Ping Sy: LED for Cathode Bias - Page 13

cogsncogs · 12th May 2006, 01:16 AM

Off-topic but...

SY!

$2.88/gal. here yesterday.

Now don't go and get 'Manson' on somebody!

I did have a bag of around 500 (minus a few) T-3 green LED's, but I gave most of them to my computer geek brother. Ordered 300 of them from MCM quite a while back and almost 600 is what I got! Too lazy to count them I guess, so somebody just grabbed a few handfuls.
I've thought of using LED's as such and to use as zeners (V-refs) many years ago. I think you guys gave me the bug again!

Cheers
Wayne

edit: My wife tells me it's going for $2.59/gal across the river in KY!

SY · 12th May 2006, 01:46 AM

Wayne, I hate you.

cogsncogs · 12th May 2006, 01:52 AM

Quote:

Wayne, I hate you.

You and a lot of everybody else! But my little 4 banger F-O-R-D wuvs me!

Now where did it go... oh yeah!

up to my eyeballs in cheap gas...

Wayne

Shoog · 17th May 2006, 06:51 PM

Hi there,
Since this thread is not to dead I thought I would ask SY a question concerning his CCS (again).
I am building a 5687 driver stage with 165V on the top. The cathode is biased with 2 red LEDs, giving a bias of 4.2V. The circuit settles at about 145V on the anode of the 5687. The plate current is variable between 18- 25mA. I have a 2K pot to adjust the CCS. I tried various things to increase the voltage drop across the CCS, but nothing helps. The interesting thing is that whatever position the pot is set in there is always just 0.6V between the collector and the base of the first transistor. There is a 9.0V drop from the base to the emitter. When the pot is at its lowest setting (ie highest current through the CCS) the voltage across the LED reference collapses and the LED's go out. At a slightly higher setting the LED's flash in time with the signal.
The CCS has a 22K resistor at the bottom of the reference chain, and 2K above that.

The question is do you think the CCS is working and i'am just not supplying adequate +B ?
The first transistor measures OK but if it failed would this cause the problems ?
Whats going on when the voltage reference is collapsing?
Would increasing the current thought the reference chain solve the problem?

Thanks

Shoog

SY · 18th May 2006, 12:53 AM

Quote:

There is a 9.0V drop from the base to the emitter.

I think there's the source of your problem. That should be 0.7V. Check that transistor. Also, your reference string is carrying 8 mA or so. That's fine, but your 2k resistor is trying to drop 16V! Substitute another LED for it, or drop its value to 470R. I'd just play it safe and sub in the LED.

One other thing- the LEDs in the cathode circuit should drop about 1.7V each. If they're dropping something north of 2V, they may not be suitable; try some super-cheap Radio Shack or similar. The older, crummier, and cheaper, the better.

BTW, this is NOT "my" circuit, it's just one I've found useful and have popularized a bit. It's an old idea, but if one were to credit anyone, it would be the guy who set me down that path, Morgan Jones.

Shoog · 18th May 2006, 08:19 AM

Thanks for that. I will substitute a Green LED (which I have).
On the cathode LEDs, it seems now days that very few Red LED's have a 1.7V voltage drop, most are rated at 2.5V.
I will check my measuremens again.

Shoog

Shoog · 18th May 2006, 06:06 PM

I worked out what was wrong. Boy am I stupid. I had installed the transistors as if they were NPN's (ie with the Emitter and the Collector reversed). Fortunately the transistors hadn't suffered. Everything is now working. Since the Red LED's have a voltage drop of 2.1V I took out one from the reference chain. I also replaced the 2K2 with a green LED.
The plate voltage now adjusts itself to the correct point for the current set.

Thanks for the help, it was your point that the base to emitter voltage should be 0.7V that got me thinking.

Shoog

lndm · 27th May 2006, 12:12 PM

I'm looking to run a string of LED's at 15mA or possibly a little more. Does anyone know how long I can expect them to last, and what will happen if they fail?

SY · 27th May 2006, 01:10 PM

Generally, a LOOOOOOOONG time. LEDs are ridiculously reliable. If you can get hold of Robert Pease's "Analog Troubleshooting," he has an interesting story about that.

