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With this approximately € 18.00 costing module you can make an electronic load (current sink) with which you can test power supplies and batteries up to 5 A current or up to 35 W power. |
Introduction to the electronic load XY-FZ35
The small module XY-FZ35
In the photo below you can see what this electronic load looks like. The front panel is 79 mm by 42 mm in size, the module has a depth of 56 mm. The device automatically clicks into a rectangular cutout in a housing by means of spring-loaded lips on the sides. On the front you see on the left a display on which two quantities are shown and on the right a rotating encoder with a small push button underneath. With the encoder you set the functions of the electronic load, with the push button you activate the device. The rear is occupied by a heatsink with a mounted fan and two PCB terminal blocks. You must connect the supply voltage to one and the input voltage to the other.
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| The front and back of the XY-FZ35. (© AliExpress) |
Manufacturer, suppliers and prices
A manufacturer is not mentioned on the device. Bad Chinese habit! On one of the manuals that can be found on the internet, the manufacturer is called 'Drok'. Drok turns out to be the trade name of Droking, a Chinese manufacturer that specializes in modules related to power supplies and that mainly sells through Amazon. However, no shortage of suppliers! Via AliExpress alone you can find dozens of shops that are willing to supply you with this module. The minimum price including shipping is approximately € 18.00.
What can you do with an electronic load?
Electronic loads are not often found in the average hobby lab. Wrongly, because such a device has a number of useful functions:
- Measuring the output stability of power supplies:
This parameter defines how stable the output voltage of a power supply remains when it is loaded with a varying current. You connect the electronic load to your stabilized power supply and increase the load current in steps from zero to the maximum current that the power supply can deliver. You measure the voltage difference between no-load and full-load. Dividing this by the current gives you an idea of the internal resistance of the power supply and of the output stability. - Observing the ripple on power supplies:
The oscilloscope allows you to monitor how the output voltage of a power supply behaves when loaded from zero to its maximum current. - Measuring the transient response of a power supply:
How does the output voltage of a power supply behave if you suddenly load it with a peak current? With an electronic load and an oscilloscope you can discover it in no time! - Checking the cooling of a power supply:
By loading a power supply with the maximum current you can check whether your heatsinks have sufficient capacity to keep all parts at an acceptable temperature. - Measuring the long-term stability of a power supply:
Does the output voltage remain stable as the circuit warms up? By loading the power supply with an electronic load, you have measured it within half an hour. - Measuring the capacity of a battery:
Load such a part with the maximum current that the equipment to be supplied will draw and you can measure how long the voltage supplied by the battery remains above the minimum acceptable value.
The specifications of the XY-FZ35
- Supply voltage: 5 Vdc ~ 30 Vdc
- Supply current: 200 mA max (with fan running)
- Input voltage: 25.0 V max
- Sink current: 5.0 A max
- Power consumption: 35.0 W max.
- Input voltage measurement resolution: 10 mV
- Voltage measurement accuracy: ±(0.5 % + 1 digit)
- Sink current measurement resolution: 10 mA
- Current measurement accuracy: ±(1.0 % + 3 digits)
- Over Voltage protection OVP: adjustable up to 25.2 V
- Over Current protection OCP: adjustable up to 5.10 A
- Over Power protection OPP: adjustable up to 35.5 W
- Minimum input voltage alarm: adjustable down to 1.5 V
- Temperature protection: +80 °C on the heatsink
- Alarm: flashing display, current to zero
- Dimensions: 79 mm x 43 mm x 55 mm
- Weight: 87 g
The manual
The module is supplied without a manual, but you can find a number of good manuals about this module on the internet. We have merged two of them into one PDF file. In the second, for the advanced hobbyist, you will find information about the UART protocol that allows this module to communicate with your PC:
➡ XY_FZ35_Manual.pdf
The electronics in the XY-FZ35
Two small PCBs
The small housing contains two PCBs that are only connected to each other by means of two PCB connectors.
