LEDs running on AC

Mike Sweeney

AK Subscriber
Subscriber
So I was going to post this in Marantz but I know these LED replacement lamps are used else where so why not here.

I know on the Marantz gear, the lamps behind the dial is around 8 volts AC. Mine actually measures at 7 volts AC. I know LEDs are intended for DC. I think I know that the LED replacements are being run on AC because they will work on a half wave of AC since it doesnt exceed the LEDs rated voltage plus it has a current limiting resistor. Where it gets interesting to me is that the half of the AC wave would be flipping the LED on and off 60 x a second ( normal AC power freq) and be reversed biased half the time. So what does this theoretical wear and tear do to the diodes lifespan? Then lets add in the non-polarized idea..

This kind of came to mine attention when I noticed in a short vide, the Dial flickering. Then down the rabbit hole I went of finding people building small full wave rectifiers from 4 diodes.. some adding a cap for smoothing etc. The geek in me loves the idea of making the small circuit and feeding the LEDs DC like they want. But is it really needed? I've seen some new LED replacement lamps now come with a SMT rectifier built into the lamp now so apparently someone else has taken notice.

I found this from some years ago.. sounds like running LEDs off AC is not really that big of deal? https://www.ledsmagazine.com/articles/2006/05/running-leds-from-an-ac-supply.html
 
I have found that without a small protecting diode, the reverse bias does break them down. the new ac/dc lights u can get thru parts express have led and limiting resistor in the package and work well, fuse type for the marantz dials, bi-pin for the function switches
 
So far what I've read says the "best" way is to run 2 LEDs in parallel but flipped. Plus the limiting resistor. This allows one of the two to be on for the half wave but also allows a path for the current so its' not banging on the diode in reverse. Which actually makes sense to me. Now I'm curious if my replacement LEDs are in fact wired up this way since there are four LEDs on the replacement. The things I do to occupy my time ;)
 
Many times I use DC-DC up converters available on the auction side for cheap prices. These small ckt boards take in 3.5v to 35v DC and output 5v DC to 35 VDC adjustable via a trimpot. I use a small 1a to 2a bridge rectifier (4 long lead type, not smt) as the input to this board. The board already has a DC filter cap on input and output so no worries about the bridge rectifier not being filtered. Connect the 2 8vac wires to the 2 ac inputs on the bridge, and the output of the board goes to the 2 wires for the lamps. Remember to adjust the dc volt output prior to installing the led's. Then install the led's. I usually run them at 10.5 to 11v to keep current way down. The brightness differenct is amazing when you run standard 12v led's colse to 12v rather than at 7 to 8v ac. Of course there are some 8v leds available now (fuse type in clear or blue) that don't need a power supply conversion

Here is the up converter I am talking about.
5A DC-DC Step Up Power Module Boost Volt Converter 3.3V-35v to 5v 6v 9v 12v 24v

I have also used the 2 amp style, but just make sure to get the ones that have the 2 DC filters.
 
I use a diode full bridge and a current limiting resistor. End up with 120hz flicker instead of 120. And is "non-polar" - and protects the LEDs. Filtering or power supply is better, but I don't see a difference.
 
Keep in mind that the 'peak' voltage is different than what you would measure with an AC voltmeter. Many 5MM LEDs have an advertised reverse breakdown voltage of 5V, but test at 15 to 20V.
Why, I don't know.

Tom
 
Found this lil beauty up on ePay a while back and have used it a couple times. Easy hookup (AC in/DC out, or vice versa) and good for something like 2-30v automatically.

led-rectifier.jpg

One nice thing - I only converted about half the lamps on my QRX-9001, because the power meter lamps especially are a real PITA to replace. Those plug directly into the stock VAC with no worries, and if I ever get around to doing the LEDs there, just move the power tap to the other side of the rectifier.
 
I've used a single diode and a capacitor, and also connected directly to AC in counter-parallel plus a dropping resistor. I like the diode/capacitor better. Here, at 50 Hz, even 2 led in counter parallel still flick a bit, not very noticeable but barely visible from some angles.

I'd fit a diode and a filter capacitor.
 
Hi Mike. That's the way I replaced the burned out vu meter lamp in my Kenwood 5500. Both LED's fit fine in the original bulb mount. Used a dropping resistor, small cap and it's working fine.
 
this is a nice hack for an LED - Doesnt take up any room at all. I found it on a model train forum since the trains run on AC but use LED lighting.

Evan Design LED ElectronicsA.png
 
If you have an old AC to DC adapter laying around like say something from those cheapie chair heat/back vibrators or an old power tool, old dac, etc. you can plug it into the switched rear AC outlet on the unit (assuming it has one) and run lighting off that. Every time the unit is turned on/off, it'll turn the lights on/off. I used to run an old AC powered ham radio cooling fan in a similar fashion, and nowadays have my dac's AC/DC connected through a switched AC outlet on whatever stereo I'm using.

The benefit is it's about as straight-forward as you are going to get (+ and -) and can leave the unit unmolested.
 
turned out a little bright as you cane see in this pic . a little bit of black sharpie on the flat face of front led took care of that , turned out nice and even and no flicker with leds running on ac in reverse polarity (like Mike had commented)304 first run.JPG
 
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