# PSU Current display level

#### PJB

Joined Oct 24, 2019
31
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 ?

#### dl324

Joined Mar 30, 2015
16,993
Is this normal or some kind of fault ?
If the 555 is operating as an oscillator, it could be normal.

A schematic and information on the current resolution on your supply would be helpful.

#### Ya’akov

Joined Jan 27, 2019
9,233
In what way does it change?

#### PJB

Joined Oct 24, 2019
31
The value of the PSU current display changes as the astable changes; the voltage is stable at 9 volts.

#### dl324

Joined Mar 30, 2015
16,993
The value of the PSU current display changes as the astable changes
By how much?
the voltage is stable at 9 volts
We'd expect this when a power supply is operating in constant voltage mode.

#### PJB

Joined Oct 24, 2019
31
By how much?
We'd expect this when a power supply is operating in constant voltage mode.
It changes between 0.014A and 0.022A, I believe this is 14 mA and 22 mA.

#### zophas

Joined Jul 16, 2021
165
Only way to be sure is to show us the 555 circuit. But 8mA variation does not seem like a fault condition.

#### Papabravo

Joined Feb 24, 2006
21,259
@PJB - A circuit diagram if you please.

#### dl324

Joined Mar 30, 2015
16,993
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.

#### PJB

Joined Oct 24, 2019
31
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.

#### Attachments

• 47.4 KB Views: 6

#### dl324

Joined Mar 30, 2015
16,993
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:

#### zophas

Joined Jul 16, 2021
165
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.