Calling any 12v Electronic Experts ?

[xeitosaphil]

I know there are lots on the forum who are very knowledgeable when it comes to electronics and will find this a simple question.

I want to put an indicator light either Neon or Led ( whichever is thought best ) into a switched circuit but not on the + side, but in the negative wiring side ( if that’s possible).
I would like to be able to break the negative feed between two points with a switch, and this break to activate a warning light to say the circuit is broken.
I know if it was on the positive side, I could use an ordinary switch with an Led indicator, but I don't think it would work an a negative circuit.

I wondered if the switch was a SP 2 way type, if the break in flow through the switch could then activate the Led / Neon light to indicate its circuit has been broken.
I have read that if using a Led, it would need a power supply, and it has to have a constant Voltage guarantee ( or within specific limits ) and also a resistor.

I also understand that onboard electric voltage can fluctuate maybe up to 15+ volts with the alternator so wondered how I would protect this sensitive circuit.

I am no wizard when it comes to electrics or electronics but can implement installations if I have the information needed. I have endeavoured to make a diagram of hopefully of the right symbols and wondered if this would work, and if not would be glad for advice for an alternative.

As ever, very grateful for any info received

Philip

 

[Beyondhelp]

Limit the current to 20ma, assume a 2v drop for red led. Worse case sinareo for battery voltage (16) gives you:

16-2. 14v

V=IR thus R=V/I

I=0.020

14/0.020 = 700ohm

700 is a non standard value nearest is 680 or 750. Choose 750ohm to be safe.

What 'wattage' resistor? Ok so power dissipated in the resistor = P(watts)=IV

I=0.020 V=14 =0.28

0.28 is just over 1/4 watts. A 1/4 watt or 0.5watt resistor will be fine.

FYI I ususally just stick a 1Kohm resistor on an LED for 12v. They always work fine, but thought I'd show the workings behind it.

Your wring diagram is correct if you want it to show when the switch is OFF in this case, and your safer putting the resistor on the +v side of the diode.

HTH

Ed

 

[elton]

You just need an LED in series with a 1K resistor, and connect them (in series) across a simple on/off switch in the negative supply. When the switch is off, current flows through the LED/resistor; when it's on, the resistor/LED is effectively short circuited so it turns off.

 

[Beyondhelp]

You just need an LED in series with a 1K resistor, and connect them (in series) across a simple on/off switch in the negative supply. When the switch is off, current flows through the LED/resistor; when it's on, the resistor/LED is effectively short circuited so it turns off.

Wouldn't work for something like a bilge pump with float switch that draws no current when not running, but would still be 'on'.

 

[bedouin]

What is on the circuit you are monitoring?

One simple approach could be to wire the LED across the switch with appropriate resistors to control the voltage (depends on what is on the circuit). A posher alternative would be to use a transistor that switches on when the voltage at the instrument side of the switch is greater than zero

 

[bedouin]

You just need an LED in series with a 1K resistor, and connect them (in series) across a simple on/off switch in the negative supply. When the switch is off, current flows through the LED/resistor; when it's on, the resistor/LED is effectively short circuited so it turns off.

That would only work with simple devices like bulbs

 

[Beyondhelp]

What is on the circuit you are monitoring?

One simple approach could be to wire the LED across the switch with appropriate resistors to control the voltage (depends on what is on the circuit). A posher alternative would be to use a transistor that switches on when the voltage at the instrument side of the switch is greater than zero

I would like to be able to break the negative feed between two points with a switch, and this break to activate a warning light to say the circuit is broken.

The OPs diagram is perfect already. I can't see how a transistor would help or make it any better.

 

[elton]

Wouldn't work for something like a bilge pump with float switch that draws no current when not running, but would still be 'on'.

That would only work with simple devices like bulbs

Fair points. Then a 300 ohm resistor between + and -, beyond the LED should draw sufficient current to illuminate the LED. And yes I know it'll be marginally dimmer, but that could be seen to indicate that nothing is drawing significant current.

 

[Beyondhelp]

Or just use the diagram the OP has done?!

 

[prv]

FYI I ususally just stick a 1Kohm resistor on an LED for 12v. They always work fine, but thought I'd show the workings behind it.

And to make it even easier, you can buy "12v LEDs" which are just a normal LED with a suitable resistor already attached.

Pete

 

[xeitosaphil]

To answer the question on what its for, the switch to break the circuit is the negative sensing wire for the VSR.

Once the VSR is disconnected I want a visible indication it has disconnected the house and engine start battery banks so I don't forget to reconnect them when any major alternator charging over 14.4v has finished.

I noticed in the week that the alternator due to low temperature was putting out something like 15v from the Adverc which will not be good for my small Red lash engine start battery. I have disconnected the sensing wire from the negative block so as not to damage the R/Flash and want a more permanent easily executed solution, but with a visible indication its disconnected so I won't forget to reconnect it.

Pete:

You wouldn't happen to have a link to the Led you mentioned?

 

[xeitosaphil]

Limit the current to 20ma, assume a 2v drop for red led. Worse case sinareo for battery voltage (16) gives you:

16-2. 14v

V=IR thus R=V/I

I=0.020

14/0.020 = 700ohm

700 is a non standard value nearest is 680 or 750. Choose 750ohm to be safe.

What 'wattage' resistor? Ok so power dissipated in the resistor = P(watts)=IV

I=0.020 V=14 =0.28

0.28 is just over 1/4 watts. A 1/4 watt or 0.5watt resistor will be fine.

