# Issue with voltage divider for DC bus voltage sensing

#### Xavier Pacheco Paulino

Joined Oct 21, 2015
728
In the picture below, you can see an array of 10 resistors used for DC bus voltage sensing. I think that the blue resistors (the original ones) are 400k/1W. The colors are: yellow, black, black, orange. One of the resistors got open, and I replaced it with the one you see different in the array, the one in the red circle. It's a 390k/1W resistor. The equipment worked well for a couple of hours, then the resistor in the white circle got open.

Do you think the new resistor could have affected the array so that a new resistor got damaged? Well, indeed we have a variation of 10k in the array with the addition of a 390k, which leads to increase the current. I did not find the 400k resistors in the market in that package (MELF). What would you recommend?

#### paulktreg

Joined Jun 2, 2008
784
Are you sure that's yellow and not green?

#### Xavier Pacheco Paulino

Joined Oct 21, 2015
728
I took out a good resistor and the meter says 400k.

#### WBahn

Joined Mar 31, 2012
25,927
The likelihood of damage from the mismatch depends on what the circuit is doing. It looks like it involves some kind of parallel matched network, so the mismatch could be very important. Replacing the corresponding resistor on the other side with the 380 kΩ resistor MIGHT be the proper thing to do. But there's really no telling with the little information available.

I don't understand why the need for 1 W resistors. To get 1 W into a 400 kΩ resistor you would need 632 V across it (or over 3 kV across the string). Are these the kinds of voltages this circuit is dealing with?

#### Xavier Pacheco Paulino

Joined Oct 21, 2015
728
The likelihood of damage from the mismatch depends on what the circuit is doing. It looks like it involves some kind of parallel matched network, so the mismatch could be very important. Replacing the corresponding resistor on the other side with the 380 kΩ resistor MIGHT be the proper thing to do. But there's really no telling with the little information available.

I don't understand why the need for 1 W resistors. To get 1 W into a 400 kΩ resistor you would need 632 V across it (or over 3 kV across the string). Are these the kinds of voltages this circuit is dealing with?
The device is fed with 480VAC, so the string is used to measure DC bus voltage of around 700V.

#### AlbertHall

Joined Jun 4, 2014
10,068
The likelihood of damage from the mismatch depends on what the circuit is doing. It looks like it involves some kind of parallel matched network, so the mismatch could be very important. Replacing the corresponding resistor on the other side with the 380 kΩ resistor MIGHT be the proper thing to do. But there's really no telling with the little information available.

I don't understand why the need for 1 W resistors. To get 1 W into a 400 kΩ resistor you would need 632 V across it (or over 3 kV across the string). Are these the kinds of voltages this circuit is dealing with?
You need to consider not only the power rating but the voltage rating of the resistors.
Also if this string is for measuring the high voltage you wouldn't want the resistors to get hot so low power dissipation would improve accuracy.