[Test-Equipment] Question Regarding Fluke 8050A

[Barry]

Correction:  Each chip's Vcc is connected to ground and the negative post of the external supply is connected to each chip's ground pin.  in other words, the displays are getting their power from the external supply.

Barry - N4BUQ

Sent from my iPad

> On Jun 10, 2015, at 10:16 PM, Barry <n4buq at knology.net> wrote:
> 
> As an update, I connected the four displays to the data bus and ran the four strobes through an inverter, each to its appropriate display. Since the onboard -5 volt supply isn't enough to handle the additional load of these displays, I connected the positive terminal of my external power supply to ground and the negative terminal to Vcc of each display (and the 7404).  The power supply is set to 5v and is floating above ground.
> 
> This is working but the meter now does something rather odd. At power up, it takes it several seconds to "settle".  In ohms mode and a random resistor connected to its probes, it starts out as if it is measuring an infinite resistance (only the leading "1" displays).  After a few seconds, the display flashes several "random" numbers.  In a few more seconds, it settles down to show the measured resistance.  From that point, the meter continues to function normally with both sets of displays showing the same sets of four digits showing the current resistance value.
> 
> Any ideas what's causing that funny behavior?  Have I violated too many good practice principles with this setup?
> 
> My intention is to implement a separate, regulated -5v supply but tried this approach until I get my 7905 regulator to see if I can get the prototype working.
> 
> Thanks for reading,
> 
> Barry - N4BUQ
> 
> 
> 
> Sent from my iPad
> 
>> On Jun 5, 2015, at 4:15 PM, Barry <n4buq at knology.net> wrote:
>> 
>> 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
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