# 680V cap

Tags:
1. Nov 14, 2016

### btb4198

I have a circuit and it has a cap that has around 680V and the high current that is pull it 4.22A. I need to add an LED to let me know when the Cap is drain. The problem I am having is that since the voltage and current are so high, I cant use a Hall effect sensor . I cant find one rated for that much voltage. I could do a voltage divider and have R1 be 600 ohms and R2 needs be 22 ohms but I cant find a resistor rated for 2911.8 Watts. Can someone help me ?

2. Nov 14, 2016

### phyzguy

Why can't you make the voltage divider resistors much larger? If R1 is 600 KOhms and R2 is 22KOhms the power will be less than 1 Watt.

3. Nov 14, 2016

### btb4198

ok to i use the power equation P = V*I to get my power and I measure my current to be at a max of 4.22 and my voltage around 680V, This is why no resistors in my resistors in my circuit, so now you are saying if I add the voltage divider to the cap, it will not burn up the resistors ? I thought any resistor i added to the circuit would have to be rated for the max watts. If I am wrong can you please explain to me ?

4. Nov 14, 2016

### Svein

Power dissipated in a resistor: R⋅I2. Since I = V/R in this case, you get P = V2/R. Since V is 680V, assume R = 1MΩ and calculate the power dissipation: P = (6802/1000 000)W = 0.46W. The current through the resistor is (680/1000 000) A = 0.68mA. This current will make a LED glow, but not very brightly.

5. Nov 14, 2016

### Staff: Mentor

Um, why are you working with a 680V circuit with so little understanding of basic electronics? Sounds like kind of a dangerous situation to me...

6. Nov 14, 2016

### btb4198

It is my 1st time with voltage this high. I normal work with embedded systems.

7. Nov 14, 2016

### Staff: Mentor

What is the system/device? What does it do, and why are you adding things to it? Please help us understand the situation.

8. Nov 14, 2016

### btb4198

it is a system and it give power to a VFD. I have 120 Vac coming in from a UPS and I convert that to 680V DC and that goes to the DC bus on the VFD.

9. Nov 14, 2016

### CWatters

What do you mean by "drain" in your post #1? Drained below a set voltage?

Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted