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  1. #1
    Join Date
    Mar 2009

    Default Making a Bidirectional 0-20 Amp DC Ammeter for Motorhomes or Workshop

    Hi everyone,

    I recently posted this on an Australian Motor Home forum, as well as other posts about my “motorhome”, and I thought that some on this forum may be interested in the concepts as well as the specific use.
    The general ideas are portable and have wider application.
    As presented it is a DC ammeter only.

    Cheerio, mike

    Kea Conqueror – Installing a Dash Mounted Bidirectional Ammeter

    My ex rental 2009 Kea Conqueror is based on a 2008 Toyota VDJ78 Troop Carrier with a 4.5 litre V8 diesel. It is the model with frig in the middle of the rear space just behind the cab.

    One of the best things you can add to your Conqueror or any motor home is a permanently switched on, dash mounted bidirectional ammeter, so that you can monitor the current going in and out of your second battery all the time. If it is dash mounted, you can watch how the battery is charging whilst you are driving. You can also use it to check the current draw of any fitted DC device and know whether it is normal.

    Here’s how I added a DIY version. Not quite a pretty unit, but OK, and very effective. This project is really aimed at those who like to make things, as you will not save a lot of money with this unit.
    If this is not you, you might prefer to just buy a unit.
    Jaycar and others have some digital ammeters.
    Jaycar QP5588 is worth a look for some applications, although I am unsure if it has bidirectional display indications.

    You will really need some electrical or electronic knowledge to interpret and construct this project. Also see caution below before you start or buy anything.

    This ammeter can be used in most 12 volt situations in most vehicles.
    Most bidirectional ammeters (ie monitoring battery charge and discharge current) require a separate floating DC supply for the display, ie with no connection to the vehicle.

    I used a K265M kit from Oately Electronics at about $30 to obtain an LCD display and floating supply. I selected this LCD type and size as distinct from a red LED unit, so as to give readable digits and minimal visual disturbance whilst driving, but any high impedance 200mv Full Scale Deflection (FSD) display could be used.
    In the Oately kit you get two displays, and parts to build a dual power supply. The kit originally was designed to power two displays, one for volts and one for current. I used only one, but there’s an opportunity here if you want to display your voltage also. I don’t bother, as my Conqueror has an analog voltmeter in the rear for battery SOC when I am not driving, and when I am driving, I know the battery charge volts must be ok if it shows positive charge current.

    Caution - my kit had details of a display mod to be carried out which is a bit tricky, so this project is probably best done by persons with some electronics knowledge.

    I mounted the display in a plastic box and ran cabling to the PSU PC board and interface in another plastic box which mounts up under the dash.
    This in turn connects to the current shunt, which is mounted on a terminal block near the positive terminal of the second battery.

    Assemble and then bench test all units in a stable cal type situation before installation. Observe sense polarities to obtain correct charge and discharge indications.

    Main Box with PSU and Connection Strips

    Install under the dash out of weather somewhere with some form of bracket.
    In my vehicle it sits on the driver’s side of the dash centre bottom “wall”. See photo.

    I added a 1N4004 type polarity protect diode on the PSU input.
    This is mounted on the PCB rear and will not show in photos.
    I connected the Battery Negative terminal to the Oately PCB internally via a short single strand of 0.1 mm copper to act as a fusible link.
    I used a 2.5 K linear pot with locking nut ex my junk box for current shunt amps calibration.
    The shunt sense signal is filtered with three 0.68MFD 100V poly caps.

    PSU Kit notes

    Refer Oately website (or a link in my end notes) for circuits and kit notes.
    PSU kit R6 is now a 1000R pot, and adjusts PSU Vout.
    I set mine to give 12.5 V with display load on, which then gives 11.9 V when battery is at my frig (Waeco CF80) low volts point of 10.1 V, so that the meter still works beyond the frig end point
    PSU kit output filter caps are now 0.22MFD ceramics (8 off).
    Freq oscillator trim adjusted for max Vout and mine is at 1818KHz.
    Current draw of PSU is 5mA, (Display 0.3mA).

    The whole ammeter is left permanently “ON”. Lift the shunt sense connectors to switch off.

    Current Shunt

    The current shunt is made from 110mm electrical length of 3.15mm dia 304 or 316 type stainless steel rod, and bent into a hairpin shape.

