The original intention was to equip the IuT voltemter with a LiPo battery and a battery shield with the view to have an isolated instrument like any digital multimeter (DMM). I found that using the shield with a quite beefy 720 mAh battery lasted for the 4 hours I specified for the design. Unfortunately this combination wasted more power than I could tolerate, and pushed the cost of the instrument up too high to my liking. I decided to look for an alternative solution.
First I would like to discuss why did I want to have an isolated instrument. I am sure for most breadboard prototyping needs it is perfectly fine to have the voltemeter powered from a power bank or even a mains adaptor. However some measurements (e.g. high side current measurements) do require isolation of the device-under-test (DUT) and IuT voltmeter's grounds. There are some inexpensive isolation DC/DC convereters available, and this log entry describes how the voltmeter performed when it was powered from the B0505 1W converter.
A B0505 part serves just this and no other purpose - to isolate ground potential of the power supply from the ground potential of the load. As electric current can only flow through some circular (closed) path, accidentally connecting two circuit nodes from the isolated power domains results in no current flow. Therefore an isolated voltmeter will happily measure differential voltage on a high side shunt resistor whereas non-isolated one will be in most cases driven to saturation at best. I used B0505 1W because I got some on a cheap from Aliexpress before; more suitable B0503, which outputs 3.3V, and could power the 3.3V raildirectly (without wasting 33% of the consumed power on the linear volatge regulator), seemed to be less common thus more expensive and difficult to get.
I wanted to figure out whether the converters I have got
- could supply enough power to drive the two modules
- whether the supplied curent is clean enough not to cause erroneous readings of the ADC
I have used a protoype shield, micro USB breakout board and B0505 converter connected as shown below
Top view above does not do justice to the awesome look of the assembeled custom shield; the picture below gives much better impression of the artifact despite the best focus I could get was not good enough.
I tested power conversion capabilities of the B0505 in a proper manner using an electronic load (pictured below)
and a very handy tool (a.k.a. USB safety tester) that I use for power measurements within its operating range (3-30V, 0-5A):
The USB safety tester was plugged into a wall adapter rated at 5 V, 3 A; its output was connected to the custom B0505 mini shield. The electronic load and a DMM were connected to the B0505's isolated power output in parallel. By regulating the load I set various currents flowing into the B0505; these currents usually fluctuated by 10-20 mA thus the recorded data can be slightly off.
|Vin,V||Iin, mA||Vout, V||Comment|
|5.09||0||5.65||no load at all; |
not permitted by the manufacturer
|4.96||250||4.68||worked for a long time without hard overheating; slightly above its specified output power maximum rating (4.68*0.25 = 1.17 > 1 (W) )|
|4.94||300||4.53||just checked because the power was notably higher than the specified rating (I have had two more modules thus could gamble; 4.53*0.3 = 1.36 >> 1 (W) )|
Wemos mini features a linear low drop out (LDO) regulator RT8013 that can tolerate the no-load output volatge of a B0505; current consumption of an ESP peaks at 170 mA at most. Therefore B0505 seems to be suitable for powering the IuT volmeter. Moreover, because of the presence of this regulator and always on CH340 the B0505 will never starve of the essential load current.
Some numbers to confirm the above estimate:
B0505 - max 1 W out, efficiency min 70%, output current 200 mA max (thus can consume up to 1W / 0.7 = 1.4 W)
RT8013 - max power dissipation 0.3 W for SC-70 package; drops 5.1-3.3=1.8 V thus can source up to 165 mA
ESP consumes 170 mA n RF transmit bursts at most
CH340 typically consumes 12 mA
SPI flash consumes ? mA (works on and off)
any other circuitry (ADC module) consunes ? mA
One cannot say from the above figures that the power supply for Wemos mini was grossly over-engineered.
Two custom modules and the mini module were plugged into the tripler base shield as shown below
The output of the B0505 was connected to the 5 V rail. It was possible to connect the mini module to a PC because the Wemos mini was equipped with a diode placed between the Wemos mini USB connector and the 5 V power rail. Therefore the 5 V rail was driven by the B0505 and not buy the host PC. A button shield was placed on the top of the mini module in order to abandon execution of the server's code in the init.lua script if the button was pressed at the power up.
The following video shows that the readings of the ADC module were not adversely affected by the output ripples of the B0505 module. Please note that the web page was reloaded more frequently (every 300 ms) than the Wemos mini printed to the terminal (measurements were conducted every 200 ms but only one in ten was printed ). That is why some of the readings visible on the web page could be absent from the terminal log.
Next hardware revision will feature a custom mini shield with all the resistors, inputs and optional B0505. If it is not required or not available, the voltmeter can simply be powered from the always available Wemos mini USB connector losing the isolation though.
Overall, B0505 1W seems to be a perfect match for the IuT voltemeter requirements, both in terms of the power it can supply and the cleaness of its output voltage.