For the electrical genius's

steptoe · Post by **steptoe** » Sat Sep 05, 2009 12:15 pm

I have an accessory power supply for automotive use that is essentially a PDA adaptor 12V down to 5V. It has a warning to unplug every time before starting car to avoid surges etc (don't that hurt at the moment!)

How do I hook up a little black box that will disconnect from the power supply once power has shut off and not reconnect until I flip a momentary switch to lock in a relay again with the relay continue to supply power until engine shut down again?

So, I need a circuit to allow 12V to switch on only after it has got its 12V supply from cig lighter AND I have flipped a switch, preferably a momentary on type so it need not be flipped off manually coz I forget and may as well do it manually everytime) to then supply to the PDA adaptor with output of 5V max of about 420mA

Any suggestions for an electronic simpleton like myself to build up
Thanks. .

Is that a latching relay that locks on once it gets a momentary switching on power and holds on until it loses all power not just the switching power?

Gannon · Post by **Gannon** » Sat Sep 05, 2009 12:29 pm

If your PDA is powered by the cig lighter, then it is automatically turned off when you start the car anyway.

dukbilt · Post by **dukbilt** » Sat Sep 05, 2009 12:46 pm

Your best bet is to get a power supply which is over-voltage protected. It will cost more, but it is designed to ensure that you get 5V independently of the supply voltage.

Ewan

BaronVonChickenPants · Post by **BaronVonChickenPants** » Sat Sep 05, 2009 1:20 pm

I believe the issue is most likely to occur when starting the car, ie: ignition from off ->acc->on->start->on, where the unit will briefly receive power during acc->on then lose power for a second during start then be powered again once back to on, this momentary loss of power can cause surges in electronic components when they are partially discharged then recieve full power again (I believe that's the theory anyway).

The simplest way to do what your after is with a N/O relay and a momentary switch. The +12v for relay coil should be on the same side of the contacts as your PDA adaptor and the momentary switch should be across the relays switching contacts, with acc power fed to the other side of the switching contacts and the remaining side of the coil to gnd, I hope that makes sense.

What will happen is when the momentary switch is pressed it will pass power to the coil, closing the contacts, when you let go of the switch the relay will be self powered holding it's own contacts closed.

Turn the ignition off and the contacts open and remain that way until the button is pressed again.

I can draw a diagram later if needed.

Jordan.

phillatdarwin · Post by **phillatdarwin** » Sat Sep 05, 2009 10:12 pm

i wood like too see a diagram if u can post one

fredsub · Post by **fredsub** » Sun Sep 06, 2009 9:12 am

BVC is on the money:p

here's a diagram

As you can see, when S2 temporarily closes it powers the relay coil,
the relay contacts then close the contacts so supplying 12V to external
AND the coil. When S1 is opened the power to external AND the coil is
switched OFF.

note that the momentary switch will briefly carry all the current to
the supplied circuit, so it should be a decent switch.

steptoe · Post by **steptoe** » Sun Sep 06, 2009 11:08 am

Thanks guys. Gannon, you tried, but the warning must be on the power supply for a reason or reasons such as those Jordan (who still looks in !) has given. A N/O relay...hmmm. I was thinking an 87, 87a relay which will switch from one thing to another, may be configured to do the job or have you a jaycar part number to suggest? Fredsubs diagram does not show up so I may have to turn on images download and have a look. I wanna wire this into my cars and have an inline one for my work car.

For some reason my server would not send the image to my email so I set it as my wall paper, instead of Jenny K Blake, so if you are googling yourself Jenny K Blake , look in and say "hi"

BaronVonChickenPants · Post by **BaronVonChickenPants** » Sun Sep 06, 2009 11:44 am

Thanks Fredsub, got caught up with the kids and didn't get a chance to finish.

In Fred's diagram S1 would be your ignition.

Steptoe an 87 87a relay would do the same job, just make sure the N/C terminal is covered so it doesn't short on anything.

This should take you directly to Fred's diagram:
picture.php?albumid=29&pictureid=195

I'm still a regular lurker, keeping an eye on everyone's projects and helping where I can.