    Mount on a very sturdy two terminal DC Terminal Block, sometimes called a Power Distribution Block, and available at Springers, Bias Boating etc. You will need to have two 6mm dia threaded terminal bolts about 20mm long, in order to attach the shunt and cable terminals.
    You can maybe scrounge an old one eighth inch solid stainless radio antenna rod, or even a 3.2mm stainless steel welding rod (SS308) with flux removed.
    Bend the two “eyes” for the shunt after heating the rod with a small gas torch to red heat. Clean the shunt after manufacture. Leave bare.
    Electrical length means the physical length of the shunt after bending the eyes and assembling onto the terminal block using small flat stainless washers.
    The current shunt using 110 mm of the stainless rod actually gives 5-10% more output mV than we need, but allows for calibration of our ammeter system.
    If your shunt rod does not allow cal, you may have an unusual rod material and you should make another 10% longer. Don’t use iron wire as it rusts.
    I have to point out now that stainless steel and iron shunts get affected by temperature more than traditional shunt materials, but don’t worry about that, as we don’t need accuracy more than 0.1 Amp in our application anyway.

    Or buy your own 0.01 ohm shunt.

    This current shunt value, combined with a required range selection on the LCD display (refer kit data sheet), will give xx.xx or plus and minus 0-20 Amps displayed (actually a maximum of 19.99). It displays a leading “-“for negative current ie going out of the battery, but not a “+” symbol; a lack of any preceding symbol means positive current, ie going into the battery.
    I chose 0-20 Amps range to suit the charge rate of my single 120AH battery, and to give some low current resolution to measure device currents, like lights, TV, frig etc. I like to see 0.1 amp changes, which means resolution to two decimal places, thus allowing for the uncertainty of the last digit. You may like to use 0-200 Amp, but you set the display to xxx.x and use a shunt of 0.001 ohm.

    My PSU and interface box under the dash contains the Oately dual PSU (we only need one section) and a calibration variable resistor, and some barrier strip type connection blocks.
    The PSU and display are DC powered via one side of the shunt sense leads and vehicle chassis earth.
    I powered up my system on the bench and calibrate it at five amps using an accurate analog current meter (French Metrix 477) a 12 Volt 10 Amp variable supply and a 0-5 ohm large load resistor.
    This five amps current gives a bit more than 50mV across the shunt, to enable calibration.
    To check my Metrix bench ammeter, I use an accurate 2.5 volt precision source and a DVM, but you can use any accurate DVM, or even any really cheap DVM which has been checked for accuracy.

    The problem with cheap meters for calibration, apart from a list of twenty things, is that you may not have accuracy at such a low voltage, and, not enough digits to display it.
    You cannot rely on a meter displaying 0.05, it must display 0.050, again allowing for the uncertainty of the last digit.

    You can also bench calibrate your ammeter using a 2.2 ohm 10 watt resistor and a 12.0 volt source to create 12/2.2 or 5.45 Amps. This resistor will get very very hot (65 watts heat), so only turn on the power in quite small bursts, no more than five seconds.
    Jaycar has these resistors, and it will probably be marked 2R2.
    Use short thickish cables and very good connections, preferably soldered where possible.
    There must be 12 volts across the resistor and shunt, not 11.9 volts.

    Wire up your ammeter shunt and the 2.2 ohm load resistor in series and place all across the 12.0 volt supply, and measure the voltage across your shunt. Adjust your cal resistor so the display reads “5.45”. Reverse your ammeter test leads, and your display should reverse its indicated polarity and show”-5.45”. Do all this quickly, so that thermal affects do not interfere, and then turn it off so nothing overheats.
    Refer to attached circuit.

    Installing the Shunt and Terminal Block

    In my Conqueror (2008 Landcruiser VDJ78), I mounted my shunt block behind the rear of my second battery, in a confined space between the battery and firewall, on a 90 deg bent piece of zincanneal steel sheet which was in turn screwed onto a clamp type battery connector.
    Steel sheet is used for rigidity and carries no current to the system, but it is “live” so beware.
    I covered mine on top with some protective clear polycarbonate sheet.
    The shunt is attached to the terminal block first with washers and nuts, then the spade connectors, and finally the battery connecting cables are attached using separate nuts and washers.
    I used modified male 6mm spade chassis connectors, (Jaycar PT4900 or similar) drilled with 6mm holes, and attached to the terminal block, so I could then use small twin cable with spade connectors to feed the shunt volts to the connection box under the dash.
    My shunt terminal block is insulated from the bent metal attachment, and my system has an 80 amp (Maxi) fuse installed between the battery positive and the current shunt as good practice.

    If you do this, you must make sure that the fuse is very “tight” in the holder, or else the unit will heat up with the constant moderately high current.

    The shunt must be in this specific electrical position, not in the second battery feed to the “house”, so as to measure both charge and discharge currents.

