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# How can I block 12v?

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#### polaris96

##### New Member
Is there a way to block 12 volts oneway but allow gnd to flow when applied as to complete the circuit. This is basically what I have. I am using a 12v relay pin 86=12v and pin 85=gnd. I want to be able to block 12v going over thru the relay coil and on to pin 85. I need to find a way to block it between pin 85 and the gnd source. Any help would be appreciated.

This kinda sounds like a diode would do what you want.

But i cant be sure ...

John

It looks like you are playing with an automotive relay.
Get a diode from your local electronic parts shop. About 100V, 1A rating.
A diode passes current in only one direction. It has a painted stripe on the housing at one end, which corresponds to the bar of the diode symbol ->|-.
the current passes in the direction of the > and gets blocked by the | .
If you connect the | end to the +12V and the other end to the relay, it will not turn on, reverse the diode and it will turn on.
Experiment with your relay and diode to see how it works.
Do NOT connect the | end to ground and the other end directly to +, your diode will instantly vaporize as it tries to pass all the Amps in your battery.
It is OK to connect it the other way round, no current passes if the | end is more positive than the other.
Klaus

Diode problem

A diode will not work for what I need. The diode will pass voltage which is what I do not want. If diode is reversed then both 12v and gnd are blocked. I need a way to block an input 12v but allow output gnd to pass when applied and complete the circuit for 30 seconds. Would a capacitor help in this situation. Not too familiar them.

The best you can do now is to show us a circuit diagram of what you want to do. Your words are unclear as to what exactly you mean by passing output and not 12V.
A capacitor will block DC and pass AC only.
Klaus

Simple ckt

GND<<<<<<<<<<<<<<<<<<<<<<<<<<12v ( Ckt )
GND-------------------- BLOCK<<<<<<<<<<12v (Block 12v)
GND>>>>>>>>>>>>Allow<<<<<<<<<<<12v (Allow gnd)

This is about as simple as I can layout the ckt. The GND is applied for 30 seconds thus completing the ckt then removed. I did not include any other components so as not to make it confussing.

Hi polaris,

I feel sure we can help to do exactly what you need.
I get the impression that electrics is unfamiliar territory to you.
When the relay coil is operated, lets call that 'engaged'

I am going to assume that you are referring to the relay's coil.
I am also going to assume that the relay will do its job when its
coil is energised, maybe work a fan, or sound a horn.

I am going to assume that you can apply a wire from ground to the
relay coil, or a wire from 12v to the relay coil.

I am going to assume that you don't want the relay to engage
when you apply the wire with 12v to the relay coil.

I am going to assume you want the relay to engage when
you apply the wire from ground to the relay coil.

Whats this bit about 30 seconds ?
Do you want the relay to remain engaged after a 30 sec connection ?

Please set out what you need in simple terms.
If there is some other machine or system that does a similar thing
to what you want, say about it.

Regards, John

Is this for operating a motorised electric aerial on a motor car ?
Maybe from an up/down switch with centre-off position ?

I'm sorry I should have mentioned what it is I'm working on. This is a newly installed car alarm which is working fine except for this one optional feature. The alarm company no longer makes these so I am unable to ask for tech assistance. The alarm sense lead in question is used to turn on the domelight for 30 seconds by applying gnd for the length of time then removed when alarm is manually disarmed. This is accomplished by using a 12v relay 85=gnd and 86=12v. This part works fine. The second alarm feature uses the same sense lead to rearm the alarm but it must not see 12v or it will not work. That is where the problem lays. 12v is coming from pin 86 thru the coil and on to pin 85 and to the sense lead. I can disconnect 12v completely from pin 86 and it works but then the domelight will not turn on due to missing 12v.

I think I know were the problem lies. By the way is this a code alarm. I've installed my share and wiring the relay is a real pain. You should have an output for the dome light that's what you hook you relay up to. So, the module turns the relay on and in return turns on the doomlight. The next is the doom lite sense wire. whitch get tapped into the driver side door switch. The one the turns on the light when the doors open. Do you have any extra wires. what's the moel number to this alarm?

The domelight output sense lead is actual used two ways. One to turn on the light during disarm and also to rearm when there is no 12 volt present. There is a seperate door trigger lead which is isolated from the domelight lead. The door trigger lead is wired to three different door switches each isolated with diodes. I have been looking at the elk-912 relay to see if it acts as a 12v auto relay but without the 12v present at the gnd side when not active. The alarm model is WH-700 and the manual can be found via this link: http://www.autoelectric.ru/autoalarm/whistler/whistler/whistler-m.htm

Sorry it took me long to response but I just returned from out of town.

Hi polaris96,

I'm not exactly sure what you need but would this work?
The secondary alarm feature would only be able to sense ground (when present).

edit... S1 not actually there, only denotes pin 85 at ground or open position.

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The ckt would work if there were two different sense leads but in this case its only one lead. In the diagham, the Gnd and seccondary lead would be one which makes it difficult to isolate both states. Its difficult when this alarm is no longer manufactured and calling tech service is out of the question.

I'M PRITTY SURE I KNOW WHAT YOU WANT. NORMALY PIN 86 AND 85 HAVE NO CHARGE. YOU WANT IT SO THAT WHEN PIN 86 GOES HIGH (+) AND PIN 85 GOES LOW (-), RELAY ONE IS ACCTIVATED, BUT WHEN PIN 85 GOES LOW (-) AND PIN 86 DOESN'T GO HIGH (OFF) THEN ANOTHER RELAY TO BE ACCTIVATED.

NORMALLY
PIN 86 (OFF) AND PIN 85 (OFF)

YOU WANT IT SO THAT STATE 1
PIN 86 (+) AND PIN 85 (-) ACCTIVATES RELAY 1

AND STATE 2
PIN 86 (OFF) AND PIN 85 (-) ACCTIVATES RELAY 2

IF THIS IS WHAT YOU WANT EMAIL ME AND I'LL SEND YOU A SIMPLE SCHAMITIC. LOPESN1464@YAHOO.COM

PS THIS IS A GREAT SITE. I'VE LEARNED SO MUCH. THANK YOU!

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