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previous: john b "but heard from people here the rea... -- 6/29/2003 10:59 AM View Thread

Re: relay popping

6/29/2003 9:01 PM
Steve A.
email
Re: relay popping
john:  
 
    The diode goes right across the relay coil contacts. Here's the text from an old post by R.G.:  
 
R.G.  
9/25/98 8:06 AM  
 
Relays - all relays - can pop from the  
normal switching problems, as well as one  
of their own. Any metal-contack switch  
will pop if the DC levels between open  
and closed are not the same. The pop is the  
current pulse as the voltages equalize.  
 
Relays have the additional problem that the  
change of voltage on the relay coil will  
induce pops into the audio path by  
capacitive coupling of the coil voltage to  
the signal path. This is especially bad if  
the designer simply allows the coil voltage  
to fly or be clamped with a diode when the  
coil is turned off. This causes a flyback  
voltage on the coil that is -very- fast, and  
couples easily to the audio path. The  
coupling is dependent on the amount of  
capacitance to the audio path and the  
impedance of the "out" side of the audio  
path. If this is low, the capacitance may not  
be able to make it pop much. If this is the  
input of a tube, even small coupling  
capacitances will cause bad pops.  
 
The solutions to this problem are two:  
slow down the voltage transition on the  
coil by ramping the voltage to turn it on  
and off, and/or shield the signal path inside  
the relay from the coil.  
 
Reed relays were invented and developed  
by the Bell Labs for switching audio in  
telephone applications. They will also pop  
if they are not shielded or coil-ramped.  
 
Reeds are fast, low power, and have low  
contact distortion and thermal noise  
voltages. Get them with shields.  
 
 
 
And here's the text from a post by RE:  
 
First off, if you input impedances are high enough I would couple  
both preamps together on the input side and select the output of  
either stage. This will eliminate the input side switch being amplified  
through the preamp(s). If you have bleed problems from a  
distortion stage short the output of the first stage of that channel to  
ground, after the coupling cap. Make sure you have coupling caps  
& ground ref. resistors on the outputs you are switching. You want  
to eliminate any dc potential shifts when switching between  
channels. This goes for the output side of the switch also. Capacitor  
couple and use a ground resistor to drain the cap. on the switch  
side. If you must switch the inputs also, capacitor couple them the  
same way. Any capacitor that may be switched must have a dc  
path to ground or it will build a charge that will give you a pop  
when switched. Also on the relay itself - add a reverse-baised  
diode and a forward biased (charged) capacitor across the relay  
coil to reduce spikes. I use a 22uF cap. on 5V relays in most  
situatioons. Use a well regulated voltage to the relay and switch the  
relay coil to ground. With a 5V 50 ohm relay coil you can draw  
enough current across the footswitch to light up an LED without  
closing the relay contacts. Then when you short the footswitch the  
LED goes out and the relay energizes. All this with just two wires  
to the footswitch. Neat trick eh?  
 
Hope this helps.  
 
RE  
 
 
Steve Ahola

 
Replies:
john b thanks for the info steve,j... -- 6/29/2003 10:13 PM