Stacked Resistors

I thought only the CIA did this...

On the back side of an infrared illuminator. I guess they populated this by hand…


  1. Peter Enmore wrote:

    This is a very interesting picture of stacked resistors. What are the resistors being used for here?

    • andy wrote:

      I assume the resistors are for current limiting, it’s an IR LED illuminator. Last time I saw that was on a piece of surplus telecom equipment from a three letter agency.

      • Leo Baldwin wrote:

        My guess is not so much current limiting as load balancing. If these strings are in parallel, one string will tend to “run away” and hog all the current to the detriment of the other strings (and eventually to itself!). Adding ohmic devices (and what is more Ohmic than a resistor, except for some extreme PCT ceramic resistors) will prevent this tendency. Not sure if the stacking is to get a low enough value, or high enough power handling. I am guessing low values. I quick touch should give a fast clue!

  2. andy wrote:

    So the NDS9410A is a 30V 7A NFET. Looks like there are strings of four IR LEDs with the resistors in question on the high side. Looks like three 0805 resistors with topcode “101” = 100 ohms. In parallel (assuming they’re all the same) are 33 ohms. Difficult to tell what the FET does. Would be strange to use it on the high side without a boostrap diode. I’ll see if I can find it again and probe it.

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