stupidly simple LED dimming from DC heater supply? - diyAudio
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Old 20th April 2013, 10:16 PM   #1
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Default stupidly simple LED dimming from DC heater supply?

Almost all the info on LDR's here on this site involves trying to get stereo to track properly in ultra hifi... I'm just looking to dim 3-4 ma worth of optocouplers, all together, as one, on the same variable resistance task in a guitar amp.

Hopefully the current will be consistent enough that they each stay put, as far as the player can tell, at resistances from the nominal 440 ohms (at 1 ma If, from the spec sheet) to about 200k-300k, which is the darkest I need them. I intend to limit the maximum resistance by putting a 33k fixed resistor in parallel with the optocouplers (NSL-26, three or four of 'em in parallel- at 1 ma each maximum).

I'd like to keep this very simple- that way I can follow it, and it doesn't matter if the LED's change color. The manufacturer tells me the LEDs are happy anywhere from 5-10 volts, and I happen to have a spare 6.6 volt 3.1 A heater supply, as well as a heavily filtered DC heater supply that's running 3 preamp tubes at .9 A in all.

The smallest required current will be somewhere between .03 and .3 ma.

I'm still grubbing around for generic information, but somebody here has doubtless been through this with a valve guitar amp or at least can see what the issues would be.
Thanks!
Ted

PS- actually I have my specs all wrong- I may need to run some optocouplers or LED/LDR combos in series as well, and may need more like 50 ma or more...

Still I appreciate anything specific to the valve transformer source!

Last edited by tapehead ted; 20th April 2013 at 10:44 PM.
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Old 21st April 2013, 01:59 AM   #2
DUG is offline DUG  Canada
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"...The manufacturer tells me the LEDs are happy anywhere from 5-10 volts..."

What LED's are these?

Red and green are around 2V and white at 3V.
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Old 21st April 2013, 02:18 AM   #3
Pano is offline Pano  United States
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Maybe the LEDs have a series resistor already inline?
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Old 21st April 2013, 03:18 AM   #4
JMFahey is offline JMFahey  Argentina
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Sorry but you are not clear.
Please post the schematic of what you intend to do and maybe we can help with some doubt.
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Old 21st April 2013, 03:55 AM   #5
Pano is offline Pano  United States
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It doesn't sound too difficult. Just put a couple of LEDs in series to get a decent voltage drop then control the current with a pot. That will dim them. Go series parallel if you need multiples. If your 6.6V supply is clean, I don't see any problems.
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Old 23rd April 2013, 11:39 PM   #6
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Quote:
Originally Posted by DUG View Post
"...The manufacturer tells me the LEDs are happy anywhere from 5-10 volts..."

What LED's are these?

Red and green are around 2V and white at 3V.
The LEDs I was talking about are the ones that come in Silonex optocouplers. I'm told the color varies with the voltage, but the LDRs in the optocouplers respond well in that range, whatever the color might be.

However, it seems I may need to use some of the LDRs that do not already come "optocoupled", but it seems that Advanced Photonix may be willing to hook me up with the housings and appropriate LEDs- if not I've seen the posts on heatshrink as a housing.

Sorry I was so very confusing- there are a lot of options and the exact voltages and currents in question remain, well, in question, but clearly 6.6v will do nicely, with voltage dropped as necessary.

I'm accustomed to dropping the voltage with a pot, not the current. It's just one of those myriad manifestations of ohm's law that I can't quite picture at the moment- I'm sure it will come clear soon enough.

Thanks again for all the help.
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