Ammeter...

Submitted: Friday, Apr 07, 2006 at 12:50

ThreadID: 32644 Views:2802 Replies:6 FollowUps:14

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Member - Blue (VIC)

Q for all the 12V guru's out there...

I purchased a 10A DC panel meter(as recommended by a 12yo sales assistant in a major electronics chain) today to measure the draw of my 110L Waeco... So knowing next to nothing about such things, but willing to learn, I embark on my journey of discovery only to be stopped before I even leave the port...

I was of the understanding that the ammeter would go in-line on the positive wire to the fridge... When I opened up my little parcel and looked at the terminals on the back, one is marked "+" and the other "-". So I cut my wires and installed the meter across both wires as indicated, "pop" blow a fuse in the lead... So before i do some real damage, does anyone know how exactly this thing should be installed...???

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Reply By: agsmky - Friday, Apr 07, 2006 at 13:02

Reply By: Mad Dog (Australia) - Friday, Apr 07, 2006 at 13:05

Reply By: Member - Wim (Qld) - Friday, Apr 07, 2006 at 13:12

Friday, Apr 07, 2006 at 13:12

Blue.

The short answer is + to the supply & - to the load.

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Follow Up By: Member - Blue (VIC) - Friday, Apr 07, 2006 at 13:18

Friday, Apr 07, 2006 at 13:18

Surely they could take the time to realise that mindless gits like me will, even though they have no clue, try to install these things at home and write "load" and "supply" on the back instead of "+" and "-"...

FollowupID: 420436

Follow Up By: V8troopie - Friday, Apr 07, 2006 at 15:09

Friday, Apr 07, 2006 at 15:09

No Blue, writing anything other than + - on the terminals would only confuse all the 'leccies' who KNOW how to connect these things.

You see, Ampmeters can be installed in either lead and as long as the terminal marked + has a more positive voltage on it than the other it would read the right way. Connect it the wrong way round and the pointer would try to go below 0 and might even bend around the little stop post if the current was big enough.

Internally, Ampmeters have a VERY low resistance, like a dead short. So if you connect them ACROSS the supply you get 'dead short' effects.

In case you are confused by now, to connect it into the NEGATIVE lead you would connect the battery negative to meter - and meter + to the load. Other load connection would go to the fuse and then battery positive.

Klaus

FollowupID: 420458

Follow Up By: V8troopie - Friday, Apr 07, 2006 at 15:21

Friday, Apr 07, 2006 at 15:21

Just to confuse you even more, there are different types of Ampmeters. The one you got is a common one and referred to as a "moving coil" ampmeter. These are polarized, hence the + - symbols.
Then there is the "moving iron" type meter which is NOT polarised, you could connect it either way round into ONE lead and the poiner always goes the correct way. It would also measure AC current directly whereas the one above would require a calibrated rectifier to be useful on AC.
However, the scala on these moving iron meters is not linear, the low readings are all squashed together. I have a 10A version and its fine over 3A but below that the scala is not easy to read accurately.

There are other types as well but they are rarer on only used for special applications.

I hope you do not mind this "lesson" but you did ask to be educated on Ampmeters :-)

Klaus

FollowupID: 420462

Follow Up By: Bonz (Vic) - Friday, Apr 07, 2006 at 17:11

Friday, Apr 07, 2006 at 17:11

Git? hehehehehe havent heard that for years

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FollowupID: 420484

Follow Up By: Member - Blue (VIC) - Friday, Apr 07, 2006 at 20:02

Friday, Apr 07, 2006 at 20:02

Bonz... My grandfather used to call me a "git", in fact he used to call everyone a git... For some reason, I'm the only one he was spot-on with...

FollowupID: 420520

Reply By: Alloy c/t - Friday, Apr 07, 2006 at 14:38

Reply By: Member - Crazy Dog (QLD) - Friday, Apr 07, 2006 at 17:39

Friday, Apr 07, 2006 at 17:39

For those who like to roll their own I found this on the internet - made by an Aussie chap.

Looks good - I am going to build one for my trailer. The devices from DSE have diagrams that are easy to follow so most people who have some skills would be able to make.

=====================================================
Needing accurate Amp and Volt readouts is a must with any DC system but unfortunately there are not many displays around that can do this without costing an arm and a leg. I was not interested in the voltage displays with clocks and thermometers as they are both inaccurate and tacky so for the same amount of money I made my own. All up cost was around $37 and little bit of time to assemble.
The setup consists of 2 LCD displays from Dick Smith (part no. Q2220 cost $9.98 ea), a 50A current shunt from Jaycar (part no. QP5412 cost $9.95 ea), a switch, resistors (to setup the displays), wire, terminals, a box and a 9V battery. The shunt is located in the battery compartment and is secured with a dob of silicon to reduce holes in the tub. The negative leads of all accessories go to one side and the batteries negative lead is connected to the other side. All accessories have their negative lead terminated at the shunt with nothing being grounded to the chassis. The LCD is then connected to the shunt to display Amps. You can then see current going into and out of the batteries (displays a negative sign when current going out). Both readouts are powered by a 9V battery and I put a switch in to turn them on and off as a battery was only lasting a couple of weeks when they were always powered on. I am extremely happy with the setup and can see what current each accessory is drawing and what the chargers are putting in.

Have fun=====

AnswerID: 165617