Re: dummy load for DC or AC

In article <heCdnaGDOYeazGLXnZ2dnUVZ8uGdnZ2d@xxxxxxxxx>,
wrightsaerials@xxxxxxx says...

Does anyone know where I can buy a ready made dummy load for testing AC and
DC power supplies? I would need to vary the resistance sufficiently to test
everything from 12V 0.5A units to 55V 5A units. This would be a 24 hour soak
test. I can check the amperage and ripple, but I need a convenient
switchable load. At present I'm making up the resistance value with various
wirewound things, old droppers from old tellys, etc, but it isn't very
convenient. I envisage something with rotary decade switches and a big fan.
Any ideas?


Ignoring, for the moment, 'ready made' and Ian's offer of high power
rheostats, how about something like this:

Get 10 x 100ohm 30Watt resistors, assuming reasonable availability
(which I haven't checked) or 20 x 200ohm 15Watt resistors or 30 x 300ohm
10Watt resistors, whatever is easily available. By paralleling the lower
powered resistors, you should end up with 100 ohm, or possibly 110 ohm,
resistors of suitable rating.

You will also need 9 x SPST toggle switches, rated at 500mA or higher
(1A would be fine).

As wire wound resistors are intended for use in free air, construction
in an enclosure with convection cooling should avoid the need for a fan.

Wire one 100 ohm resistor directly across the supply, then the remaining
nine, via the SPDT switches, in parallel.

Monitor the current and then switch in a suitable number of parallel
resistors to make up the desired load. The following table shows current
into the load by voltage and the number of load resistors used - note
that, even with your worst case 55V supply, no switch is required to
handle more than about half an amp. The column at the far right shows
the power dissipated in each 100 ohm resistor for each applied voltage
(must be viewed, of course, using a fixed font.)

R = 100 ohms

V/n 1 2 3 4 5 6 7 8 9 10 W/R
12 0.1 0.2 0.4 0.5 0.6 0.7 0.8 1.0 1.1 1.2 1.4
15 0.2 0.3 0.5 0.6 0.8 0.9 1.1 1.2 1.4 1.5 2.3
20 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 4.0
25 0.3 0.5 0.8 1.0 1.3 1.5 1.8 2.0 2.3 2.5 6.3
30 0.3 0.6 0.9 1.2 1.5 1.8 2.1 2.4 2.7 3.0 9.0
35 0.4 0.7 1.1 1.4 1.8 2.1 2.5 2.8 3.2 3.5 12.3
40 0.4 0.8 1.2 1.6 2.0 2.4 2.8 3.2 3.6 4.0 16.0
45 0.5 0.9 1.4 1.8 2.3 2.7 3.2 3.6 4.1 4.5 20.3
50 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 25.0
55 0.6 1.1 1.7 2.2 2.8 3.3 3.9 4.4 5.0 5.5 30.3

If you find 220 and 330 ohm (preferred values) for the parallel
scenario, you will end up with 110 ohm resistors.

These still cover the range you originally stated, with slightly lower
dissipation:

R = 110 ohms

V/n 1 2 3 4 5 6 7 8 9 10 W/R
12 0.1 0.2 0.3 0.4 0.5 0.7 0.8 0.9 1.0 1.1 1.3
15 0.1 0.3 0.4 0.5 0.7 0.8 1.0 1.1 1.2 1.4 2.0
20 0.2 0.4 0.5 0.7 0.9 1.1 1.3 1.5 1.6 1.8 3.6
25 0.2 0.5 0.7 0.9 1.1 1.4 1.6 1.8 2.0 2.3 5.7
30 0.3 0.5 0.8 1.1 1.4 1.6 1.9 2.2 2.5 2.7 8.2
35 0.3 0.6 1.0 1.3 1.6 1.9 2.2 2.5 2.9 3.2 11.1
40 0.4 0.7 1.1 1.5 1.8 2.2 2.5 2.9 3.3 3.6 14.5
45 0.4 0.8 1.2 1.6 2.0 2.5 2.9 3.3 3.7 4.1 18.4
50 0.5 0.9 1.4 1.8 2.3 2.7 3.2 3.6 4.1 4.5 22.7
55 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 27.5

You say that you already monitor the current, so no built-in meter is
needed. Operation is simple, you don't even need the table above - all
switches OFF; measure current; switch ON additional load as required.

This meets your original specification of 12V/500mA to 55V/5A but may
require a little fine tuning to suit your needs. E.g.: @ 12V, current is
only adjustable in 120mA steps (100ohm) or 110mA(110ohm).

For finer control, replace one of the the 100 ohm resistors with 2 x 200
(or 220) ohm 15W resistors and an extra switch. Current @ 12V can now be
adjusted in ~60mA steps.

Any use?

(You're welcome to the Excel spreadsheet I used to produce these tables,
if you want to play ...)
--

Terry
.

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