Discuss 12v Switch in the Auto Electrician Forum area at ElectriciansForums.net

Welcome to ElectriciansForums.net - The American Electrical Advice Forum
Head straight to the main forums to chat by click here:   American Electrical Advice Forum

D

dave21

New here so I say hello.


Some advice on car 12v please.


I want to fit a dual volt and amp meter in my dash, and have one with the appropriate shunt. My expectations are, when the engine is off it will show the state of the battery. When the engine is on it will show the active volts and amps?


I assume I can wire to the cigarette lighter socket with is always on?


As I don’t want the meter ‘always on’, I’d believe I can fit a simple on/off switch on the positive side. However, would a common accessory switch do the job or do I need a beefy high amp type?


Thanks, Dave
 
just fit a voltmeter to the battery with a 2A fuse in line. the meter will show battery volts ( 12. summat ) when engine off, and 14.2V or thereabouts with engine running.
 
The ammeter, if I remember correct, needs to be in series with the load you want to measure. So if you wanted normal running current would have to break into the main battery feed. Start up amps could be a lot have a look at the max output of your battery could be over 150 amp. Which is why they have large main cables. If you connected to cig lighter, it would show current draw from whatever is plugged into cig lighter. Althou I may just be talking rubbish lol.
 
think OP isa confused between ammeter and voltmeter.
 
No you right, the shunt is usually a 0.5 ohm resistor, and needs to be in series with the main positive cable to the battery, the amp meter then measure the voltage across the shunt, usually +1v to -1v which is proportional to the current being drawn, and shown in Amps
 
No you right, the shunt is usually a 0.5 ohm resistor, and needs to be in series with the main positive cable to the battery

Don't believe so. Starter motor draw from the battery is many tens of amps, depending on engine size, compression (diesels draw shedloads), oil gloopiness etc. Call it 40 amps ballpark though this will vary massively. You can't get 40 amps through a 0.5 ohm resistor off of 12V, and if you could it would dissipate 800 watts. Bang.
 
Don't believe so. Starter motor draw from the battery is many tens of amps, depending on engine size, compression (diesels draw shedloads), oil gloopiness etc. Call it 40 amps ballpark though this will vary massively. You can't get 40 amps through a 0.5 ohm resistor off of 12V, and if you could it would dissipate 800 watts. Bang.
IT is a shunt...rated at 150 amps...very common on cars and boats for measuring amps...been used for years....google before making rash comments, or I`ll set Glenn on you....!!!
 
Why do you want the meters? If you just like gizmos then fair play to you. But you almost certainly already have a battery voltage indicator of sorts, i.e. the battery warning light, which comes on when the battery voltage when being charged by the alternator drops significantly below 14.4V. When the battery voltage is being kept at this level then it is being charged / kept topped up by the alternator so any current in the main battery lead will be going in not coming out. I'm not sure this is actually the current you want to be measuring. If you really want to measure some current somewhere I'd go for the Hall effect sensor someone mentioned or some other sort of external current clamp which works at DC.
 
IT is a shunt...rated at 150 amps...very common on cars and boats for measuring amps...been used for years....google before making rash comments, or I`ll set Glenn on you....!!!

Do you mean milliohms or something? If what I've put is wrong, someone has rescinded Ohm's Law.
 
Nick, a shunt is made of carbon steel, able to withstand high currents, but to measure high currents is not practical......so the shunt has a resistance to its total load. ie 0.5 ohm....the voltage across the the shunt is measures in mV, which is proportional to the current being drawn....full 100amp = 1v forward, therefore 500mV = 50amp, and so on....even tho the amp meter shows amps, it actually measures mVolts
 
100 amps through a 0.5ohm resistor entails voltage drop across it of V = IxR = 100 x 0.5 = 50 volts.

100 amps through a 0.5milliohm resistor entails voltage drop across it of V = IxR = 100 x 0.5x10^(-3) = 50mV.

