I discovered a Constant Voltage/Constant Current
(CV/CI) battery charging module that can be repurposed
into a power supply capable of delivering up to 30V at up
to 5A in a size that would astound you. It measures 2” x 1”
at one inch deep. It also includes a three-digit DVM and a
three-digit ammeter. It includes three LEDs for current
limiting, for battery charging, and for charge complete. It
has two trim pots: one to adjust the voltage and another to
adjust the current. It costs less than $12 on eBay, including
a heatsink and shipping. This heatsink might cost about a
$1.50 more than the average price but it is well worth it in
this application. The finished power supply is in Figure 1.
Some of these Chinese bargain modules have hidden
problems that we will fix. For example, it has two voltage
regulators that exceed worst-case limits and it uses a
ground side current monitoring resistor, meaning that the
input ground and the output ground have a current
monitoring resistor in between them. So, if they are shorted
together, the ammeter and current limiter will not work.
This also means that adding a second module will require
another 32V DC input power supply.
The first problem is the voltage regulators for +5V and
+3V. The 5V regulator (for the current monitor) uses a
78L05 and draws about 5. 25 mA. The input voltage is rated
at 30V absolute maximum; we are using 32V. I have
already blown one 78L05 regulator in the module.
The display board (schematic not shown) uses an
HT7130-1 3V regulator and draws about 9. 5 mA. There are
a couple of problems here. First is the input voltage is rated
at 26V absolute maximum (we are running it at 32V).
Second, the power dissipation is rated at 250 m W and we
are running it at over 300 m W. We will fix these later.
For full dynamic range, the module requires a 32V DC
input power source. The module that I selected was a used
HP 0957-2093 AC power adapter.
Theory of Operation
Let’s begin with the HP 0957-2093 AC-to-DC power
adapter. Its input voltage is 100V-240V AC ( 50-60 Hz); refer
to Figure 2. The output voltage is 32V DC, with a current
output capability of 2.5A DC, or 80 watts. Figure 3 shows
two views of the CV/CI module. It is a step-down module
with an output voltage of 0.8V-30V at a rated current of up
to 5A. It incorporates a red LED panel meter with a three-digit voltmeter and current meter display.
May 2014 27
; FIGURE 2. HP 0957-2093
AC power adapter.
; FIGURE 3. CV/CI module.