Is it safe to run 12v DC LED from 8v AC bell transformer?

[haggis999]

I have bought a rather classy and expensive engraved stainless steel bell push fitted with a Bulgin-style pushbutton that has an integral LED rated at 12v DC. I would want the LED to be lit at all times (except perhaps while pressing the pushbutton).

It has been difficult to get much information from the German supplier, but they have told me that most users in Germany use a 12v AC bell transformer and the small number of LED failures they have investigated are due to transformer outputs of 14v or higher.

My problem is that MK, the manufacturer of my 8v AC bell transformer, does not think it suitable for a 12v LED. Bulgin has offered similar advice. The open circuit output from my MK transformer is 16v. If I measure the voltage while my chimes are in operation then the voltage constantly swings up and down in the range 8-15v AC. However, it seems unlikely that a British 8v bell transformer is a riskier power source than a German 12v bell transformer.

I could buy a dedicated 12v DC LED power supply for about £20, but is this really necessary?

David

 

[Mikefromlondon]

LEDs can tolerate a fair amount of abuse, unlike incandescent bulbs, so you should have no problem with connecting your 8v transformer that is giving you 16v dc unloaded voltage, when you connect your LED, the voltage may drop down to roughly 12v so you should be OK, if for any reason you blow your LED, they are not difficult to replace, they are widely available and any competent technician should be able to replace one easily if need be.

 

[endecotp]

An LED supply might not successfully work the bell.... some kinds take quite a lot of current.

Can you see how the LED is wired? Is there just a series resistor, or is it more complicated?

 

[Mikefromlondon]

If a bell button has been supplied with an LED, it will have a current limit series resistor or one that is built into LED, such as constant current LEDs, and the bell button contact across the LED with the series resistor shorts the LED out and full current then flows into the solenoid striker for a ding dong chime bell.

What the LED's don't like is the reverse conduction, as they only have a very low threshold for reverse voltage and so by connecting wrongly they can blow, but if they have included a forward conduction diode then that should not be a problem if wired up wrong polarity, as it will not illuminate though the bell may still chime when button pressed.

 

[ericmark]

An LED is around 3 volt for white, so it will have some extra components and we have no idea what is inside the package to allow a 3 volt device work on 12 volt. It could be a simple resistor or it could be a pulse width modulated chip. So in real terms one can't really answer the question.

I also had problems finding a 12 volt DC power supply for my mothers door bell. The electrician who did the job fitted a 12 volt AC power supply which did not work, and after being called to correct he ended up with a wallmart power supply.

There are it would seem voltages from 4 to 16 volt both AC and DC used for door bells, in the main DC door bells do more than simply ring a bell, often a voice or door release system.

The current used by a door bell also varies, the electronic type is normally in the milliamp range where some of the simple ding dong bells take amps to work.

There is no reason why the AC supply can't be made DC for the light and the voltage reduced, but simple way is get a button designed for AC.

 

[haggis999]

Many thanks for all the rapid and useful responses so far.

I have attached a couple of photos that show the sort of illuminated pushbutton I am talking about. The circular LED-powered light is white. I'm not sure if there is any way to dismantle it and replace a blown LED, so I am not planning to connect any power source until I am convinced it is safe. Replacing the entire pushbutton would be quite expensive.

So far, I have failed to establish the make and model of pushbutton, which makes it difficult to be certain about the internal components. The supplier's website claimed it was a Bulgin part but when I sent my photos to Bulgin for confirmation they told me it was not one of theirs.

 

[haggis999]

Let me also correct a misunderstanding by ericmark. Finding a 12 volt DC LED power supply is not my problem. I have already identified the following item as appearing to be an exact match for my needs. However, I don't wish to introduce added cost and installation complications if they are not really necessary. Using my existing bell transformer as the power source for the LED would obviously be a simpler and cheaper option.

http://uk.farnell.com/xp-power/dle15ps12-v/led-driver-15w-12v-1-25a/dp/2365028

 

[sparkymarka]

Solder a bridge rectifier on to the button led light terminals .. I use these on AC door entry systems when a DC device is required .

 

[Echo the husky]

Does the indicator actually say DC? I can only see 12V on the marking.

