[volt-nuts] Fluke 845

[Dick Moore]

On Mar 25, 2010, at 8:44 PM, volt-nuts-request at febo.com wrote:

> Message: 2
> Date: Thu, 25 Mar 2010 22:50:56 +0100
> From: "Dr. Frank Stellmach" <drfrank.stellmach at freenet.de>
> Subject: [volt-nuts] HP 419 & Fluke 845
> To: volt-nuts at febo.com
> Message-ID: <4BABDAC0.1010505 at freenet.de>
> Content-Type: text/plain; charset=ISO-8859-15; format=flowed
> 
> On those units, the opto chopper, i.e. the neon bulbs were the crucial 
> parts, always.
> 
> I suggest trying to replace the neons by LEDs, assuming neons are also 
> red emitters.
> 
> Their very low leakage currents (not specified) perhaps rely on the CdS 
> switches.
> So a replacement by a 7650 would be sufficient, but perhaps worsen 
> parasitic currents.
> 
> Frank

Frank's idea of using LEDs for the opto source is a good one -- bright and long-lived. But remember that these neon bulbs were driven by relatively high voltage AC signals, so some kind of divider would be needed. I've seen blue, white and green LEDs that would shine fairly brightly on 10-100 microamps of current, so a simple series resistor would suffice for a divider/current limiter, although a red or orange LED might be better -- I can't remember the curve for CdS cells.

Best,
Dick Moore