# Simple diode circuit help

Discussion in 'General Electronics Chat' started by roybk, Aug 23, 2011.

1. ### roybk Thread Starter New Member

Aug 23, 2011
1
0
Excuse me, my circuit skills just got rusty.
Can anyone please give me a clue what would the voltage rating be at the purple node? appreciated!

R.

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2. ### SgtWookie Expert

Jul 17, 2007
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Here's a datasheet for the BAV99; a dual diode:
http://www.fairchildsemi.com/ds/BA/BAV99.pdf
It's being used as a clamp to limit the voltage excursions above and below ground.

The AUTO_IN - whatever that is - limits the current through either diode in the BAV99 in case the signal goes above 5v or below ground. The clamp at low current values (1mA) is the positive supply +~.7v to ground - ~0.7v

3. ### t_n_k AAC Fanatic!

Mar 6, 2009
5,448
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Do you mean voltage rating or range?

Presumably input AUTO_IN can have a somewhat unspecified range (within limits). This bit of circuitry seems to be designed to ensure the volatge at the node in question does not go outside the nominal range of 0 to 5V.

The two diodes are orientated & biased to achieve that outcome.

Dec 26, 2010
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Aug 20, 2011
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That voltage at that point, if we assume the fvd of diode 0.7V, will not be less than -0.7V and greater than +4.3V

6. ### t_n_k AAC Fanatic!

Mar 6, 2009
5,448
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Greater than 5V+Vf ..... The node has to climb over 5V to get the top diode forward biased.

Last edited: Aug 23, 2011