fuzzy behaviour of logic gates

[desperadogear]

hi every body, i am facing the following problems with digital logic gates (I.C'S)

CD 4072( a 4 I/P or gate) , CD 4030 (AN EXOR GATE).

The circuit in all uses around ten digital ICs,(CD4072,CD4030,CD4015,74ls04, 74ls08, 74ls154,DTMF transmitter,reciever IC). Im using a power supply rated 5V 500mA DC.

Instead of logic 1 (5v) and logic 0(0v) , i got some strange values(1.90V, 2.24V.,etc) as output of 'some' gates.Has anyone else experienced this fuzzy behaviour..if yes what could have been the problem.
Im using a constant DC voltage of 5V. But after connecting supply to the circuit, The voltage across is shown as 4.53 V. I wonder where the rest of voltage dropped.

help greatly appreciated...

 

[dch222]

Sounds like you have outputs which are switching (by design) or oscillating (due to unconnected inputs?). If you are just using a voltmeter it will give some average value like you are seeing.

If these are 'scope observations, then it does sound like an output is shorted to ground somewhere dragging down internal circuitry, which would explain the volt drop on the supply.

If the supply is delivering 500mA as you suspect, the 2.5W being dissipated should be easy to find from the smoke.
I assume the 0.6V is not being dropped by a series protection diode

 

[philba]

Or it could be several chips fighting over the output state - one driving it high, the other driving it low. I'd inspect for shorts or connection errors. Check the actual current draw (briefly). You could also feel for which chips are getting hot. though do be carefull, I once had a microchip logo burned into my thumb - it was still legible after a week. ouch.

Baring that, I'd start by progressively powering different sections until you discover the error. generally a methodical approach will reveal the problem(s).

 

[desperadogear]

excellent.. i guess u found the culprit... 'unconnected inputs'
thanks for feedback @dch222 ,i appreciate your professional treatment to the problem.

by design it certainly should not oscillate . it is meant to give stable values of 0 or 1 and do password checking using some xor gates and shift registers. mine were voltmeter observations.

If these are 'scope observations, then it does sound like an output is shorted to ground somewhere dragging down internal circuitry, which would explain the volt drop on the supply.

even this could be possible... i will have to check all the outputs.There is no series protection diode used in the circuit, which forces me to consider possibility two that you have mentioned too.

There's one more symptom i forgot to mention. This fuzzy behaviour is exhibited for a few seconds and for a few seconds the circuit acts innocent and is fine..
Il do my best to find if any of those output pins is grounded. In the mean time any suggestions/views..

 

[desperadogear]

You could also feel for which chips are getting hot.

i can find 54ls154 getting warm but not really hot.I believe this is natural.isnt it?

I'd start by progressively powering different sections until you discover the error.

I cannot do this unfortunately as this is a connection on a pcb. This circuit is take from an electronics magazine and is supposedly tested.

 

[philba]

that kind of logic should run totally cool unless you meant for the chip to drive a load (kind of unlikely based on what you said).

by the way, unconnected CMOS inputs are a no-no. You will pick up all kind of astral vibrations (ok, stray emi but it does seem like voodoo when you experience it) causing the outputs to dance around. I'm not sure that would pull that much current, though.

 

[AllVol]

This may sound old hat, but are your circuit rails properly bypassed?

The liberal use of 100uf electolytics between + and - rails can eliminate a lot of problems, especially with digital work, as has been mentioned before.

 

[Roff]

that kind of logic should run totally cool unless you meant for the chip to drive a load (kind of unlikely based on what you said).

by the way, unconnected CMOS inputs are a no-no. You will pick up all kind of astral vibrations (ok, stray emi but it does seem like voodoo when you experience it) causing the outputs to dance around. I'm not sure that would pull that much current, though.

My first CMOS breadboard was a UART, before you could but IC UARTs, around 1970. That's when I learned about floating inputs - the hard way. This thing had 20 or 30 ICs, and it was wirewrapped, so the individual pins were kinda hard to see. Since it wasn't working, I started checking chip temps with my finger. Several were noticeably warm. I used a remote-reading IR thermometer to find almost all of the chips with floating inputs. IIRC, several of the chips were 5 to 10 degrees C above ambient. Of course, I went back to the schematic eventually, but my point is, floating inputs can definitely cause a CMOS chip to draw DC current, which can be a real problem on a battery-powered circuit. Unused gates, flip-flops, etc., are especially insidious, because the don't interfere with operation.

 

[desperadogear]

I found that all the inputs to be given from the DIP switches were hanging. I used pull down resistors to all of them, fixed a 100uF capacitor across the rails and IT WORKS!!!!.
so the problem was floating inputs.

Lesson learnt: check for floating inputs in digital ICs and never leave them without connecting proper pull up/pull down resistors.

Thanks to all of you who replied to my thread!!!!
by the way is there any special reason/therory why we should use 100 uF in digital circuits

 

[AllVol]

Iby the way is there any special reason/therory why we should use 100 uF in digital circuits

Basically, it is frequency dependent. I've learned that the lower the frequency of operation, the more capacitance needed, as a rule of thumb.

The late TJ Byers, in his last column in Nuts and Volts said,
"Many schematics that you find published in magazines and books leave the bypass capacitors out. It's assumed you KNOW to put them in."

Good advice.

 

[philba]

Basically, it is frequency dependent. I've learned that the lower the frequency of operation, the more capacitance needed, as a rule of thumb.

I've been curious about this for a long time but have never been able to find a discussion that isn't deeply theoretical. I presume that the bypass cap and the impedence of the power rail form a low pass filter. however, I have no way of determining the actual impedance of the rail so all my bypass caps are sized by spitting in the wind. 100n seems to work well for most stuff. I can sleep at night with that. lol

 

[Hero999]

The main reason smaller capacitors are use for higher frequencies is because larger capacitors behave as inductors (or even worse tuned circuits) at higher frequencies.

In general I imagine that providing the tuned circuit formed by the PCB trace and decpoupling is lower than the frequency of the circuit then there won't be any problems.

 

[audioguru]

A 555 creates a current surge of 400mA from the power supply when its output switches.
An old TTL logic output creates a current surge of tens of mA when its output switches.
A 74HCxx high speed Cmos logic circuit creates a current surge of about 60mA when its output switches.
An ordinary Cmos logic output creates a current surge of a few mA when its output switches.

You need to have a high frequency capacitor near the switching IC to provide the current for the surge or the supply voltage will drop and mess up other circuits.