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 Post subject: LED Question
PostPosted: Dec Sun 09, 2018 8:13 pm 
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I need to replace all the bulbs on a Yamaha CR-440 receiver. The lamp voltage supplied is 13vac, and all lamps to be replaced are in parallel with a common tap on the power transformer.

To save space and parts, any issue using one current limiting resistor, and one 'reverse diode' to supply all 6 LED's? The LED's are 5mm, 3.1v, 20mA and I'd calculate about a 560 ohm resistor.

I guess my real question is, if I strap my reverse diode across 0v and 13.1v (after the fuse) of the power transformer, and with the limited view I'm providing with the rest of the PS to the right of F402, would this cause any concern - IE, with diode D404? Anode of my reverse diode to 0 transformer tap?

Or maybe an even simpler way to ask after writing this all out - what would be preferred for connection of the reverse diode (anode/cathode) to the 0 and 13v taps?

I would then use one one 560 ohm (3 watt?) to feed all the LED's.

Attachment:
Screen Shot 2018-12-09 at 1.40.20 PM.png
Screen Shot 2018-12-09 at 1.40.20 PM.png [ 181 KiB | Viewed 1101 times ]



Essentially what I'm doing, but with 6 LED's in parallel:

Attachment:
LED2.jpg
LED2.jpg [ 10.21 KiB | Viewed 1101 times ]


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 Post subject: Re: LED Question
PostPosted: Dec Mon 10, 2018 12:39 am 
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How about a Zener regulator for smoothing to prevent/control visible pulsing effects (if you end up with any)?
Hopefully, you won't need a filter.

Is there somewhere in the unit you might could tap the correct DCV and just beef up that portion of circuit to handle the extra LED load?


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 Post subject: Re: LED Question
PostPosted: Dec Mon 10, 2018 12:45 am 
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I don't think the brightness will be even, and LED voltage drop has a negative thermal coefficient, so the warmest one will hog the current. Series would be better, two strings of 3 each would need two resistors (330 Ohms or so) and two reverse diodes.

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 Post subject: Re: LED Question
PostPosted: Dec Mon 10, 2018 1:07 am 
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Location: Houston, TX
I would use a resistor per LED and mount all the supply components on a tiny protoboard. Six standard 1/8W R's side by side will take up less room than a standard 3W. Mount the board using hot glue.

Of course, if the reason you want to do it the way you suggest in the OP is because those are the parts you have on hand and don't want to order anything, that's another matter.


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 Post subject: Re: LED Question
PostPosted: Dec Mon 10, 2018 1:09 am 
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Thanks guys - good ideas. I'll ponder a little more on this.


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 Post subject: Re: LED Question
PostPosted: Dec Mon 10, 2018 2:27 am 
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Why bother with the reverse diode? Use leds for the reverse path.
Two strings of 3 leds, one string cathodes up, one string cathodes down. One resistor, dropping 4 volts at current of 20 ma = 200 ohms at .08watts, so use a 1/4 watt or larger R.
You will, of course have to rewire the lamp wiring to configure the two strings of 3 leds is series.

-Chuck


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 Post subject: Re: LED Question
PostPosted: Dec Mon 10, 2018 7:10 am 
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If you want to preserve the OEM look, maybe this would do the trick: https://www.ebay.com/itm/Yamaha-Receive ... SwSSdb73MD

Or these for much less: http://www.bulbtown.com/8640_Miniature_ ... p/8640.htm
Or these, longer life, but may be too large diameter: http://www.bulbtown.com/DW07_JKL_T1_1_4 ... p/dw07.htm

If your colored bulb condoms are in bad shape or missing, nude bulbs might not be much help.
Green caps are available, but may not exactly match OEM: http://www.bulbtown.com/Green_Colored_B ... -06-4a.htm

If you can't get a good color match, you might be able to coat the bare bulbs with glass paint of the correct color, if it'll withstand the temp.

Or: http://www.bulbtown.com/2182G_GREEN_MIN ... /2182g.htm
May be acceptable diameter w/o the rubber cap, but may not be a good color match.


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 Post subject: Re: LED Question
PostPosted: Dec Mon 10, 2018 10:15 pm 
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I will see if the guys I'm fixing this up for wants to do incandescent bulbs, but he was liking the idea of LED's. Each lamp location has a colored cover so I am using white LED's.

One quick question - any current limiting resistor considerations based on this being AC?


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 Post subject: Re: LED Question
PostPosted: Dec Fri 14, 2018 3:14 pm 
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Done. I did 3 white LED's per string in series and used a 16v DC source off the power supply. Looks great!

Attachment:
led3.jpg
led3.jpg [ 15.39 KiB | Viewed 877 times ]


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 Post subject: Re: LED Question
PostPosted: Dec Sat 15, 2018 10:12 pm 
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<edit> the following commend missed the change from AC to DC supply. Disregard this comment if the LEDs are supplied from DC.

Please reverse polarity of one of the two strings, and connect the right hand end of the two strings. Then use only one 330 ohm resistor. This configuration allows for the unknown reverse breakdown characteristics of your LEDs/

-Chuck


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Last edited by K7MCG on Dec Sun 16, 2018 1:02 am, edited 1 time in total.
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 Post subject: Re: LED Question
PostPosted: Dec Sat 15, 2018 10:32 pm 
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But it looks like he's now using DC.

K7MCG wrote:
Please reverse polarity of one of the two strings, and connect the right hand end of the two strings. Then use only one 330 ohm resistor. This configuration allows for the unknown reverse breakdown characteristics of your LEDs/

-Chuck


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 Post subject: Re: LED Question
PostPosted: Dec Sun 16, 2018 12:59 am 
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I hadn't noticed that + sign he sneaked in on the 16volt supply. I was assuming the schematic, above, that showed the lamps powered with AC. If the source now is DC, disregard my post.
-Chuck


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 Post subject: Re: LED Question
PostPosted: Dec Sun 16, 2018 3:06 am 
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I probably confused things a little. Originally I was looking to use the AC lamp tap from the transformer. Now using a 16v section of the DC power supply.


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