Film-Tech Forum ARCHIVE: 24VAC transformer - will one of a higher amperage than needed hurt the apppliance?

This is topic 24VAC transformer - will one of a higher amperage than needed hurt the apppliance? in forum Film-Yak at Film-Tech Forum ARCHIVE.

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Posted by Leo Enticknap (Member # 534) on 11-26-2013, 05:52 PM:

Irritrol RD-900 backyard sprinkler timer/controller - started playing up last week, the symptoms being a flickering LED display and chattering solenoids when actuated. I discovered two faults. The power supply transformer's output was fluctuating between roughly 15-22 volts, not the 24 it should have been delivering, and a 220-microfarad capacitor on the motherboard was done (it just gave a steady resistance reading, not a decaying one after the meter probes were attached).

So I replaced the capacitor and the transformer, and the thing is now fixed and working perfectly (I'm guessing that as the transformer died, it took the capacitor with it). However, one little worry remains at the back of my mind. The original, deceased transformer unit was rated at 24v AC, 1.25 amps. The replacement (the only 24v AC one Radio Shack had) is 24v AC, 2 amps.

Will the higher current capability of the new transformer cause a problem, e.g. frying something on the motherboard over an extended time period, or will the controller simply take a lower current from the transformer than it could deliver if asked for, and remain happy? There's a 2-amp slow-blow fuse in the controller itself, which has not blown. I'm just a bit worried that I could be slow-nuking the thing. Many thanks in advance.

Posted by Louis Bornwasser (Member # 3063) on 11-26-2013, 06:22 PM:

Voltage must be the same. By using a higher current transformer, you are lowering the operating temperature on the transformer. This is good, conservative design. AC vs DC matter, as well as polarity on the DC. Otherwise, you are better off than before the failure. Louis

Posted by Leo Enticknap (Member # 534) on 11-27-2013, 08:21 AM:

Many thanks - much appreciated. The lady at Radio Shack said that these transformers are normally used to replace blown ones in burglar alarm systems, but the voltage and current type is the same.

Posted by Frank Cox (Member # 6258) on 11-27-2013, 12:48 PM:

If someone tries to steal your lawn, now you're all set!

Posted by Paul H. Rayton (Member # 1817) on 11-27-2013, 09:01 PM:

The rating of a power transformer is the theoretical maximum output it is designed for, meaning that if is is driving a load greater than that rated output, the transformer will be at risk of overheating or burning out. Using LESS than the rated amperage is very safe.

On the other hand, voltage output is what you do not want to raise very much. If you put a 2A/48V transformer on that line -- that is, one with a 50 volt output, and up to 2A load -- you very much run the risk of frying your sprinkler controller circuit, but the transformer itself would be fine. Kind of like putting 120v lightbulbs into a circuit that's 220v: "POOF!"

Posted by Gordon McLeod (Member # 33) on 11-27-2013, 10:05 PM:

Just remember that a transformer under low load will have a higher output voltage than at full load

Posted by Leo Enticknap (Member # 534) on 11-29-2013, 09:06 PM:

Many thanks. I've just checked the output from the transformer, and it's 26.79. According to this site (see picture at the bottom), anything from 24-27 is OK. So I'm currently close to the upper limit of OK (if the authors of this site are correct), but still within it. I will keep checking it every now and again, to make sure it doesn't go any higher, though it was hardly fluctuating at all when I put the meter on it just now.

I actually have a complete replacement, working RD-600 motherboard now (it was listed on Ebay as for spares or repair - got it for $7 and the fault turned out to be the same as with ours: one dead capacitor. That circuit design obviously kills them pretty quickly.), and so if the worst comes to the worst and the original one fails again, I'll swap it out and get a 1.25 amp transformer to power the replacement at that point. Should have done that in the first place. I was just impatient: Radio Shack had the 2 amp one in stock and ready to buy, whereas an actual OEM replacement would have taken a week or two to arrive.

And ironically, just as I finished repairing and reinstalling the whole sprinkler setup, the Inland Empire has had its first serious rain over the last week since I arrived here in July!