Hi Alex:
> We use a linear (basically a transformer with a PTC thermistor) class II
> SELV O/P stand alone power supply rated 10Vac, 5A. One of the labs tested
> the O/P Isc with a direct application of the sc. The other lab tested the
> O/P Isc by applying a load of 8A. The results were therefore interpreted
> differently.
> 1. What is the correct method to test for a Limited Power source in
> determining what enclosure is required?
Limited Power Source is a combination of a number
of criteria:
output voltage;
output current;
output volt-amperes.
These are independent criteria.
Note that Isc is defined as:
"Maximum output current after 60 s of operation with
any non-capacitive load, including short-circuit."
Both labs are wrong.
If a power supply shuts down due to a short-circuit, then
it is necessary to determine the maximum current where the
power supply does not shut down. Typically, this is about
twice rated output. Since your power supply is rated 5 A,
then I would expect the overpower to take effect at about
10 A output.
When testing a power supply output for LPS, start at rated
load and gradually increase the load. If the output
exceeds 8 amps or any other criteria, then the output is
not LPS.
Then, introduce the single, worst-case fault in the
regulating circuit, and repeat the test. In your case,
this means shorting the PTC, in which case the output will
certainly exceed 8 amperes.
By the way, your output also fails the output volt-amperes
criteria. If the open-circuit output voltage is 10 volts,
then the maximum VA is 50, or 5 amps.
Since one lab only did the output short-circuit test, the
results are not correct.
Since the other lab did a single test at 8 amps, this is
more-or-less okay, but for the wrong reasons.
Neither lab did the VA test! The VA test would have proved
the unit failed at 5 A output.
Neither lab introduced a fault in the regulating network!
(But, the didn't need to since the unit failed under normal
operating conditions.)
Both labs are wrong, but one came up with the correct
determination. (Of course ONE had to be correct!) :-)
If you use a PTC, then your output need not necessarily be
considered "inherently limited." In this case, your
maximum output current is 100 amps and your maximum VA is
250, i.e., 25 amps (with the PTC shorted). The trick is
to determine if your test houses can class the PTC as an
"overcurrent protective device."
A colleague claims that very few test houses really know
how to do LPS testing. I concur, and your report confirms
this again.
Best regards,
Rich
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