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Jack
Jack, Technician
Category: Consumer Electronics
Satisfied Customers: 8349
Experience:  35 years experience as audio/video repair and installation tech.
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I want to know if you can help me a little problem I have

Customer Question

Hello!... I want to know if you can help me a little problem I have uncounted…. I have a two-in-one device a volt meter and a amp meter “DROK® Voltmeter Ammeter Volt Amp Multimeter DC 4.5-30V 150A Voltage Current Monitor Ampere Testing Gauge V/A Panel Meter Red/Blue Dual LED with Current Shunt Car Auto Motorcycle Battery Power”.. It’s a digital led display unit volts displayed on top and the amp on the bottom….. Now it has 3 wires one for the dc voltage in also for the voltage display…one for the negative for the device and voltage measurement… and the last wire is used for the amps…. Now the shunt is connected on the load at the negative wire.. Therefore the volt drop for the meter must be from battery negative and 175mv = (150 amps) across the shunt into the third wire…The volts and amps has the negative wire as common… Can you help me connect this work on a car alternator since I can’t isolate the negative of the alternator..
Submitted: 1 year ago.
Category: Consumer Electronics
Expert:  Steve G. replied 1 year ago.

Hello there and welcome to Just Answer. My name is ***** ***** I will be assisting you today. I am an engineer with over 30 years of electrical and electronic training, repair and installation experience. I will try and answer your question accurately and precisely so that we can get you on your way.

The case of the alternator is the 'negative'. The alternator grounds through it's connection to the engine block. So you can take the negative off literally ANY ground on the car.

Does that help? If not, let me know.

Steve

Customer: replied 1 year ago.

OK so how do I measure the current from the alternator with out having the electrics and the starter motor currents past trough the 150A shunt resistor?....Did you understand the notes and questions posted?...Check out this link : http://ecx.images-amazon.com/images/I/61CUVF%2B%2ByOL._SL1200_.jpg

There is a diagram of the connections.. how do you get the shunt resistor to work on the positive wire and feed the current sense wire with no more than 175mv to ground (-)?

Expert:  Steve G. replied 1 year ago.

What is it that you are trying to measure? The amperage of your audio system?

Customer: replied 1 year ago.

No just the current of the alt.

Expert:  Steve G. replied 1 year ago.

I do not see a reason why you would not put this on the positive side of the circuit. The meter will not experience the starter current - the shunt will but it is designed to. I doubt you would see more than 150A on a starter and even if you did, the shunt will handle much more than that. The instructions for installing on the negative side is a safety concern on the part of Drok. By keeping the shunt at a ground potential, the chance of a short circuit is reduced.

Customer: replied 1 year ago.

if you put the shunt on the POS. side the voltage input to the meter would be the battery voltage this would destroy the meter the meter needs no more that 175mv into the sensor wire (Yellow)...

Expert:  Steve G. replied 1 year ago.

Hi, that is incorrect (electrical engineer here). The voltage is always 12V.

When you start the car, the starter motor runs on 12 volts. There can never be more than 12-13 volts.

What does increase is current. Most starter motors will pull about 100A, which is why you have a shunt. The shunt causes only a small amount of current to flow through the yellow wire.

You will be fine.

Let me know if you have any questions.

Steve

Customer: replied 1 year ago.

The question is how can you produce no more than 175mv between black wire and yellow wire while the shunt is fitted on the alternator's positive out lead to the battery?

My starter motor peaks at 235 amps, My inverter also runs over 200 amps, My amplifiers go above 150 amps....I have 4 15 plate batteries in parallel..

Please Read and understand my question....

Expert:  Jack replied 1 year ago.

Hello , my name is Jack
Perhaps I can help you with this problem but first i would have a few questions so could you please send me a reply (if you still need help with this ) when you are ready to proceed ?

Customer: replied 1 year ago.

Yes I still would like to have a work and practical workable answer to my Qus..

Expert:  Jack replied 1 year ago.

Hello , this is actually an easy answer , had all the specs ready yesterday but if you`ll give me a minute I will review this and give you your answer

Expert:  Jack replied 1 year ago.

O.k , first , this little volt/amp monitor would be used for monitoring the output of the alternator , is that correct ?

Customer: replied 1 year ago.

Take you time...

Expert:  Jack replied 1 year ago.

Did you see my question above ?

Expert:  Jack replied 1 year ago.

Hello ?

I was asking ;

this little volt/amp monitor would be used for monitoring the output of the alternator and charging current, is that correct ?

Customer: replied 1 year ago.

