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Author Topic: 24VAC transformer - will one of a higher amperage than needed hurt the apppliance? Leo Enticknap Film God Posts: 7474 From: Loma Linda, CA Registered: Jul 2000 posted 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.  |  IP: Logged Louis Bornwasser Film God Posts: 4441 From: prospect ky usa Registered: Mar 2005 posted 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  |  IP: Logged Leo Enticknap Film God Posts: 7474 From: Loma Linda, CA Registered: Jul 2000 posted 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.  |  IP: Logged Frank Cox Film God Posts: 2234 From: Melville Saskatchewan Canada Registered: Apr 2011 posted 11-27-2013 12:48 PM                        If someone tries to steal your lawn, now you're all set!  |  IP: Logged Paul H. Rayton Expert Film Handler Posts: 210 From: Los Angeles, CA , USA Registered: Aug 2003 posted 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!"  |  IP: Logged Gordon McLeod Film God Posts: 9532 From: Toronto Ontario Canada Registered: Jun 99 posted 11-27-2013 10:05 PM                     Just remember that a transformer under low load will have a higher output voltage than at full load  |  IP: Logged Leo Enticknap Film God Posts: 7474 From: Loma Linda, CA Registered: Jul 2000 posted 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!  |  IP: Logged

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