Message-ID: <01BDE9A1.FAD03F00.ed@testelectronics.com>
From: Ed Armstrong <ed@testelectronics.com>
Reply-To: "ed@testelectronics.com" <ed@testelectronics.com>
To: "Lancair [lancair.list@olsusa.com] (E-mail)" <lancair.list@olsusa.com>
Subject: Protecting Switch Contacts
Date: Sat, 26 Sep 1998 23:04:12 -0700
Organization: Test Electronics
Mime-Version: 1.0

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Marv asks: the question is what can we do to protect those relay contacts 
to prevent arcing when we fire-up/shut-down those motors?
Yes good point, electric motors are also inductive loads, and you can use 
the same flyback circuit on the motors to protect the relay contacts. So, 
first use the low current diode circuit to protect the switch from the 
flyback that the relay coil produces. Next, use the high current varistor 
circuit to protect the relay contacts from the flyback the motor produces. 
This will help eliminate welded contacts. Make sure to physically place the 
flyback circuits as close as possible to inductive loads which produce the 
flyback, this will help reduce noise in your electrical system. The diode I 
added in series with the MOV is only for noise reduction if you have noise 
sensitive instruments. You could try it first without this diode and then 
add it later if you need it. The schematic for both these circuits is at 
http://www.testelectronics.com/ftp/flyback.gif

The term "welded contacts" is a very good description of what makes the 
switches stick. This just made me think of an arc welder. A simple arc 
welder is nothing more than a large coil in series with a power supply. 
This may help more of you understand just how important a flyback circuit 
is.

Regards,
Ed.