I am replacing a transformer - Page 1
Forum section: electronics / beginners

author: Hextejas
author_rank: Frequent Contributor
timestamp: February 25, 2024, 03:53:23 pm
content: in a desk lamp and the specs are not quite the same so I am wondering if I am going to get into trouble.Old device Input: 120VAC 60 Hz (RED)New device Input: AC100V~130V 50/60HzOld Device Output: 12VAC 1.67A20VA(BLUE)New Device Output: AC1260W`The 20VA vs 60W is what I am not sure of.thanksOh yeah, there are 2 red wires and 2 white wires both sets on the same side of the xformer.

author: Philip S Williams
author_rank: Newbie
timestamp: February 25, 2024, 04:23:07 pm
content: Hextejas - You should be fine, if I have correctly understood what you have written.The new transformer, apparently rated to handle a 60 Watt load, should have a greater power handling than the 20VA transformer being replaced. Technically the "60W" rating indicated the designed load for the transformer where as the "VA" rating is the "Volts times Amps" the transformer is designed to handle - with a purely resistive load the two are the same - there can be other conditions where the two measures would not be the same.Since the new device says 50/60Hz you should be fine (assuming you are using it in a 50 or 60Hz mains power feed) - If the original had been rated for 50Hz and the new one 60Hz it would not have been suitable as a transformer designed for use on 60Hz only is likely to overheat if used on 50Hz.In terms of the "2 red wires and 2 white wires" - the key question is whether you have been able to identify which wires are the primary (120V) and which are the secondary (12V) - you definitely don't want to connect them the wrong way round (bang!).

author: tooki
author_rank: Super Contributor
timestamp: February 25, 2024, 04:27:35 pm
content: There is one potential problem, which you may just need to test after assembly: how much higher the actual voltage is.Transformer voltages are specified under full load, but under less load, the voltage is higher. So by having a transformer with 3x the power, assuming your load is unchanged, then you’re loading the new transformer to a much smaller percentage of full load.If you’re post-regulating, this is likely irrelevant. But if it means you’re running a lamp at, say, 13V instead of 12V, it could reduce its life.

author: tooki
author_rank: Super Contributor
timestamp: February 25, 2024, 04:28:59 pm
content: Quote from: Hextejas on February 25, 2024, 03:53:23 pmThere are no standards for transformer winding colors. Check the datasheet.

author: Hextejas
author_rank: Frequent Contributor
timestamp: February 25, 2024, 06:05:02 pm
content: Regretfully, I cant find a datasheet. Maybe the sticker on the device is it. notice the110V ---Oin the upper right .I am guessing that means that the 110V is the red wires.And similar in the lower right is 12V-----O in black.There is actually no black wires, only white and red.Soooo, I could wire up my 120V to the red and see if there is smoke.

author: johansen
author_rank: Super Contributor
timestamp: February 25, 2024, 06:05:13 pm
content: A 60 watt 12v transformer will be at least 13vac at no load, maybe 14. The 20 watt light bulb will be ok.

author: tooki
author_rank: Super Contributor
timestamp: February 25, 2024, 06:24:35 pm
content: Quote from: johansen on February 25, 2024, 06:05:13 pmRunning a 12V lamp at 14V could significantly reduce its lifespan.Not that it matters here since it turns out it’s actually a power supply, not an actual transformer.

author: tooki
author_rank: Super Contributor
timestamp: February 25, 2024, 06:26:21 pm
content: Quote from: Hextejas on February 25, 2024, 06:05:02 pmAh! “Electronic transformer” means it’s not a transformer at all, it’s actually a power supply. So the output voltage under low load shouldn’t be an issue at all.Yes, the diagram is telling you to use the red wires for the input and the white for the output.Don’t fret the 110 vs 120V — the nominal voltage has been 120V for the better part of a century, but many still write 110V out of habit. Either way it’ll be within its tolerances.

author: Zero999
author_rank: Super Contributor
timestamp: February 25, 2024, 06:46:46 pm
content: Quote from: tooki on February 25, 2024, 04:27:35 pmIt's true lightly loading a transformer will result in a higher secondary voltage, than given on the name plate, but larger transformers have better regulation and have an unloaded voltage, closer to the one specified, so it might not be that bad. It's definitely a good idea to check the voltage, just in case. A series resistor is the easiest fix, if the voltage is too high.I would also advise the original poster changes the lamp to LED. A decent LED will have a current regulator and should be able to take a slightly higher voltage, without damage.

author: Hextejas
author_rank: Frequent Contributor
timestamp: February 26, 2024, 04:50:35 pm
content: the lamp is an LED

author: IanB
author_rank: Super Contributor
timestamp: February 26, 2024, 04:56:26 pm
content: Quote from: Hextejas on February 26, 2024, 04:50:35 pmThe device in the picture is an electronic "transformer" designed for halogen bulbs. These devices in general are not suitable for anything other than a halogen lamp. It is not likely suitable for an LED.

