Cruisers Forum
 


Join CruisersForum Today

Reply
 
Thread Tools Rate Thread Display Modes
Old 11-10-2013, 11:09   #1
Moderator
 
Dockhead's Avatar

Cruisers Forum Supporter

Join Date: Mar 2009
Location: Cowes (Winter), Baltic (Summer) (the boat!); somewhere in the air (me!)
Boat: Cutter-Rigged Moody 54
Posts: 19,744
Question about Relays

One for you electrical wizards:

I am having difficulty installing this relay:



I don't know whether (a) I've got the wires connected wrong; or (b) I've got the wrong relay for the job; or (c) the relay is defective.

I have a constant 24v power supply on 30. I have my load (alternator exciter wire, a 24v alternator) on 87. I have a 12v switched power supply (from my ignition switch) on 85. 86 is connected to ground.

When I switch on the ignition, 30 and 87 are supposed to be connected so that the alternator gets excited.

Instead I get blown fuses in the power supply connected to 85.

I notice that there's a diode between 85 and 86 with some kind of connection to the the 30-87 circuit -- maybe that's putting 24v power where it shouldn't be? Maybe I need a relay without the diode and interconnection?

As always I will be grateful for your sage advice.
Attached Thumbnails
Click image for larger version

Name:	relay.jpg
Views:	120
Size:	112.2 KB
ID:	68483  
__________________

__________________
Dockhead is online now   Reply With Quote
Old 11-10-2013, 11:14   #2
Registered User
 
MARC D's Avatar

Join Date: Oct 2009
Location: Canada
Boat: Shopping...
Posts: 239
Re: Question about Relays

You must apply positive side of 12 v to 86. Neg. to 85. Then normally open contact should close.
__________________

__________________
Sur le même bateau, l'homme de terre et l'homme de mer ont deux buts différents. Le but du premier est d'arriver, le but du deuxième est de repartir.
La terre nous tire vers le passé, la mer les pousse vers le futur.- Albert Londres, 1927
MARC D is offline   Reply With Quote
Old 11-10-2013, 11:28   #3
Moderator
 
Dockhead's Avatar

Cruisers Forum Supporter

Join Date: Mar 2009
Location: Cowes (Winter), Baltic (Summer) (the boat!); somewhere in the air (me!)
Boat: Cutter-Rigged Moody 54
Posts: 19,744
Re: Question about Relays

Quote:
Originally Posted by MARC D View Post
You must apply positive side of 12 v to 86. Neg. to 85. Then normally open contact should close.
I just tried that -- reversed the 12v circuit so that 12v power is on 86 and negative is on 85. Blows the fuse as before. Even with just those two wires connected, and the 24v wires disconnected.
__________________
Dockhead is online now   Reply With Quote
Old 11-10-2013, 11:40   #4
Registered User
 
transmitterdan's Avatar

Join Date: Oct 2011
Boat: Valiant 42
Posts: 4,019
Quote:
Originally Posted by Dockhead View Post
I just tried that -- reversed the 12v circuit so that 12v power is on 86 and negative is on 85. Blows the fuse as before. Even with just those two wires connected, and the 24v wires disconnected.
Probably blew out the diode when it was wired backwards. Measure resistance between 85-86 in bothe directions. One way should be more than the other. If same the New relay is in order.
__________________
transmitterdan is offline   Reply With Quote
Old 11-10-2013, 11:46   #5
Senior Cruiser
 
AfterHoursNLCT's Avatar

Cruisers Forum Supporter

Join Date: Nov 2010
Location: Quaker Hill, CT (just above the US Coast Guard Academy)
Boat: Silverton 34 Convertible
Posts: 191
Re: Question about Relays

Wouldn't you want the positive and negative sides to be completly isolated? What is the purpose of the diode?...LL
__________________
AfterHoursNLCT is offline   Reply With Quote
Old 11-10-2013, 11:48   #6
Senior Cruiser
 
AfterHoursNLCT's Avatar

Cruisers Forum Supporter

Join Date: Nov 2010
Location: Quaker Hill, CT (just above the US Coast Guard Academy)
Boat: Silverton 34 Convertible
Posts: 191
Re: Question about Relays

Like this one
12VDC/40A SPST Automotive Relay : Automotive Relays | RadioShack.com
__________________
AfterHoursNLCT is offline   Reply With Quote
Old 11-10-2013, 12:08   #7
Registered User
 
jstevens's Avatar

Join Date: May 2006
Location: On board Sarah, currently lying in Jacksonville, FL
Boat: Pearson, 424, 42', Sarah
Posts: 582
Images: 4
Re: Question about Relays

I'm not familiar with this relay and 24v alternators in general, but... Normally the ignition circuit (12v) is connected to the AUX terminal (and ground) on the alternator (I'm only familiar with 12V alternators), through an idiot light on the instrument panel. When 12v power is applied through the ignition switch it closes the relay switch to provide excitation current to the alternator, and the idiot light is illuminated. As soon as the alternator output has started, the AUX terminal raises to normal output voltage and no current flows through the ignition circuit, turning off the idiot light and opening the relay circuit shutting off the excitation current.

