basic relay question - Hot Rod Forum : Hotrodders Bulletin Board
Hotrodders.com -- Hot Rod Forum



Register FAQ Search Today's Posts Unanswered Posts Auto Escrow Insurance Auto Loans
Hot Rod Forum : Hotrodders Bulletin Board > Tech Help> Electrical
User Name
Password
lost password?   |   register now

Reply
 
LinkBack Thread Tools
  #1 (permalink)  
Old 11-22-2008, 07:57 AM
Registered User
 
Last photo:
Join Date: Oct 2008
Location: louisiana
Posts: 145
Wiki Edits: 0

Thanks: 0
Thanked 0 Times in 0 Posts
basic relay question

alright fellas,
iv wondered this for quite a while, but never really bothered me till now. what is a relay good for, and why use them on everything??? I always hard wire everything, fuel pump, fans, gauges, stuff like that. but when you look at wiring diagrams, everything has relays. whats the advantage? it seems like hard wiring is more simple and less wires.jus a simple question i had to get off my chest

    Advertisement
Reply With Quote Quick reply to this message
Sponsored Links
Advertisement
 
  #2 (permalink)  
Old 11-22-2008, 08:07 AM
Member
 
Last photo:
Join Date: Feb 2003
Posts: 642
Wiki Edits: 0

Thanks: 0
Thanked 0 Times in 0 Posts
The purpose of a relay is so the electrical circuit in question pulls the load from the battery itself as opposed from the rest of the electrical system.
Reply With Quote Quick reply to this message
  #3 (permalink)  
Old 11-22-2008, 08:19 AM
Shelby1's Avatar
Steel Dreams
 
Last photo:
Join Date: Nov 2006
Location: Lillian,AL
Age: 69
Posts: 317
Wiki Edits: 0

Thanks: 0
Thanked 1 Time in 1 Post
basic relay question

Quote:
Originally Posted by 350cruiser
alright fellas,
iv wondered this for quite a while, but never really bothered me till now. what is a relay good for, and why use them on everything??? I always hard wire everything, fuel pump, fans, gauges, stuff like that. but when you look at wiring diagrams, everything has relays. whats the advantage? it seems like hard wiring is more simple and less wires.jus a simple question i had to get off my chest
Relays are used to handle higher current loads(amperage)than the switches them selves could handle. Also relays switching is/can be done remotely to eliminate bring heavy current wiring into the passenger area which will reduce the risk of fire,running lighter gauge wire for control of the relays reduces length of the heavy gauge wire runs, (read costs) and to certain extent weight of the vehicle.


Kenny

Lost your job yet?
Keep buying foreign.
Reply With Quote Quick reply to this message
  #4 (permalink)  
Old 11-22-2008, 08:44 AM
Registered User
 
Last photo:
Join Date: Oct 2008
Location: louisiana
Posts: 145
Wiki Edits: 0

Thanks: 0
Thanked 0 Times in 0 Posts
hey guys thanks for the infor, i figured it was something like that but jus had to make sure...
Reply With Quote Quick reply to this message
  #5 (permalink)  
Old 11-22-2008, 09:20 AM
Registered User
 
Last photo:
Join Date: Jul 2008
Location: Northeast PA
Posts: 205
Wiki Edits: 0

Thanks: 3
Thanked 7 Times in 5 Posts
An example of a relay your car would already have is the starter solenoid. Although commonly called a solenoid, one of it's functions is that of a relay. Imagine how large your ignition switch would have to be to handle the several hundred amperes of current your starter needs to spin over the engine. The solenoid needs only a few amperes at most to energize, to control the large current that the starter needs.
Reply With Quote Quick reply to this message
  #6 (permalink)  
Old 11-22-2008, 11:23 AM
Member
 
Last photo:
Join Date: Nov 2004
Location: Lakeland FL
Age: 65
Posts: 4,110
Wiki Edits: 0

Thanks: 0
Thanked 3 Times in 3 Posts
once upon a time cars didn't have any accessories relays and only needed a 40 amp alt to power everything....
(no electric door locks/electric windows/in glass defrosters/AC/electric fan/etc/etc/etc)

the ign switch and primary wire feed was heavy duty and had reserve amps capability so you could fuse wire in additional small amps items like the radio to the ign switch post provided.....
the headlights harness was usually a second independent feed and relied on the breaker in the heavy duty contacts lamps switch for fire protection....

