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FYI on relays!


 
3/31/1999 7:55 AM
Benjamin Fargen
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FYI on relays!
To all:  
Here's a great site for understanding how relays work:  
http://www.euroseek.net/page?ifl=uk&page=sitejumper&url=http%3A%2F%2Fwww.howstuffworks.com%2Frelay.htm  
 
It helped me out.......Now maybe I can make a real channel switching amp!  
 
Benjamin.
 
3/31/1999 9:14 PM
Steve A.
Website
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Benjamin:  
 
    I'm really impressed that you typed in that address without making a single mistake. [g] And people used to complain that my old URL over at GeoCities was hard to type in...  
 
    On a more serious note (C#), remember to add in a flyback diode across the relay coil. (Search the archives here for the keywords: relay diode). I believe it was Don S. who recently explained why it is necessary to add in that diode and how it is often omitted in channel switching designs.  
 
Steve Ahola  
 
P.S. Designing complicated HVAC switching circuits with relays is one of my favorite tasks in my day job! When I get to do that, it's not like work- it's too much fun! Unlike audio circuits where you have to deal with noises that might be amplified, with HVAC controls it is all pure logic...  
 
    I finally figured out how to access my free homepages at ATT and I'll be putting up a website devoted to HVAC circuits and troubleshooting guides:  
 
http://home.att.net/~hvactech/  
 
(I was surprised that no one had already used that name over at ATT— ~blueguitar was already taken so I set up one of my five free 5meg homepages there as ~blue_guitar. I also reserved ~amptech and ~guitartech in case I run out of run at my regular site for schematics, etc.)
 
 
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4/1/1999 7:09 AM
Don Symes
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The flyback diode is too cheap and effective to leave out.  
 
Use a 1N4001 or something similar. 1N914s may not stand enuf current to really last.  
 
Anode to relay_coil_minus, cathode to relay_coil_plus, solder directly to the terminals if possible.
 
4/1/1999 11:01 PM
Steve A.
Website
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Don:  
 
    FWIW Peavy uses a 1N4148 reverse-biased flyback diode across the coil for their Classic 30 and 50 amps, while using 1N400x diodes for the rectifiers. Any idea why they used the 1N4148? As a "switching" diode (whatever the heck that means!) it is faster than a simple rectifier- right? But the 1N400x can handle more current. (I used a 1N4007 on one relay board and a 1N4148 on the next, but they both seemed to work equally well.)  
 
    Thanks!  
 
Steve Ahola
 
4/2/1999 9:12 AM
Don Symes
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The 4148 is faster, and as a switching diode, gets used in RF applications like switches and mixers.  
 
The 400x is most likely overkill in terms of current-carrying capacity, but at least you KNOW it will never give up in the middle of a gig or recording session.  
 
Coil voltage and inductance come into an equation I don't feel like dealing with to give a definitive answer. Fortunately, RG's next post does.
 
4/2/1999 7:04 AM
R.G.
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FWIW - The flyback diode clamps the coil to the forward voltage of the diode. Since V=L di/dt, di/dt is way small, so the current circulates for a relatively long time until it gets down to the dropout ampere-turns of the relay.  
 
You can speed things up a bit at the price of introducing a larger reversal transient by using a resistor in series with the diode. If you size the resistor equal to the coil resistance, then the recovery voltage will initially be equal to the ON voltage, and your drop out times should be similar to the turn on times. If you're really good, you can calculate a cap to slow down the transient edge and keep the resistor to damp things so that you get little ringing. This can keep turn on/off transients from being capacitively coupled to the signal contacts.
 
4/1/1999 8:27 AM
jason
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The flyback diode gives the coil a path to discharge.  
jason
 
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