[Test-Equipment] Question Regarding Fluke 8050A

[Barry]

Yes, that would be my preference as well.

As an update, I connected all five displays on a breadboard, powered it with an external supply, and watched the current demand.  The maximum I could get out of all those was about 310 mA.  I tested them with the four primary digits connected together and stepped it through 0000, 1111, 2222, etc. (the first digit the +/-/1 and it doesn't pull much current).  Oddly, 4444 drew the most power.

It turns out I had an SN7400N that I was using as an inverter and I was testing with that out of the circuit.  When I plugged that in, though, the current demand shot to over an amp!  I think something may be wrong with that chip and was the primary reason I was seeing such a high power demand.

I have secured some SN7404N inverters and am going to see what one of those does to the circuit.  Hopefully that was the main problem and it all just might still work as-is (with a 7404).  If not, I may still see if the problem is the regulator being pushed to its limit and using a separate regulator might still be an option.

Unfortunately, I can't get the plastic cover off of the transformer to measure the wire diameter of the various windings.  I may still try to get that off, though, as I don't really think its on there all that securely but didn't want to take a chance damaging things at that moment.

Thanks again,
Barry - N4BUQ

----- Original Message -----
> From: "David" <davidwhess at gmail.com>
> To: "Discussion of Electronic Test Equipment" <test-equipment at mailman.qth.net>
> Sent: Thursday, June 4, 2015 8:19:10 PM
> Subject: Re: [Test-Equipment] Question Regarding Fluke 8050A
> 
> I think the simple way would be to use a small solid state relay
> powered from the DC side after the switch to control the second
> transformer.  Alternatively leave the auxiliary transformer powered
> and switch the secondary side.
> 
> This brings up something I have wondered about; is it best to switch a
> transformer using a zero crossing switch or switch it at the peak of
> the input voltage?  With a zero crossing switch, the transformer will
> see a peak flux higher than normal but if switched at the peak, the
> surge current into the secondary will be maximum.
> 
> On Wed, 3 Jun 2015 14:34:17 -0400 (EDT), you wrote:
> 
> >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
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