Brian Beck · 27th May 2006, 02:05 PM

This LED biasing topic has got me interested. Years ago I bought a bag of 1000 rectangular-shaped red LEDs for $3. Talk about CHEAD red LEDs! I wondered what I would ever do with that many LEDs, but I couldn’t pass up the bargain. 1000 pilot lights for a 1000 projects? Not in my lifetime. Maybe now I know…

12th May 2006, 01:46 AM	#122
SY diyAudio Moderator Join Date: Oct 2002 Location: Austin, TX Blog Entries: 1	Wayne, I hate you. __________________ "...we stumble and get up, we are sad, confident, insecure, feel loneliness and joy and love. There is nothing more; but I want nothing more.” - Christopher Hitchens 1949-2011

27th May 2006, 01:10 PM	#129
SY diyAudio Moderator Join Date: Oct 2002 Location: Austin, TX Blog Entries: 1	Generally, a LOOOOOOOONG time. LEDs are ridiculously reliable. If you can get hold of Robert Pease's "Analog Troubleshooting," he has an interesting story about that. __________________ "...we stumble and get up, we are sad, confident, insecure, feel loneliness and joy and love. There is nothing more; but I want nothing more.” - Christopher Hitchens 1949-2011

27th May 2006, 02:05 PM	#130
Brian Beck diyAudio Member Join Date: Dec 2004 Location: South Florida, USA	This LED biasing topic has got me interested. Years ago I bought a bag of 1000 rectangular-shaped red LEDs for $3. Talk about CHEAD red LEDs! I wondered what I would ever do with that many LEDs, but I couldn’t pass up the bargain. 1000 pilot lights for a 1000 projects? Not in my lifetime. Maybe now I know… __________________ Brian

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12th May 2006, 01:16 AM	#121
cogsncogs diyAudio Member Join Date: Jul 2003 Location: Wayne, West Virginia	Off-topic but... SY! $2.88/gal. here yesterday. Now don't go and get 'Manson' on somebody! I did have a bag of around 500 (minus a few) T-3 green LED's, but I gave most of them to my computer geek brother. Ordered 300 of them from MCM quite a while back and almost 600 is what I got! Too lazy to count them I guess, so somebody just grabbed a few handfuls. I've thought of using LED's as such and to use as zeners (V-refs) many years ago. I think you guys gave me the bug again! Cheers Wayne edit: My wife tells me it's going for $2.59/gal across the river in KY!

17th May 2006, 06:51 PM	#124
Shoog diyAudio Member Join Date: Aug 2002 Location: Eire	Hi there, Since this thread is not to dead I thought I would ask SY a question concerning his CCS (again). I am building a 5687 driver stage with 165V on the top. The cathode is biased with 2 red LEDs, giving a bias of 4.2V. The circuit settles at about 145V on the anode of the 5687. The plate current is variable between 18- 25mA. I have a 2K pot to adjust the CCS. I tried various things to increase the voltage drop across the CCS, but nothing helps. The interesting thing is that whatever position the pot is set in there is always just 0.6V between the collector and the base of the first transistor. There is a 9.0V drop from the base to the emitter. When the pot is at its lowest setting (ie highest current through the CCS) the voltage across the LED reference collapses and the LED's go out. At a slightly higher setting the LED's flash in time with the signal. The CCS has a 22K resistor at the bottom of the reference chain, and 2K above that. The question is do you think the CCS is working and i'am just not supplying adequate +B ? The first transistor measures OK but if it failed would this cause the problems ? Whats going on when the voltage reference is collapsing? Would increasing the current thought the reference chain solve the problem? Thanks Shoog

18th May 2006, 08:19 AM	#126
Shoog diyAudio Member Join Date: Aug 2002 Location: Eire	Thanks for that. I will substitute a Green LED (which I have). On the cathode LEDs, it seems now days that very few Red LED's have a 1.7V voltage drop, most are rated at 2.5V. I will check my measuremens again. Shoog

18th May 2006, 06:06 PM	#127
Shoog diyAudio Member Join Date: Aug 2002 Location: Eire	I worked out what was wrong. Boy am I stupid. I had installed the transistors as if they were NPN's (ie with the Emitter and the Collector reversed). Fortunately the transistors hadn't suffered. Everything is now working. Since the Red LED's have a voltage drop of 2.1V I took out one from the reference chain. I also replaced the 2K2 with a green LED. The plate voltage now adjusts itself to the correct point for the current set. Thanks for the help, it was your point that the base to emitter voltage should be 0.7V that got me thinking. Shoog

27th May 2006, 12:12 PM	#128
lndm diyAudio Member Join Date: Mar 2006 Location: nsw	I'm looking to run a string of LED's at 15mA or possibly a little more. Does anyone know how long I can expect them to last, and what will happen if they fail?

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