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Want to install LED lights on receiver problem is AC voltage

M

mycity_33

New Member
This is a vintage Harman Kardon 430 where i installed LED's with resistor by each LED. The problem is that voltage remained the same after LED was installed and it shows AC on my DVM. I would expect LED to burn out shortly (voltage is over 6 volt and LED voltage should be 3,2 volt) and light is too bright- What to do? get a regular diode and install in series in wires feeding voltage

Would a Zener diode be better, I have this one: http://parts.digikey.com/uk/1/593081-diode-zener-6-8v-500mw-5-do-35-1n754a.html

What specs should diode have to work as intended?

Voltage being fed is 6,7 volt. LED's I am using are 3,2 V and 50 MA
 
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This online calculator works for DC supply.
If you have a supply of resistors/pots, you could just do some experimenting to get that LED dimmed down.
 
resistor is installed but AC remains the same

Thanks for calculator VictoriaGuy however resistor is installed and it shows same voltage over 6 volt AC. I don't want AC to power LED's because it is damaging, only DC voltage. Is it possible that resistor does not reduce AC but would reduce voltage once I made it to DC circuit?

By looking at service manual it shows power coming straight from main power transformer therefore it is pure AC.
 
mycity_33 said:
I don't want AC to power LED's because it is damaging, only DC voltage.
Click to expand...

LEDs are not damaged by low voltage AC as long as the max reverse voltage is not exceeded- only by too much current.
They are diodes after all.
But if you are concerned, you could just put a diode (1N400x) in the AC line supplying your LED.
There's probably some DC around that you could tap into as well.
 
VictoriaGuy said:
LEDs are not damaged by low voltage AC as long as the max reverse voltage is not exceeded- only by too much current.
They are diodes after all.
But if you are concerned, you could just put a diode (1N400x) in the AC line supplying your LED.
There's probably some DC around that you could tap into as well.
Click to expand...

There is an led lighting up my front door lockset. It's been working for about 15 years. It is powered by the doorbell transformer (26 vac?) and a series resistor to limit current flow.
 
There ARE LEDs made for AC - they contain two LEDS inside, wired reversed in parallel. You can connect LEDs in pairs this way, with a single resistor. Forward conduction on one limits the reverse voltage to 3V on the opposite one.
 
You need to take your ac voltage and multiply it by the square root of 2 to get your actual peak voltage. Once you have this, get your resistor value, put that and a 1 n4001 in series for conducting one way only. If you want I can sketch a little diagram, I installed LEDs for dial lighting on 1010's , 1980 using 360 degree LEDs ' and even a small hk unit just like yours using 2 LEDs and a resistor and a diode for each fuse lamp I replaced. What we are concerned with is the current and forward voltage of the led. Keep the current around 10 to 15 ma for a 20 ma super bright it will last forever.
 
That large AC that you're measuring is the negative polarity part of the waveform. Don't worry about that in your diagnosis.

In the forward direction, you will only see the Vf, or forward voltage. The meter cannot distinguish what part of the waveform is being measured.

It would be better, in my opinion, to put a reverse polarity diode across the LED, to prevent any possibility of a peak reverse voltage breakdown issue.

One better is to use a full wave bridge. This would double the flicker rate, to ensure that the strobing light was not very noticeable. One better is to use a full wave bridge and a small filter cap.
 
Why make things harder for yourself? Just take your feed for the LEDs from somewhere there is DC present. Get a schematic for your receiver, and take a feed off the main power supply cap, via a suitable dropping resistor of course.

I did something similar to my Kenwood KT-7500, I removed the AC from the original board for the lamps, and used this as a junction point for my six LEDs (three blue, three green).

Lee.
 
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