FYI I ususally just stick a 1Kohm resistor on an LED for 12v. They always work fine, but thought I'd show the workings behind it.

Your wring diagram is correct if you want it to show when the switch is OFF in this case, and your safer putting the resistor on the +v side of the diode.

HTH

Ed

Hi Ed,

Forgot to say thanks for your very descriptive and informative reply most appreciated

Thanks Philip :encouragement:

 

[pampas]

Also Maplins do a 12v Red flashing led, might make life a bit simpler.

 

[xeitosaphil]

Thanks Pampas,

That would be good if it meets the spec needed?

Philip

 

[BabaYaga]

I have done something broadly similar for a similar purpose. But instead of a warning light to remind me to reconnect the batteries, I use a momentary switch and a small five pin relay set up so that it stays open as long as the engine runs.
So when I want to disconnect the Red Flash battery, I operate the switch and the main relay is de-energized, separating the batteries. Now only the house batteries get charge. This remains the case until the engine is shut down. Then the small relay also releases and when the engine is started again, the main relay will work as normal.
Might be something to consider.

 

[xeitosaphil]

I have done something broadly similar for a similar purpose. But instead of a warning light to remind me to reconnect the batteries, I use a momentary switch and a small five pin relay set up so that it stays open as long as the engine runs.
So when I want to disconnect the Red Flash battery, I operate the switch and the main relay is de-energized, separating the batteries. Now only the house batteries get charge. This remains the case until the engine is shut down. Then the small relay also releases and when the engine is started again, the main relay will work as normal.
Might be something to consider.

That sounds a good suggestion as well, with an element of automation, which could be better than my option as it is automatically disconnected when engine is off and reverts back to normal VSR operation until pressed again.

Would you happen to have circuit diagram maybe I could follow, or maybe a list of suitable parts you used? Sounds very interesting.

Thanks for the suggestion anyway,

Philip

 

[BabaYaga]

That sounds a good suggestion as well, with an element of automation, which could be better than my option as it is automatically disconnected when engine is off and reverts back to normal VSR operation until pressed again.

Would you happen to have circuit diagram maybe I could follow, or maybe a list of suitable parts you used? Sounds very interesting.

Thanks for the suggestion anyway,

Philip

As a relay I use something similar to this:
http://www.in-car-stuff.com/shop/12v-30-amp-pin-relay-spco-30a-p-59.html
The momentary switch can be any type of NO, e.g. push button. I use a toggle switch on the distribution panel that doesn't stay in the closed position.
The difference between your setup and mine is that you use a VSR while my combining relay is a "simple" heavy duty relay controlled by alternator D+. So you need to operate on a negative lead, while mine is a positive one. Because of this, the circuit diagram will not be identical, but I think it will work with some modification. Exactly how to do it I have not quite figured out yet, PM me if you would like a suggestion. Perhaps someone one the forum knows?

 

[xeitosaphil]

I have done something broadly similar for a similar purpose. But instead of a warning light to remind me to reconnect the batteries, I use a momentary switch and a small five pin relay set up so that it stays open as long as the engine runs.
So when I want to disconnect the Red Flash battery, I operate the switch and the main relay is de-energized, separating the batteries. Now only the house batteries get charge. This remains the case until the engine is shut down. Then the small relay also releases and when the engine is started again, the main relay will work as normal.
Might be something to consider.

As a relay I use something similar to this:
http://www.in-car-stuff.com/shop/12v-30-amp-pin-relay-spco-30a-p-59.html
The momentary switch can be any type of NO, e.g. push button. I use a toggle switch on the distribution panel that doesn't stay in the closed position.
The difference between your setup and mine is that you use a VSR while my combining relay is a "simple" heavy duty relay controlled by alternator D+. So you need to operate on a negative lead, while mine is a positive one. Because of this, the circuit diagram will not be identical, but I think it will work with some modification. Exactly how to do it I have not quite figured out yet, PM me if you would like a suggestion. Perhaps someone one the forum knows?

Is there anyone that might be able to come up with a solution to what BabaYaba is suggesting - using a relay of some sort, but using it to break my Negative sensing wire rather than how he has his wired.

Or is this not feasible?

It just seemed a good solution and being automatic in it's operation once activated, just means that it's something less for me to have to be aware of - re-activating the VSR, although it's no great shakes to do it.

Philip

 

[xeitosaphil]

Forgot to add the Diagram so deleted this post and made another -Sorry

 

[xeitosaphil]

BabaYaba has kindly supplied me with a Diagram and explanation of its use in an attempt to solve my proposed circuit installation option. He has given me permission to post it here for any relative feedback.
Could someone just confirm that his diagram and explanation of use will actually work as suggested Please, and if not make any further adjustments or recommendations if thought necessary?

Explanation of use :

In normal operation the negative feed will pass right through the relay via 30 and 87A.
However, when the momentary switch is pressed, the relay coil (85 and 86) will be energized meaning the connection inside the relay will change to 30 to 87. The relay will remain closed even when mom. switch is released, by the connection 87 to 86.
On engine shut down alternator D+ will lose its voltage, making the coil de-energized, opening the relay.

Would you say that this is a standard 12v 5 pin relay or do they call it something else, also would it have to be a special spec as the sensing wire only carries approx. 3Amps, also would the momentary switch have to be a special rating?

Once again thanks for any replies

Cheers Philip

Sorry for duplicating the reply , I forgot to add the Diagram and you can't add it to post when editing :redface-new: