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Old 6th October 2014, 12:33 AM   #1
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Default capacitance of LED dark

I want to clamp the input voltage of my linear amp board to +- 1.6 v with a led. I have some Lumex LX3059srsgw led. the data sheet doesn't show anything like that. Power dissipation and a 30 ma current limit. They are bidirectional power supply red and green so they would clamp voltage 2.0 v either way.
I have a pi filter on amplifier input: 10 uf cap in series with 2 ea 4.7k resistors, then the input transistor base. 27 pf cap is between 4.7k resistor center and input transistor emitter. Max amp output supposed to be at 1.5 VAC. I want to parallel the 27 pf cap with the led. I don't know the capacitance. 6 ea 1n4148 would do it, 3 back to back either way but I don't have room. 1n4148 are showing 4 pf capacitance. Back to back bzx79-2v4 zeners would be close, clamping at 3 v, but those have 450 pf of off capacitance.
So does anybody have a red/green two lead low power led and a capacitance meter that goes below 100 pf? Can you find out the off state capacitance ? thanks.
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Last edited by indianajo; 6th October 2014 at 12:55 AM.
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Old 6th October 2014, 12:53 AM   #2
Art M is offline Art M  United States
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LX3059srsgw is a 3 lead device, common cathode according to data sheet ?
Looks like up to a 0.5V difference in forward voltage for the Red/ Green diodes.
This will not work for even handed clamping ?
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Old 6th October 2014, 12:58 AM   #3
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The transistor has back voltage Vbe of 6. I just don't want to blow anything up if I turn the disco mixer up too loud- it could put out 6 VAC pretty easily. I can parallel the two LED outer (hot) leads pretty easily. the output of the amp will clip somewhere between 1.5 v and 2, I don't have to worry about the sound, it would be clipping badly with this much input.
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Old 6th October 2014, 01:47 AM   #4
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The common-cathode LED won't be bidirectional. Only half the AC wave will be clamped. You want the two-lead "tricolor" type of LED.
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Old 6th October 2014, 01:51 AM   #5
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I don't want to wait a week and pay $6 freight. Actually, since the transistor conducts forwards, I could use one led backwards across b-e. The armstrong 621amp used one silicon diode backwards across the input be junction, I don't see the sonic wisdom of this unless they selected for 0.7 v diodes and 0.6 v transistors. But a led doesn't conduct until way above 0.6 v. It should be inert at 1.5 vac full power input except for capacitance.
Does anybody know the capacitance of red low power LED's? I've got some old HP datasheets and they never specified dark capacitance.
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Old 6th October 2014, 02:04 AM   #6
rayma is offline rayma  United States
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Quote:
Originally Posted by indianajo View Post
Does anybody know the capacitance of red low power LED's? I've got some old HP datasheets and they never specified dark capacitance.
The capacitance is nonlinear with voltage, but will be around 30-50 pF.
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Old 6th October 2014, 02:07 AM   #7
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The capacitance could well be dynamic, dependent on applied voltage and polarity. A Googled EDN article stated a typical LED capacitance of 50pF. Perhaps doubled with paralleled pn junctions.
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Old 6th October 2014, 04:30 AM   #8
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50 pf thank you. I will just leave out the 27 pf cap on the print (a non-linear x7u) and put in a non-linear red led the opposite way from the b-e junction. Had it been 450 pf like a zener, that might have been a sound problem.
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Old 6th October 2014, 06:56 AM   #9
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Quote:
Originally Posted by sofaspud View Post
The capacitance could well be dynamic, dependent on applied voltage and polarity.
Like any PN junction the LED capacitance WILL depend on the applied voltage - maximum capacitance as the diode starts to break into forward conduction, decreasing as the REVERSE bias voltage increases. For practical purposes there is little change in the capacitance for reverse voltages beyond about 3 - 4 volts.

I believe the basic device physics gives a pretty accurate prediction of the junction capacitance, so a decent SPICE model of the LED could be used to determine the capacitance-versus-reverse bias curve in simulation. (Unfortunately, the one time I tried to verify an LED's published SPICE model against measured behavior the agreement was rather poor but, of course, that doesn't prove ALL models for LED's are bad.)

Quote:
A Googled EDN article stated a typical LED capacitance of 50pF. Perhaps doubled with paralleled pn junctions.
Is it the article "An LED's intrinsic capacitance works in a 650-mV LRC circuit" at < http://www.edn.com/design/led/436839...mV-LRC-circuit >? It doesn't give any reference for the "typical" value they quote, but the circuit described in the article seems to suggest some clever ideas for measuring the capacitance.

Dale
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Old 6th October 2014, 08:53 AM   #10
Elvee is offline Elvee  Belgium
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Quote:
Originally Posted by indianajo View Post
I've got some old HP datasheets and they never specified dark capacitance.
Philips publishes that data: for the CQT24 (an ordinary, 5mm bicolor LED) the typical capacitance @0V & 1MHz is 160pF
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