[QRO] Thank you Mr. Lay

[Robert Lay (W9DMK)]

Dear Chad,

A careful reading of your posting seems to suggest that you will be able to
do everything you want to do with the bucket of oil, although for the life
of me, I have no idea what is a "vdc load". If you mean a load that is
intended for use with dc voltages, then that is what you should say.

The design for any dummy load should consider two things:
1)   the maximum instantaneous power level
2)    the long term average power level, where "long term"  is dependent
upon the thermal time constant of the dummy load system.

The max. instantaneous power level, along with the impedance will determine
which stress is the dominant stress - voltage or current. The dummy load
must be able to survive that stress.

The dummy load system will have a thermal time constant, which is a measure
of how rapidly the temperature approaches the steady-state temperature of
the system. The shorter that time constant, the quicker you reach steady
state.

Generally speaking, the life of the dummy load will depend on how it is
stressed. Its deterioration with use will depend on the voltage and current
stresses at max. instantaneous power, but it will also be stressed by the
steady state temperatures at which it operates.

There are very few sources of information on the stress limits for the
materials that might be used for a dummy load. Two sources of reliable
information are the manufacturers of the oil used and the resistance
elements used. If you seek technical support information from the suppliers
of transformer oil and the suppliers of heavy duty resistors, you may find
what you need. However, just purchasing a supply of nichrome wire and using
whatever oil you find readily available  will provide you with the basics
for running your own experiments. The important thing is to conduct your
experiments in a professional manner, taking accurate data, and taking
nothing for granted.

For what you are intending to do, the most likely error would be in
underestimating the temperature rise of the dummy load over time. I
recommend that you put a thermometer into the oil bath so that you can keep
an eye on how high it is getting. Obviously, you want to stay about 20 deg
centrigrade below the boiling point of the oil, and make sure that whatever
container you use for it is vented. Have you ever seen a pole peg explode?

73 de W9DMK (Bob Lay) in Dahlgren, Virgina
E-mail: w9dmk@crosslink.net
Web: http://www.qsl.net/w9dmk
www.geocities.com/w9dmk/W9DMK.html
www.fortunecity.com/meltingpot/albania/784