A noisy output...

Thread Starter

ScorpFire

Joined Jun 4, 2009
13
I have a modem which has a (0v when low, 2.8v when high) digital output.

I have a an external relay module that i want to control with this output. Basically turn on the relay when the output is high.

However i see that whenever i turn my modem output to high, the relay produces a buzzing sound instead. The contacts do not close. I suspect that the signal may be noisy, is there any way to reduce the noise? perhaps with capacitors or something.

Here is my circuit:

 

bertus

Joined Apr 5, 2008
22,270
Hello,

It can be that the voltage on C1 is not stable.
Change it to 1000 uF / 25 Volts for a try.
The voltage on C1 will be around 15 volts as you use a 12 volts AC.

Greetings,
Bertus
 

Paulo540

Joined Nov 23, 2009
191
If the ripple isn't sufficiently filtered out, the relay won't fully 'kick over' because the voltage is dropping below the minimum pull-in. every pulse.

As bertus mentioned, you need a bigger cap, based on the current the relays will be drawing.
 

russ_hensel

Joined Jan 11, 2009
825
Do you have a scope? If so look at the modem output to make sure it is dc. If not use a blocking cap and meaure the output voltage with an ac meter. Also check the power supply voltage for the relay, it should be 90% dc.
 
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