The circuit shown on this page allows inexpensive digital multimeters to measure currents in DCC systems. The DCC ammeter circuit is designed to use a DC milliammeter to monitor the current but other types of meters could also be used.
Using a milliammeter as the readout means that the circuit does not require re-calibration if a different meter is connected to the circuit. The meter will normally be set to the 20 milliamp range which allows the highest output reading range.
The DCC Ammeter uses two - ZETEX - ZXCT1009 - three terminal 'Current Monitor' integrated circuits to measure the current.
The DCC Ammeter can operated in circuits of between 7 and 20 volts. The 20 volt maximum can be increased if needed, see the diagram further down the page.
The 2010 DCC ammeter circuit is designed for a 5 amp continuous current but can operate at up to 7 amps for short periods.
Current Monitor - IC 1 produces an output when the current is flowing from terminals A to C.
Current Monitor - IC 2 produces an output when the current is flowing from terminals B to D.
The outputs from IC 1 and IC 2 flow through the meter which is normally set to either the 20 milliamp range.
For the values given, R1 and R2 equal 0.1 ohm, when the load current is 1 amp the current from the output terminal of a ZXCT1009 will be 1 milliamp. This gives a ration of or 1000:1. (See the ZXCT1009 datasheet for more information.)
Therefore, the output current from the ammeter is 1 milliamp per ampere of load current.
The milliammeter will normally be set to the 20 milliamp range which allows the highest output reading range. If the meter is set to the 2000 microamp (2 mA.) range the readout will only indicate as high as 2 amps but gives better resolution for low current measurements.
No external calibration is required for this circuit. The accuracy is mostly dependent on the 1 percent tolerance of the 0.1 ohm resistors.
With no load on the circuit, the meter will indicate a current of about 40 milliamps. This is due to the internal bias currents of the ZXCT1009 current monitor ICs.
The output of the circuit always has the same polarity regardless of the polarity of the input current.
Diodes D1 and D3 prevent current from flowing into the output terminals (pin 3) of the current monitors when they are reverse biased.
Diodes D2 and D4 prevent short circuiting through the meter's common return portion of the circuit.
The design operating voltage range for the ammeter circuit is between 7 and 20 Volts.
The ammeter circuit is bipolar and can be checked using a DC supply and a separate ammeter. The circuit can be checked with both polarities to test for balance.
If you would like to make use of this circuit, refer to the manufacturers data sheet at; ZETEX - ZXCT-1009 Data Sheet (PDF)
Installation Note: If the circuit board is to be used in a permanent installation the terminal blocks can be omitted and the supply and load wires soldered directly to the circuit board. This will reduce the overall cost of the circuit as well.
This circuit has been tested using 60 Hz AC but it is less accurate due to the much slower transition between polarities of a sine wave when compared to square waves from a DCC power source.
|Part Number||Description||Digi-Key Part #||Qty|
|IC1, 2||-||IC Current Monitor SOT-23||-||ZXCT1009FCT-ND||-||2|
|D1, 2, 3, 4||-||Diode 100V 4.0Ns DO-35||-||1N4148FS-ND||-||4|
|R1, 2||-||01. Ohm resistor 1% - 5W||-||LVRC-.10RCT-ND||-||2|
|-||-||2 Position Terminal Block - 3.5mm||-||ED1514-ND||-||2|
The following options are available for the DCC Ammeter (2010) circuit board.
1 - Bare Circuit board is 6.00 dollars US plus postage. (Each additional bare circuit board is 5.50 dollars.)
1 - Circuit board with 2 - ZXCT1009 current monitors and 4 diodes mounted is 10.00 dollars US plus postage. (Each additional circuit board is 9.50 dollars.)
1 - Circuit board with 2 - ZXCT1009 current monitors and all components including terminal blocks mounted and with 8 inch meter leads attached is 15.50 dollars US plus postage. (Each additional circuit board is 15.00 dollars.)
1 - Circuit board with 2 - ZXCT1009 current monitors and all components mounted and with 8 inch meter leads attached but without terminal blocks is 14.00 dollars US plus postage. (Each additional circuit board is 13.50 dollars.)
NOTE 1: Option 4 can be used for a permanent installation where the ammeter does not need to be disconnected. The input and output leads would be soldered directly to the circuit board.
If you are interested in printed circuit boards please send an email to the following address: firstname.lastname@example.org
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The circuit board measures 2.9" X 0.9" and and mounts the current monitor ICs on the copper side of the board.
The board is shown with terminal blocks but can be used without them.
The ZXCT1010 is a very small, its package outline is approximately 1.5mm (0.060") by 3mm (0.120"). The photo shows the ZXCT1010 Current Monitor ICs mounted on the circuit board. For scale, the width of the copper traces is 0.030 inches. The width of the pins on the IC is about 0.020 inches.
The DCC ammeter can be used for systems that operate at greater than 20 volts by adding a voltage drop to the meter's circuit.
A voltmeter can also be used as a display if a calibrating resistance is placed across the outputs of the current monitors.
The 1K variable and 470 ohm resistors are used to calibrate the voltmeter circuit against a known DC or DCC load or another DCC capable meter. The voltmeters reading is adjusted the reading is 1 volt per amp measured.
The disadvantage of using a voltmeter as the display is that if a different meter is used the ammeter circuit may have to be recalibrated.
A digital panel meter with a 200 millivolt range can also be used with the ammeter circuit.
Here is an example of this type of meter from Marlin P. Jones & Assoc, Inc. These meters require a separate power supply for their displays.
Due to the low resistance of the 10 ohm shunt resistor, it does not need a 1 percent tolerance resistor if one is not readily available.
The circuit uses a full wave bridge rectifier and a zener diode to replace the multimeter's battery. Capacitors C2 and C3 isolate the meter's circuitry from the track circuit.
The parts values given in the circuit above are based on the digital multimeter having a current draw of approximately 1 milliamp from its 9 volt battery. The parts values were determined using actual circuit conditions.
A 1N757, 9.1 volt zener type diode is used in the circuit above but any combination of zener's, forward biased ordinary diodes or LEDs that can produce a voltage drop of between 8.0 and 9.5 volts at 1 milliamp would also work.
When the power to the track is ON, the optoisolator will act as a closed switch and the meter will have power.
The following is a Dual - DC Ammeter circuit made with the DCC Ammeter circuit board shown at the top of this page.
The only changes needed to the original ammeter circuit are the removal of the four blocking diodes and connecting the meters directly to the outputs of the current monitor ICs.
A toggle switch could be used to select outputs so that one meter could be used for both outputs.
The circuit board could also be used to make two separate DC ammeter circuits.
The explanations for the circuits on these pages cannot hope to cover every situation on every layout. For this reason be prepared to do some experimenting to get the results you want. This is especially true of circuits such as the "Across Track Infrared Detection" circuits and any other circuit that relies on other than direct electronic inputs, such as switches.
If you use any of these circuit ideas, ask your parts supplier for a copy of the manufacturers data sheets for any components that you have not used before. These sheets contain a wealth of data and circuit design information that no electronic or print article could approach and will save time and perhaps damage to the components themselves. These data sheets can often be found on the web site of the device manufacturers.
Although the circuits are functional the pages are not meant to be full descriptions of each circuit but rather as guides for adapting them for use by others. If you have any questions or comments please send them to the email address on the Circuit Index page.
09 February, 2012