LED Question

The people here seem nice.

I am building a tube amplifier. I have a 5VAC tap on my Power transformer that I wish to use for some LEDs as a Power "on" switch indicator. I have a peice of etched glass that I want to light up using a set of LEDs, this array of LEDs will be the "on" indicator when the switch is closed.

I imagine running these in a parallel line (probably 8 LEDs) would be better than series but then I am not sure.

I was thinking of using a full wave bridge network from the 5VAC tap using these LEDs as well...I just dont know the best way to go about this.

What would you recommend?

Thanks for your input.

Reply to
Xtrchessreal
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One solution would be to use 4 series circuits in parallel (each series circuit consists of 2 LEDs + a 150 ohm resistor). A half-wave rectifier should be OK.

Reply to
Charles Schuler

These are InGaN Blue SuperBright 8mm with 40,000mcd luminous intensity.

3.6Vf max and 20mA Forward DC Current. I just found these specs, would anything change in your recommendation based on this?

Also, what is this luminous intensity spec. what does "mcd" stand for and how can I learn more about it.

BTW: I made a purchase of 50 of these on eBay for a small price $6.00 USD though the shipping was another $11.00. Plus they claim to be sending me resistors for these LEDs based on a 12 VDC use. Compared to RS this was a steal for the number of LEDs...36 cents per LED. I found a single Blue LED at RS for $1.79 at a nearby store.

Thanks

Reply to
Xtrchessreal

--
I recommend you tell us what the forward voltage of the LEDs is.  Or
the color.
Reply to
John Fields

I was writing that info while you were responding. Vf and If max stated above.

The wavelength is between 465-475 nm

Thanks for your help.

Mark

Reply to
Xtrchessreal

Seem nice? - OK Pollyanna

Series LEDs are better from an efficiency point of view - but with a tube amp you probably aren't concerned with efficiency.

The LEDs want to see DC voltage so two leds back to back for AC works well. With 5 VAC and RED Leds dropping 1.9 volts each that would be a back to back pair of LEDs running with a 155 ohm resistance in series to the 5 VAC transformer winding.

Or two pair of two series LEDs "back to back" with a 60 ohm resistor to 5 VAC

For 20 milliamps per led . . . your unloaded 5 VAC may be higher than

5 VAC. So go 10% higher resistance if you have no 5 volt toobs on the same supply. 5 VAC is normally the 5Y3 or 5U4 filament supply - and most of those have been replaced with diodes these days.

Avoid running LEDs on AC voltage without using either a rectifier or back to back LED connections (the reverse breakdown of LEDs is often on the order of 4.5-5 volts)

Green, blue, and white LEDs have higher voltage requirements than RED

- what I calculated for. Look up the specs and figure out what resistor you need if you use gbw leds. Red orange amber - ~1,8 - 1.9 V

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Reply to
default

mcd is milli candela, i.e. a thousandth of a candela. It is a measure of luminous intensity. A candle is about 1cd (which I suppose is where the name comes from) and a 100W light bulb is about 120cd.

Note that this is intensity not total light output, so if you focus the same light into a narrower beam the intensity will increase.

see:

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See also:

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RS can be expensive, but if you want something in a hurry it's worth paying the extra.

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Reply to
Gareth

Stated above what? Most of us don't use googlegroups. You have to include context for your post to make any sense in a real newsreader.

Thanks, Rich

Reply to
Rich Grise

News==----

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My Power transformer has 6.3VAC and 5VAC windings. The 6.3V for the heaters and the 5V for my LEDs otherwise not used. My amp faceplate is a new idea to me so I need to play around with these LEDs to determine just how many I will need for the desired effect. The Glass is clear except for the etched portion so you can see the red glow of the power tubes. My hope is that the red glow with the Blue 465-475 nm LED will provide a near Ultra violet effect when the stage lights are off.

If the spec sheet is stating 3.2 Volts for typical forward voltage is this the same as the minimum operating voltage to cause a forward current in the LED? Similar to 0.7V for a 1n4007 diode? I am writing and figuring this out at the same time. Vf is Vf no matter what diode you are talking about.