You mean 0.5 milliohms, dude.
 
Sorry, that comes up as blank to me.

Shunt (electrical) - Wikipedia, the free encyclopedia

"In this case the shunt, a manganin resistor of accurately known resistance, is placed in series with the load so that all of the current to be measured will flow through it. In order not to disrupt the circuit, the resistance of the shunt is normally very small. The voltage drop across the shunt is proportional to the current flowing through it and since its resistance is known, a voltmeter connected across the shunt can be scaled to directly display the current value.
Shunts are rated by maximum current and voltage drop at that current. For example, a 500 A, 75 mV shunt would have a resistance of 0.15 milliohms, a maximum allowable current of 500 amps and at that current the voltage drop would be 75millivolts. "

Similarly a 100A, 75mV shunt would have a resistance of 0.75 milliohms. A 100A, 750mV shunt would have a resistance of 7.5 milliohms.
 
Why do you want the meters?....... the battery warning light, which comes on when the battery voltage when being charged by the alternator drops significantly below 14.4V.

Because I want to know the state of my battery, the charge and the effect various plug-ins are having - like a cooler as I'm making a 1000k run. I already have pretty pointless lights that came with the car. i.e. When I had a beautiful Scorpio Exec it had a terrific diagram gauge where if a door or boot/bonnet was not closed properly it advise which one it was. Not wildly useful but what was was the bulb-out advice which showed which cluster it was. Now I have a more 'modern' Ford, one light advise something is not closed - guess what it is, and more useless one light to advise a bulb is out - go find it. It's doubly useless because it's always on despite all my 'nickable if out' bulbs are completely functioning.

Yes there's also a bulb to advise if the charging is low. Great, but what if the regulator's not regulating and the system is overcharging? Had that once, blew bulbs and did the batter no favours.

The gauge I have is duel readout showing volts and amps, and uses a 50amp 0.75mV shunt. I don't want/need it on all the time so I'm going to make it switchable. However, since posing the what switch question, it occurs to me a heavy duty switch is not needed as the meter itself is drawing negligible current, and, if wired to the lighter socket the socket circuit is already fused @15amps.

Thanks for the various replies.
 
NickD stop being an argumentative git, all Tazz is pointing out is the principal use of what and how a shunt works not the actual design parameters. Way your going you'll be in the naughty boys room again.
 
All I'm concerned about is the OP and anyone else reading the thread being misled and confused and thinking they've got their understanding of basic electrical theory wrong or wasting their time building circuits with the wrong values. Is this so unreasonable? On an electrical discussion forum I would have thought that was important. I find it odd anyone thinks it's not.
 
No you right, the shunt is usually a 0.5 ohm resistor, and needs to be in series with the main positive cable to the battery, the amp meter then measure the voltage across the shunt, usually +1v to -1v which is proportional to the current being drawn, and shown in Amps

... all Tazz is pointing out is the principal use of what and how a shunt works not the actual design parameters.

'0.5 ohm' sounds like an 'actual design parameter'. In this case, it's clearly an inappropriate value for the shunt, given the level of current to be expected.
 

Reply to 12v Switch in the Auto Electrician Forum area at ElectriciansForums.net

OFFICIAL SPONSORS

Electrical Goods - Electrical Tools - Brand Names Electrician Courses Green Electrical Goods PCB Way Electrical Goods - Electrical Tools - Brand Names Pushfit Wire Connectors Electric Underfloor Heating Electrician Courses
These Official Forum Sponsors May Provide Discounts to Regular Forum Members - If you would like to sponsor us then CLICK HERE and post a thread with who you are, and we'll send you some stats etc
This website was designed, optimised and is hosted by Untold Media. Operating under the name Untold Media since 2001.
Back
Top
AdBlock Detected

We get it, advertisements are annoying!

Sure, ad-blocking software does a great job at blocking ads, but it also blocks useful features of our website. For the best site experience please disable your AdBlocker.

I've Disabled AdBlock