[Test-Equipment] Question Regarding Fluke 8050A

[Barry]

Hi David,

I had thought about that as well.  Since mine is the model without the batteries, there is indeed room for another transformer and I might just go that route.  I was thinking that since it does not have the batteries and, hence, no charger current required, there might be extra juice available but they used a different transformer for the batter-powered models.

I think the entire unit is rated at 4W and this retrofit could nearly double that.  Unless they built in a fair amount of breathing room, I'm thinking the transformer will be overloaded as well.

Thanks,
Barry - N4BUQ

----- Original Message -----
> From: "David" <davidwhess at gmail.com>
> To: "Discussion of Electronic Test Equipment" <test-equipment at mailman.qth.net>
> Sent: Wednesday, June 3, 2015 1:11:17 PM
> Subject: Re: [Test-Equipment] Question Regarding Fluke 8050A
> 
> 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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