    The displayed current is the arithmetic sum of input and output battery currents flowing at the shunt position, and including sign.
    This means if your Conqueror has the motor stopped, solar charging "off" and is mains charging at 6 amps and you turn on a 2 amp 12 volt TV, you meter will display “4.00” amps, ie going into the battery.
    If you are in camp, motor stopped, solar charging at 2 amps, and your 4.5 amp frig comes on, your meter displays “-2.50” amps.
    When you are alternator charging at more than 19.99 amps, your display will display “-“ or similar, as overscale, and your battery probably is charging at approx 25 - 40 amps, due to cabling size and other system losses. (Some LCD displays may have a slightly different way of indicating this “overscale”).
    No damage to your ammeter or shunt will occur, and when the charge current falls below 20 amps, the ammeter will again display “19.99’ amps and falling lower as battery state of charge increases. The shunt will carry a very high current, and in a fault situation, your other protection devices will activate before your shunt gets damaged.

    Display (Oately type DPM1)

    Follow the kit instructions to carry out any mods and set the decimal point to xx.xx .
    I added a BAV21/1N914/1N4148 type polarity protect diode on the PSU input
    Attach wires for DC supply and V measure, and mount in box.
    I used a Jaycar box cat HB6068, and cut off the tabs, and made other cuts and holes to make the display fit.
    I mount my display on the RHS of the dash using Velcro.
    My display has a current draw of 0.3mA, and works down to a supply voltage of 7 Volts.
    The display usually will “auto zero”, and have an adjustment for the 200mv “Full Scale Deflection”, but in most cases you can ignore this, leaving it set as per ex-factory; see the following adjustment if you have sleepless nights about it. The adjustment range is normally very small anyway.
    My display box internals are a bit "rough" as it is still my prototype.

    Full Scale Deflection Check and Adjustment

    The calibrating DVM must be 4.5 digits, ie you must be able to manually select a voltage range so that it can indicate 190.0 mV. Do not do this FSD adjustment if your DVM cannot do this.

    Wire up a 15k 1 watt 1% resistor and a 500 ohm linear potentiometer in series and set the pot to half range.
    Solder connection wires from the pot wiper (centre tab) to LCD display “V in” positive, and the “V in” negative to the end of the pot which is not connected to the 15 k resistor.
    Connect a 12 volts DC supply positive to the free end of the 15k resistor, and negative to the pot tab which has the display “V in” negative connected.
    Power up the display with a separate 12 volt supply.

    Place an accurate DVM across the display “V in” terminals, and adjust the pot so that you read 190.0 mV on the DVM (not 199.9, and there is a reason). Adjust the display FSD cal pot so the display indicates “190”.
    If you have set the display decimal point to be xx.xx, then it will indicate “19.00”.
    Refer to attached circuit.

    Parts list

    Oately kit K265M – “Panel Meters and Alternative Energy Battery Charge Monitor”
    Maxi Fuse Holder – Narva, from Autobarn, Repco, or Supercheap
    Box for PSU and connections – Jaycar HB6075
    Box for display - Jaycar cat HB6068
    Terminal Strips – Terminal Barrier Strips Panel Mount Four Way, Jaycar HM3166
    Spade lugs for shunt –Jaycar Chassis Mount PT4900 modified
    Resistor 2R2 10 Watt for cal – Jaycar RR3344
    Cal Resistor variable 2K5 linear – multiturn or locking type

    I have no association with any company.

    Edit - no damage to shunt with high charge current

    1 dash display P1130401 lo.jpg2 PSU and Conn box P1130387 lo.jpg3 PSU and Conn box internals P1130390 lo.jpg4 shunt on block P1130385a lo.jpg5 shunt terminal block P1130395a lo.jpg6 Ammeter cct scan0002.jpg7 ammeter cal scan0001.jpg

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  3. #2
    Join Date
    Mar 2009


    Hi again

    I spotted a (fatal) error today whilst I was playing around with my unit.

    The ammeter circuit is incorrect in the area of battery polarity to the Oately PSU module. Apologies.

    Corrected circuit attached.

    Incidentally, there are minimal components to suppress RFI and transients in this circuit.
    If you have a suspected problem, where your unit works on the bench, and is erratic in the vehicle, then this could be the problem. You may have to add some filters on all leads, and possibly a zener on the supply.

    And, this type of ammeter typically shows a small offset with no current through the shunt due to "common mode currents". It arises because we are powering the circuit from the circuit we are measuring. My unit shows an offset of +0.07 amp when actually the current is zero.
    This is of no importance (especially when you are charging at say 15 amps), and can be ignored or allowed for when not charging.
    The circuit was arranged this way for simplicity.

    If this offset bothers you, then you can power up the ammeter circuit from the starter battery (in a motorhome), which is a separate battery electrically when you are not charging from the alternator and your VSR is not operated.
    Use a fused source. Mine also has a disaster fuse (single strand of 0.0076 inch tinned copper) in the battery earth lead inside the box.

    This is what I have done in my motorhome, as I am using it to measure the current drawn by appliances and gadgets etc, and with some accuracy.

    cheerio, mike

    Ammeter cct scan0002 corrected lo.jpg

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