Panel ammeter-voltmeter and its use.

My order arrived in a plastic bag, with a small bundle inside containing the item itself.


The device was packed in a resealable bag—simple and neat.

For this, I put together a small test setup consisting of:
Two multimeters: one measures current (on the left), the other voltage (on the right).
Two bench power supplies: one operated as a current source (top), the other as a voltage source (bottom).
A power supply that powered the panel meter itself (sitting separately)


The first test showed that the current measurement wasn't quite right.
More precisely, the measurement accuracy was well within acceptable limits for an ammeter, but if there's a chance to make it better, why not take it.
Four measurements before adjustment —
On the power supply operating as a voltage source, I simply set voltages in multiples of 4 V, while on the one operating as a current source, I set the current according to my multimeter.


After this stage, I made a small adjustment. It looks better to me.

I decided to use this little meter in an interesting and useful device—an electronic load.
I can't say I really need this device; it might come in handy, but initially I just liked the idea and wanted to make something for my readers.
There's nothing unusual about this device; all the design choices are well covered in the literature. I simply put it together my own way.
At first I made it very compact and used SMD components, but then decided to redesign it so it would be easy to build even for beginners in amateur radio.
There are no microcontrollers or rare, expensive components here. The most expensive part is the panel meter itself, and even all the other components are through-hole.

The circuit uses an operational amplifier. The IC contains two identical op-amps.
One is used in a comparator circuit that sets the voltage at the power transistor's gate so that the current in the circuit matches the set value.
I used the second amplifier as a current buffer for the reference voltage source. I could have done without it, but it was there, and I didn't want it sitting idle.
The reference voltage source is needed to keep the set current from drifting during operation, for example as the circuit warms up.
The whole circuit is powered from 8 V, regulated by a 78L08 linear regulator.

The circuit can set the load current up to 5 A. The input voltage must be such that the power dissipated by the power transistor does not exceed 30-40 W.
With a more powerful transistor and active cooling, the power can easily be increased to 50-100 W.
The board has pads for a fan-control thermistor and its connection. Since I was making a demonstration version, I didn't install these components.
Almost any MOSFET can be used, or even a bipolar transistor (but that needs to have high gain, such as a KT827).
Almost all components can be used with values that vary by as much as 50-100%. The only exceptions are the R2 + R3 resistor pair and the current-sense resistors R7 and R8.

Circuit diagram
The input connector has three contacts so that the load power can be increased by connecting an external load resistor. There is a jumper between two contacts on the board; it isn't actually needed there and should be installed on the connector itself.


Printed circuit board.


The board is very easy to make, with no bottlenecks, thin traces, etc.
The board also has an outline for cutting out a window to mount the ammeter-voltmeter.


Here are all the components.
All resistors are rated at 0.125–0.25 W, except R7 and R8, which should be rated at 1 W.


And here is the assembled board. The heatsink is a little too large, but I used the first one I found that more or less fit.
The power transistor is insulated from the heatsink. This isn't necessary when installing the board in an enclosure, but it's advisable if using it exposed, since the transistor case is connected to the board input and carries the voltage of the circuit being measured.


A closer look at the board.
On the left is the connector for whatever we want to load: a power supply, battery, or something else.
Next to the display is the potentiometer knob. It's best to use a potentiometer with a linear taper: type A for domestic models and type B for imported ones.
On the right are the power switch and the input for the device's power supply. I used a standard 12 V power supply.


There's nothing particularly interesting on the back. I routed the display wires over the top edge of the board; it seemed neater and more reliable that way. You can actually just solder them to the underside.
The board was designed for a transistor in a TO-247 package, but I used one in a TO-220 package.


First power-up. If all the components are working, the device should work right away. The only component that might need adjusting is resistor R2. I didn't need to adjust anything.


I used a bench power supply for testing. The load current adjustment starts at 20 mA, as shown on the device's display; the potentiometer won't let you set it any lower.

Here are a few photos from testing the finished device.
The power supply's output voltage stayed the same. The lower voltage shown on the device's display under load is normal; it's due to voltage drops across the wires, terminals, etc.
1. Load current: 1 A.
2. Load current: 2 A.
3. Load current is almost at the maximum: 5.07 A.
4. Maximum load current: 5.1 A. The power supply switched to current limiting, and the device's display shows the minimum voltage drop. In practice, this is the minimum input voltage at which the device can supply a load current of 5 A.


The voltage at the input terminals matches the voltage shown on the ammeter-voltmeter, within its measurement error.


Finally, just for fun, I checked how well the load current stays regulated as the input voltage changes. It works perfectly.
Input voltage: 3 and 12 V; load current: 2 A.


Additional materials—the schematic, board layout, and a couple of datasheets—download.

That seems to cover everything about the device. If you have any questions, I'll be happy to answer them :)