For my next project (converting an ATX PSU 580W into a laboratory one), I bought the above indicator. Not immediately and not in time it turned out that his power input is galvanically connected to the minus input of the shunt. This introduces a noticeable error when the indicator is powered from the same source from which the current is measured (error up to an ampere with my 50A shunt!). It was possible, of course, to pile up another duty room and power the indicator from it, but it seemed to me too bold and I decided to crack the indicator itself.

Searching the Internet I found his twin brother YB27VA and his typical circuit. I must say right away that the circuit of my device is slightly different. The essence of the alteration is to untie the differential input of the ad8605 operational amplifier (marked as B3A) from the common power wire. Rework will require initial skills in reverse engineering (to make sure that the circuit is the same), soldering small parts and knowledge of Ohm's law :)

Schematic before modification:

Cut tracks are marked in red. I decided to abandon the resistor R6, since it seems that it is needed only so that the ammeter shows “0” when the shunt is off. Also, transferring power to the ad8605 (2 legs) is not necessary (judging by the tests in the simulator).

The second alteration solves the problem associated with the fact that the indicator does not “see” the first ~ 180mA of current, that is, when 1A is applied to the shunt, the device shows 0.8A, if 0.2 is applied, then zero, etc. This is due to the offset of the input of the op-amp and the ADC. It can be calculated by knowing the resistance of the shunt and the amount by which the device "lies". I got 270uV at the input of the op-amp. This offset is easy to create artificially by adding one resistor to the circuit, as a result, the device will begin to measure from zero.

In my case, it was necessary to add a 1140kΩ resistor from the 3V integral stabilizer to the "+" input of the op-amp. This resistor, together with R7 and the shunt, forms a divider that sets the initial offset.

The composite resistor turned out exactly as much as needed, due to the error of one of them :)

As a result, it now measures from 50mA up to 50A in minimum increments of approximately 20mA (0 also shows). Linearity also did not let us down, but sometimes it skips one, for example, it jumps from 0.12 immediately to 0.14.

The achieved accuracy pleasantly surprised me, it turned out to be a real measuring device, which can be used in the laboratory BP as the main indicator. Which you can even trust :) (this applies, at least, to the current). It is not clear why the Chinese decided to save on a couple of cheap parts. Their cost is clearly an order of magnitude lower than other components, the same ad8605, for example. Use good tools :)

More photos with measurements:

P.S. I already wanted to publish an article, but I decided to check - but how are things going with tension? It turned out that things are not good either - the device is lying at 0.1V, and it cannot be fixed elegantly, because the lower resistor is a tuning resistor. But I still soldered a 20MΩ resistor there and the result suited me)

At present, from various electronic devices, which for one reason or another are decommissioned, there remain various power supplies, both pulsed and assembled on step-down transformers. Their use by novice radio amateurs as a laboratory power supply is difficult because they have a certain stabilized voltage at the output. However, the inexpensive miniature modules of voltage and current regulators that have appeared on sale make it possible, together with the same miniature digital voltmeters and ammeters, to successfully convert them into laboratory blocks power supply, sometimes even without the manufacture of a new, more spacious building.

There was a power supply unit that gave a stabilized voltage of 5V at the output. Naturally, there was a desire to use it more intensively in their amateur radio needs. Moreover, there was already a voltage adjustment of about 5.5 volts to the maximum, which could be done using a tuning resistor. And the output current easily reached almost one ampere.

To achieve the desired, it is necessary to install a measuring device on the front panel - a voltammeter, a voltage regulator ( variable resistor instead of a trimmer), a switch for the type of measurement (voltmeter - ammeter) and connecting terminals.

It turned out not to be difficult at all. A Chinese-made voltmeter modified by this method to the possibility of measuring current, too, for smoother and more accurate settings, a PK-1 pushbutton switch and two types of connecting terminals - standard for power supplies and an RCA “tulip” connector - as it proved to be very convenient in this quality.

Block connection diagram

The scheme for connecting additionally introduced devices is not at all complicated, and its implementation takes even less time than drawing. It is better to make the power supply of the voltammeter separate, through an integrated 5-volt stabilizer, as an option from suitable batteries or accumulators, then the output voltage indication will start from zero. The switch for the type of measured value PK-1, on which the necessary additional electronic components scheme. A fuse is required.

Everything fit, except I had to slightly file the edge printed circuit board and a module with a rectifier and a voltage stabilizer, from an additional winding of a regular transformer, place in an isolated "box" (it orange color) and give him a place inside the radiator (it does not heat up).

The adjustment of the voltmeter and ammeter readings went without complications. The readings of the voltmeter are adjusted by a trimming resistor located on its board, and the readings of the ammeter by changing the resistance of the measuring resistor, indicated in the diagram as “R measuring resistor 0.2 Ohm”. Current readings are in amps. The readings relative to the exemplary meter are set quite accurately, but there is still a nuance not fully understood: I set the voltmeter readings and they match the exemplary ones perfectly, but after I set the ammeter readings, the voltmeter readings go astray. And vice versa. Therefore, I had to choose whose testimony would correspond, and whose “read” would have to be corrected.

This is how the power supply turned out in the end: with a display of an adjustable output voltage, with the ability to find out the current current consumption (you need to press the non-latching button of the PC-1 switch) and two types of connecting terminals. A novice radio amateur should not assemble his first PSU from scratch, the best option is to refine it to fit his needs. Written by Babay from Barnaula.

