[Test-Equipment] Question Regarding Fluke 8050A

[Barry]

I had the 7400 plugged in backwards. :(

That explains the massive current draw.  Silly little notches!  Sigh...

Thanks,
Barry - N4BUQ

----- Original Message -----
> From: "Barry" <n4buq at knology.net>
> To: "Discussion of Electronic Test Equipment" <test-equipment at mailman.qth.net>
> Sent: Thursday, June 4, 2015 8:33:05 PM
> Subject: Re: [Test-Equipment] Question Regarding Fluke 8050A
> 
> 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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