The bottom PCB
The bottom PCB contains the display, rotating encoder and push button on one side and the microcontroller and display driver on the other side. What is immediately noticeable are the remnants of solder paste, the large parts are sloppily soldered in by hand at the manufacturer! On this PCB you will discover a Nuvoton 8051 compatible 8-bit microcontroller, a Holtek HT1621B 32x24 LCD controller and an XLSEMI XL1509-5 DC to DC converter.
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| The two sides of the bottom PCB. (© 2022 Jos Verstraten) |
Tho top PCB
The second PCB of the same size is clicked onto this bottom PCB with the two connectors as the only mutual mechanical fixing. The current control component is located on this PCB. That's a TIP122 NPN darlington in TO-220 housing. According to the specifications, it can be loaded up to a maximum of 100 V and 8 A. This semiconductor is attached to the heatsink on the other side of the PCB via a cut-out in the PCB.
In addition to this transistor, an LM324 quadruple op-amp is present on this side of the PCB. Finally, you can notice a large SMD resistor, an R025. That is without any doubt the current sensor resistance. On the left you see the PCB terminal block L- and L+, to which you must connect the input f the electronic load. On the right an identical PCB terminal block V- and V+ is meant for the supply voltage of the module.
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| The two sides of the top PCB. (© 2022 Jos Verstraten) |
Fan operation
The fan starts automatically when the power consumption exceeds 10 W or the temperature of the heatsink exceeds +40 °C.
Working with the XY-FZ35 electronic load
The power supply of the XY-FZ35
You can power the module with a DC voltage between 5 V and 30 V. The power consumption is minimal, so there are not many requirements. We used a cheap (less than ten euros) Chinese mains plug adapter of 12 V. We do not recommend a 5 V USB power supply. After all, this is the minimum specified supply voltage and some USB adapters deliver slightly less than 5.0 V. The question is whether the XY-FZ35 can handle this.
Switching on
After switching on the power, the following picture appears on the display. The voltage indication is at zero, because nothing is connected to the device yet. The current indication is at the value you last set, so it is kept in a memory. You can now use the rotary encoder to set the current to the desired load current. Unfortunately, this only works per 10 mA per click of the rotary switch. So it takes a while to turn up the current from 1.00 A to 5.00 A!
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| The display after switching on the device. (© 2022 Jos Verstraten) |
Connecting the load
Connect the load to the XY-FZ35. The display now shows the voltage of the load and, moreover, the set current is immediately drawn from the load. The IN symbol in the top left corner indicates that the device is active. Briefly pressing the yellow ON/OFF button will cause the current to become zero and the current display will flash. If you, from the ON position, long press this ON/OFF button, a symbol ├ will appear next to the current display, see the photo below. The current setting is now locked at the set value. Only after a long press on this button again this check mark disappears and you can set the current to a different value with the rotary encoder.
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| The display during the operation of the device. (© 2022 Jos Verstraten) |
The other data on the display
By briefly pressing the rotary encoder, you scroll through the other data that the XY-FZ35 measures:
- The power consumption in W.
- The total ampere-hours supplied by the load in Ah.
- The total time of the measurement in hours and minutes.
The last two data can be reset to zero by long pressing the ON/OFF button.
Setting the parameters
The XY-FZ35 can store seven values of adjustable parameters in its memory. You can select this by long pressing the rotary encoder. 'SET' appears in the display at the bottom left and you can use the rotary encoder to set the value of the parameter. By briefly pressing the encoder, the device moves to the next parameter.
- OVP, Over Voltage Protection:
The maximum voltage that the XY-FZ35 wants to feel between its two input terminals. You can set this parameter up to 25.2 V. If the input voltage increases, the current switches off and the display flashes. - OCP, Over Current Protection:
The maximum current, adjustable to 5.10 A. - OPP, Over Power Protection:
The maximum power consumption, maximum 35.50 W. - LVP, Low Voltage Protection:
Allows you to set the voltage at which the XY-FZ35 will stop drawing current from the load and end the measurement cycle. The minimum value is 1.5 V. - OAH, maximum capacity:
Sets the amount of Ah that the XY-FZ35 can draw from the load before it stops sinking current. Can be set to zero by briefly pressing the ON/OFF button. - OHP, maximum discharge time:
Sets the maximum time that the device is allowed to draw current from the load. Can be set to zero by briefly pressing the ON/OFF button. - DAT, data set:
If you select '0' in this option, the Ah indication and the duration of the discharge will be reset to zero at the start of each measurement. If you select '1', this data will be cumulated over the various successive measurement cycles.