Let us know how it goes or if you have any more Q's

Jordan.

steptoe · Post by **steptoe** » Sun Sep 06, 2009 11:57 am

Thanks again Jordan. Did you ever get around to fitting the EJ....oh, no, of course you didn't, you mentioned kid plural

. And now in a climate that is more forgiving most of the year to work with cars and tools...... the link took me there too THANKS, where did you get the link from?

this was an option developed by some who just love relays....

http://www.the12volt.com/relays/page5.asp#lsp

steptoe · Post by **steptoe** » Sun Sep 06, 2009 12:57 pm

Jordan, if you can supply a basic sketch of how to do this with an 87, 87a relay (only if you have it in your head as to how it works and Amy and ankle biters give you the time

) for the benefit of myself and others to follow it would be greatly appreciated, due to my laziness not to work it out on a board first, or make electrronic diagrams to post up. Jaycar do not list 87, 87a relays and the only NC relay seems to wanna switch high loads

Ta , Jonno
87, 87a relays are so marked & look just like a normal Bosch relay for lights, horns etc but when coil is energised one terminal out of 87, 87a gets the switched power, when the coil not energised the other will get the power switched through. One use is in LPG conversions where either LPG circuit is energised or the petrol pump & its solenoids or injector cut, one being connected to 87, the other 87a

BaronVonChickenPants · Post by **BaronVonChickenPants** » Sun Sep 06, 2009 2:05 pm

http://www.snides.net/images/relay-circuit.jpg

Here's an updated version of Fred's diagram with relay pins marked.

http://www.the12volt.com/relays/page5.asp#lsp

Good god, talk about over complicating something, I would hate to try fault finding on some of those circuits.

For the link I just grabbed the direct path to the image, firefox allows you to right click and "copy image location"

The EJ never eventuated, more important things came up, you know how it is.

Brisbane is a much more suitable climate for working on these things but it just doesn't have the same charm as the southern highlands.

Jordan.

fredsub · Post by **fredsub** » Sun Sep 06, 2009 5:22 pm

lol thats needing to be a bit pedantic ain't it?

If it were me I wouldn't be using a one of those 30amp+ 87, 87a relays anyway,
goin by the OP that type of relay will consume more power than what your trying to power.......

get whatever 12V and more efficient relay from Jaycar or DSE, it ain;t no rocket science to understand relays.
Should get a relay with an energising current of < 50milliAmps.

Don't know which terminal is which?
use a pair of wires from a 9V transistor battery (or similiar current limited source, NOT your car battery!:twisted:) and start probing pairs of terminals - listen for the click and you got the coil terminals. Use a lamp continuity test for the relay contacts......

On the otherhand I wish i had more sense mechanical wise.....did something silly today:rolleyes: urrrr

dukbilt · Post by **dukbilt** » Sun Sep 06, 2009 7:28 pm

Steptoe,

using a relay won't fix power surges. Did your last power surge happen when starting the car, or after the engine had been running for some time? What are you trying to power? Is the 12V to 5V adapter a cheapie?

If you really want to fix future power surge problems, you need to provide some voltage regulation. The alternator has a regulator built in, but it may be on the way out. The battery acts as a regulator - it will dampen voltage spikes because it's essentially a really, really big capacitor. Now that you've changed the fusible links, the battery will be more able to dampen the voltage spikes. Before, with the corroded link to the battery, the alternator was probably connected to the battery via a high resistance path. The alternator voltage could wander and the battery wasn't able to dampen it. That shouldn't happen if you've now changed the alternator/regulator.

If you really want to put in some surge protection, buy a (new!) suppression capacitor (called suppressors). They were used in older cars with points and were fitted across the points (electrically speaking) so that the spikes generated when the points opened and closed went through the suppressor and not through the points. Without the suppressor, the arcing across the points would quickly burn the point contacts out. If there is a voltage spike, the suppressor will shunt the spike to earth, and not through the rest of the circuit. Try to wire it in as close as possible to the cigarette lighter itself.

Note - this will stop spikes, but not a high DC overvoltage situation (but then, neither will the relay!). Also, this appears to be a band-aid solution to a problem which shouldn't be occurring anyway. I don't know what the cause is!