John
__________________
jstevens is offline   Reply With Quote
Old 11-10-2013, 12:19   #8
Registered User
 
Tymadman's Avatar

Join Date: Jun 2013
Location: Brisbane, Australia
Boat: Farr 6000
Posts: 113
The diode will be fried now due to being connected backwards as previously mentioned. The relay will need to be replaced.

The purpose of the diode is to prevent a high voltage appearing across the coil of the relay when the relay is switched off. When the current through the coil is suddenly interrupted by opening the switch in the coil circuit the collapsing magnetic field of the coil (an inductor) wants to cause current to flow in the opposite direction to normal. Without the diode to allow that current to flow there is no place for the current to go due to the switch being open so the voltage across the coil rises until an arc occurs at the switch contact (normally) and the current is discharged.
With the diode present the current is able to flow through it and 'free-wheels' around the circuit formed by the diode and coil until the energy is dissipated in the coil. Because the current has somewhere to go the voltage across the coil only rises very minimally and arcing is prevented at the switch.

The diode is known as a free-wheeling diode in this application.
__________________
Cheers,

Neil
http://www.madmanmarine.com
Tymadman is online now   Reply With Quote
Old 11-10-2013, 13:20   #9
Moderator
 
Dockhead's Avatar

Cruisers Forum Supporter

Join Date: Mar 2009
Location: Cowes (Winter), Baltic (Summer) (the boat!); somewhere in the air (me!)
Boat: Cutter-Rigged Moody 54
Posts: 19,744
Re: Question about Relays

OK, that makes sense. I connected it backwards and blew the diode, whereupon the relay started blowing my fuses. Ekh.

So I'll get another. Thanks greatly for your help
__________________
Dockhead is online now   Reply With Quote
Old 11-10-2013, 13:23   #10
Moderator
 
Dockhead's Avatar

Cruisers Forum Supporter

Join Date: Mar 2009
Location: Cowes (Winter), Baltic (Summer) (the boat!); somewhere in the air (me!)
Boat: Cutter-Rigged Moody 54
Posts: 19,744
Re: Question about Relays

Quote:
Originally Posted by jstevens View Post
I'm not familiar with this relay and 24v alternators in general, but... Normally the ignition circuit (12v) is connected to the AUX terminal (and ground) on the alternator (I'm only familiar with 12V alternators), through an idiot light on the instrument panel. When 12v power is applied through the ignition switch it closes the relay switch to provide excitation current to the alternator, and the idiot light is illuminated. As soon as the alternator output has started, the AUX terminal raises to normal output voltage and no current flows through the ignition circuit, turning off the idiot light and opening the relay circuit shutting off the excitation current.

John
That's exactly how the regular 12v alternator, the one which came with the engine, works. That alternator powers the engine start battery, which is 12v.

The relay in question here is for exciting the second, large, school-bus alternator which provides power to my house batteries, which are 24v.
__________________
Dockhead is online now   Reply With Quote
Old 11-10-2013, 13:34   #11
Marine Service Provider

Join Date: Oct 2011
Posts: 1,205
Re: Question about Relays

Quote:
Originally Posted by Tymadman View Post
The diode will be fried now due to being connected backwards as previously mentioned. The relay will need to be replaced.

The purpose of the diode is to prevent a high voltage appearing across the coil of the relay when the relay is switched off. When the current through the coil is suddenly interrupted by opening the switch in the coil circuit the collapsing magnetic field of the coil (an inductor) wants to cause current to flow in the opposite direction to normal. Without the diode to allow that current to flow there is no place for the current to go due to the switch being open so the voltage across the coil rises until an arc occurs at the switch contact (normally) and the current is discharged.
With the diode present the current is able to flow through it and 'free-wheels' around the circuit formed by the diode and coil until the energy is dissipated in the coil. Because the current has somewhere to go the voltage across the coil only rises very minimally and arcing is prevented at the switch.

The diode is known as a free-wheeling diode in this application.
Excellent answer!

