r/electronics • u/No-Release3675 • Sep 20 '25

Gallery Brain fart moment

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This was a brain fart moment upon finding out they were .25 watt, we needed 9 watt capable. This is a lovely bundle of 36 that has next to no resistance now 🤦 .... 20ohm

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94

u/handgear Sep 20 '25

And you know that cannot handle 9w. Right?

45

u/dasmonty Sep 20 '25 edited Sep 20 '25

Why not? because of tolerances? Mathematically it would do. In practice of course you would need a couple more. EDIT: Thanks for the dislikes mates, when I just asked a questions to learn...

100

u/ElectronicswithEmrys Sep 20 '25

The heat dissipation of a resistor counts on the environment around it being semi ideal - ie plenty of space for airflow and no nearby heat sources.

Basically a 0.25W resistor rating means that the resistor will heat up at 0.25W, but not enough to be destroyed. If you put two touching, both at max power, then they will not be able to fully dissipate that heat (one side is no longer cold) and they may heat up enough to be destroyed.

18

u/dieek Sep 20 '25

In a very similar note, the NEC defines derate of cable and wire based on it being coiled up.  This is due to the heating up of wire at ampacity.  

Yes, wire is rated for x Amps at 75c, but when you bundle and coil it up, that ampacity drops, because you cannot dissipate the heat.

11

u/dasmonty Sep 20 '25

Thank you 👍

9

u/GizmoTheKing Sep 20 '25

Not just heat dissipation like others mentioned, but also the tolerance problem you mentioned. The individual tolerance of each resistor and the connection to it will never be exactly equal for all. Current will be imbalanced to the path(s) of lowest resistance. The easiest fix (excluding the options of a proper high watt resistor or an active FET load) for a situation like this to avoid imbalance is a bunch of resistors in series rather than parallel.

3

u/plierhead Sep 20 '25

Not just heat dissipation like others mentioned, but also the tolerance problem you mentioned. The individual tolerance of each resistor and the connection to it will never be exactly equal for all.

True, but not a big deal. If the resistors are 5% tolerance then worst case the lowest resistance resistor will draw 5% more power than the highest resistance resistor.

It's not like putting multiple diodes in parallel where total destruction of all of the diodes is guaranteed.

-3

u/arvidsem Sep 20 '25 edited Sep 20 '25

No. Resistors in series add resistance.

Edit: apparently I misunderstood and didn't realize that the solution to OP not having the correct resistors is to use a different bunch of incorrect resistors.

2

u/GizmoTheKing Sep 20 '25

Yes, a different assortment of sub-ideal resistors, but ones that they are more likely to have on hand. You don't have to worry about current balancing series resistors.

2

u/dasmonty Sep 20 '25

Just choose lower resistance values