[Test-Equipment] Question Regarding Fluke 8050A

[Ian Wilson]

Perhaps use a repurposed wallwart, add a voltage regulator
and you're done.

73, ian K3IMW

On Wed, Jun 3, 2015 at 11:11 AM, David <davidwhess at gmail.com> wrote:

> You could test it but my guess is that even with a separate regulator
> the transformer will be overloaded.
>
> Is there room for a second transformer?
>
> On Wed, 3 Jun 2015 09:36:41 -0400 (EDT), you wrote:
>
> >I have a Fluke 8050A that's showing early signs of LCD disease.  I know
> there are several options out there for retrofitting this with 7-segment
> LEDs, programmable displays, etc., but my thought was to use some HP
> 5082-7032 "dot-style" LEDs (the kind with the built-in decoder/latch that's
> seen in gear like the 8640B, 8655A, etc.).  Instead of using the existing
> decoder/latch ICs, these can be driven directly from the 4-bit data lines
> and strobe them with the existing strobes (just need to invert those) and
> powering them from the existing -5V line.
> >
> >I put together a breadboard with the last two digits working in
> conjunction with the LCD and all was fine.  When I added the third display,
> the system stopped working.  The LCD began to show all zeros and the LEDs
> were showing somewhat dim zeros as well.
> >
> >It turns out these little LEDs are pretty power hungry and adding the
> third one was enough to drag the Vcc line down to about 3.5V as well as
> causing that power rail to start oscillating significantly.  Looking at the
> specs, those LEDs need 170mA in a "worst case" scenario (depends on how
> many individual LED dots are needed - assuming an "8." is worst case) with
> typical consumption at 94mA.
> >
> >The -5V rail is delivered via an LM7905 and my first thoughts were that
> the additional power required for these LEDs were too much for that (along
> with everything else it's being called on to provide power for).  Would
> that be a reasonable assumption?  If so, I'm thinking I could incorporate
> another 7905 just to supply these LEDs; however, that gets into whether the
> transformer can handle the additional load (counting on needing about 3/4
> amp more for all these new displays) and I don't know the specs on the
> transformer.  Of course, I can lose the old decoder/latches and the LCD
> will be gone so there's some power saving there but I think that may be
> small compared to these LEDs.
> >
> >I can always go with some of the previously-mentioned options to retrofit
> this thing but I really like those HP displays and they fit perfectly in
> the limited space.  I'm thinking I'm approaching the limit of what the
> transformer can be called on to deliver and so may be wasting my time but
> not sure.  Any thoughts?
> >
> >Thanks,
> >Barry - N4BUQ
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