[Test-Equipment] Question Regarding Fluke 8050A

[Barry]

A good suggestion.

With another transformer, I'll need a way to turn its output off.  The 8050A's transformer is "on" anytime it is plugged in.  The + and - lines from the rectifier are switched on and off with the power switch so I'll need to add something to turn on and off the new power supply.  More stuff to add...

I plan to see if this is all going to work by powering the LED array with an external supply.  If that works, then great, but if not, I think I'll resort to normal 7-segment displays and dropping resistors.

Thanks again,
Barry - N4BUQ

----- Original Message -----
> From: "Ian Wilson" <ianmwilson73 at gmail.com>
> To: "Discussion of Electronic Test Equipment" <test-equipment at mailman.qth.net>
> Sent: Wednesday, June 3, 2015 1:18:44 PM
> Subject: Re: [Test-Equipment] Question Regarding Fluke 8050A
> 
> 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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