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74HC132 testing results

earckens

Member
50 74HC132 SOP-14 werd tested as freerunning oscillator. Two thirds failed: one or more gates of 4 per IC failed to act as oscillator (1M and 1uF). They were obtained from 2 or 3 sources, unfortunately I can't distinguish from which.
What is the forum's comment on this?
 

ChrisP58

Well-Known Member
You should know by now that the first question the forum members will ask is: Where's the schematic?

Not just the simple oscillator, but what are the rest of the connections.
 

earckens

Member
You should know by now that the first question the forum members will ask is: Where's the schematic?

Not just the simple oscillator, but what are the rest of the connections.
The same schematic as audioguru shows in his reply plus a LED from each gate output on a 500ohm resistor to ground.
Nothing else.
 

Les Jones

Well-Known Member
Did you try NOT loading the output of the oscillator outout with the LED and resistor but buffering the output using one of the other gates to drive the LED ?

Les.
 

earckens

Member
Did you try NOT loading the output of the oscillator outout with the LED and resistor but buffering the output using one of the other gates to drive the LED ?

Les.
Good idea! The spec sheet shows max load per output to be 20mA, but I try your suggestion tomorrow, I will feedback asap.
 

earckens

Member
Did you try NOT loading the output of the oscillator outout with the LED and resistor but buffering the output using one of the other gates to drive the LED ?

Les.
That made a difference: all pass now. Thanks too for ChrisP58: schematics from now on.
 

audioguru

Well-Known Member
Most Helpful Member
With a supply of 6V a 74HC IC has an output current of 26mA when it is shorted. But it will not oscillate when its output is shorted and some will barely oscillate in your circuit when the output current is 8.7mA.
With a 4.5V supply then the output current is only 15mA when shorted and some barely oscillate when the load is 5mA.
With a supply of 2V or 3V then guess how little output current will prevent oscillation. The LED won't light up anyway.
 

gophert

Well-Known Member
Most Helpful Member
That made a difference: all pass now. Thanks too for ChrisP58: schematics from now on.
It works much better when the oscillating gate Has essentially no load (output sent to another gate on the chip to act as a buffer) to power the LED.
 

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