Schlagwort-Archive: Keithley

Small craft project for the summer time

solar module
As a mini – craft project for the summertime I call the following tinkering. A small monocrystalline solar module called „SM 6“ from a well-known large electronics distributor starting with „C“ and ending with „d“ plays the main role in the project. The module has a nominal power of 6Wp with a maximum current of 320mA. The rated voltage is 17.3V. The open circuit voltage is 20.8V. The silicon cells are embedded in an EVA (ethylene vinyl acetate) plastic sheet and are UV and moisture resistant. The whole module is about 25cm x 25cm in size. It is thus ideally suited to provide the power to power USB devices. For example, I thought about WIFI IP cams. It should also be possible to charge smartphones or tablets.

In order to be able to do this, the operating voltage of the USB standard (5V) must be generated from the rated voltage of the photovoltaic cell. You could do that easily with a 7805 controller and convert the difference into heat. But this is the least efficient way to get the panel’s energy into a cell phone. Firstly, the internal resistance of the panel depends on the light intensity, which has a major impact on the efficiency of unmatched load resistors. On the other hand, a series regulator is a power shredder, since the difference between input voltage and regulated output voltage is converted into power loss, ie heat, during the flow of current. Here you are better served with a switching converter (buck converter).

 

 
In a simple laboratory setup, the behavior of the panel can be examined. For this purpose, the open circuit voltage of the panel is measured at different illuminance levels. Subsequently, the panel is loaded with different resistance values ​​and the current through the load as well as the voltage at the panel are measured. The measured values ​​are recorded and the Ri (internal resistance of the source) is calculated. The following circuit diagram shows the measurement setup:
measurement setup – schematic
The ammeter is an Agilent and Voltmeter Keithley 2701 table multimeter. These gauges can both be controlled via SCPI commands. The interface is a LAN port. This makes it easy to implement an automated measurement process via a PC and a suitable script. And since Matlab offers a very convenient way to script, it’s also used right now. In order to be able to measure in a laboratory and have approximately the same environmental conditions, a table lamp with halogen bulb is used instead of the sun. The brightness of the lamp is easily adjusted by supplying it with a laboratory power supply of 0-13V. Of course, the laboratory power supply can also be controlled by Matlab.
measurement setup with lamp as „sun“

The lamp is placed at a distance of 25cm in the middle of the panel. In order to get a feeling of which illuminance is achieved with the lamp, a reference measurement is taken with a luxmeter. That is, the lamp goes through the power ramp of 0-13V and the lux meter measures the illuminance at a distance of 25cm under the lamp. The whole thing is resolved in 0.5V steps. This results in a curve that looks like this:

Voltage on the lamp results in illuminance

Now the measurement can begin. Resistors are manually connected to the panel as a load resistor and current and voltage are measured at each brightness level. There are eleven load resistance values ​​ranging from 4.7 ohms to 220 ohms connected in sequence. An idle measurement is then of course made without load resistance. The following graph shows the calculated internal resistance for two loads of the panel over the brightness curve of the lamp in lux and in the other graph over the voltage at the lamp (for better scaling). The internal resistance of a source is calculated from the open circuit voltage of the source minus the voltage under load, divided by the current. With the difference between the no-load and load voltage, the voltage drop at the internal resistance is obtained. Since the load is also known as the current, it is only necessary to use Ohm’s law to obtain the resistance value …

Internal resistance vs. illuminance
Internal resistance vs. Voltage on the lamp

Since some clarifications about the behavior of the PV cell have now been eliminated, I can briefly report on the structure of the voltage converter. As previously announced, a switching converter is the more efficient way to adapt the energy to the consumer. Here comes an LM2596S used. The LM 2596 is a „Simple Switcher Power Converter“ that switches at 150kHz and can supply a load with 3A.) Here is an overview of the functions:

  • 3.3-V, 5-V, 12-V, and Adjustable Output Versions
  • Adjustable Version Output Voltage Range: 1.2-V to 37-V ± 4% Maximum
    Over Line and Load Conditions
  • Available in TO-220 and TO-263 Packages
  • 3-A Output Load Current
  • Input Voltage Range Up to 40 V
  • Excellent Line and Load Regulation Specifications
  • 150-kHz Fixed-Frequency Internal Oscillator
  • TTL Shutdown Capability
  • Low Power Standby Mode, IQ, Typically 80μA
  • Uses Readily Available Standard Inductors
  • Thermal Shutdown and Current-Limit Protection

(source: datasheet from vendor TEXAS Instrument)

With this switching converter and a few other components can quickly assemble a circuit and transform with the layout tool „Eagle“ into a board. However, this circuit is so simple that it only works as efficiently as possible with the advantages of the LM2596, but does not perform any power tracking. This means that the load representing the circuit for the solar cell is not adapted to the internal resistance of the solar cell.

