, and it never sees that 833W. But if the FET fails, or the uC pulsing the FET glitches, or there's a programming bug (never happens, we all write perfect code, and unicorns poop rainbow ice cream), the you may not want the failure to cascade, and take out the resistor, too.

#### Dodgydave

Joined Jun 22, 2012
9,654
can someone tell me formulas or as to how to select the resistor R2(value and wattage) in the given circuit.C4 charges via R2 from supply and apply to load.
Thankyou
W = VxV/R
Or
I x I x R

#### nisha p

Joined Dec 26, 2017
20
The supply is glassman power supply which can supply 0-40KV with output current as 1 mA ,so is this valid in the given scenario too?Thankyou for your help

#### AnalogKid

Joined Aug 1, 2013
8,687
There is no "scenario". Your question is missing a *lot* of basic information.

What is it you are trying to do?
What are the minimum and maximum frequencies of the "12 V pulses"?
What are the minimum and maximum pulse widths of "12 V pulses"?
What do you want the voltage across the load to be?
What do you want the current through the load to be?

ak

#### kubeek

Joined Sep 20, 2005
5,733
The supply is glassman power supply which can supply 0-40KV with output current as 1 mA ,so is this valid in the given scenario too?Thankyou for your help
If you load a 1kV supply with 1200 ohm, you will have 0.83 amps flowing and the supply is either gonna turn of or blow up. If the load is a very fast pulse, and you sufficiently decouple the power supply so it doesn't see that peak current, then it may be possible. I don't see why you want to use such a resistor and not use some better scheme to achieve what the circuit is supposed to ultimately do.

#### nisha p

Joined Dec 26, 2017
20
There is no "scenario". Your question is missing a *lot* of basic information.

What is it you are trying to do?
What are the minimum and maximum frequencies of the "12 V pulses"?
What are the minimum and maximum pulse widths of "12 V pulses"?
What do you want the voltage across the load to be?
What do you want the current through the load to be?

ak
am trying to electroporate a sample in a cuvette which is the load.minimum pulses range in 50microseconds and maximum 60 milliseconds.as for the voltage across the load i want the maximum possible applied voltage applied to develop across the load.I know losses will occur but pulses have to appear across load with max possible voltage without much distortion.

#### AnalogKid

Joined Aug 1, 2013
8,687
Based on post #6, is this correct:

You have a high voltage power supply set to a maximum output voltage of 1.0 kV. It also has current limiting that is set to 1.0 mA. Is the current limiting the constant current type or foldback type? Lets assume constant current. So the worst case current through anything is 1 mA.

If all of that is correct, then the worst case power dissipated in R2 occurs if Q1 is on constantly, and is determined by Joule's Law:

P = I^2 x R = 0.001 x 0.001 x 1200 = 1.2 mW

C4 and C5 form a voltage divider. Their effect on the current through R3 is a function of the pulse width and frequency.

ak