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the12volt`s installbay - Mobile Electronics Forums the12volt's install bay / Relays

Subject Topic: Diode across coil on a relay? (Topic Closed Topic Closed)

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blazo
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Posted: April 02, 2003 at 1:43 PM - IP Logged  

on alot of the diagrams i see on this site, a diode is placed across the coil of the realay. I have been installing for over 3 years, and i've never seen or heard of this. what effect does it have on the relay, what is the outcome, and what would it be used for??? if someone could explain this to me, i would appricate it. thank you
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scuba1954
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Posted: April 02, 2003 at 2:16 PM - IP Logged  

Here is a link from this site about diodes and their purposes. At the very bottom is specifically addresses why diodes are used across relays.

http://www.the12volt.com/diodes/diodes.asp

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blazo
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Posted: April 06, 2003 at 7:11 PM - IP Logged  

it reads.....
Diode across the coil of a relay:
The diode provides a path for current when the current path to the relay is interrupted (i.e. switched off). This allows the coil field to collapse without the voltage spike that would otherwise be generated. The diode protects switch or relay contacts and other circuits that may be sensitive to voltage spikes.

this helps me little, i'm trying to see how it would be applied. does it make the relay stay open longer, closed longer, does it change anything about the relay??? does it make it require more current to open, less current?? what is an example of when you would use this?? can anyone esplain this to me?
thanx
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blazo
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Posted: April 06, 2003 at 7:26 PM - IP Logged  

what does putting a diode across the coil of a relay do? how does it change the relay, and what applications are possible by doing this that arnt possible without using a diode across the coil contacts??? i'm loosing my mind trying to figure out this concept. i've been installing in car audio for 4 years, and i've never seen a diode across a relay, does it serve no purpose, or am i missing out on something sweet???
thank u for any help
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cool_greg
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Posted: April 06, 2003 at 7:30 PM - IP Logged  

it's there simply to protect any senestive electronic devices that may be connected, it does nothing to improve or degrade preformance of the relay.
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blazo
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Posted: April 06, 2003 at 7:42 PM - IP Logged  

thank you
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2G Sport
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Posted: April 06, 2003 at 10:34 PM - IP Logged  

Diodes are most often used across the coil to provide a path for current when the current path to the relay is interrupted (i.e. switched off, coil no longer energized). This allows the coil field to collapse without the voltage spike that would otherwise be generated. The diode protects switch or relay contacts and other circuits that may be sensitive to voltage spikes. (JimR, contributor, install bay member)

http://www.the12volt.com/diodes/diodes.asp

I am still learning about these myself...
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Zeke
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Posted: April 07, 2003 at 7:15 AM - IP Logged  

To put it more technically:

The coil of the relay is basically a large inductor. Basic eletromagnetics tell us that the voltage induced across such a coil will be proportional to the change in current.

When you de-energize a relay, the current basically goes to zero very rapidly, which is in effect a very fast change. The coil of the relay will then induce a very large voltage (hundreds of volts?) across itself. This voltage effect is called "back EMF".

If the relay is simply connected to a dry contact and the battery, this large voltage is applied to the battery and does not do much damage (although it could impact other electronics connected to the battery). In many occassions, the relay is connected to the ouptut of electronic drivers, such as solid state transistors. In the latter case, the back EMF would damage the delicate electronics.

The effects of back EMF can be reduced by connecting a diode across the coil. The diode's electric characteristics are such it will short out the back EMF, thus protecting any equipment connected to the coil.

Hope this helps.
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jgold47
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Posted: April 21, 2003 at 5:35 PM - IP Logged  

does it change the coil amperage..if a normal relay throws at 150Ma, and diode is good for like 1a, (or is it  .7a whatever....) what does that do to the amperage for throwing the coil...
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JasonL
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Posted: April 21, 2003 at 5:54 PM - IP Logged  

As long as the diode is oriented correctly, it won't affect relay operation at all. The diode will be reverse-biased and won't do anything (all current will still flow through the coil).

If you hook it up backwards, the relay will never activate.
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