555 False Triggering

Thread Starter

nevis2010

Joined Apr 28, 2016
7
Hi, i have the following circuit. When the 25W DC motor or the Air Compressor is turned on the current to the 555 circuit drops and causes false triggering of the 555 monostable circuit and issues with the 555 astable circuit. Does anyone have any ideas?
 

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AnalogKid

Joined Aug 1, 2013
8,537
Your circuit shows no decoupling or other power management. Since the 555's drive relatively compliant loads (a relay and an LED), Consider this:
Place a rectifier diode such as a 1N4004 between the +12 V supply and the 555 circuit.
Place a 10 uF capacitor from the diode cathode to the 555 circuit GND.
Place a 0.47 uF capacitor across each 555's power and GND pins.

This gives the 555 circuits a stable power source even when the main power drops out momentarily.

ak
 

hp1729

Joined Nov 23, 2015
2,304
Hi, i have the following circuit. When the 25W DC motor or the Air Compressor is turned on the current to the 555 circuit drops and causes false triggering of the 555 monostable circuit and issues with the 555 astable circuit. Does anyone have any ideas?
Isolate the power to the LM555s from the 12 V line. Stick in a couple of hundred ohms and a big capacitor (1,000 uF) on the 555 side.
 

AnalogKid

Joined Aug 1, 2013
8,537
Delete the 200 ohm resistor and connect the 1000 uF capacitor directly between the diode cathode and GND. The suggestion in post #5 was for a *series* resistor, in place of the diode. I think the diode gives more consistent 555 performance.

ak
 

hp1729

Joined Nov 23, 2015
2,304
Delete the 200 ohm resistor and connect the 1000 uF capacitor directly between the diode cathode and GND. The suggestion in post #5 was for a *series* resistor, in place of the diode. I think the diode gives more consistent 555 performance.

ak
Agreed, the diode is a better idea than the resistor.
 

Thread Starter

nevis2010

Joined Apr 28, 2016
7
I have add the new parts as suggested. The issues is resolved by the 1000uF capacitor & diode as well as the clamp diodes on the AC pump. How do clamp diodes work?
 

Thread Starter

nevis2010

Joined Apr 28, 2016
7
How would i put a clamp diode on the AC pump? The diode on the 12vDC works well. Not sure if i can use a diode on 240vAC? Any ideas?
 

Thread Starter

nevis2010

Joined Apr 28, 2016
7
Its the AC Air compressor thats causing the issue. Its connected before the AC-DC SMPS. The solutions above have stopped the 12vDC pumps causing any issues, its just the AC air compressor. Any ideas would be most welcome.
 

Thread Starter

nevis2010

Joined Apr 28, 2016
7
This is the present circuit, the air compressor is controlled by a pressure switch, when the blow gun is pressed it turns the pump on. In addition sometimes the 555 circuits trigger.
 

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dannyf

Joined Sep 13, 2015
2,197
Your capacitor is of no use for those two pumps. Take the power electronics out of power domain from which the 555s are attached.
 

ronv

Joined Nov 12, 2008
3,770

Thread Starter

nevis2010

Joined Apr 28, 2016
7
Thanks, the RC network looks like the right part. Any idea how i size an RC network? the AC compressor is 230vAC, 550W.
 
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