eliminate all the electrical "stuff" that requires a 130amp+ alt in todays cars and you don't need relays....

it is "possible" but not practical to wire a car with no accessories relays.....
use independent/isolated/heavy duty copper wire grounds batt direct sources to each device with a heavy duty switch and independent/isolated/heavy duty/fused positive wiring back to the batt positive)
(which is how your house 120vac is wired, about 20 independent wiring breaker circuits)

one more relay reason:
items like the AC compressor or electric fan constantly turn on/off/on/off which causes a amps spike on make and break...
the large relays contacts surface area and armature are designed to last thru a gazzillion cycles worth of electrical contacts arcs before it does fail....

those are excellent explaination posts above...
Reply With Quote Quick reply to this message
  #7 (permalink)  
Old 11-22-2008, 01:06 PM
Registered User
 
Last photo:
Join Date: Oct 2008
Location: louisiana
Posts: 145
Wiki Edits: 0

Thanks: 0
Thanked 0 Times in 0 Posts
hey guys thanks for all the info. 65 mustang.. you give good info for a ford guy..lol.. j/k

but the reason i ask is due to me re wiring my cj7. the only relays I have in the entire jeep are 2 for the headlights. high and low beam. but being its a jeep, i dont have much inside either. just the wipers, radio, gauges, and fuel pump are the only thing on switches and the radio and gauges are together, so i only have 4 switches countin the lights. i also wired my ignition to a heavy duty switch. but should i have used relays for some of the stuff? it works great for what its used for. its a jeep with a modified 350 and the only off road it sees is the grass in the front yard when company is over, and only driven in rain if im caught in it on the way home.
Reply With Quote Quick reply to this message
  #8 (permalink)  
Old 11-22-2008, 01:22 PM
Registered User
 
Last photo:
Join Date: Oct 2007
Location: Portland,Or
Posts: 838
Wiki Edits: 0

Thanks: 0
Thanked 43 Times in 37 Posts
Why Relays?

First case, your headlights. On an older car or truck, the headlights sit in the front of the car, and so does the battery. The headlight power circuit goes from the positive battery post, through a circuit breaker, through the firewall to a light switch, back through the firewall, to the dimmer switch, and back to the front of the vehicle to the lights, and then to a ground, and back to the battery. All this wire has to be a large gauge wire to handle the current the headlights draw, without a voltage drop. Use a relay, and you can locate the relay in the front of the vehicle. Now the headlight power circuit goes like this. From positive battery post, through a circuit breaker, to the headlight relay, to the headlights, and then to ground. you can use two relays, one for low, and one for high beams. much less wire, much less voltage loss, much brighter headlights.
Second case, fuel pump on a modern car. It is controlled by the engine's computer. You do not want the computer chip to get hot, you have to limit the current that goes through it. You let the computer control the fuel pump relay, and the computer is cool and happy.
Third case, electric radiator fan. This fan can draw 20 or 30 amps when it is running, and even more when it is switched on. The temperature switch that controls the fan could not handle the current the fan motor wants.
Think of a relay as a heavy duty switch you can mount remotely.
You can also use two relays to run a motor that reverses either way, and you can then just run small control wires to the switch.
I know parts left off a vehicle cost nothing, and create no service problems, but in many cases, with the cost of upgrading the entire electrical circuit to handle the heavy currents, it costs less to install relays.

Last edited by DanielC; 11-22-2008 at 01:27 PM.
Reply With Quote Quick reply to this message
  #9 (permalink)  
Old 11-22-2008, 02:05 PM
Member
 
Last photo:
Join Date: Nov 2004
Location: Lakeland FL
Age: 65
Posts: 4,110
Wiki Edits: 0

Thanks: 0
Thanked 3 Times in 3 Posts
as long as the primary power feed wire into the car is a size #10 (which can handle 40amps total plus some) or larger,,,
nah.....
you just add up the total amps for all the devices to tell if the positive primary wire amps capability is a big enough size wire....

wipers 7amps?
radio 7amps?
guages 5amps?
fuel pump 15amps?
=34amps max "if" everything is turned on at once....