If that is so, the better solution is as explained above by Default, where I have a series resistor and LEDs back to back. The connection to the next bridge (LED/Resistor/LED) in the network is on the anode terminal of the previous bridge. This should be right. I figured 160 ohms to keep it a 20mA which is typical. Absolute max 40mA (I mistated that earlier). The only issue here is the PIV which is 5V but while one is conducting it is protecting the other from over voltage as

5-Vf is 1.8V.

I hope I am thinking right.

Thanks for all of your input. Clearly I have not used LEDs until now.

Reply to
Xtrchessreal

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No. What it means is that when you force 20mA through the LED the
voltage dropped across the LED will be, typically, 3.2 volts.
Reply to
John Fields

Then, use 4 parallel circuits with each leg consisting of an inverse parallel pair of LEDs in series with a 180 ohm resistor. No rectifier needed with this configuration.

Reply to
Charles Schuler

Thanks for your information John. Can you tone down the cheer in your writing and just get to the point. The circuit was very helpful in Courier as you suggested. Thanks

Reply to
Xtrchessreal

One issue to consider: Without a rectifier and filter cap, those LEDs will be pulsing at 60Hz. This is annoying to some people.

Reply to
Kurt

I had thought of that initially and using the LEDs as the rectifier in a bridge but there is not enough voltage to make it happen. I do have some misc diodes sitting around but calculating the recified voltage I would only have 3.18 Pulsating DC which is not enough for the average Vf at 3.2 for these diodes. So I will have to put up with the strobing effect. With each half cycle causing a LED to light it will be at 60 Hz x 2. Since the LEDs will be very close to each other the overall effect to the eye should be the same as pulsating at 120Hz but it could be worse and actually amplify the effect at 60Hz. Oh well I'll see when its done, might be a cool effect.

Reply to
Xtrchessreal

They make violet leds too...might help your ultraviolet effect...lol....and they also make ultraviolet leds too.....

what sort of tube amp are you building.....??? dont tube regulators or some tube rectifiers give off a purple glow ??? might be neat to stick one of those in there using the 5 volt winding.....at least them it would be all tube instead of a tube amp bastardized with some led effects...

just a thought....

Happy Hanukkah......Merry Christmas...... whatever kwanzaa

Reply to
cornytheclown

your ultraviolet

-- Show quoted text -

Well its hardly a bastard since its my baby. I haven't played around with building any of my amps with a tube rectifier but I might in the future. So far I have only built 2204s since thats the sound I like though i put KT88s in them. Theyre really bright and have a great distortion at loud volumes. Perfect for my style of Blues and Rock.

Have a happy (Corporately run, Government sanctioned, Religiously guilted, Scientifically negated) holiday.

Reply to
Xtrchessreal

Not been following this thread. I use four Led in a bridge across my door bell push (12 Volts 50Hz) There is no visible flicker at all. It makes the bell push easy to see in the dark.

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Best Regards:
                      Baron.
Reply to
Baron

If you filtered the 5 volts you'd get ~7.1 - 7.6 enough for a pair of leds in series. Converting from RMS to peak when you filter and if you could see the flicker at 60 HZ (I can't - unless my eyes are moving) the filtering would cure that. ( ~200 microfarad, 10 volt for four leds)

Even without the filter - the full wave bridge you mention should work, because the peak voltage will still be there even if the meter only reads the RMS value.

In any event from what you've said, the LEDs are behind a frosted panel so the light would blend and any flicker by one diode in a FWB rectifier would be canceled by the other diode conducting.

I don't see why the full wave bridge wouldn't work with the filter cap across the bridge - on one half-cycle, the cap would charge through two diodes and on the other half cycle the other two LEDs would be discharging the cap - but maybe someone smarter than I am could poke a hole in that theory.

If the 5 V transformer winding was for powering a tube filament it is

5 volts under load, and can be ~10% higher no load.

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Reply to
default

Using LEDs in a tube amplifier seems incongruous, neon-bulbs would be more fitting. These are operated from mains voltage via a current limiting resistor.

If however you really want the LEDs 5 V AC is equivalent to 7 V eff, so you could operate 3 LEDs in series with a single current resistor (red LED 1.8 V, so you need a resistor of (7-5.4) V / 20 mA = 80 Ohm, the next higher standard value is 82 Ohms). You can either use 2 such diode chains anti-parallel, or you need to rectify first:

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Reply to
Dr Engelbert Buxbaum

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Bill Bowden

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