how to calculate wattage of resistor in circuit

[shahbaz.ele]

dear all

I am using power supply of 1KV for my circuit, its current would be 8.2mA.
what should be the wattage of resistors I should use for my circuit, because when I used 1/4watt resistor, it burnt up.

should it be more wattage, if yes then how much ?

 

[kamranravian]

its looks simple to me
P = VxI = V'2xR = I'2xR

P = 1000x0.0082 = 8.2

resistor should be 10 watt

 

[godfreyl]

1KV * 8.2mA = 8.2W
So the resistor should be rated at least 10W.

For high voltages it is better to use many resistors connected in series so their voltage rating is not exceeded.

Ten 2 Watt resistors connected in series would be good.

 

[shahbaz.ele]

thanks dear
I would purchase and check it .

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thanks dear
I would purchase and check it .

 

[bigdogguru]

I am using power supply of 1KV for my circuit, its current would be 8.2mA.
what should be the wattage of resistors I should use for my circuit, because when I used 1/4watt resistor, it burnt up.

should it be more wattage, if yes then how much ?

Both FvM and Alex are correct, unless the entire 1kV drops across the resistor in question, knowledge of either the actual value of the resistor or its voltage drop are required before calculating it's required power rating.

P = V x I = I² x R = V² / R


Resistor Power Rating


BigDog

Thanks FvM and Alex, point well taken, incorrect assumption revised.

 

[FvM]

You didn't say that the resistor voltage drop is actually 1 kV. (Seriously speaking, I can hardly imagine that the resistor's only pupose is to burn electrical power, but there may be special cases).

In any case, you can't avoid to calculate the power dissipation (in normal circuit operation and expectable maximum value) for eaxh resistor and choose a respective part.

 

[alexan_e]

The info of the first post only mention that the circuit is powered by 1KV and consumes 8.2mA.

Based on that we can't answer what resistors to use in the circuit unless you provide specific info for each resistor.

 

[shahbaz.ele]

I have resistor divider network
1KV is divided through following figure resistor divider
ten (10) watt resistor is also getting hot.

 

[godfreyl]

AAAaarrrrgh!!!!!
That bitmap is almost a Megabyte! 8-O

For the love of bandwidth........
Here's a GIF of it.
(and no, I didn't shrink or compress it - this is really all there is in the bitmap!)



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ten (10) watt resistor is also getting hot.

Yes, it will get hot. That's OK. Just make sure it has good ventilation, and nothing else is close enough to get burned.

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btw, What software did you use to draw the picture? Was it MS Paint? If so, look in the toolbox - there's tools that make it easy to draw straight lines, neat circles etc.

 

[FvM]

Any serious reason for not using a higher voltage divider resistor level?

 

[lalgpt]

Your voltage 1 KV = 1000 V
Current 8.2 mA = 0.0082 A

Resistor value = V/I=1000/0.0082=121.951kohm
Power rating = I² R= 8.2Watts approx 10Watts can be used


If the resistor value is 1000ohm it will take 1A if resistor value decreases current will increase.

 

[shahbaz.ele]

dear godfreyl
pic was drawn in paint.


Actually I want to use 10V rather 1Kv for rest of circuit.
for comparison of voltages.

 

[FvM]

pic was drawn in paint.

Yes. There's an option to save the drawing as *.gif instead of uncompressed *.bmp

Actually I want to use 10V rather 1Kv for rest of circuit.
for comparison of voltages.

Clearly. But do you also need the low resistance values? How about increasing them by a factor of 10? If the connected load (measuring device, whatsoever) resistance is the reason, it may be better to place a buffer amplifier behind the voltage divider instead of burning so much power.

Of course it depends on the application. In power electronics, e.g. for electric traction, nobody cares for a few ten watts power dissipation in a voltage transducer.

 

[shahbaz.ele]

dear FvM
I can not understand what you want to say ?

 

[alexan_e]

You mean the resistor part?

All he said is to use higher value resistors so that the current is lower and the power consumption also lower.
It depends on what it is that you are going to connect in the 10v output, what is the expected load there?

 

[leekc_84]

I have the similar question. Let say if voltage 3.3V, load current 0.5A... now I increase voltage to 3.8V, with series resistor 1 Ohm, to create a voltage drop of 0.5V. Thus, the 1 Ohm resistor power rating should be at least 0.25W?