content/ZPB30A1.rst

=========================
 ZPB30A1 Electronic Load
=========================

:Author: David Douard
:Category: Electronics
:Tags: ZPB30A1, electronic load, test equipment

A few weeks ago, I purchased a cheap electronic load from AliExpress
(`this one`_ to be precise). It's a ZHIYU model ZPB30A1. It's
specifications are:

- constant current mode 
- max voltage: 30V
- discharge current: 0.2A to 9.99A (+/- 0.7% + 1 digit)
- max dissipated power: 60W
- voltage measurement error: 1% + 2 digits

It supports 2 operation modes:

- electronic load: in this mode, you can set the current and the
  minimum voltage above which it beeps; in this mode, it display the
  measured voltage and set current.
- battery capacity test mode: in this mode, you set the discharge
  current and the lower voltage limit at which the discharge test
  stops. It displays the total discharged capacitu (in Ah) and energy
  (in Wh).

.. image:: {filename}images/ZPB30A1/zpb30a1.jpg
   :alt: Overall view of the ZPB30A1 electronic load
  
The device looks pretty nice and reasonably well built. It can do 4
wire measurements.

The device itself is built around a single ST W60N10 N channel MOS
transistor (100V, 60A, 200W) and a STM8S005K6 microcontroller.

.. image:: {filename}images/ZPB30A1/zpb30a1_board.jpg
   :alt: Main PCB of the ZPB30A1 electronic load

The shunt resistor is a 10mΩ wire. A very small value that explains
the rather big minimal current of 200mA (which means a voltage drop as
low as 2mV).

.. image:: {filename}images/ZPB30A1/zpb30a1_mosfet.jpg
   :alt: Main transistor of the ZPB30A1 electronic load

One nice thing with this device is the fact that every component value
is written on the PCB: no need to use the magnifier to read the SMD
parts values.

The bottom side of the PCB is pretty empty besides the ground plane.

.. image:: {filename}images/ZPB30A1/zpb30a1_bottom.jpg
   :alt: Bottom side of the PCB of the ZPB30A1 electronic load


The most annoying thing is the minimum current of 200mA, which pretty
hight. It's obviously not designed for small battery or PSU
testing.

Another thing is that its limited to CC mode only (does not provide
constant resistance mode nor constant voltage). It's a bit unfortunate
since it's probably only a matter of software.

Also, the usability is quite rough. For example, to choose the
operating mode (between CC charge and battery capacity test mode), you
have to power off the device and power it on holding the start/stop
button down.

The last missing feature, for me, is that there is not "output" port
allowing to capture the values when doing a battery discharge
test. The good news is that there is unpopulated port. Looks like some
ISP/serial port directly connected to the microcontroller.  Pins are
labelled as G, R, T, L, F and Vc. G stands for ground, and Vc is
obvious.  R and T might be Rx and Tx of a serial port. F and L are not
obvious.

So I've probed a bit to see if there are any signals.

The overall (messy) setup was:

.. image:: {filename}images/ZPB30A1/overall_setup.jpg
   :alt: Overall testing setup for the ZPB30A1.

During this test, the electronic load was sunking almost 1A at 30V,
thus dissipating around 30W. The MOSFET transistor was getting a bit
hot. I measured 65°C with my cheap IR thermometer.

The F pin shows a square signal at 50kHz, and there is some activity on
the T pin. I've used my new Rigol DS1054Z scope to try to find out if
the signal is meaningful. And after playing a bit with the Serial
decoder, I finally found that:

.. image:: {filename}images/ZPB30A1/DS1Z_serial.png
   :alt: Serial signal on the T pin.

When the ZPB30A1 was actually set up like this:
	
.. image:: {filename}images/ZPB30A1/zpb30a1_setup.jpg
   :alt: Testing setup for the ZPB30A1.

So the measured voltage is constantly written on the serial port at
115200 baud, 8N1. The third value is 1 when it is normally operating,
and 0 when the voltage is below the limit.

However, in battery discharge mode, there is nothing on the serial
line.

Now, I have to solder a socket in there. Then the next step will be to
see if we can send commands to the ZPB30A1. I may need to learn a bit
about the ST microcontroller...


.. _`this one`: http://fr.aliexpress.com/item/New-Constant-current-electronic-load-9-99A-60W-1-30V-battery-capacity-teste-free-ship/32571919265.html