Orpington Astronomical Society

Astronomy => Technical => Topic started by: mickw on Oct 20, 2010, 15:40:41

Title: Resistors in Parallel
Post by: mickw on Oct 20, 2010, 15:40:41
OK, brain broken  :(

e.g.
If you have a 100 ohm resistor and a 200 ohm resistor and connect them to a supply using one plug, do they remain individually at 100 and 200 or do they behave as resistors in parallel.

I'm thinking of 2 dew heaters sharing the same plug.

The numbers 100 and 200 are meaningless
Title: Re: Resistors in Parallel
Post by: Mac on Oct 20, 2010, 15:49:26
their individual resistance dosent change, the overall resistance does.

if you have them in series its the same as a single 300 ohm resistor.

If you have them in parallel its the same as a single 120 ohm resistor.

Title: Re: Resistors in Parallel
Post by: MarkS on Oct 20, 2010, 15:59:40
Quote from: Mac
If you have them in parallel its the same as a single 120 ohm resistor.

If they're in parallel it's like a 67 ohm resistor not 120 ohm.

Mick,
a 100 ohm dewheater will consume 1.44 amps
a 200 ohm dewheater will consume 0.72 amps
Put them in parallel and together they consume 1.44+0.72 = 2.16 amps
Mark
Title: Re: Resistors in Parallel
Post by: mickw on Oct 20, 2010, 16:57:55
So 2 dewheaters sharing the same plug won't burn one of them out  :o

Makes sense now, all dewheaters are connected to the same battery which is the same as one plug anyway  :roll:

I think you know why I am so cautious now  :cry:
Title: Re: Resistors in Parallel
Post by: Mac on Oct 20, 2010, 18:37:25
QuoteIf they're in parallel it's like a 67 ohm resistor not 120 ohm.
:oops:

i'll use a calculator next time,
Title: Re: Resistors in Parallel
Post by: Ian on Oct 20, 2010, 23:17:28
bear in mind that the equivalent resistance only matters to the battery. As far as each resistor is concerned, it's seeing the same voltage and will draw the same current as it would if it was on it's own.

that is, it will heat just as efficiently connected in parallel to another as it would on it's own. the difference being your battery will run down sooner.

Note, this is assuming internal resistance is negligible, which it is, until you start to draw scary (car starting) sort of amps.