Low Voltage Transformer for the House - Page 1
Forum section: electronics / projects

author: SbassCaAz
author_rank: Newbie
timestamp: April 28, 2016, 11:29:33 pm
content: Hello,I have a project going on here at the house that I’d like some feedback on. One of the projects that I’m working on involves the low voltage transformer that is used for both the doorbell and address light. I started this project because I noticed that the address light just seemed way too warm. Plus, a couple of the plastic numbers have developed bubbles in them.The low voltage transformer is susposed to take an input of 120 VAC and drop it down to 16 VAC, 10 VA. When I measured the voltage at the address light, I got a meter reading of 18.2 VAC. When I talked with the manufacturer of the address light, it is not designed to take more than 16 VAC. So, I found the reason for the excess heat.So, I went ahead and replaced the transformer with a new one with the same specs. While I had the old transformer out, I measured the voltage at the source and it measured 122.1 VAC. After installing the new transformer, I measured the output, which is now 21.9 VAC. I’ve talked with a couple of people and they seemed to think this is normal (this made no sense to me). So, it appears that maybe the original transformer was not bad at all but that’s another issue to deal with.My problem is that I can’t have an output greater than 16 VAC (that’s the maximum voltage for both the doorbell and address light). So, using Ohm’s Law, my first thought was to put a 5 Ohm resistor (this would be with the original transformer in place) in line at both the doorbell and address light. To meet the specs that I think I should use (5 ohm, 1% tolerance, 10 watts), I found a wirewound type of resistor. I know this seems fairly simple, but does anyone see any problems with this solution? Thanks in advance for your help.SbassCaAz

author: Seekonk
author_rank: Super Contributor
timestamp: April 28, 2016, 11:43:34 pm
content: The added voltage is to compensate for resistance loss of long leads. A better solution is to get a 24V transformer and wire it as an auto transformer or buck transformer. and feed your transformer lower voltage.

author: SbassCaAz
author_rank: Newbie
timestamp: April 29, 2016, 05:16:30 pm
content: Seekonk,Thanks very much for your response, I appreciate it. Unfortunately, I have a single junction box from which to work from and that's where my transformer is connected to the 120 VAC source. While your suggestion might be a more elegant solution, would my suggestion of using the wirewound resistor work without causing any problems? Thanks again for your help.SbassCaAz

author: Photon939
author_rank: Regular Contributor
timestamp: April 29, 2016, 05:29:03 pm
content: What about replacing the light bulb with an LED module? That would help to cut down on the heat.

author: SeanB
author_rank: Super Contributor
timestamp: April 29, 2016, 06:27:05 pm
content: Not sure of the USA, but here the standard doorbell transformer ( when they were still an installed item, not many around these days, with the most common doorbell being an intercom) has 3 secondary taps, giving 4VAC, 8VAC and from this you can get 4, 8 or 12VAC from it.

author: Zero999
author_rank: Super Contributor
timestamp: April 29, 2016, 06:53:19 pm
content: Did you measure the voltage with the address light connected?Transformers only ever output their rated voltage when fully loaded. With a no load or a light one, the secondary voltage will be 10% or more higher.A resistor can be used to lower the voltage but why not simply use a 12V transformer?

author: qwaarjet
author_rank: Regular Contributor
timestamp: April 30, 2016, 01:46:19 am
content: Yes transform voltage is rated at max load so if it is under loaded the voltage will be high.Possible solutionsadd series resistor with your loads fine for const on loads like light but transient loads will see spikes in voltage at lower currentadd a parallel resistor to bring the the load up on the transformer max rating and the voltage will drop to speca get a new transformer that has a rating much closer to your actual load.

author: SbassCaAz
author_rank: Newbie
timestamp: April 30, 2016, 06:05:06 pm
content: Thanks to all who responded, I really appreciate it!Photon939: I like the idea of LEDs but unfortunately, I don’t have many options when it comes to varieties of low voltage address lights. But, also, I think I would rather try to find a way to reduce the output voltage. Thanks very much for the suggestion.SeanB: You’re absolutely right, they do make a doorbell transformer with 3 secondary taps. Unfortunately, they are 8, 16, and 24 VAC (rather than 4, 8, and 12) so I’m kind of in the same spot. Thanks very much for your suggestion.Hero999: I wasn’t thinking when I took the old address light down and did not measure the voltage with it connected. I still have the old light fixture but for some reason, I can’t find the bulbs that were used with it. The new address light is on order and has not arrived yet so I’m not able to connect it up. I’ll search around here locally for some replacement bulbs. I should be able to find something that I can plug in. As for a 12 V transformer, all of the doorbell transformers that I’ve found use either a single output of 16 VAC or a triple output option of 8, 16, or 24 VAC. So, I think I’ll stick with the single output of 16 VAC (18.2 VAC actual) and play around with a resistor to get where I need to be. Thank you very much for your suggestion.Qwaarjet: I like the simple idea of using a resistor in series. I’ll see if I can find some replacement bulbs for the old fixture and then do some experimenting. Thank you for your suggestion, this should get me where I need to be.Thanks again to everyone for taking the time to respond. Your responses have been very helpful.SbassCaAz

author: IanB
author_rank: Super Contributor
timestamp: April 30, 2016, 06:14:20 pm
content: As far as heat is concerned, incandescent bulbs get hot. That is what they do. The cure for that is to use a different light that uses LEDs instead of ordinary bulbs.You can tell if the address light is getting too much voltage because the bulbs will burn out and need replacing much more frequently. If you have never had to replace burned out bulbs in the address light it is likely the voltage is/was fine.If you want to make the bulbs run cooler (or not burn out so often) you can replace the 16 V bulbs with a higher voltage like 20 V or 24 V. You should be able to find such bulbs online if you can't find them in local shops.

author: IanB
author_rank: Super Contributor
timestamp: April 30, 2016, 06:19:13 pm
content: I have, out of curiosity, just measured the voltage on my doorbell transformer and it reads 11 V. So lower voltage transformers were apparently available when it was installed. This suggests they should still be available somewhere still.

author: Seekonk
author_rank: Super Contributor
timestamp: April 30, 2016, 07:56:35 pm
content: You can just put a diode in series with the lamp, instant dimming.