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how to reduce noise.

sid2286

Aug 24, 2011
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I have a design, wherein the circuit has a 24v input,

I drive a buzzer in a astable mode for a certain condition on 5v.

Also i drive a relay for another alarm condition. this relay is 12v(coil supply)+230vA/c 5A.

Now the problem i'm facing is I'm driving another relay from the exixting relay and some manual switches ahead of it.

But whenever there is a change in condition(switching action on 2nd relay). MY buzzer on the card start buzzing. :(

I tried using an optocoupler for isolation but that too isn't helping.

Please suggested some solution.


Regards,
Sid
 

GreenGiant

Feb 9, 2012
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If Im understanding this correctly I think that this is either connections in the switch(es) and/or the relay(s)

do you have a circuit diagram that we can look at?
That would help immensely
 

CDRIVE

Hauling 10' pipe on a Trek Shift3
May 8, 2012
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What an odd title you chose. In our world "How to reduce noise" has totally different connotations. As previously stated..post a schematic.
 

CraziestOzzy

Jul 4, 2012
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as everyone said....photo or drawing of circuit needed.
Usually an electrolytic capacitor on the relay prevents voltage spikes in circuit that may trigger other devices (such as normally on or off rellays) sharing the same power supply and also helps preserving relay contacts
Its obvious you need a filter of some sort...but schema/mud-map will help
You may have a trace or wire in the wrong place but you say you have isolated with an opto....so go with filtering as a possible fix
 
Last edited:

sid2286

Aug 24, 2011
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Aug 24, 2011
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102
I managed to sort it out using an Emc filter at the output end of the relay.
though I'm using an optocoupler, the ground planes are not getting isolated, so the noise factor still in picture.

Is there a better way to isolate the output from the circuit, coz using an EMC filter everytime will be an expensive affair.

I have attached the drawing, please have a look.

regards,
Sid
 

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Harald Kapp

Moderator
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Nov 17, 2011
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Your problem sounds like a matter of bad or missing supply decoupling.
Add a 100nF ceramic capacitor between Vcc and GND as near as possible to every IC.
Add a 10µF...100µF electrolytic capacitor between Vcc and GND near the power supply input.
 

sid2286

Aug 24, 2011
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Aug 24, 2011
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Hi, I have connected the decoupling capacitor across the ICs, though not shown in the circuit.

however haven't connected the cap across the power supply source.

will give it a shot and update the results.


thanks for the advice.

Sid
 

gorgon

Jun 6, 2011
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Jun 6, 2011
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Which logic family do you use for this? If it is TTL or LSTTL, the design could be iffy.

If U4 is a LM555 you should review the design again.

TOK ;)
 

sid2286

Aug 24, 2011
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yes its LM555 and reading about TTL and LS-ttl i guess TTL can cause glitches.

should i replace it with LS-ttl? or is there any other alternate solution for U4 IC.

regards,
Sid
 

sid2286

Aug 24, 2011
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ok, I tired with the capacitor at the input of power supply.

At the output end of the regulator of 12v and 5v I used a 100uF capacitor, and also decoupling capacitor of 0.1uF at the Vcc and Grd of all the ICs.
 

gorgon

Jun 6, 2011
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What's the value of R4 and the other pulldown resistor? For TTL pulldown is generally a bad idea, and depends on the type of gate (U1,2 &3). If it is a Schmitt trigger NAND it could be ok. A TTL input need 1.6mA current to pull it low.

TOK ;)
 
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