I was about to say the same thing , but decided I would read the other responses first.

Relays come in three varieties basic no resistor or diode, with a resistor, or with a diode. Th basic and the suppression resistor type are not polarity sensitive, and the diode type is.

In a diode protected relay they are polarity sensitive, (+) power goes to the cathode side pin, and ground goes to the Anode side pin. Look for a relay diagram or a relay schematic that is printed on the side of the relay to determine.

ON most Bosh Relays in the States 85 is the cathode side pin, ie positive. But this is not the case with all relays some use 86 as the cathode side.

I just got a batch of Cole Hersey Diode/Relays the diagram clearly showed 85 as the cathode. When I installed them, pop goes the fuse in the circuit, as well as the diode in the relay. I was pissed, So I used a little trick to test the rest of the relays in the batch before I installed them.

Trick='s nine volt battery to jump the relay, the cathode side will be evident as it will only turn on when the + is connected to the cathode side. And will not work when connected opposite polarity, neither will the 9 volts, pop the diode when connected reversed polarity. I tested 18 relays in this batch and all were ms-marked as to the cathode/Anode.

Also, if you are using the 30, and 87/87A to power and inductive coil, you better put in a free wheeling diode in that circuit as well.

Lloyd
__________________
FlyingCloud1937 is offline   Reply With Quote
Old 11-10-2013, 13:48   #12
Moderator
 
Dockhead's Avatar

Cruisers Forum Supporter

Join Date: Mar 2009
Location: Cowes (Winter), Baltic (Summer) (the boat!); somewhere in the air (me!)
Boat: Cutter-Rigged Moody 54
Posts: 19,744
Re: Question about Relays

I sure know a lot more about relays, than I did before. Thanks to all of you for the education
__________________
Dockhead is online now   Reply With Quote
Old 11-10-2013, 14:13   #13
Registered User
 
Tymadman's Avatar

Join Date: Jun 2013
Location: Brisbane, Australia
Boat: Farr 6000
Posts: 113
Quote:
Originally Posted by Dockhead View Post
I sure know a lot more about relays, than I did before. Thanks to all of you for the education
You've actually learnt a bit about inductive loads and about one way to prevent arcing, all of which is routine stuff if you're in the electrical game.

The point made about also installing a diode across the relay contact is to prevent arcing at the contact caused by an inductive load (such as your alternator exciter coil) when the relay contact opens. This will extend the life of the relay.
__________________
Cheers,

Neil
http://www.madmanmarine.com
Tymadman is online now   Reply With Quote
Old 11-10-2013, 14:28   #14
Marine Service Provider

Join Date: Oct 2011
Posts: 1,205
Re: Question about Relays

Quote:
Originally Posted by Tymadman View Post
You've actually learnt a bit about inductive loads and about one way to prevent arcing, all of which is routine stuff if you're in the electrical game.

The point made about also installing a diode across the relay contact is to prevent arcing at the contact caused by an inductive load (such as your alternator exciter coil) when the relay contact opens. This will extend the life of the relay.
Yep, not only saving the relays, but also any other ECU's and discrete built electronics from voltage spikes in both the 12 and 24 volt wiring.

I've seen expensive units taken out from the voltage spikes, many don't have on-board suppression.

Lloyd
__________________
FlyingCloud1937 is offline   Reply With Quote
Old 12-10-2013, 09:18   #15
Marine Service Provider

Join Date: Dec 2003
Location: South Carolina
Boat: Philip Rhodes Custom
Posts: 395
Re: Question about Relays

The reverse connection may or may not have fried the diode but the relay should still work. When the diode is fried by over-current in the forward direction it acts like a fuse and OPENS, it is extremely unlikely it would short.

In the past we have deliberately removed the internal diode in relays by blowing them out with forward current. It worked 100% of the time.

When connected the "correct" way round do you still hear it "click". I suspect you have something else wrong with the wiring and replacing the relay will not cure it.
__________________

__________________
Andina Marie is offline   Reply With Quote
Reply

Thread Tools
Display Modes Rate This Thread
Rate This Thread:

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off




Copyright 2002- Social Knowledge, LLC All Rights Reserved.

All times are GMT -7. The time now is 01:45.


Google+
Powered by vBulletin® Version 3.8.8 Beta 1
Copyright ©2000 - 2017, vBulletin Solutions, Inc.
Social Knowledge Networks
Powered by vBulletin® Version 3.8.8 Beta 1
Copyright ©2000 - 2017, vBulletin Solutions, Inc.

ShowCase vBulletin Plugins by Drive Thru Online, Inc.