author: mariush
author_rank: Super Contributor
timestamp: February 26, 2024, 05:10:32 pm
content: You can get proper transformers from around 15$ (if you get 1) :https://www.digikey.com/en/products/filter/power-transformers/164?s=N4IgjCBcoKwBxyqAxlAZgQwDYGcCmANCAPZQDa4ALAMxwDsAbCEWDDAEwPvPgwCc7OtR5g4lBpUoi%2BlOA2Es%2BQpT3ZgB61Z1aIi7WnUOrKMMOyl66MBnW576DCSAC6RAA4AXKCADKHgE4AlgB2AOYgAL4sDGB0SCCokJi4hCTkILSUAAyUfDyZsjAu7l6QvgEh4RFRIEzQIIEAJt4AtGBZECXePB4Anm543hg4qNVAABest to get one with wires preinstalled...Alternatively, You can probably replace the whole transformer with a 15-16.5v wallwart / laptop adapter style power supply, something that can supply 1.5A or more current.Your lamp probably used classic transformer because of its weight, to keep the lamp from flipping over... internally, most likely there's a bridge rectifier converting the 12v AC voltage to around 15.5v DC and from there either a led driver or some current limiting resistors will send this voltage to your leds.So if you connect 15-16.5v DC where the AC wires are.. the bridge rectifier will just pass through DC voltage and work just fine.

author: tunk
author_rank: Super Contributor
timestamp: February 26, 2024, 05:15:21 pm
content: And it says "output:AC12V", the LEDs may not like the AC.Test it with a multimeter and see if it's AC or DC.

author: Zero999
author_rank: Super Contributor
timestamp: February 26, 2024, 07:50:45 pm
content: Quote from: IanB on February 26, 2024, 04:56:26 pmThere are halogen replacement LEDs which are specifically designed to work off an electronic transformer. I installed them in my parents' bathroom, which has 12V fittings, each with an electronic transformer, about 10 years ago and they're still going strong.I suspect that's not the case for the original poster.Quote from: Hextejas on February 26, 2024, 04:50:35 pmWhat sort of LED? Is it a halogen replacement LED?Are you having problems getting it to work?As mentioned above, the replacement transformer you've selected is an electronic transformer. Rather than a big heavy low frequency mains transformer, it uses a high frequency oscillator to drive a small, ferrite cored transformer. The output will be a high frequency, typically 20kHz to 100kHz, modulated by double the mains frequency. The LED must be specially designed to work with an electronic transformer. If you can't find any data on the LED, then it's safe to assume it needs 50Hz to 60Hz to work and is incompatible with your electronic transformer. If replacing the LED with a suitable one isn't an option, then you need to find a mains frequency transformer.One helpful hint in looking for mains frequency transformers, is to avoid anything which is designed for halogen lighting and if you do find a lighting transformer, check it's toroidal and the weight. Electronic transformers are much lighter. A 40VA mains toroidal transformer will weigh about 500g. If it's much lighter, then it's definitely an electronic transformer.

author: Hextejas
author_rank: Frequent Contributor
timestamp: February 28, 2024, 04:00:35 pm
content: My word, what an exercise. My wife got me a desk lamp at the Goodwill store for $5.00 and the xformer burned out so I thought that I would try and repair it as a training project.I really, really, liked this lamp.Now I am tempted to throw the beast away. It sounds as if every replacement part that I have bought will not work.Sheesh !!!

author: ejeffrey
author_rank: Super Contributor
timestamp: February 28, 2024, 04:56:21 pm
content: Quote from: IanB on February 26, 2024, 04:56:26 pmSure it is. For instance, if it is for MR16 base lamps, LED replacements are widely available, they work fine. I have several using both electronic transformers like this and traditional magnetic transformers.Does / did the LED bulb work with the original transformer? If so it will almost certainly work with the new one. If you tried to replace a halogen bulb with an LED and were told the LED wouldn't work with the old transformer, it's likely the new one won't either. That would likely only be the case if you did something more DIY than buying replacement bulb of the same style.Is the lamp dimmable? That does make it more complicated. Electronic vs. magnetic transformers work best with different dimmers.

author: themadhippy
author_rank: Super Contributor
timestamp: February 28, 2024, 05:10:27 pm
content: Quotenot always,some electronic trannys have a minimum load and if the load aint enough the dam thing thinks its a strobe.

author: IanB
author_rank: Super Contributor
timestamp: February 28, 2024, 05:37:48 pm
content: Quote from: ejeffrey on February 28, 2024, 04:56:21 pmCool, thanks for the correction. I hadn't really thought about LED replacements for halogen capsule lamps. I was thinking more along the lines of desk lamps with integrated LEDs rather than replaceable bulbs.I certainly see how MR16 replacements should work with electronic transformers.

author: IanB
author_rank: Super Contributor
timestamp: February 28, 2024, 05:39:52 pm
content: Quote from: Hextejas on February 28, 2024, 04:00:35 pmThe bulb you pictured looks like it should work with the transformer you pictured. Sorry for any confusion I might have caused.

author: Zero999
author_rank: Super Contributor
timestamp: February 28, 2024, 06:26:16 pm
content: Quote from: themadhippy on February 28, 2024, 05:10:27 pmThat's not a problem with the LED lamps which are designed for electronic transformers. The circuitry has some tricks which enable it to get round the minimum load requirement.Quote from: Hextejas on February 28, 2024, 04:00:35 pmForget the transformer. Buy a 12V DC power supply, with sufficient current capacity to power the LED bulb. You should be able to pick one up from a charity shop.