Dissimilar Resistors in Series

[TheN00b]

Hi all,

Wow, it's been a while since I posted. I am in university studying an engineering discipline, and I wanted to come up with a project to further my understanding of circuits. I have decided to make a dynamic electronic load (that can dissipate 50W to 1kW in very roughly 50W steps). To do this, I had to come up with a resistance that would, when put in parallel, roughly equal the 1kW load in 20 individual parts. So I went with a 2.9 ohm load per part, which I know will not let me have exactly 1kW but I can get close enough for my liking. One issue I have run into is heat, but with 3x dissimilar resistors in series (1 ohm, 1.5 ohm, 0.4 ohm), I am having a bit of an issue calculating the heat dissipated per resistor in order to find the correct heatsinks for the resistors. With similar resistors, it is very easy, as I would call on my old friend Ohm to just use

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p=i^2r

per every resistor. However, with different values of resistors, I run into a bit of an issue. Knowing that current throughout a series circuit is unmodified, do I just use that same equation? I feel like there is a complication to it and before I go out and buy heatsinks, wanted to confirm that my logic is sound.

Thanks in advance!

[unitedatoms]

There is no complication. Each resistor Ra, Rb, Rc in series will dissipate Ra*I^2, Rb*I^2 and Rc*I^2.

Go ahead, and let the basic safety be with you (1kW is not a joke, no matter how small the voltage is)

[james_s]

Ohms law is your friend. With anything in series you know the current is the same through all nodes. Since you know the current and you know the resistance of each resistor you can easily calculate the voltage across any one of them and from that you can calculate the power.

[atmfjstc]

How would this electronic load work, exactly? From your description it seems like you would have 20 sets of 3 resistors each (1+1.5+0.4 ohm) in parallel, each set dissipating 50W, and that you could somehow switch the sets in and out of circuit. Is this correct?

[TheN00b]

Ah, got it. Thanks everyone! I will be careful definitely.

@atmfjstc, it uses relays on switches to control each channel that is connected to a bus bar (parallel) so I can "step" up using the 20 switches on a panel.