Re: Pete Lefferts LED current source

On Tue, 21 Oct 2008 08:25:18 -0700 (PDT), miso@xxxxxxxxx wrote:

On Oct 21, 7:51 am, Walt Jung <test...@xxxxxxxxxxxx> wrote:
I am trying to establish who and when first used an LED (RED or GRN) as
a voltage reference to bias the base of a bipolar xstr as a current
source/sink. To my best knowledge, this was Pete Lefferts of NSC, back
in 1975.

1.      Peter A. Lefferts, ?LED Used as Voltage Reference Provides
Self-Compensating Temp Coefficient,? Electronic Design, Feb. 15, 1975,
pp. 92.

Comments welcome.

Walt Jung

LEDs were used in some audio power amps for biasing if you want a
commercial application.

An interesting side note, when the high efficiency LEDs hit the
market, some chips had difficultly driving them due to the higher "on-
voltage" (forward bias). That is, you drove them with some chips, and
the light was dimmer due to the current source running out of
compliance. When I did the MAX7219, I put in more headroom to handle
these higher voltage devices.

I had the opposite problem. We use blue led's on module front panels
as VMEbus activity indicators. The early Cree SiC parts neeeded 50 mA,
so I drove them with a couple of 74F38's, 5 volts, 27 ohms. As the
blue led's kept getting better, my customers started complaining about
being blinded by them. We're down to about 4 mA now.

Which brings up another issue... for later.

Gotta go.

John


.

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