Hope this helps,

Ewan

steptoe · Post by **steptoe** » Sun Sep 06, 2009 7:44 pm

Jordan, thanks for the link to your labeled diagram.

Fredsub, I am OK with relays in normal situations as in one wire to this terminal, another to that and always remember 85 to earth for diode protected relays and try to follow in that fashion with all others. Where my brain gets stuck is using interconnecting wires as in fredsubs great diagram. I will connect up some wires and fuses to play with an 87 87a relay first with push button as per diagram , before moving on. I have found 87 is NO and 87a is NC. I'll check out a few electrical places for NC relay, as jaycar don't list any 12v dedicated NC.

Ewan, just a coincidence of sorts, not trying to prevent damage I got with my recent and current headache with the turbo car. Just got a warning on a new thing that says to unplug before starting every bloody time, so with recent experience and the warning just another task to tackle if I can prevent well and good. Also not a bad way to protect anything modern we stick in our lighter sockets of the cars of yesteryear. Been thinking that a USB power socket or two in the dash or console giving out protected switchable 5V would be handy.

When you consider what the cig lighter socket was used or designed for in the 60's, 70's and 80's - rather basic electrical power supply rather than for delicate electronic $tuff we are using in them today, switchable and better protected power supply is not a bad idea for the modern jonny. I have replaced the 15A cig fuse with a 2A blade fuse too

The old points condensor huh? I remember hoking one of those up on a Champion plug cleaner/tester in 1983, one hand holding it on the base, lead clipped on and pressed the metal spark test button with a Victa part didn't blow the switchboard, or earth leakage detector (coz they did not exist) but lived to tell the tale

I was set up
thanks for all comments and suggestion and diagrams.

When completed I will come in and finish off with a progress report

Jonnoo

fredsub · Post by **fredsub** » Sun Sep 06, 2009 8:55 pm

just read what you wrote there

as jaycar don't list any 12v dedicated NC.

N.C. - Normally Closed, means contact closed when relay NOT energised.

you want N.O. - Normally Open contacts.

just had a look through the jaycar catalog.

this would be my pick
http://www.jaycar.com.au/productView.as ... BCATID=754

CAT. NO. SY4029

SPST - Single Pole Single Throw, gotta assume its a N.O. contact
be extremely strange if it were otherwise.

------------

oh remembered that my 12V->15V Laptop DC converter also has label saying to switch off whilst starting engine,
I have done so tho on a few occasions with no drama, but of course it is a likely time for something to fail if it was to due to failing components.....
but I feel its more about covering any flaw in their product and warranty claims. Failed -> ah you left it on while starting engine ?
fail! so no warranty......

steptoe · Post by **steptoe** » Mon Sep 07, 2009 4:18 pm

Oh, fredsub , watcha doin' to me? I read this latest after I have constructed MARK1 on the bench using an 87, 87a relay I had in reserve. Going by Jordans numbers added on his link at snipes.net , (87a and 30 are linked) it works just as I wanted !! Really s'prised how the relay takes things different to the basic relay connections I worked out a long time ago. I am happy, I am in here to say IT WORKS - CHEERIN'. BIG thanks to Jordan for understanding and explaining my want, fredsub for the diagram. Jordan mentions earlier about insulate one relay terminal - scratch that, as all five are in use. I will insulate them all with heatshrink tube.

BaronVonChickenPants · Post by **BaronVonChickenPants** » Mon Sep 07, 2009 4:52 pm

In the diag, 87a refers to the top relay contact not connected to anything, in theory it "shouldn't" effect anything they way you have it....

87a = Normally Closed Contact
87= Normally Open Contact

Glad to hear it works and even better that it does what you wanted, what's the next challenge

Jordan.

steptoe · Post by **steptoe** » Mon Sep 07, 2009 11:51 pm

OK. One less wire and crimp

Made up one box to take between different cars with a male and female lighter socket, and also hard wired into the GLTA using one of the 22A stock subie relays used all over the old subies. Used a small meomentary ON switch discreetly stick in dash and does the job nicely.

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