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

[haggis999]

... Can I now give you a Diploma in electronics

I am embarassed to tell you that I already have a degree in electrical engineering. However, I graduated 50 years ago and have never designed an electrical circuit in my life. My training at university was more theoretical than practical and preceded the wide availability of LEDs, which Wikipedia tells me were invented in 1962 (the year I went up to university). A degree got me a decent job, but I never had an opportunity to directly exploit my studies, which were thus gradually forgotten. It was a prime example of 'use it or lose it'.

After 50 years, pretty much all I remember is Ohms Law, the meaning of words such as resistor, capacitor and inductor and the effect of combining resistors in series and parallel. This forum conversation has also dredged up long lost memories of half and full wave rectification. None of this was of much relevance in a career as an IT Manager.

Nothing beats hands-on experience, and there you beat me hands down!

 

[Mikefromlondon]

LOL! you sounded a lot younger, but at the same time I had my suspicions about your abilities, at times I thought you knew well enough and were just trying to tease us, with your cad cam Autocad hands on experience, drawing symbols accurately, I thought you knew well enough, and since you interpreted my words into a circuit diagram, well there you go, I remember I had to teach my Physics teacher how to use an oscilloscope and I had to rig an experiment using a thermionic valve to perform half wave rectification which our Physics teacher still know him by his name Mr Lobo! that was back in late 60s and early 70.

I also had a classic argument with a teacher in my community college where the teacher teaching us about Electronics told us how mains 50Hz switches on and off 50 times per second, poor chap was mislead and I tried to correct him but he wouldn't have it, he insisted it switches on & off 50 times per cycle, despite my demonstrating that it switches off and on 100 times a second, indeed he must have been getting mixed up with polarity change that occurs 50 timers a second.

Any way another happy chappy!

 

[haggis999]

You have reminded me that I have had an ancient Tektronix oscilloscope in my attic since Adam was a boy. It was being disposed of by my employer at the time and I got it for £10, but it has never been used for any practical purpose. Time has probably taken its toll on the capacitors, etc, and I would be wary about connecting to the mains again. My wife is rather keen for me to take it to the council dump!

I may head to Maplins later today to get a 100mF 25v capacitor. Would it be a good idea to get some other values as well? If so, what would you recommend?

 

[Mikefromlondon]

My first oscilloscope was a Gold Advance, a 10Mhz oscilloscope I bought in 1975, after which I bought used 100Mhz again Gold Advance, company near me in Hainault manufactured these, I then also bought a 200Mhz Colour Digital Oscilloscope for £2,500.00 with a printer and a serial port to connect to pc, in my trade Oscilloscope is very essential, in fact I had to use one today to diagnose fault on a switching transistor that seem to work Ok when not on load and yet when subjected to load its output collapses.

As for capacitors, they should still work well even after many years lying on shelves, they are well sealed and should not lose electrolyte, but may need reforming, i.e. charge and discharge a a good few cycles and they should be good as gold.

You may try 220uf 25v just to give you a bit longer duration for your LED to continue to glow when someone presses on button for a while longer.

But before connecting a cap, disconnect one leg of your LED, measure off load voltage and if it is high like 19-20v, then use 1K resistor , we worked out that you would need something like a 500 ohm resistor to get reasonable amount of brightness, so have a 500R (or nearest would be 470R) just in case you find 1k is not giving good brightness.

 

[JohnW2]

My first oscilloscope was a Gold Advance, a 10Mhz oscilloscope I bought in 1975

Ah. My first one was a Heathkit one (although, for some reason labelled as "Jason kit"), bought (and constructed) around 1963/4. I still have it (along with much more modern alternatives), and it still worked when I last tried it - albeit now rather dusty/rusty! ....

Kind Regards, John

 

[Mikefromlondon]

Good lord, that is memorabilia piece, I do remember Heathkit, along with things like world's first micro miniature radio, it was called Sinclair Micromatic, didn't have a speaker and used earphone piece. I used to buy Practical Electronics, Everyday Electronics, Practical Wireless, Wireless World, Elector, every month.

 

[haggis999]

I have now obtained those two capacitors, but when I started to work out exactly how I would assemble all the additional electronic components, I realised that I had deluded myself that my Option D wiring diagram would need only two conductors routed to the illuminated pushbutton. Four conductors are actually required. However, it is not a problem for me to run two parallel lengths of bell cable.

Oscilloscopes are powerful tools, but I just never seemed to find a reason to use one at home. Mine comes with a huge separate unit containing the power supply. It was the capacitors I assume are contained in this box that I thought might have degraded over time.

 

[stillp]

My first one was a Heathkit

I used to have a friend who was building his way through the Heathkit catalogue - I used to borrow the documents from him and beg/borrow/otherwise acquire the parts to make a copy! Finding the CRT for the 'scope was a problem, but I managed a multimeter, signal generator, transistor tester...

 

[Mikefromlondon]

I have now obtained those two capacitors, but when I started to work out exactly how I would assemble all the additional electronic components, I realised that I had deluded myself that my Option D wiring diagram would need only two conductors routed to the illuminated pushbutton. Four conductors are actually required. However, it is not a problem for me to run two parallel lengths of bell cable.

Oscilloscopes are powerful tools, but I just never seemed to find a reason to use one at home. Mine comes with a huge separate unit containing the power supply. It was the capacitors I assume are contained in this box that I thought might have degraded over time.

That depends, you really haven't deluded yourself, if you were able to install all components behind the switch, then you only need the two wires from the transformer secondary. Unfortunately i haven't got the time to do a little sketch but you know it can be done if there is a space behind the switch.

 

[haggis999]

The stainless steel backplate is designed for attaching directly to the wall, rather than to any form of back box. I will therefore have to drill a hole through to the inside wall, where I intend to fit a recessed junction box to contain all the other components. Four wires are therefore required.

I always realised that this would be the arrangement, so I was indeed deluded to think that I could get away with feeding only two wires through the wall. It was just another sign of advancing senility.

The attached diagram shows what I hope is the final plan for this project. Somebody else might find this useful in the future (though I forgot to add add 'Copyright MikefromLondon')

Edited version of wiring diagram uploaded on 10 Nov to make more realistic use of terminal block

 

[Mikefromlondon]

I can't wait to hear it! LOL

 

[endecotp]

I may head to Maplins later today to get a 100mF 25v capacitor.

Why not get a 78L12 voltage regulator too, while you're there?

 

[haggis999]

Why not get a 78L12 voltage regulator too, while you're there?

I have to give you credit for persistence, but MikefromLondon gave some persuasive reasons why this might not be such a great idea. I don't remember seeing any response from you on this...

 

[bernardgreen]

Why not do away with electrical illumination and use a tritium powered Beta Light. May not be the cheapest option but no wires and a half life of over 10 years. ( samples may be available )

http://www.betalight.nl/en/light-source.html

 

[endecotp]

I have to give you credit for persistence, but MikefromLondon gave some persuasive reasons why this might not be such a great idea. I don't remember seeing any response from you on this...

Ah, OK - I'm ignoring MikeFromLondpn. I forget why, he must have posted something rude or otherwise annoying at some point in the past.
I now see his post number 61 where he makes various comments, which I guess I should respond to:

- It clearly won't get any hotter than a resistor doing the same job.
- With a resistive constant load like an LED it will work fine without a decoupling capacitor.
- People niormally use series resistors to regulate LEDs because they have a known supply voltage and a known load current. In this case we have an unknown supply voltage and an unknown load current, but a known load voltage.
- These devices cost pennies, and the implementation is very straightforward with no need to guess or even measure anything - so it's far from overkill.