Do I need a diode here?

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Ramdude
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Do I need a diode here?

Post by Ramdude » Sun May 23, 2004 11:16 am

In the 12VDC relay circuit (below) where the trigger is a simple toggle switch supplying power for the relay and the Neg output is a low current load..
Do I need to install a diode for protection across terminals 86 & 85?<p>Image

Silveradogold
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Re: Do I need a diode here?

Post by Silveradogold » Sun May 23, 2004 12:23 pm

Hello Ram, I liked your part in TRON; (kidding) I think if the relay specs do not show diode protection across the coil, to be on the safe side you should put one in.
On another note how do you put schematics in the posts? (I always wanted to do that)
Thanks --- Silveradogold :cool:

Vincent
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Re: Do I need a diode here?

Post by Vincent » Sun May 23, 2004 1:51 pm

Just right click on a picture and go to properties.<p>Select the address for that particular picture and insert it where the X is...<p>[IMG]X[/IMG.]<p>The period after the second "IMG" is NOT suppose to be there. I only put it there to keep it from performing the image action.<p>[ May 23, 2004: Message edited by: Vincent ]</p>

cato
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Re: Do I need a diode here?

Post by cato » Sun May 23, 2004 3:29 pm

What are you hoping the diode will protect? The toggle switch? Shrug..I think its a big boy and can take care of itself.

dnszero
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Re: Do I need a diode here?

Post by dnszero » Mon May 24, 2004 12:38 pm

I'm new to this so hence my question:<p>What would putting a diode across there do? What could possibly happen without it?
Thank God I'm not too early!

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haklesup
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Re: Do I need a diode here?

Post by haklesup » Mon May 24, 2004 6:43 pm

You can probably bias the coil on that automotive relay either way and it will pull in the arm and make the switch. A doide is certainly not needed to protect the relay, the switch or most automotive power systems. I would encourage a fuse on the coil loop and the load loop (switch contact side) seperately though. (you will burn your fingers yanking flaming wires off a miswired relay as I have had the pleasure of)<p>Putting a diode in series with the coil would prevent reverse bias and any damage associated with putting the opposite power on a circuit. Probably not needed here. A series diode (or 2)may also be used to allow a DC relay to be closed by AC voltage as in a rectifier<p>Putting a diode in parallel across the coil (such that it is reverse biased when the coil is properly biased to turn on) Will shunt any reverse voltage spike (caused when the magnetic field in the coil collapses as the power is turned off) to ground and prevent the damage that may cause. However, since you didn't say you are interfacing with any other active silicon components, I would guess the diode is unnessary in that mode also. <p>A parallel diode would also protect the coil from reverse bias by providing a lower impedance path around the coil. Though it is likly the coil can handle more current than a typical 1W diode.<p>You might choose either diode arrangerment to protect circuitry you may be using to activate the relay especially if the coil has a high inductance (parallel diode) or if a removable supply might be used (series diode)<p>[ May 24, 2004: Message edited by: haklesup ]</p>

Ramdude
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Re: Do I need a diode here?

Post by Ramdude » Mon May 24, 2004 10:26 pm

Thanks everyone for the advise, I didn't think I needed the diode but thought I would ask anyway.
The toggle sw contacts would be the only thing it would protect.

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MrAl
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Re: Do I need a diode here?

Post by MrAl » Tue May 25, 2004 12:05 pm

Hi there,<p>Actually, a reverse connected diode is a good
idea with most relays unless they already have
one built in.<p>The reasons for this include:<p>1. protect the switch contacts from arcing
2. eliminate emf and high power amp speaker 'pop'
3. protect the enamel insulation of the relay
coil, which can arc over and short<p>
Take care,
Al
LEDs vs Bulbs, LEDs are winning.

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