FNIRSI SG-002 Signal Generator

[Dave_g8]

For evaluation purposes, I recently purchased a FNIRSI SG-002 (with battery).

The device provides the following main features:
- DC voltage output “AVo” with respect to “GND” between -12V and +12V in 0.01V steps, with a maximum specified current of 20mA.
- DC current output “AIo” between 0mA and 24mA in 0.01mA steps, which can be connected in “Active” or “Passive” configurations (see manual for details).
- Powered from internal Lithium battery (3.7V, 1100mAh), Micro-USB (5V), “24V” terminal (15V to 30V). The internal battery is charged from the USB or “24V” inputs.
- The ON/OFF switch is for battery power only, the device is powered if the USB or “24V” inputs are present.



For such a low-cost device it performs reasonably well as a voltage and current generator.

Manual reference:
https://img.wqdres.com/res/0/20231229/366b286d9fbd412ebdde4ef171aa6da9.pdf

Based on limited testing, there are a couple of points to note.

When the voltage output “AVo” is set to a positive level, switching the unit ON shows a short burst of negative voltage.
The example shown below is with a set voltage = +10V.


When a load is initially connected between “AIo” and “GND” (active configuration), there is a short burst of high current.
The example shown below is with a 100Ω load resistor.


The peak current is 24V/100Ω = 0.24A and decays to the set current after approximately 10mS.
The initial high current is due to the capacitor that is on the output circuit, which will be charged up to 24V when there is no load connected.
This may not be a problem, but if something like an LED is connected directly, it is likely to be damaged. In this case a current limit resistor should be added to limit the initial peak current. I managed to blow an LED before I realised what was happening.

Voltage and Current:
I do not have a precision voltage or current reference and I have only compared the device (with default calibration settings) against my DMM.




For general interest, an image of the SG-002 PCB is shown below.


Much of the circuitry is for the generation of the various supply voltages +5V (2 off), +24V, +14V and -14V.
There is a standard “4056” Lithium battery charger IC. The charger input is fed from both the 5V (USB) and 5V derived from the external “24V” input terminal.
The “AVo” output terminal appears to be driven from an Op-Amp IC with some current limit detection.
The “AIo” output terminal appears to be driven from a voltage to current IC with some external current limit protection.

[Kean]

I have one of these that I bought last year for use in the field when testing PLCs and similar.  I actually just bought another two (one with battery, one without) for general use on the bench when I need stimulus voltages that I can easily step through manually.  I normally use my Advantest R6142 for this, but I sometimes need more than one stimulus.

Interesting note about the voltage/current spikes.  I'll need to keep those in mind.  Otherwise they seem to be quite handy little units for the price.  I bought one without the battery as I suspect it should be easy enough to add myself if I later feel it is needed.

[Dave_g8]

Hi,
Yes, you should be able to add a battery as required.
There is no protection circuitry on the SG-002 PCB, but it is built into the battery as shown attached.


One other minor point that I have noticed on the unit that I have is that the charging cycle does not terminate correctly. It does reach more or less full charge.
This appears to be due to the diode load sharing arrangement that feeds the voltage to the 4056 charger IC and the 24V boost regulator.
A simplified diagram of the inferred PSU arrangement is shown below.

[Kean]

OK, battery was installed easily.  As I suspected all the battery support circuitry is installed.  For now I've used a old cell I pulled out of something else.  The cell capacity is probably down to about 500mAh due to age, but still usable for something non critical like this.  I  just had to change the connector to an XHP-2.

Everything was the same between the three units except for the screws and battery.  Weirdly they used very different (undersized) screws for mounting the PCB on the non battery model.

I forgot to mention that I also have an SG-003A that I bought during a recent sale.  It is a much fancier version with graphical display, banana jacks, and USB Type-C, but also higher precision and input measurement modes.

A simplified diagram of the inferred PSU arrangement is shown below.

Is that diagram something you put together from some reverse engineering?

[Dave_g8]

Is that diagram something you put together from some reverse engineering?

Hi,
Yes, the power supply arrangement is based on the unit I received. The diagram may not be 100% accurate, but it’s hopefully representative.

For what it is, there is quite a lot going on in the little unit and the parts alone would probably cost more than the cost of the delivered unit.
The absolute accuracy is not going to be fantastic, but for a basic voltage and current source for hobbyist use it seems to be good value.
The reference voltage appears to be derived from a TL431 or similar, it is fitted under the display on the hardware version I have, but that’s what seems to be used on other similar devices.

A few more diagrams are attached for interest.


Current Control:


Voltage Control:

[Dave_g8]

Hi,
For information, I did modify my SG-002 to increase the voltage out of the “24V” port to 5V buck regulator (U10).
Although it is a fixed voltage “XL1509-5.0E1” type, by adding an external resistor it is possible to increase the voltage.
It required a track cut to isolate the FB input (Pin 3) and with an added 1kΩ resistor (R2ext) the voltage is increased to 1.23 x (1 + 8.6/2.5) = 5.46V.
This compensates for the Schottky diode drop in the load sharing circuit and the battery charging circuit now works correctly when using the “24V” port.


If the USB (5V) voltage is set to around 5.4V, the charge circuit will also work as expected.