Please check this for me - resistors/parallel ?

[MartyMart]

I need a 10 Ohm 1 Watt resistor for the Shaka tube and only have lots of
100 Ohm 1 Watt.
So 2X 100 Ohm in parallel gives me 50 Ohms
    3X 100 Ohm in parallel gives me 25 Ohms
    4X 100 Ohm in parallel gives me 12.5 Ohms ( approx )  ??
0r does 3 X give me 12.5 .......
Do I have this correct, or am I going "nuts" ??

Thanks,
Marty.

[davebungo]

1/Rx = 1/Ra + 1/Rb + 1/Rc

so if you require Rx=10 then 1/Rx = 0.1

Sum of 1/Ra + 1/Rb etc = 0.1 and as 1/100 = 0.01 you need 10 parallel resistors of 100 ohms to make 10 ohms

[petemoore]

Edit...100 divided by 10 = 10 [the equation I see], but the DMM Sees All !!!

[corbs]

http://www.electronics2000.co.uk/calc/

watch out: pop-up city (unless you have a good web browser like firefox )

[MartyMart]

Thanks chaps, i was thinking that each equal parallel resistor was halving
the value !!
I need to buy a few 1Watt resistors then !

Cheers,
Marty.

[Mark Hammer]

I think so.  Bad enough finding space for a single parallel 1/4W.  Stuffing over a half dozen 1W resistors onto the board starts to get just plain clumsy.

Incidentally, keep in mind that the wattage is for heat dissipation.  If you have some suitable values in the 1/4W or 1/2W range, you can parallel them up and the heat dissipation will be divided up amongst them.  So, a trio of 33R-1/4W resistors will have a parallel resistance of 11 ohms, and a combined heat dissipation capability of 3/4W.  This is, in fact, how one can stick a bunch of 30W Celestions in a stack powered by a 100W Marshall, and still have functioning speakers at the end of the day: the heat dissipation is divided among components that individually are not up to the task, but in ARE when in combination.

Having said that, unless it involves a 2hr drive and $20 worth of gas, maybe it's time to pick up a few common 1W values.

[MartyMart]

Mark - "You're a Star"  
I was just about to ask "can I use 4X47 Ohm 1/4 watters" .... so I have,
which gives me about 11.7 Ohms, with 1 watt  !!!   close enough

Cheers,
Marty.