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Old 4th May 2010, 05:30 PM   #1
phrarod is offline phrarod  United States
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Default Need confirmation on LED bias for circuit

I need bias voltage of 2.2 - 2.5v so according to the threads I should pick a green LED and the orientation I've seen in schematics have the - leg on the cathode and + on ground (which seems weird).

I am only using 1/2 of the tube and its transformer output in a very simple linestage.

So if I remove the cathode bias cap/resistor and just put in the green LED as outlined above it will work? I need between 11-13 mA of bias current. Right now a 222 Ohm resistor provides that.
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Old 4th May 2010, 05:48 PM   #2
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Sounds about right. You may want to test polarity with a batery and resistor. I have on several occasion gotten it backwards after cutting the leads of to fit the component.

Not all Green LEDS are created equal so you may have to try a few different ones. You can measure the potential it will regulate at by putting a 600R - 1k resistor in series with a 9volt battery. Then measure the potential across the LED.
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Old 4th May 2010, 05:53 PM   #3
phrarod is offline phrarod  United States
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Great! That's also what I was searching for is a way to easily measure them before just slugging them in.

What I've read is the longer leg is positive. If you forget or cut the leads the larger section within the LED is the negative.

Running to a surplus store today so I was hoping to get a answer before I went. In the threads I've read here they said to get the 'dimmest' LED possible as they will have the lowest resistance.
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Old 4th May 2010, 05:55 PM   #4
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I still on occasion get them backwards!
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Old 4th May 2010, 05:58 PM   #5
phrarod is offline phrarod  United States
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Trust me I've seen that unwanted 'flash' from LEDs.
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Old 4th May 2010, 06:17 PM   #6
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Phrarod, the longer led is indeed the anode (+). It should go to the tube cathode. Often times the underside of the led body is flattened at the cathode (-) which can help when the leads are cut.

BTW, did you know: if you forget the series resistor on a red led, you get an orange led. No kidding!

Kenneth
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Old 4th May 2010, 06:18 PM   #7
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Quote:
Originally Posted by phrarod View Post
LED and the orientation I've seen in schematics have the - leg on the cathode and + on ground (which seems weird).
To be clear, it goes this way.
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File Type: gif led_bias.gif (998 Bytes, 351 views)
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Old 4th May 2010, 06:33 PM   #8
phrarod is offline phrarod  United States
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But in the schematic showing orientation of the diode. I guess I assumed the line connecting the point of triangle is the + of that diode. At least when I put diodes in a circuit that's what always worked.

Can you tell I'm a little confused?
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Old 4th May 2010, 06:42 PM   #9
Arnulf is offline Arnulf  Europe
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Diode - Wikipedia, the free encyclopedia

If you want diode to conduct in forward direction you'll have top connect anode to positive and cathode to negative supply, just like kavermei told you and just like what dsavitsk's picture shows. This is just like vacuum tubes, nothing to be confused about here
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Old 4th May 2010, 06:45 PM   #10
phrarod is offline phrarod  United States
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Thanks everyone!
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