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Registered Member #621
Joined: Sun Apr 01 2007, 12:37AM
Location:
Posts: 119
I'm curious about seriesing and paralleling resistors and how it affects their power ratings.
Say I have two 1 ohm 1 watt resistors and I series them. I know I will have 2 ohms resitance but will I have 2 watts capacity also?
Say that instead of seriesing I parallel them. I know I would have 1/2 ohm resistance but will I have 2 watts capacity also?
The reason why I ask is that I am designing a resistor network for an 8kV Meter I bought, and it has a Full Scale rating of 1ma. Using ohms law I calculated that I need 8 Megohms of resistance to give the meter 1ma of current at 8,000 volts so it reads correctly. To make things a bit safer, I decided that rather than just putting 8 Megohms of resistance on one of the meter's leads, I want to put 4 megohms of resistance on each lead, 4 Megohms on the - lead and 4 Megohms on the + lead in order to make the panel mounted meter's case more safe should anything form a path between it and the oppositve polarity. This was suggested by Steve Connor a while back and I'm glad for it. I will put the resistors near my cap bank and just run High voltage cables to the meter off the ressitors, therefore making BOTH of those high voltage cables current limited if you see what I mean since there's no way around sending the high voltage to the darn meter. Since I can't limit the voltage to it, at least limit the current going both ways i figure.
Here's the problem, I found Military Spec precision resistors to use, and they are 1 Megohm each with a 1/2 watt rating each. So if I series 4 of them on each side of the meter, and my thinking is correct, I will get my 4 Megohms desired resistance per side (for a total of 8) with a power handling of 2 watts (4 watts total). This isn't sufficient as 8,000 volts at 1ma gives me 8 watts of power to dissapate and I want to exceed this for safety margin. So if I instead make 4 sets of 8 seriesied resistors, and then parallel two sets on each terminal of the meter and my thinking is correct I will once again have my desired 4 Megohms per side but this time POSSIBLY a power handling of 4 or 8 watts per side (8 or 16 watts of capacity total)....can't figure this out darnit.... Keeps frying my brain and it should be simple.
Registered Member #621
Joined: Sun Apr 01 2007, 12:37AM
Location:
Posts: 119
So then it seems it would mean that as far as power capability of multiple resistors is concerned as long as they all have the same watt rating it doesn't matter if it is series, parallel, or both in any sort of combination, the total power handling is the sum of that of each resistor....
Registered Member #191
Joined: Fri Feb 17 2006, 02:01AM
Location: Esbjerg Denmark
Posts: 720
no, if you put two different resistors in parallel, say one is R and the other one is 2R, they won't be sharing the current equaily. the resistor with a smaller value is going to take more current, adn will be dissipating more power. for P = VI.
same thing with putting two differnt resistors in series, the larger one is going to take more voltage adn hence more power.
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Question on resistor power handling (series/parallel)
