Voltage balancing resistor calculation

[siddarth.ghaste]

HI

Can anybody tell me how to calculate voltage balancing resistor....

 

[BradtheRad]

Does this have to do with balancing loads through parallel led's or diodes or transistors?
Which is to say, components which are liable to have slightly different current-vs-voltage profiles?

 

[siddarth.ghaste]

Okay.....these are in parallel with diodes(current limiting diodes)

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Resistors connection is shown in this diagram...

 

[BradtheRad]

None of the diodes must carry greater than its rated amperes.

The volt level can be the same...
Yet current flows can be unbalanced. (Left-hand schematic.)



1.

If you only need to install one resistor, then you choose its value to create a voltage drop equal to the maximum observed difference between the two diodes.

Example, you install two diodes in parallel, and you plan to make each carry up to 3 A max.
By experiment you find their forward V differs by .1 V.
Therefore choose a resistor value that will develop .1 V at 3 A. This makes .033 ohm.

(Center schematic.)

2.

On the other hand, suppose you are building many modules, and you cannot test every component...

Then you must put an equal balancing resistor on every diode, and the value must be high enough to solve the greatest observed difference between the diodes.

Calculating the value is not easy. You reduce current through one, and it raises current through the other. Diodes are not linear devices, so I'm not sure the process can be predicted mathematically. So I cheated and used a simulator. (Right-hand schematic).

 

[godfreyl]

@Brad: He's not talking about normal diodes, but "current limiting diodes" (which act like constant current sources). Why anybody would want to connect two of those in series is beyond me, maybe to increase the maximum voltage?

 

[siddarth.ghaste]

@godfrey: yeah to increase voltage only.......I got something like, R = rated voltage/leakage current.
...............and value of balancing resistor will be 10% of this R.

 

[BradtheRad]

@Brad: He's not talking about normal diodes, but "current limiting diodes" (which act like constant current sources).

Yes, now I find it is a technique of arranging an FET and resistor to create a constant current device.

From Wikipedia:
"Constant current diode (also called CLD, current limiting diode, constant-current diode, diode-connected transistor or CRD, current-regulating diode)."

 

[FvM]

Does the manufacturer suggest balancing resistors? To keep the current limiting diode's voltage drop in rated voltage range, I would primarly think of zener diodes.

 

[siddarth.ghaste]

yeah they recommended balancing resistor for uniform distribution of voltages, because diode should not get over voltage than it can handle.

 

[pplus]

Series resistors killed the idea of ​​constant current device. Series zener and constant-current diode killed the dinamic diapason of ​​constant current device. As they say, use the field-effect transistor and a resistor.

 

[betwixt]

The problem they are trying to overcome, for reasons which are somewhat vague, is that constant current diodes have a limited maximum forward voltage. For the FET to work it has to be a depletion mode device, the 'normal' high voltage ones are enhancement mode devices. I've never come across a high voltage (>350 V) high power JFET and I'm not sure such a thing exists.

As pointed out, if you add resistors across a constant current device it negates it's purpose as it can only regulate current through itself, not through the resistor as well. It's worth remembering that constant current diodes work by dropping a voltage across themselves in order to limit the current from the available voltage source, if they are joined in series and are not exactly matched, they will each drop different voltages and dissipate different amounts of heat.

Brian.

 

[FvM]

A high voltage depletion mode MOSFET could be used, there's e.g. Infineon BSP135. In contrast to dedicated constant current diode,you have to adjust the current set resistor for Vth variations.

Series zener and constant-current diode killed the dinamic diapason of ​​constant current device.

Due to spelling problems, I don't understand which problem you expect with zener diodes? As a disadvantage, the voltage drop across the CC diodes will be different, but the current will be independent of voltage variations, which isn't the case with balancing resistors.

Nevertheless can balancing resistors work if they are dimensioned so that the balancing current is only a small part of the total current.

 

[pplus]

This is guesswork without values ​​of working voltage, the desired current and dynamic range of the output voltage. Give the scheme.

 

[betwixt]

Thanks for that part number FvM, I didn't realize they were available but I've now added the data sheet to my collection.

Assuming this thread is related to another one by the same OP, the purpose of the current regulator is to allow an LED to run directly from rectified mains AC, I think it was 230V in the original thread and the current needed was 20mA. The OP didn't want to use resistive or capacitive voltage droppers.

Brian.

 

[FvM]

This is guesswork without values ​​of working voltage, the desired current and dynamic range of the output voltage. Give the scheme.

You are absolutely right that theinformation is missing fromthis thread. Looking at the initial post, the author has apparently favour for guessing games...

Well-informed readers know some of the requirements and the design purpose from other threads, as betwixt mentioned. Of course that's no excuse for omitting essential information.
https://www.edaboard.com/threads/280237/
https://www.edaboard.com/threads/279776/

Assuming this thread is related to another one by the same OP, the purpose of the current regulator is to allow an LED to run directly from rectified mains AC, I think it was 230V in the original thread and the current needed was 20mA. The OP didn't want to use resistive or capacitive voltage droppers.

For some reason,the indicator LED should work with low to high DC voltage with constant brightness respectively current. Many solutions have been suggested in the previous threads. Unfortunately simple questions like intended current and acceptable power dissipation have been never answered. 20 mA would be out of reach for some of the solutions, also the MOSFET I suggested.