PSU Current display level

Thread Starter

PJB

Joined Oct 24, 2019
13
I have a power supply unit connected to a 555 charging circuit. The input to the circuit is 9 Volts. When the circuit operates
my PSU current display level keeps changing. Is this normal or some kind of fault ?
 

Thread Starter

PJB

Joined Oct 24, 2019
13
The value of the PSU current display changes as the astable changes; the voltage is stable at 9 volts.
 

dl324

Joined Mar 30, 2015
13,846
It changes between 0.014A and 0.022A, I believe this is 14 mA and 22 mA.
8mA doesn't seem unreasonable to me; a bipolar 555 can sink/source 200mA. What is the load on the timer? What is it driving? A schematic of the circuit involved would still be helpful.
 

dl324

Joined Mar 30, 2015
13,846
8mA still seems reasonable to me. The load may appear to be constant regardless of the timer output level, but the output stage can have a spike when transitioning between HIGH and LOW.

This is from a Philips AN170:
1637264580086.png
 

zophas

Joined Jul 16, 2021
157
I would not worry about a fault in that circuit, @PJB . You are switching between two leds and one of them (or it's resistor) is drawing just a little bit more current than the other. Leds and other components are all made to certain tolerances and you will seldom find any that perform exactly the same. Some amplifiers have matched output transistors but that is not something you come across too often.
 
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