Hoffman Amplifiers Tube Amplifier Forum

Other Stuff => Solid State => Topic started by: John on July 08, 2011, 10:27:58 am

Title: relay question
Post by: John on July 08, 2011, 10:27:58 am

The more I read the more confused I get, so please tell me - is this the proper way to wire a diode across the relay terminals? The banded end is the cathode /negative side, so it should get soldered to the negative side of the relay.... right?

Thanks!
John

Title: Re: relay question
Post by: RicharD on July 08, 2011, 12:59:47 pm

Points to the positive.

Title: Re: relay question
Post by: OldHouseScott on July 08, 2011, 01:02:49 pm

Other way 'round. The banded end should be tied to the positive terminal of the relay.

In normal operation, with positive voltage applied between the '+' and '-' terminals of the relay, conventional current will flow in the relay coil. The diode is reverse biased so no current will flow in the diode. When the voltage to the relay is removed, the current in the coil will want to continue to flow momentarily, due to the inductance of the coil, as the magnetic field collapses creating a negative voltage spike. The diode is now forward biased with respect to the voltage generated in the relay coil, creating a "flyback" path for the current until it is dissipated by the diode and coil resistance.

Title: Re: relay question
Post by: phsyconoodler on July 08, 2011, 01:07:54 pm

You have two pictures of two different relays.
  The banded end definitely needs to go to the positive terminal.Usually relays have power all the time when the amp is on and you get the coil to switch by adding a ground on the other end of the diode.(-)

Title: Re: relay question
Post by: Willabe on July 08, 2011, 01:17:49 pm

Quote from: phsyconoodler on July 08, 2011, 01:07:54 pm

You have two pictures of two different relays.

Look again, top/bottom view. It's from Dougs store.


           Brad       

Title: Re: relay question
Post by: John on July 08, 2011, 01:23:40 pm

Thanks to everyone! I would read one blurb and think I understood, and then read another one and be foggy again. Argh!

*edited picture to show correct direction*
Thanks again!!

Title: Re: relay question
Post by: RicharD on July 08, 2011, 02:38:59 pm

Diodes ALWAYS point towards the more positive in a power supply.  In a rectifier it points to the +.  Even a neg bias supply where it points to ground, we tap off the anode.  Even though the cathode is at zero volts, it's still more positive.  Used as a regulator protection diode, it points to the source which is potentially more positive than the regulated output.   Here's why you want a protection diode on a relay coil.  When you switch a coil in and out, it's gonna try to make AC just like a distributor and coil on a car engine.  This makes for a nasty pop.  Having a diode in parallel with the coil blocks the momentary current flow in the reverse direction.

If you're gonna point a diode negative in a DC circuit, then you need a resistor in series to limit the current otherwise you have a direct short and something is gonna glow.  Ah... LED.  It's cathode grounded like a vacuum tube.... which also glows.   :icon_biggrin: 

Title: Re: relay question
Post by: EL34 on July 08, 2011, 07:55:59 pm

You plucked that diagram off my relay switching pages but maybe did not look at all the other info?

This page shows the diode in place across a coil
Same page as the diagram you linked to.

http://www.el34world.com/projects/relay_switch5.htm (http://www.el34world.com/projects/relay_switch5.htm)

Title: Re: relay question
Post by: John on July 08, 2011, 09:01:28 pm

Doug, yes I did see the other stuff. But I wanted to know, for sure, which side the band goes on. Before I have to order yet 2 more relays from ya.  :laugh:

Title: Re: relay question
Post by: EL34 on July 09, 2011, 05:27:21 am

Yes, you cannot trust my diagrams. :l2:

Title: Re: relay question
Post by: John on July 09, 2011, 09:12:26 am

Ha ha. It wasn't your diagrams giving me problems, it was my brain!