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Remote turn on relay

Old 03-10-2009, 02:41 PM
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Originally Posted by 95highline View Post
i like the post but wouldn't you want to have terminal 30 going to your electronic device and have the +12 volt going in through terminal 87A? that way your not makeing a loop from terminal 30 to terminal 87 running the chance of shorting out the center terminal?
Close. I think you just got your 87s backwards. Many people, me included, wire the +12v to 87 and use 30 out to the equipment so that when the relay is 'off' there are no live terminals.
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Old 03-10-2009, 02:45 PM
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yes actually i have seen the inside of those relays as well as use them. perhaps i just mis-directed you as to what i was saying. with the relay at rest then the contacts on the inside of the relay are resting on terminals 87a (which would open the circuit for terminal 87) once you apply power to the coil the electo-magnet then pulls the contacts which close terminal 87 (which would open the circuit for terminal 87a) correct me if i've made a mistake at this point.

with knowing that the relay's contacts would sit at rest on terminal 87a and it's connected to terminal 30 through the "bar" as you mentiond that would provide a path for the current of electricity to pass through terminals 87a and 30 (not 87 becuase the coil's not energized) but once the coil is energized the contacts then move to terminal 87 (opening the circuit to terminal 87a) there for creating a path for the electricity to travel from terminal 87 to terminal 30. the way i was always taught about relays is that terminal 30 goes to what your trying to energize (or shut off) and that if there's a constant voltage on terminal 30 there's a chance shorting because the current will always be going to another terminal (either 87 or 87a depending on the situation)
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Old 03-10-2009, 02:47 PM
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Originally Posted by Dukk View Post
Close. I think you just got your 87s backwards. Many people, me included, wire the +12v to 87 and use 30 out to the equipment so that when the relay is 'off' there are no live terminals.
very possible. i always look at the relay befor i use to them to double check the wireing and make sure i'm connecting it right.
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Old 03-10-2009, 02:55 PM
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Thanks, I misunderstood your suggestion the first time . . . and I agree with what DUKK wrote on this one - if you have no purpose in anything being connected when the circuit is off, 87 to power supply and 30 to equipment is the safest option.

A few years back, I did have my stereo wired in such a way that I wanted a contact made to different things when the relay was energized vs. idle. I found that 87A pin very helpful for that. But for most applications, 87A can be more of a liability than an asset for the potential of something shorting out, as you noted.
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Old 03-10-2009, 03:03 PM
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Crap! Would a mod be able to edit my above post (post #10) to include the letter "A" as I've noted in red in my quote below. I reread it now and realized I missed an "A" that is quite important for distinguishing the two 87 terminals.

And if you could delete this post when that change has been made, that would be great.


Originally Posted by maltesechicken View Post
If you did that, your electronic device would be active when the relay is at rest. Then the moment the relay activates, your device connected to 30 would lose power because 87a would no longer be connected to the circuit.

I'm guessing that you haven't seen the guts of one of these relays. 87 and 87a are 2 "options" for 30 to be connected to - 30 and 87A when the realy is at rest (when there is no voltage at 86); and 30 and 87 when voltage is present at 86. The way they work is there is wire that goes up from 30 and is connected to a "bar" that is hinged above points 87 and 87A. There are springs that hold this bar against 87A. So there is continuity between 87A and 30 when the relay is at rest. When the relay activates, it activates an electro-magnet that pulls the bar away from 87A and makes the bar have contact with 87. Now 30 and 87 have continuity.

Because this bar is hinged between both 87A and 87 there is never a point when 87 and 87a can have continuity with each other or short each other out.

I hope this helps.
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Old 03-10-2009, 05:02 PM
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here is the definitive answer on relays.
Single Pole Double Throw (SPDT), Single Pole Single Throw (SPST) Automotive Relays
30 is always the common wire. meaning your power from the battery always goes to 30. if you were to use the wire harness for relays you would see that 30 is a much bigger wire (a.w.g) and it is this way for that reason!
you don't have to put your power to 30 however, it will work safely either way. just if you are using the wire harness you will have to. BTW the harness is the safest way to be sure there are no shorts and if/when the relay fails (which happens to everything electronic at some point) you simply unplug the bad and pop in a new, good one!!

oh buy the way nice post!


Last edited by nghtrdr; 03-10-2009 at 05:03 PM. Reason: forgot to say thnx
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Old 11-20-2009, 04:13 PM
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Ummm.... This relay setup doesn't provide any more power than the remote wire itself... The +12 volt wire on 30 (I'm assuming you would connect to ACC) is simply closing the circuit to allow power to flow between 86 and 87. The power flowing between 86 and 87 is simply the power the remote wire can provide, hence not solving the initial problem. Having the remote wire connected to 30 and the ACC wire connected to 86 provides a full +12 volts to the components you are trying to power up, and it will only do so when the car's ACC is energized, and the system is turned on (thus energizing the remote lead)
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Old 11-21-2009, 10:29 AM
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i have to disagree with you romanticmoments, as far as more power using the relay setup. the remote output of a head unit is naturally a 12 volt source but depending on the head unit the remote wire is very limited in the amperage output. that's where the relay setup works over the straight remote wire. the relay is able to handle alot more amperage. as for the power flowing between 86 and 87...wont happen. 86 is part of the coil which is connected to terminal 85. which you would connect the remote wire to terminal 86 and ground 85 so when you turn on your head unit it will trigger the relay. as for connecting to the ACC wire. you can connect it straight to your power wire feeding you amp providing you have a proper terminal (distribution block for example) and put a fuse inline from the power wire to the relay. (usually 5 amp fuse is more then enough, i personally would use a 2.5 amp fuse if you could find one)...wiring it like that all you need to do is have the head unit on to have the rest of your system turned on.
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Old 11-23-2009, 12:38 PM
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Ooops Good call, my bad
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Old 01-18-2010, 06:46 PM
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Back when the earth was still cooling (sorry, my old-fart-itis is kicking in again) we got around the problem of inadequate current from the remote turn-on lead by using the relay diagram shown at the start of this thread. Occasionally, you could hear a nasty click or pop, which was generated by the relay coil. Next fix was to wire a diode (like an IN4007) across terminals 85 and 86. But then an alternative came to light; using a transistor for a relay.
Either a TIP 120, or a 2N6387. One little transistor, two resistors, and safely 5 amps of current available to turn on various devices. Being part of a busy car audio shop, this saved us a lot of time, valuable install space, and to the owner's delight, MONEY. Also, no problems with coil noise from a relay.
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