Voltage regulator question

Thread Starter

mbird

Joined Jan 29, 2009
35
Hi --

I made a simple 555 Timer Astable Multivibrator Circuit that pulses every 1 second.

When I run this with two 1.5VDC batteries it works fine.

I wanted to experiment with voltage regulators so I got a UA78M33CKCSE3 (Jameco Datasheet) Voltage regulator. I also had one of those common wall plug power supplies (it says it is 6VDC 300mA). I connected the power supply to the voltage regulator (input and common) and I connected the output to a 22 Ohm resistor (so I expect 3.3VDC 150mA and that is what I see).

Now I use the output (with the 22 Ohm) and common to power my 555 circuit. The result is that pin 3 of the 555 is always 3 volts! If I switch back to my battery it goes 0 - 3 volts over over 2 seconds as expected.

Every where I check the voltage regulator seems fine (it has 3.3 volts and the current is around 150mA).

I must be missing a key idea about using a voltage regulator??? If you can help with any ideas it would be appreciated!

Thank you!

 

bertus

Joined Apr 5, 2008
20,888
Hello,

Did you put the two capacitors, 0.1 μF on the input to ground and 0.33 μF on the output to ground.
These capacitors are needed to prevent the regulator from oscillating.

Greetings,
Bertus
 

SgtWookie

Joined Jul 17, 2007
22,220
Adding to what Bertus said, you will also need a larger capacitor on the output of the regulator, in the range of 100uF to 330uF. 555 timers create a momentary short from Vcc to ground when they are changing output states. Unless you have a fairly large "bypass" capacitor, the timer can appear to get "hung up".
 

Thread Starter

mbird

Joined Jan 29, 2009
35
Thanks -- your info fixed the problem (I did not have any capacitors in my original experimenting!).
 
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