Finally, there is a non-adjustable parameter:
- OTP, Over Temperature Protection:
Current sinking stops when the temperature of the heatsink rises above +80 °C.
When you have set all parameters, you have to press the rotary encoder again for a long time. You will then return to the mode where the device is ready to sink current from the connected load.
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| Setting the adjustable parameters. (© 2022 Jos Verstraten) |
What happens if one of the parameters is exceeded?
Very little! Current sinking is aborted and the display will flash. Unfortunately, there is no built-in acoustic alarm. In such a situation, the total number of Ah delivered and the sink current time are stored in the memory and you can read them out without any problems.
A practical application of the XY-FZ35
It is certainly possible to let the device work unattended. In this way, for example, the device can measure how long it takes before the voltage of a charged battery pack has dropped from 5.0 V to 4.5 V at a given discharge current.
Set the maximum capacity OAH and the maximum discharge time OHP to zero. Set the discharge current and the low voltage protection LVP to the desired value. Start discharging the battery pack. The moment the display flashes, you know that the measurement is complete. Then disconnect the battery pack from the electronic load. You can read the time and the number of Ah.
Testing the XY-FZ35 electronic load
The accuracy of the voltage measurement
The reading on the XY-FZ35 is compared to the reading on our Fluke 8842A. To avoid voltage drop across cables, measurements are taken without current sinking. The results are summarized in the table below. To calculate the percentage error, the reading on the Fluke is taken as a 100% reference.
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| The accuracy of the voltage measurement. (© 2022 Jos Verstraten) |
The accuracy of the current measurement
Because the 8842A can only measure up to 2 A, we use our second reference meter, the ET3255 from East Tester. It is switched in the positive connection between the power supply and the XY-FZ35. The measurement is performed at a supply voltage of 5.00 V.
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| The accuracy of the current measurement. (© 2022 Jos Verstraten) |
The stability of the current
With a electronic load, the current you set must be completely independent of the input voltage. Of course we also tested this by setting the current to 1.00 A and varying the input voltage between 5.00 V and 25.00 V. The table below shows that the electronics in the XY-FZ35 works excellently.
We also measured the minimum voltage at which the current remains stable. For a current of 1.00 A, this is 1.8 V. For a current of 5.00 A, this minimum voltage rises to 2.7 V.
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| The stability of the set current. (© 2022 Jos Verstraten) |
The long-term stability
Finally, we measured the effect of heating up the electronics on the stability of the output current. In the same measurement, we observed the temperature between two ribs of the heatsink with a thermocouple. These measurements are performed at the maximum power of 35 W that may be dissipated in the XY-FZ35: 2.3 A at a voltage of 15 V. Both measurement results are summarized in the table below. It is clear that the device perfectly keeps the current constant. The heating of the heatsink remains well within the safe limits, of course with the fan switched on.
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| The long-term behavior of the XY-FZ35 at maximum load. (© 2022 Jos Verstraten) |
A nice housing for the XY-FZ35
Finally, we would like to give you a hint. If you want to build the XY-FZ35 in a housing, a ready-made case is available. Google 'Riden DP and DPS metal housing' and you will find a nice metal enclosure with accessories for about € 20.00.
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| The Riden case that neatly fits the XY-FZ35. (© 2022 Jos Verstraten) |
Our verdict on the XY-FZ35 electronic load
There is in fact nothing bad to say about this small device. It does what the manufacturer promises and that for a very low price. When working with small voltages and currents, the accuracy is slightly lower than specified, but not so low that the error becomes unacceptably for hobby use. The only thing we miss is an audible alarm that beeps when the XY-FZ35 has completed a discharge cycle.
XY-FZ35 electronic load
