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Re: Wire-wound resistors as dummy test load



Original poster: "davep by way of Terry Fritz <twftesla-at-qwest-dot-net>" <davep-at-quik-dot-com>

>>Sorry, nope, but in series, each resistor will still be able to
>>dissipate its own 200 watts, quite independently of the other three, so
>>four in series will give an 800 ohm, 800 watt resistor, with maximum
>>dissipation when you put 800 Volts at one amp through the string.  A
>>single resistor would dissipate 200 watts when carrying 200 volts at one
>>amp.

	Correct.

 
> The wattage rating of resistors in series is only as high as the
> weakest link which in this case is 200 watts.

	indeed, 200 watts per resistor.  4 200 watt resistors
	will handle 800W, more or less forever.  (assuming
	nominal current, cooling.)

> Each resistor is still being asked to pass an 800 watt total load

	Each resistor passes the same CURRENT.
	Power (watts) is different.
	With no specification as to voltage, need to do some
	numbers.

> no matter what each is dissipating separately and it won't work

> for long.

	It will work more or less forever.

> If you splice 4 pieces of wire together in series they still will

> only handle what a single length is rated for in amperes.

	Just So.  IN AMPERES.  Watts are different.

> In parallel the 200 watt resistors could handle 800 watts because

> each resistor handles 25% of the load.

	Indeed.  Just as they will in series.  Each will dissipate
	25%.

=============================================

Numerical example:
	I disrecall if the resistance was specified.  I'll
	Assume 100 ohms.  (it makes no difference, simply
	varies the limiting actual currents and voltages.
	Watch the maths, correct as needed.

		200W, 100 ohm

		200W=(v**2)/R == V**2/100

	rearranging and computing:
		20,000 == V**2
		Vmax == [sqrt]20,000 = 141.4 V

		Imax = V/R ==141.4/100 ==1.41 amps.

	(Checking:
		Pr = V*I =141.4*1.414 =199.xxxxW.

	Now.  The same _current_ (series case) flows thru
	each resistor, and each dissipates ~200W.  4 of
	these resistors will dissipate 800W 'forever'.
	Each will dissipate its 200W.

	(assuming that 566 V be available... and appropriate
	cooling....)

	best
	dwp

...the net of a million lies...
	Vernor Vinge
There are Many Web Sites which Say Many Things.
	-me