Hi Fi front panel bulbs

[bsr]

Hi, I can't read the front panel of my radio. According to the schematic, the display is lit with 3x 8V bulbs. I've measured 8.3vAC across the tracks. The soldering doesn't look great and it looks like maybe one of the bulbs has been replaced with a green LED. Supposedly this was new when I bought it's albeit 20 years ago!

I thought a green LED was about 2v, how can it survive 8V AC? How would you fix?

I've found these online in USA, can't find anything similar in UK. Do you think they are basically an LED soldered in line with a resistor and covered in heat shrink?
https://lighthouseleds.com/pre-wire...-wired-led-white-ultra-bright-15-000-mcd.html

The display is currently green on black, there is a green filter between the backlights and the LCD. I'm happy to change the colour to whatever.

Thanks in advance.

Bsr

 

[ericmark]

One would normally use a diode to feed the LED so DC feed and a resistor to limit the current, red, green, blue, yellow and white LED's all have a different threshold voltage, red typically 1.2 volt and white typically 3 volt, so three red LED's would need 3.6 volt in series, so with an 8 volt supply and say for example 30 mA needed for bulbs, 8 - 3.6 = 4.4 volt and 4.4/0.030 = 146 Ω resistor required to control current. Clearly with white needs more than 8 volt, so would need to have them in parallel so 8 - 3 = 5 so 166 Ω.

Does that help.

 

[bsr]

Yes I understand V=IR but that doesn't explain what is going on. Good point on the diode but I can't see one in the circuit (schematic provided).

1. How does the LED survive 8V AC?
2. What should I replace all three with? Should I leave the LED and put in new bulbs, replace all three with new incandescents, or something else?

Thanks

 

[bsr]

I've found these, maybe I just replace all three including the LED?

https://www.ebay.co.uk/itm/10-X-8V-...=p2349624.m2548.l6249&mkrid=710-127635-2958-0

 

[bsr]

Right fixed. I used white obscured 20mA LEDs, cut the positive lead short and soldered on a 300R resistor.

The LEDs were flickery to look at but I installed them opposite polarity to reduce flickering and it seemed to have worked, the display looks fine.

I was concerned about breakdown voltage since I didn't use a diode as above, but it doesn't seem to be a problem. So despite the spec sheet saying breakdown voltage 5V they seem to cope with 8V AC so a peak of 11.3V.

 

[BS3036]

With the 300k resistor, the breakdown current will be limited to well below a damaging one, so you are safe on that score. Breakdown voltage is how zener diodes work in their normal mode of operation.

 

[Adam_151]

300k sounds quite a bit too high? possibly 300ohm rather than k-ohm?

 

[BS3036]

LEDs use MUCH less current today than the early ones. You can light a modern LED by holding one lead and touching the other lead on a random piece of metal. Every time we buy a new batch of LEDs we have to raise the resistor values in the circuit to avoid burning out the customers eyes.

 

[bsr]

Good point to those above I meant 300 ohm. Now corrected.