 

Circuit diagram of the DC-DC converter

From this circuit, a simple layout was created, a board etched and equipped. A USB socket on the output allows the direct connection of USB devices. To make the whole thing look a bit reasonable, I have donated the board still a small plastic casing …

Measurement of illuminance

measurement setup
Switchable load resistors
Layout on the computer
Foil for creating the printed circuit board
Etched PCB
Etched PCB
Finished circuit
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Long-term measurements with Keithley and Matlab

Keithley2000 desk multimeter

The bench multimeter from Keithley is an old companion in the field of measuring instruments. The types of the 2000 series are predominantly used in our laboratories. They are available in different equipment variants with regard to the interfaces to the outside world. Here GBIP bus is of course a standard, as is RS232. The newer devices now have a LAN interface with which communication via the Internet protocol is possible. Each of these interfaces can be used to communicate with the device using „Standard Commands for Programmable Instruments“ (SCPI). In this example I will control the Keithley 2000 via Matlab and read out measured values ​​cyclically over a longer period of time, save them in Matlab and finally output them in a plot – virtually configure a simple data logger. The purpose of this setup is to record the voltage curve (or current) of a rechargeable battery or battery of a low-energy device.

backside of the Keithley 2000

GPIB Interface (IEEE488)

RS232 interface

In this example I will use the serial data transmission via the classic RS232 interface, as this is completely sufficient for my application. In addition, I can save myself the installation of the driver packages for the GPIP-USB interface. 🙂 Since many of the current computers and laptops no longer have any RS232 ports, a USB-RS232 adapter (e.g. FTDI232 etc.) is required.

USB-RS232 Adaper am Keithley2000

Once the connection between the multimeter and the computer has been established, communication can take place via a Matlabscript, as in this example. The Keithley only needs to be told that it should „talk“ over the serial interface. The following code snippets show how you can easily read out data via SCPI:


serialObject = instrfind('Type', 'serial', 'Port', 'COM26', 'Tag', '');
%serialPort = 'COM23';
%serialObject = serial(serialPort,'BaudRate',9600, 'DataBits',8);

if isempty(serialObject)
serialObject = serial('COM26','BaudRate',57600, 'DataBits',8);
else
fclose(serialObject);
serialObject = serialObject(1)
end

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
%
% Sourcemeter 2000 setup
%
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
fopen(serialObject)
% fprintf(serialObject,':*RST')

time = now;
voltage = 0;
%%
figureinstanz = figure('NumberTitle','off',...
'Name','Spannungslogg',...
'Color',[0 0 0],'Visible','off');
plotinstanz = plot(voltage,time,'Color','red');

%% Messzeit und evtl Messintervall
stoptime = 10; %60 seconds
timeInterval = 1; % brauch' ma jetzt nicht

% Messgeraet einstellen
fprintf(serialObject,':SOUR:FUNC:MODE CURR'); % current source selection.
fprintf(serialObject,':SOUR:CURR:MODE FIXED'); % changes voltage mode to fixed
fprintf(serialObject,':SOUR:CURR:LEV 0'); % sets current to 0

fprintf(serialObject,':SENS:FUNC "VOLT"');
fprintf(serialObject,':SENS:VOLT:PROT 4');
%fprintf(serialObject,':SENS:CURR:RANG:AUTO ON');
fprintf(serialObject,':SENS:VOLT:RANG 10');
fprintf(serialObject,':FORM:ELEM VOLT');

% %fprintf(serialObject,':TRAC:COUN 1');
% %fprintf(serialObject,':TRAC:FEED:CONT NEV');
%
%
% fprintf(serialObject,':TRAC:CLE');
%
% fprintf(serialObject,':TRAC:POIN 10');
% fprintf(serialObject,'TRAC:FEED:SENS');
% fprintf(serialObject,'TRAC:FEED:CONT NECT');
% fprintf(serialObject,'TRIG:COUN 10');
% fprintf(serialObject,':OUTP ON');
%
% fprintf(serialObject,':INIT');
% fprintf(serialObject,':TRACE:DATA?');

%% Daten abholen
count = 1; voltage(1)=4
tic;
time=toc;
% while time<=stoptime
while voltage>=1.5
% fprintf(serialObject,':INIT');
% fprintf(serialObject,':TRAC:FEED SENS');
% fprintf(serialObject,':TRAC:DATA?');
%
fprintf(serialObject,':READ?');
voltage(count) = fscanf(serialObject,'%f');
time(count) = toc;
set(plotinstanz,'YData',voltage,'XData',time);
set(figureinstanz,'Visible','on');
pause(timeInterval);
count = count +1;
end

figure(1);
plot(time,voltage);
grid on; hold on;
xlabel('Zeit [s]'); ylabel('Batteriespannung [V]')
title('Spannungsverlauf Batterie 3V Lithium (2032 mit Modul) im default mode');

% fprintf(serialObject,':OUTP OFF');
%% Put the instrument in local mode
fprintf(serialObject,'SYSTEM:LOCAL');
fclose(serialObject);

The following plot shows what such a data log looks like. Here, the voltage curve of a nearly discharged battery is recorded over time until the consumer is switched off.

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