Assembling a high-quality laboratory power supply or charger is impossible without accurate monitoring of output parameters. It is for this purpose that radio amateurs and professional engineers use combination meters that display voltage and current simultaneously. Correct volt-ampere connection diagram is the foundation for the stable operation of your entire power supply system, be it a simple battery charger or a complex converter.
In this article we will analyze in detail the nuances of installing digital indicators, such as the popular model DSN-V288 or TK1382. You'll learn how to avoid common switching errors, why the screen goes blank when a load is connected, and how to provide galvanic isolation of the measuring circuit to improve safety.
Incorrect installation can lead not only to incorrect readings, but also to failure of the measuring module itself or even the main power source. Therefore, before starting work, it is necessary to clearly understand the purpose of each output and understand the physical principles of operation. shunt in a DC circuit.
Operating principle and types of digital indicators
Modern digital volt-ampere meters are compact devices that combine two measuring channels. The voltmeter is connected in parallel to the circuit being measured and has a high internal resistance so as not to introduce distortion. The ammeter, on the other hand, is connected in series and uses a low-resistance calibration resistor for the voltage drop, which is then converted into a current value.
The most common models are those with a three-wire connection, where two wires are used to power the display itself, and the third (or group of wires) is for measurement. There are also modifications with external shunting, allowing you to measure currents up to 100 Amps or more. In such cases, an external shunt, and the device itself only reads the voltage drop across it.
⚠️ Attention: Most cheap Chinese volt-ampere meters do not have galvanic isolation between the display power circuit and the measuring circuit. This means that the “minus” of the device’s power supply and the “minus” of the measured circuit are internally connected.
If you plan to use the device as part of a complex system where the control circuit ground and the power circuit ground must be decoupled, you will need either an isolated power supply model or a separate voltage source to power the display itself. Ignoring this fact may result in a short circuit through the measuring circuits.
Why is the screen flickering?
If there are sudden surges in current consumption or insufficient capacitor capacity in the power supply, the voltage at the display input may drop below the threshold for stable operation, causing flickering or complete blanking of the display.
Required tools and components for installation
Before you begin assembly, make sure you have all the necessary components on hand. The quality of connections directly affects the accuracy of measurements, especially in circuits with high currents. To work you will need a set of tools and materials:
- 🔧 Tool set: side cutters, wire stripper, soldering iron with a thin tip and a set of screwdrivers.
- 🔌 Conductor materials: copper wire with a cross-section of at least 1.5 mm² for power circuits and 0.5 mm² for signal circuits.
- 🔋 Test power source: stabilized power supply or battery for initial testing and calibration.
- 📏 reference multimeter: necessary for checking readings and making adjustments to the calibration of the device.
Pay special attention to the selection of wires for connecting the shunt. If you are using the device's built-in shunt, the wires should be as short and thick as possible to eliminate additional resistance that will introduce an error in the measurements. amperage.
Hot melt adhesive or silicone sealants that can withstand heat are ideal for securing components inside the case. However, it is better to secure the indicator circuit board itself with screws or double-sided tape with a foam base to avoid vibrations.
Use stranded wire for all connections inside the enclosure. Monocores may break due to vibration, which will lead to unstable contact and jumps in readings on the display.
Classic connection diagram for a 3-wire volt-ampere meter
Let's look at the most common connection option using the example of a popular module DSN-V288. This device usually has five or six wires coming out of the housing. Two thick wires (red and black) are designed to measure current and voltage in the load circuit, and three thin wires are used to power the display itself.
The thin red wire is connected to the positive of the power source (before the voltage regulator, if there is one, or parallel to the output), the thin black wire is connected to the common negative. The yellow or third thin wire is often the voltage sense input, but in simplified circuits it may be combined with the red power wire. It is important to carefully study the datasheet of a specific model.
Power wires (thick) are included in the open load circuit. The red thick one goes from the plus of the source to the load, and the black thick one goes from the minus of the load to the minus of the source (or through a shunt). It is through these wires that all the current flows, so their cross-section must match the power of your unit.
☑️ Checking connections
When assembling the circuit, try to minimize the length of current paths. Long loops of wires act as antennas and can introduce interference, which is especially noticeable on cheap models with sensitive electronics. Stability of readings directly depends on the quality of installation.
Connecting models with an external shunt
To measure currents greater than 10 Amps, standard wires and internal board traces may not be able to handle it. In such cases, volt-ampere meters with an external shunt are used. The connection diagram here has its own characteristics and requires increased care.
The external shunt is a calibrated resistor with a very low resistance (usually in the region of 0.001–0.005 Ohms), which is installed in the negative circuit. The device itself is connected to the shunt through thin signal wires, which are included in the kit. These wires cannot be lengthened or shortened, since their resistance is taken into account during calibration.
