In TELECOM Digest V23 #500, TELECOM Digest Editor <ptownson>
wrote (in part):
> I have two transformers: One is Radio Shack, 'clean' DC output, 13.8
> Volts at 3 Amps.
> The other transformer is an Intermatic, model is 'Malibu 88-T' and
> it does output of 12 Volts and 1 Amp. ... Are these two power
> supplies interchangeable...?
The Radio Shack power supply is DC [direct current] at 13.8 V (the
output of a "12-volt" automotive charging system).
The Intermatic Malibu light power pack is simply an AC transformer
with a timer on the primary (line voltage) side. I think the "1 Amp"
rating you saw was the line side draw (120 VAC 1 A).
> How do you calculate volts/amps to watts?
For DC, it's pretty simple for simple loads on a linear power supply,
which is almost certainly what the RS unit is: watts = volts x
AC calculations are trickier -- that's likely why Intermatic gives
their power consumption figures in watts (the 88-T is rated at 88
For a rough calculation, though, you start with 120 VAC at 1 amp (120
volt-= amps) supplying a 10:1 power transformer (120:12 volts) with a
typical transformer power factor of about 0.75 (0.733 in this case):
120 V x 1 A = 120 VA x 0.7333 PF 88 watts (output)
When you calculate how many Malibu lights the 88-T will handle, power
factor is practically 1.0, since lamp filaments are almost pure
I'm not going to further flaunt how long it's been since I've done any
real AC calculations ...
Paul A Lee Sr Telecom Engineer <email@example.com>
Rite Aid Corporation HL-IS-COM (Telecomm) V: +1 717 730-8355
30 Hunter Lane, Camp Hill, PA 17011-2410 F: +1 717 975-3789
P.O. Box 3165, Harrisburg, PA 17105-3165 W: +1 717 805-6208
[TELECOM Digest Editor's Note: My problem is *something* caused a
short on the line which caused the Malibu 88-T to get fried. That
something was probably the all-day drizzle/rain we had the other day
in the one spot where the cord strung to the various lights was not
properly protected. I will endeavor to find/correct that problem, but,
wouldn't it be good to put an external fuse in the line to prevent
that from happening in the future? I am thinking of one of the little
glass fuses and fuse holders you can wire in series with the line.
I *assume* (correct me as needed) such a fuse in the line would stop
any short from reaching the Intermatic power supply. I would rather,
next time, blow one of the little five in a box for two dollars fuses
rather than be in bed asleep when the short develops (if in fact I
get it cured) and the Intermatic silently fries away all night until
I wake up, or worse, have my backyard shed burn down (unlikely, I
know). What are your thoughts? What *size* fuse should I use of the
little glass ones that fit in a holder from RS? Not to small to not
allow the little lights to work correctly, but not too big so it won't
do a prompt job of stopping any shorts. Ideas? PAT]