For the series resistor connection:
Given a series connection of EQUAL valued resistors, the power dissipated
is shared EQUALLY among them.
Given a series connection of UNEQUAL valued resistors, the power dissipated
in EACH resistor is proportional to its resistance. (I^^2R, where I^^2 is
the same for all elements in a series circuit.)
Thus, a series combination of ten 5 ohm 1-watt resistors would be rated ten
watts.
A 10 ohm 1-watt resistor and a 20 ohm 1-watt resistor in series would
together be rated 1.5 watts. In this case, the limit is determined by the
20 ohm resistor; the 10 ohm resistor runs cooler at half the power
dissipation.
Don't forget that you should not expect a resistor to be happy dissipating
its full power rating. It will get hot enough to burn a circuit board.
Improving the rate of heat transfer away from the resistors improves the
power handling capability of the dummy load. The usual ways to do this are
large heat sinks and heat-conducting fluids (oil).
Since most leaded 'non-inductive' resistors are slightly inductive at HF
frequencies, the best combination of resistors for an HF dummy load is a
parallel arrangement, which reduces the inductance proportionally to the
number of resistors. (Remember calculating parallel inductance?) This
serendipitous result can be nullified if the connections to the resistors
have too much inductance, so use wide, short connections.
For a good dummy load:
Obtain a number of equal-valued non-inductive resistors. Mount them with
minimum length leads between two slabs of PC board material (low inductance
interconnection). Space the resistors apart about half a resistor
diameter. Use short copper strap to connect them to a coax connector.
Mount the whole assembly inside an empty paint can (You can purchase
unused paint cans from a paint store). Mount the coax connector to the
lid. Fill the can will mineral oil (Helps conduct the heat away from the
resistors).
Mike K1MG
----------
> From: Marty Watt <mdwatt@usit.net>
> To: Low Power Amateur Radio Discussion <qrp-l@Lehigh.EDU>
> Subject: Re: Dumb Dummy Qestion
> Date: Tuesday, February 18, 1997 8:01 PM
>
> On Tue, 18 Feb 1997 06:37:33 GMT, mdwatt@usit.net (Marty Watt)
> wrote:
>
> >Dan, N7VE, remarked that the power would be the sum of the power
> >ratings individually (in parallel). I would assume that series
> >resistance power ratings would be that of the lowest resistor in the
> >series?
>
> I have gotten an interesting varitey of responses to this query.
>
> Some say the lowest power rating would be the max input. Others say
> the power capability is multiplied for resistors in series (i.e., 2
> 10W resistors in series could handle 20 watts, 4 10W resistors could
> handle 40 watts, etc.).
>
> This question appears moderately important, and I think the list
> would benefit from a public discussion, given the variety of
> responses.
>
> RS does sell an 8 Ohm, non-inductive, 20W resistor. If I put 6 of
> these in series, does that mean it would handle 20W or 120W? 10W or
> 60W (half of calculated power under the various methods)?
>
>
> 72 es 73 de
> Marty, KM7W (x-KN4BH)
> ____________________________________________________________________
> Jackson, Tennessee e-mail: mdwatt@usit.net
> http://www.public.usit.net/mdwatt
> "The Curmudgeon's Corner"
> ARRL VE - QRP ARCI #7514 - QRP-L #953 - AK/QRP #098 - Grid Sq EM55oq
> ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
>
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