The signal wires from the shunt are connected to the corresponding inputs on the indicator board. They are often labeled as I+ and I- or they just come in a separate connector. By reversing the polarity of the shunt signal wires, you will get negative current readings or zero on the display.
| Connection type | Maximum current | Shunt resistance | Installation feature |
|---|---|---|---|
| Internal shunt | up to 10 A | Built into the board | Thick power wires |
| External shunt (50A) | up to 50 A | ~0.0015 Ohm | Thin signal wires |
| External shunt (100A) | up to 100 A | ~0.00075 Ohm | Solid copper bar |
| High precision | up to 20 A | Precision | Requires calibration |
When installing an external shunt, make sure it is securely attached and has good heat dissipation. When large currents flow, the shunt can heat up, which will lead to a change in its resistance and, as a consequence, to a drift in the ammeter readings. Use heat resistant materials for fastening.
The external shunt is always installed in the gap of the negative line, closer to the load, in order to eliminate the influence of transient contact resistances on the accuracy of measurements.
Calibrating readings and eliminating errors
Chinese measuring instruments often have a factory error, which can reach 5-10%. This is not suitable for laboratory work or fine tuning of batteries. Fortunately, most models have trim resistors on the back of the board.
There are usually two of them: one for calibrating voltage (Voltage), the other for calibrating current (Current). To configure, you will need a reference multimeter. Connect it in parallel with the load to measure voltage and in series to measure current, then rotate the “micrics” with a screwdriver until the readings match.
- 🔍 Find the tuning resistors on the board, they are usually marked as V-adj and I-adj.
- 🔌 Apply voltage and connect a load that creates a current in the middle part of the range (for example, 2-3 A).
- 🔧 Carefully rotate the screws, monitoring changes on the display and checking the standard.
Sometimes users are faced with a situation where the readings “float” or the display goes dark when a powerful load is turned on. This often indicates a lack of power for the display module itself. In this case, it is recommended to power the display from a separate 5-12 Volt source, not connected to the circuit being measured.
If you notice that the ammeter shows a reading other than zero even when the load is off, check for stray leakage currents in your circuit. The cause may also be interference from power wires that are laid too close to the measuring inputs.
⚠️ Attention: When calibrating the current part, do not exceed the maximum allowable current for your wiring and shunt. Prolonged flow of current above the nominal value will lead to overheating and an irreversible change in the shunt resistance.
Typical errors and troubleshooting
Even following the instructions, beginners often make mistakes that lead to incorrect operation of the device. One of the most common problems is the absence of a common negative between the power supply circuit of the device and the measured circuit in models without galvanic isolation. As a result, the display may not light up or show erratic values.
Another common mistake is using too thin wires for the power section. At a current of 5 Amps on a wire with a cross section of 0.5 mm², a noticeable voltage drop and heating will already begin. This will cause the voltmeter to show one value at the output of the power supply, and the voltage at the battery terminals will be significantly lower.
If your volt-ampere shows negative current values, check the direction of the shunt connection. The current must enter one shunt contact and exit the other in accordance with the markings. If you mix up the inputs, the sign of the readings will change to the opposite.
In cases where the display lights up but the segments are not displayed or are dim, check the supply voltage on the thin wires. It must be in a strictly specified range, usually from 4.5 to 30 Volts depending on the model. Excessive voltage will cause the stabilizer chip inside the device to burn out.
What to do if the shunt burns out?
If the built-in shunt burns out (often happens when the current is excessive), it can be replaced with a piece of nichrome wire or manganin, selecting the resistance experimentally, but it is easier to buy a new module.
Questions and answers (FAQ)
Is it possible to use this volt-ampere meter in a 220 Volt network?
No, absolutely not. These devices are intended exclusively for direct current (DC) circuits with voltages typically up to 100 Volts. Connecting to a 220V AC network will instantly damage the device and can be life-threatening.
Why does the ammeter show current when there is no load connected?
This can be caused by several reasons: the presence of leakage currents in the power supply, incorrect zero calibration, or interference. Try shorting the output terminals and adjusting the current regulator (I-adj) to zero.
How to increase the current measurement limit?
To increase the measurement limit, it is necessary to use an external shunt of the appropriate rating. The internal traces of the board are not designed for high currents. You will also need to recalibrate the device, knowing the exact resistance of the new shunt.
Is it possible to extend the wires going to the display?
The display power cables can be extended without problems. However, it is not recommended to lengthen the signal wires coming from the shunt (in models with an external shunt), as this will introduce an additional error in the measurements due to the resistance of the wires themselves.