Power supply for 12v lamp - Page 1
Forum section: electronics / beginners

author: Jake87
author_rank: Newbie
timestamp: December 11, 2022, 03:13:23 pm
content: I have a lamp that uses 5x10w g4 bulbs at 12v. Years ago the inline dimmer failed and have since tried two different inline dimmer switches and they both blow the bulbs as soon as they switch on. I tried an adjustable 12v power supply as shown in the picture but at max setting, there is not enough to power all five bulbs. I'm really struggling to find something I can plug into 240v mains without it blowing the bulbs immediately. I've spent more than the lamp is worth in monetary value and more time than I'd like to admit trying to find a solution but I know very little about electronics so if someone here could help it would be much appreciated

author: wasedadoc
author_rank: Super Contributor
timestamp: December 11, 2022, 03:31:13 pm
content: A 10 Watt 12 Volt bulb takes 10/12 (10 divided by 12) Amps. Five of them take 50/12 Amps which is more than 4 Amps. Your power supply label clearly shows 600mA (=0.6 Amps). Obviously not going to work.Without knowing anything about the original "in-line dimmer" it is difficult to advise. I guess it had a 240 to 12 volt transformer in addition to dimming circuitry. You cannot replace that with a dimmer which is intended for use with 240 volt bulbs.

author: Jake87
author_rank: Newbie
timestamp: December 11, 2022, 04:01:58 pm
content: Quote from: wasedadoc on December 11, 2022, 03:31:13 pmThanks for the reply. You are correct, the original dimmer did both jobs, there was not a separate transformer. I found an adjustable power supply with an ac240v input and dc9v-24v 3a output, would that work? Thanks

author: mariush
author_rank: Super Contributor
timestamp: December 11, 2022, 05:30:02 pm
content: The easiest and safest would probably be to repurpose a 12v 47-65w laptop adapter as a power supply.Then, you could have a small dc-dc converter hidden inside the lamp or on the cable in a nice box to convert 12v to some voltage set by adjusting a potentiometer.Here's one such example :https://www.ebay.com/itm/272834659085- desolder the potentiometer and replace it with an external one or one mounted on the lamp.Another potential idea would be to replace the 10w incandescent / halogen / whatever G4 bulbs with replacement bulbs that use LEDs, to get less power consumption.Here's one of the first results on Amazon :https://www.amazon.com/BAILIDA-Halogen-Replacement-Daylight-5-Pack/dp/B07C69Y7MS/The bulbs are 1.5W ones, so 5 of them would make the lamp consume less than 10 watts, so you could power the whole lamp with a cheap 12v 1A wallwart adapter.The downside is that it's unlikely you'd be able to use a dc-dc converter to change brightness by adjusting the voltage - those leds will probably stop working at around 10v or so, so only 10-12v is kind of limited range.

author: BeBuLamar
author_rank: Super Contributor
timestamp: December 11, 2022, 05:52:09 pm
content: Quote from: Jake87 on December 11, 2022, 04:01:58 pmThat might work but I wouldn't use it because it's rated at 3A and your 5 lights would draw 4A. You would draw more current than the power supply is rated for and it might work but not something I want to do. In fact I would get a power supply with the rating of at least 5A.

author: inse
author_rank: Super Contributor
timestamp: December 11, 2022, 06:11:24 pm
content: As switching power supplies don’t like overloading and your halogen lamps will draw a considerably higher inrush current, even the 5A version may be problematic.

author: CaptDon
author_rank: Super Contributor
timestamp: December 11, 2022, 08:04:11 pm
content: One of the posters calculated your current draw at OVER 4 AMPS, and another mentioned the high inrush current. NO, your 3 amp supply will fail miserably. You probably need a 240vac to 12vac transformer and a special version of a dimmer that normally controls ceiling fan speed since they are designed to control inductive loads. The 240 vac to 12vac transformer could be one of the easily found on the used market 'Control Transformers' with dual 120 vac primaries and dual 12 vac secondaries. Series the primaries for 240 and parallel the secondaries for higher current at 12 vac. Be careful with phasing. A 50va may be big enough but very close to its limit. I would probably go with a 100va. You may find a 0-20 volt power supply with a 10 amp capacity. You could also use a 50va or 100va variac with the control transformer I mentioned above. Cheers!!

author: wasedadoc
author_rank: Super Contributor
timestamp: December 11, 2022, 08:44:47 pm
content: When making suggestions for the OP please do not overlook what was included in the first post: "... but I know very little about electronics ..".

author: mikerj
author_rank: Super Contributor
timestamp: December 11, 2022, 09:52:40 pm
content: "Dimmable transformers" are easily available for this type of application. They can be driven from a conventional 240v light dimmer, and are designed for driving 12V halogen lamps. You will need at least a 60 Watt device e.g.https://internationallamps.co.uk/product/electronic-dimmable-transformer-60va/

author: themadhippy
author_rank: Super Contributor
timestamp: December 11, 2022, 10:39:30 pm
content: Quotecan they? looking at the specsQuoteyour conventional 240v light dimmer isn't a resistive dimmer,

author: IanB
author_rank: Super Contributor
timestamp: December 11, 2022, 11:33:34 pm
content: Quote from: themadhippy on December 11, 2022, 10:39:30 pmAt the same time, domestic resistive light dimmers do not exist, so there is probably some confusion in the use of words somewhere.However, clearly what this application needs is a standard 12 V halogen transformer, 60 W or greater, electronic or magnetic. There may be halogen transformers with a built-in dimmer control, or it could be a "dimmable" transformer that can be used with a standard light dimmer switch.I am amazed that at the very top of this thread someone didn't say "halogen transformer". These things are standard, off-the-shelf items.

