TELECOM Digest OnLine - Sorted: Re: Help Me Identify/Repair/Replace a Power Transformer

Re: Help Me Identify/Repair/Replace a Power Transformer

Jeff Spidle (
Thu, 21 Oct 2004 20:49:20 -0500

Pat, a 10 Amp fuse should be sufficient. 10 Amps * 12 Volts = 120
Watts. If you only have 88 watts of lights it leaves you with some
headroom for small surges and inrush current when you turn on the
lights. 10A fuses are common and inexpensive.

Jeff, KC9EII

<> wrote in message news:telecom23.504.14@

> 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
> amps.

> AC calculations are trickier -- that's likely why Intermatic gives
> their power consumption figures in watts (the 88-T is rated at 88
> watts).

> 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
> resistive load.

> 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 <>
> 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]

[TELECOM Digest Editor's Note: That is what someone else said to me
tonight on the phone. A 10 Amp fuse, wired in series on the output
side of the line should protect future Intermatic transformers in
the event of a short. It should hold up for the light string but
pop quickly in the event of a short. PAT]

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