Strange amperage reading on multimeter

[JJBladester]

Hello everyone,

I created a 4-Digit multiplexed display.  It is supposed to light up one of four seven-segment displays at a time.  The displays are common-cathode with an "on current" of 5 mA.  

With a Vcc = 5 V and using 1 kΩ resistors, I should be able to get a current of 5 mA.

However, when I hook up a multimeter between my decoder and my seven-segment display, I get a current in the pico-Amp range.

Also, placing a measurement probe in the location indicated on my circuit (see attached file) causes a simulation error to occur.

Any ideas would be very  much appreciated!

[Tom__Sedlack · Accepted solution]

Your LED current calculation is off.  You are not taking into account the voltage drop across the LED when calculating your resistor to get 5mA.  Try (5V-VFwd)/5ma.  VFwd is normally around 2 volts or so.

The 74154 are not tri-state output parts which means that you can not wire-or them as you have done.  The reason you are getting the error is because the two parts are fighting against each other to control the logic state of the line.  You need to find a different method of connecting the data selectors.

[JJBladester]

Thank you.  I have added tri-state buffers to the two multiplexers which share a common data bus.  This fixes the unknown voltage issue.  Your suggestion about voltage drop was helpful.