author: TimFox
author_rank: Super Contributor
timestamp: December 12, 2022, 12:02:08 am
content: Quote from: themadhippy on December 11, 2022, 10:39:30 pmI assume that the spec meant "dimmable with standard dimmers for resistive loads".Getting syntax right in a foreign language can be difficult.

author: mikerj
author_rank: Super Contributor
timestamp: December 12, 2022, 11:40:13 am
content: Quote from: TimFox on December 12, 2022, 12:02:08 amYes, this is exactly what it means. These "transformers" can be driven by a conventional phase controlled dimmers and are used to add dimming capability to existing 12v incandescent lighting systems.

author: Jake87
author_rank: Newbie
timestamp: December 12, 2022, 02:48:07 pm
content: Thanks for the replies although some of them may as well have been written in hieroglyphs. The reason i suggested a 240v to 9-24v at 3a dimmer is, if I powered the bulbs at the minimum setting on the dimmer and draw half the max power of the bulbs, for example, then it would draw just under 2amps according to an online calculator I used. In all the years we had the lamp it was only used at the minimum setting - more for decoration than proving light to the room.

author: IanB
author_rank: Super Contributor
timestamp: December 12, 2022, 03:00:44 pm
content: Quote from: Jake87 on December 12, 2022, 02:48:07 pmWhat you want is something like this:https://www.amazon.co.uk/YAYZA-Constant-Dimmable-Switching-Transformer/dp/B081QN8STYYou want a 12 V DC output. Since you have 5 x 10 W bulbs you need at least 50 W output. Since you want it dimmable you need it to say "TRIAC dimmable" (this means you can control it with a standard light dimmer). Since you want to run G4 halogen bulbs you need it to say "constant voltage" (not constant current).These days the label may say "LED driver" rather than "halogen driver". As long as it is a dimmable, 12 V, constant voltage output with sufficient power, that doesn't matter. It will work.

author: Jake87
author_rank: Newbie
timestamp: December 12, 2022, 04:45:31 pm
content: Quote from: IanB on December 12, 2022, 03:00:44 pmIn UK, we'd only use something like that for led spotlights in ceilings and that transformer would be tucked away above the ceiling never to be seen. I believe I've found a solution, albeit not as straight forward as the original dimmer/transformerhttps://www.ebay.co.uk/itm/163964665683?mkcid=16&mkevt=1&mkrid=711-127632-2357-0&ssspo=Ko4YDz65Spa&sssrc=2349624&ssuid=QoVPVHRUQ8O&var=463578046796&widget_ver=artemis&media=COPYUse the 6a 72w model to convert 240 to 12vAnd run a simple 12v inline dimmer switch between the above, and the lamp fixture

author: IanB
author_rank: Super Contributor
timestamp: December 12, 2022, 05:12:21 pm
content: Yes, that power supply would work. But I'm not sure what solution you have for the simple 12 V inline dimmer switch?Also, when you say lamp "fixture", that word implies a fixed installation, unlike (say) a table lamp. If it is a fixture, the item I linked would be appropriate. If it is something like a table lamp, then your linked item would be right.

author: themadhippy
author_rank: Super Contributor
timestamp: December 12, 2022, 05:34:22 pm
content: Quoteplenty to chose from,for examplehttps://lightingparts.co.uk/relco-low-voltage-12v-inline-dimmer-4991318/one thing to watch out for,is some electronic transformers give out high voltage spikes that can destroy dimmers,especially at the cheaper end of the market.

author: Jake87
author_rank: Newbie
timestamp: December 12, 2022, 05:51:06 pm
content: Quote from: themadhippy on December 12, 2022, 05:34:22 pmThat might explain why the original dimmer failed. It would randomly pulse the bulbs from dim to bright. Thanks

author: IanB
author_rank: Super Contributor
timestamp: December 12, 2022, 06:36:05 pm
content: Quote from: themadhippy on December 12, 2022, 05:34:22 pmThe product page for that dimmer is strange. The text clearly says "Use only with 12 V systems. Using with mains voltages will destroy the device." Yet the photos clearly show a mains cable connected to the input with blue and brown conductors.

author: Jake87
author_rank: Newbie
timestamp: December 12, 2022, 07:07:00 pm
content: Quote from: IanB on December 12, 2022, 06:36:05 pmIt also says input voltage 240v on the spec. Does it look like there's a transformer in there?

author: IanB
author_rank: Super Contributor
timestamp: December 12, 2022, 07:16:11 pm
content: Quote from: Jake87 on December 12, 2022, 07:07:00 pmNo, it doesn't look like it is supposed to take a 240 V input. There is no visible isolation or separation between mains and low voltage and no double-insulated transformer. I think it is meant to take 12 V DC on the input and do PWM dimming on the output.That 240 V on the description seems to be wrong.

author: themadhippy
author_rank: Super Contributor
timestamp: December 12, 2022, 09:23:51 pm
content: Quotenope it shows an AC input,its not uncommon to use "mains " cable for low voltage lighting circuits,but any decent sparky will flag the cable