Discuss the article DIGITAL VOLTAMMETER CONNECTION DIAGRAM

I have already done a couple of reviews of a similar thing (see photo). I ordered those devices not for myself, for friends. A handy device for homemade charging, and not only. I also envied and decided to order already for myself. I ordered not only a voltammeter, but also the cheapest voltmeter. I decided to assemble a power supply for my homemade products. Which of them to put was determined only after I assembled the product completely. Surely there will be people who are interested.
Ordered November 11th. There was a small discount. Even though the price is low.
The parcel went for more than two months. The seller gave the left track from Wedo Express. But still the package arrived and everything works. Formally, there are no complaints.
Since it was this device that I decided to implant in my power supply, I’ll tell you a little more about it.
The device came in a standard plastic bag, “bubbled” from the inside.


AT this moment item is not available. But this is not critical. Ali now has a lot of offers from sellers with good rating. Moreover, the price is steadily decreasing.
The device was additionally sealed in an antistatic bag.

Inside the actual device and wires with connectors.


Key connectors. On the contrary, do not insert.

The sizes are just tiny.

We look at what is written on the seller's page.

My translation with corrections:
- Measured voltage: 0-100V
- Circuit supply voltage: 4.5-30V
- Minimum resolution (V): 0.01V
- Consumption current: 15mA
- Measured current: 0.03-10A
- Minimum resolution (A): 0.01A
Everything is the same, but very briefly, on the side of the product.


I immediately took it apart and noticed that minor details were missing.


But in the previous modules, this place was occupied by a capacitor.

But their price was also different.
All modules look like twin brothers. Connection experience is also available. The small connector is designed to power the circuit. By the way, at a voltage below 4V, the blue indicator becomes almost invisible. Therefore, we follow technical specifications devices, less than 4.5V do not serve. If you want to use this device to measure voltages below 4V, you need to power the circuit from a separate source through a “thin wire connector”.
The current consumption of the device is 15mA (when powered by 9V "crown").
Connector with three thick wires - measuring.


There are two accuracy controls (IR and VR). Everything is clear in the photo. Resistors are dark. Therefore, I do not recommend twisting it often (you will break it). The red wires are the leads for voltage, the blue ones are for current, the black ones are “common” (connected to each other). The colors of the wires correspond to the color of the glow of the indicator, do not get confused.
The head chip is unnamed. It once was, but it was destroyed.


And now I will check the accuracy of the readings using the exemplary installation P320. I applied calibrated voltages 2V, 5V, 10V, 12V 20V, 30V to the input. Initially, the device underestimated by one tenth of a volt at certain limits. The error is insignificant. But I adjusted myself.


It can be seen that it shows almost perfectly. Adjusted the right resistor (VR). When the trimmer is rotated clockwise, it adds, when rotated counterclockwise, it decreases the reading.
Now I'll see how it measures the current strength. I power the circuit from 9V (separately) and supply an exemplary current from the P321 installation


The minimum threshold from which it starts to correctly measure the current is 30mA.
As you can see, the current measures quite accurately, so I won’t turn the adjusting resistor. The device measures correctly even at currents greater than 10A, but the shunt starts to heat up. Most likely, the current limit is for this reason.


At a current of 10A, I also do not recommend driving for a long time.
More detailed calibration results are summarized in a table.

I liked the instrument. But there are shortcomings.
1. The inscriptions V and A are painted, so they will not be visible in the dark.
2. The instrument measures current in one direction only.
I would like to draw attention to the fact that it would seem that the same devices, but from different sellers, can be fundamentally different from each other. Be careful.
On their pages, sellers often publish incorrect connection diagrams. In this case, there are no complaints. That's just a little of it (scheme) changed to a more understandable eye.

With this device, in my opinion, everything is clear. Now I'll tell you about the second device, about the voltmeter.
I ordered on the same day, but from a different seller:

Bought for US $1.19. Even with today's exchange rate - funny money. Since in the end I did not install this device, I will go through it briefly. With the same dimensions, the numbers are much larger, which is natural.

This device does not have a single tuning element. Therefore, you can only use it in the form in which it was sent. Let's hope for Chinese good faith. But I'll check.
The installation is the same P320.

More details in the form of a table.


Although this voltmeter turned out to be several times cheaper than a voltammeter, its functionality did not suit me. It does not measure current. And the supply voltage is combined with the measuring circuits. Therefore, it does not measure below 2.6V.
Both devices are exactly the same size. Therefore, replacing one with another in your homemade product is a matter of minutes.


I decided to assemble the power supply on a more universal voltammeter. The devices are inexpensive. There is no burden on the budget. The voltmeter is still in stock. The main thing is that the device is good, and there will always be an application. Just from the store and got the missing components for the power supply.
I have been idle for several years now with such a set of homemade.

The scheme is simple but reliable.

It is pointless to check the completeness, a lot of time has passed, it is too late to make claims. But everything seems to be in place.

The trimmer resistor (complete) is too dumb. I see no point in using it. Everything else will fit.
I know all the disadvantages of linear stabilizers. I have neither the time, nor the desire, nor the opportunity to fence something more worthy. If more is needed powerful block power supply with high efficiency, then I'll think about it. Until then, what has been done.
First I soldered the stabilizer board.
I found a suitable case at work.
I rewound the secondary of the torroidal trance to 25V.


Picked up a powerful radiator for the transistor. All this stuffed into the body.
But one of the most important elements of the circuit is the variable resistor. I took a multi-turn type SP5-39B. The output voltage accuracy is the highest.


Here's what happened.


A little unsightly, but the main task is completed. I protected all electrical parts from myself, I also protected myself from electrical parts :)
It remains to "retouch" a little. I will paint the case from a spray can and make the front panel more attractive.
That's all. Good luck!