"but!!!!"

you forgot the heater blower which is another 15amps+ approx...
and the dist ign itself which is likely up to 7amps more added
and a 4amp(?) electric choke?
and the 15amp cigarette lighter?
=75amps max if all on!!!!!

at that point, if it is the normal #10 size primary feed wire, with the heater amps added in,,, it is wise to put a relay on the blower or the fuel pump for a second primary batt wire feed to that device

or you can wire one or the other with a heavy duty toggle switch and fuse wire to the batt direct,,,

advantage of using the relay is "everything IS off" when the key is off....
(my custom mega cooling under-dash AC is batt direct, turn the motor off and you can for sure hear the fan blowing to know to turn it off before getting out of the car)

for all the devices to last as long as possible and work at their best it is wise to have more than enough amps carrying capability based on the wire size amps rating....

energy flow is from the batt neg to the positive so "0" ohms resistance copper wire grounds supply to the device direct from the batt neg can only help especially on the higher amps devices....
(that's partly why the head lamps are on a relay,,,a direct copper wire ground power source from the batt neg to the lamps )
Reply With Quote Quick reply to this message
  #10 (permalink)  
Old 11-23-2008, 09:13 AM
Registered User
 
Join Date: Aug 2006
Location: Alexandria, VA
Posts: 1,758
Wiki Edits: 0

Thanks: 4
Thanked 66 Times in 64 Posts
My '75 GMC truck has only a few stock relays for high amperage items:
-- Horn relay (used in nearly every vehicle in the last 40 years or more)
-- AC compressor relay (used on nearly all AC equipped vehicles)
-- High speed blower relay - puts blower on high speed when the AC is on max
-- Dual battery/camper relay - disconnects second battery from charging circuit when vehicle is off

I could remove every relay except the horn relay, and still have a fully working vehicle, so your Jeep does not really need many relays. However, I have considered adding relays on the headlight circuits, so I could switch to halogen sealed beams and keep them good and bright.

Bottom line - if you don't have anything that draws very many amps, then you probably don't need relays. However, if you have electric fans or fuel pump, off-road lights, a winch, or any other mods, then relays are definitely the best way to go. They provide the most direct feed of power, without any need for long runs of large gauge wire to switches or sensors.

Bruce
Reply With Quote Quick reply to this message

Recent Electrical posts with photos

Quick Reply
Message:
Options

Register Now

In order to be able to post messages on the Hot Rod Forum : Hotrodders Bulletin Board forums, you must first register.
Please enter your desired user name (usually not your first and last name), your email address and other required details in the form below.
User Name:
If you do not want to register, fill this field only and the name will be used as user name for your post.
Password
Please enter a password for your user account. Note that passwords are case-sensitive.
Password:
Confirm Password:
Email Address
Please enter a valid email address for yourself.
Email Address:

Log-in

Human Verification

In order to verify that you are a human and not a spam bot, please enter the answer into the following box below based on the instructions contained in the graphic.




Currently Active Users Viewing This Thread: 1 (0 members and 1 guests)
 
Thread Tools

Posting Rules
You may post new threads
You may 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 On
Pingbacks are On
Refbacks are On


Similar Threads
Thread Thread Starter Forum Replies Last Post
Can I build this wireless led brake light cheap? cboy Electrical 11 07-06-2007 11:16 PM
Omg, Car Fire!! Malibu1980 Electrical 21 06-16-2004 12:34 PM
Hi All I just Joined, Questions Re:454 rebuild Gay-Shark Engine 8 11-29-2003 10:25 AM
Basic question on 69 Camaro rayNmiami Hotrodding Basics 6 10-29-2003 09:44 AM
basic engine question - 305 and 350 chevy antirice Hotrodding Basics 4 09-27-2003 12:24 PM


All times are GMT -6. The time now is 10:44 PM.


Powered by vBulletin® Version 3.8.7
Copyright ©2000 - 2014, vBulletin Solutions, Inc.
Search Engine Optimization by vBSEO 3.6.0 PL2
Copyright Hotrodders.com 1999 - 2012. All Rights Reserved.