# Designing Balancing Resistors for Series Capacitors: Factors Affecting IR

• Ntip
In summary, you need to consider the RC time constant when balancing resistors in series for capacitors.
Ntip
TL;DR Summary
I am trying to understand factors that cause a capacitors insulation resistance to decrease over time
I am lookin designing balancing resistors for series capacitors and understand that I need to consider the leakage current from the capacitors. I am trying to determine factors that would case the insulation resistance to decrease over time so I can design around that.

I should add that I do realize heating and aging are two factors. Is there some way to estimate the percentage the leakage current would increase based one either of these factors? I'm not sure how I would use them in the design stage.

I know that a capacitors lifetime is reduced as you operate closer to the rated voltage. How does this effect it?

Ntip said:
Is there some way to estimate the percentage the leakage current would increase based one either of these factors?
Some of the US manufacturers show graphs of leakage current for some products. I don't recall which particular ones, so you will have to search around a bit.

Here is an online calculator to help for lifetime:
https://www.illinoiscapacitor.com/tech-center/life-calculators.aspx

Also try this search, and look at the suggestions at the bottom of the results page.

Cheers,
Tom

Ntip
Well think of other materials and how heat can age them?

Also - when voltage is applied - this applies a mechanical stress to the material at the molecular and bonding levels - so that is why as you approach the rated Voltage they also age faster.

Keep in mind - the capacitor by design wants the thinnest dielectric possible to increase the capacitance.

Look at how the manufacturer defines the lifetime - the leakage current they give MAY be the EOL leakage. So as long as you stay within the aging parameter limits and time - you should be fine.

Ntip
Ntip said:
I am lookin designing balancing resistors for series capacitors and understand that I need to consider the leakage current from the capacitors. I am trying to determine factors that would case the insulation resistance to decrease over time so I can design around that.
You have not specified the capacitor technology you will use. Are you using electrolytic capacitors?

I think of electrolytic capacitors as being more like living things that must be cared for, and that can chemically learn about operating voltages, or die in a hot environment. In many respects, other types of capacitors are more robust.

We discussed various aspects of resistive balancing for chains of capacitors in a recent thread.
https://www.physicsforums.com/threa...-my-high-voltage-circuit.994058/#post-6396764

hutchphd
Baluncore said:
You have not specified the capacitor technology you will use. Are you using electrolytic capacitors?

I think of electrolytic capacitors as being more like living things that must be cared for, and that can chemically learn about operating voltages, or die in a hot environment. In many respects, other types of capacitors are more robust.

We discussed various aspects of resistive balancing for chains of capacitors in a recent thread.
https://www.physicsforums.com/threa...-my-high-voltage-circuit.994058/#post-6396764
I was thinking about electrolytic because it seems like I can get a high value capacitance pretty easily.

Tom.G
Ntip said:
I actually had a question about the matching RC time constant part but I'll make another thread about that since this one is titled as insulation resistance.
Fixed ratio "broad band potential dividers" are an interesting subject.

Ntip

## 1. How do I calculate the value of the balancing resistor for a series capacitor?

The value of the balancing resistor can be calculated using the formula R = 1/(2πfC), where R is the resistance in ohms, f is the frequency in Hertz, and C is the capacitance in Farads. This formula takes into account the reactance of the capacitor and ensures that the resistor will balance the current flow in the circuit.

## 2. What factors affect the value of the balancing resistor?

The value of the balancing resistor is affected by the frequency of the circuit, the capacitance of the capacitor, and the desired amount of current flow. Higher frequencies and larger capacitance values will require a lower resistance value to balance the current flow, while lower frequencies and smaller capacitance values will require a higher resistance value.

## 3. Can I use any type of resistor for balancing series capacitors?

Ideally, a resistor with low inductance and low capacitance should be used for balancing series capacitors. This will minimize any unwanted effects on the circuit and ensure accurate balancing. A metal film or carbon composition resistor would be a good choice for this application.

## 4. What happens if the balancing resistor is too small?

If the balancing resistor is too small, it will not be able to effectively balance the current flow in the circuit. This can result in an uneven distribution of current and potentially damage the capacitors or other components in the circuit. It is important to calculate the appropriate resistance value to ensure proper balancing.

## 5. Is it necessary to use a balancing resistor for series capacitors?

In most cases, it is necessary to use a balancing resistor for series capacitors. This is especially important in high frequency circuits where the reactance of the capacitor can significantly affect the current flow. However, in some low frequency applications, a balancing resistor may not be necessary if the capacitance values are small enough and the current flow is well controlled.

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