 

[haggis999]

It does indeed simply say 12V. The supplier's rather limited installation instructions also state "The voltage range for the LED is based between 8 and 12 volt. You can use either continuous or alternating current. Both are possible". Elsewhere in those instructions it says "You will need a power requirement of 8 - 12 volt, which will be supplied by the in-house transformer for the doorbell, so you won't have to install additional lines".

On the other hand, the supplier's incorrect claim that a Bulgin pushbutton was used caused me to look at the Bulgin catalogue, where the most likely match appeared to be offered only with an LED requiring a DC power supply. I am left with the thought that the use of a 8-12v AC supply may work for most people, but carries some risk of early LED failure. As ericmark observed, not knowing if my pushbutton contains any more than a current limiting resistor makes it difficult to quantify that risk.

I like sparkymarka's suggestion of a bridge rectifier. That's a far cheaper and simpler solution than the dedicated LED power supply I mentioned in my last post.

 

[Echo the husky]

It says 8V AC is OK, in which case it will already have a rectifier and suitable current limiting built in, as you are already at the lowest end of the indicators working voltage any less and it may not work at all.

 

[endecotp]

You're lucky that it has 4 terminals, so you can add components to affect the LED but not the switch.
I'd add a bridge rectifier and/or a 7812 12V voltage regulator.

 

[Mikefromlondon]

The switch has 5 pins, I am presuming the middle 3 pins in a row are c/o contacts (i.e. NC, C, NO) and the two other pins are thus LED feed, If the LED is rated to run on 12v DC, it would mean it has built in current limit, so one can apply 12 directly to it, with correct polarity if it is DC, if you want to run it off ac, you can use a single diode in series like the 1N4001 or a small bridge rectifier. No big deal here. Even if you were to apply 8v ac directly, LED would sustain no damage at this low voltage, however since you mentioned your transformer can reach as high as 16v when not on full load, but for an 8v transformer to reach 16v off load appears that its output is already rectified and smoothed by a smoothing electrolytic capacitor, so it may well be outputting a DC voltage, it is for you to confirm if your transformer really has a dc output or is it simply a true ac output. ( If you have a multimeter you can check this, or if you can open it up and see if can see any components inside like a few diodes forming a bridge rectifier and a smoothing capacitor.

 

[flameport]

Replacing the entire pushbutton would be quite expensive.

Somewhere from £2, perhaps a bit more if you don't want to wait for delivery.

http://uk.farnell.com/xp-power/dle15ps12-v/led-driver-15w-12v-1-25a/dp/2365028

That 15 watt item is grossly overpowered, any 12V supply will do, actual power required less than 1W. Readily available for less than £5.

 

[haggis999]

The switch has 5 pins, I am presuming the middle 3 pins in a row are c/o contacts (i.e. NC, C, NO) and the two other pins are thus LED feed, If the LED is rated to run on 12v DC, it would mean it has built in current limit, so one can apply 12 directly to it, with correct polarity if it is DC, if you want to run it off ac, you can use a single diode in series like the 1N4001 or a small bridge rectifier. No big deal here. Even if you were to apply 8v ac directly, LED would sustain no damage at this low voltage, however since you mentioned your transformer can reach as high as 16v when not on full load, but for an 8v transformer to reach 16v off load appears that its output is already rectified and smoothed by a smoothing electrolytic capacitor, so it may well be outputting a DC voltage, it is for you to confirm if your transformer really has a dc output or is it simply a true ac output. ( If you have a multimeter you can check this, or if you can open it up and see if can see any components inside like a few diodes forming a bridge rectifier and a smoothing capacitor.

My bell transformer is as shown below. The specs state clearly that the nominal output is 8v AC. I have also confirmed with my ancient multimeter that there is no significant DC output. The no-load output is still 16v AC. However, our mains voltage is at the legal maximum of 254.4v (240v + 6%).

https://www.mkelectric.com/en-gb/Products/CP/sentry/Belltransformer/Pages/5711s.aspx
https://www.mkelectric.com/en-gb/Products/CP/sentry/Belltransformer/Pages/5711s.aspx
In one of your earlier posts you stated "...the bell button contact across the LED with the series resistor shorts the LED out and full current then flows into the solenoid striker for a ding dong chime bell". That implies a different wiring arrangement than I had assumed. The attached diagram is my best guess as to what circuit connections you meant. Please confirm if this is correct.