YES!...However only the current from the alt. should pass trough the shunt resistor.

Expert:  Jack replied 1 year ago.

Yes I`m getting to that

Expert:  Jack replied 1 year ago.

Now the load of your amps ,inverter and starter do not really matter as they are feeding themselves on the batteries .

Just making some comments on what I read prior to my coming in and could not figure what was the deal herev .

Getting to that shunt now

Expert:  Jack replied 1 year ago.

That shunt is rated at 150 amps and from its specs when the load reaches that 150 then the voltage across it is 175 mv being picked up by the meter .

Nothing can change that and if you want to try and change that value then you will only get false readings .

More to come please be patient I`m a slow typist

Customer: replied 1 year ago.

Jack... The links that you email me does not work and it looks like this :

http://www.justanswer.comhttp/www.justanswer.com/electronics/9a924-want-know-help-little-problem.html?jats=FFC7FA2133C7464A909EC6ACB93F01DE&utm_source=sys&utm_medium=email&utm_content=viewanswer_bu&utm_campaign=answer_notify&r=nte%7C2015-09-04_pdcJaUsTrxAnswerNotify%7Cln=viewanswer_bu

Expert:  Jack replied 1 year ago.

Sorry but I have no idea what you are talking about here , I did not email you anything , I don`t have your email address, the website does , not me

Expert:  Jack replied 1 year ago.

to get back to your problem ;

Now this will monitor the load and voltage between the alternator and batteries so the current there is very unlikely to go over 150 amps unless you are connecting a welder to charge your batteries .

A heavy duty alternator will put out 90 amps and say that you have an out of the ordinary alternator then perhaps 120 amps coming out at max and again that would be with completely dead batteries and lots of them so I cannot see how you would go above 150 amps =175 MV and if you do then you cannot limit that 175v without getting false readings at the meter , in that case what you would need is a bigger shunt

Customer: replied 1 year ago.

And also keep in mind that the voltage must swing from 0 - 175mv so I think a circuit may be built to allow the current sense wire to be at zero volts when no current is passing...and what I see if the shunt is on the POS. side of the alt. it would have no less than 12v on the shunt to ground and 175mv is the max into the sense wire..

Expert:  Jack replied 1 year ago.

Not sure how you figure that but I think you got the wrong configuration here .

For example that little drawing supplied with the meter shows a motor and how to measure its drain and feed voltage , not an alternator

Expert:  Jack replied 1 year ago.

Let me rework that drawing a bit for your setup and I will upload this here , was waiting for you to reply before getting into that

Customer: replied 1 year ago.

No it's for DC and that motor is just a representation of the load ( it's chines)..got figure!..

But you can't come up with a something or device to make it work... Now if the black wire was able to be separated and have two wire for currnt and two for volts maybe I mite gen it to work... but it difficult to get 0-175mv from something connected on a live/hot wire...

Customer: replied 1 year ago.

could you let JustAnswer know that there links are not work...

Expert:  Jack replied 1 year ago.

About that link , I `m not even sure what you are talking about but I will take a look .

Was just looking at that again and realized that either the amp of voltmeter could work on the positive side but not both .

The amp meter negative side could be the out of the alternator and positive the battery side but then that would not give you a good reading on volts .

This is made to be connected on the ground side of a load not an alternator

You`d be better off with automotive amp and volt meters , just an example ;

http://www.autozone.com/gauges-and-gauge-accessories/amp-volt-gauge/auto-meter-0-to-250-amp-cobalt-digital-ammeter-gauge/703725_0_0/

Expert:  Jack replied 1 year ago.

Just checked that link and it doesn`t give me anything either so I will give you a working link and phone number where you can get all the answers for this website and even a refund ;

http://ww2.justanswer.com/help
1.***-***-****

Expert:  Jack replied 1 year ago.

That was fun , too bad that little gizmo is not made for that

Customer: replied 1 year ago.

So what now?

Customer: replied 1 year ago.

Not a full refund... A discount would surface to be fair!

Expert:  Jack replied 1 year ago.

You use the link and/or phone number i give you and get a refund , that thing will not work unless you are prepared to do some heavy ground rework on your vehicule to isolate the alternator from that chassis and then again would end with heavy static through the audio system .

Just saw your reply and that is cool and very appreciated , that can be worked out as well , use the phone number or link there then and tell them what you think is a fair price

Expert:  Jack replied 1 year ago.

This is very feasible , just the wrong equipment , good thing it wasnt very expensive and you could still use the voltmeter part