Basic Circuit Block Diagram
Three Voltage Power Supply
This
circuit provides a +/- 12V and a +5V supply.
+12V: Relay Coil
Voltage Amplifier (741 op-amp)
Multiplexer (MAX DG508A)
-12V: Voltage Amplifier (741 op-amp)
Multiplexer (MAX DG508A)
+5V: AtoD converter (MAX187)
+12V Supply ripple under full load
with relay coil energised
Load Control Circuit
This
circuit consists of the following:
Test Battery.
For the circuit shown below, this would typically be any
Lead Acid battery with a rating of >1Ah.
With different loads, it will be possible to test other batteries.
Load (RL).
This is provided by a load box providing a number of
different loads depending on the battery being tested.
Shunt Resistor (Rs)
This enables the circuit current to be measured. This is a Bare Element Resistor, chosen for
its excellent Temperature Coefficient.
The circuit current is equal to : I = Vs / Rs. In practice I will measure the circuit
current using a DMM at different levels and convert the measured voltage into a
current reading using software.
Relay
The switching of this relay will be computer controlled;
therefore the computer can remove the load when the battery voltage drops below
a certain value.
Vb= Battery Voltage.
Vs = Shunt Voltage (Proportional to
Circuit Current)
The Load is controlled via the
Relay.
EG. 12v, Test Battery
6R Load
Nominal
Circuit Current I = Vb/(RL+Rs)
I= 12/6.1 =
2A (24W)
The voltage
developed across Rs should be : V = I * R
V = 2 * 0.1
= 0.2V (200mV)
The computer
closes the relay to start the discharge cycle.
In practise we won’t allow the battery to discharge below a certain
value.
Test set-up of circuit diagram.
The DMM was
used to measure Shunt Voltage (Vs).
It can be
seen that with a 12V (1.3Ah) battery the Shunt voltage is 0.160V.
The load
current measured with the DMM was 1.500A.
We can now
calculate the value of the Shunt Resistor.
Rs = Vs / I
Rs= 0.160 /
1.5 = 0.107ohms.
The nominal
value of the resistor is 0.1ohms.
In reality the correlation between
Vs and the load current should be checked at different current and voltage
levels, as it may vary. This can be compensated for in the software.
Voltage Amplifier.
The voltage
measured across the shunt resistor will vary from approximately 0v to 200mV
depending on the load used. The MAX187
AtoD has an input voltage range of 0 to 4v.
The non-inverting amplifier below has a gain of 11, which will give a
maximum output voltage of 200mV * 11 = 2.2v.
Gain = (Rf+Rin)/Rin
Gain = (47000+4700)/4700
= 11
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The
amplifier was constructed on breadboard and tested as follow:
Input Voltage Output
Voltage Gain
50.0mV 0.568V 11.4
123mV 1.362V 11.0
So, in
practise the 741 gives a gain of approximately 11.
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Relay Control Circuit (Load
Control).
The relay
is triggered by the output of b5 of the parallel port. The relay coil requires 12v to energise. The resistance of the coil is 200ohms,
therefore a current of 12v / 200ohms = 60mA will be drawn. The parallel port output of 5v at a maximum
of 12mA isn’t sufficient to energise the relay coil. The circuit below will convert the +5v signal
from the parallel port to +12v.
The diode
protects the transistor from back EMF which will be generated when the relay
coil is switched off.
Relay Coil Load Current (Ic) = 12v/200R = 60mA
Load Current / Max parallel
port current
= 60 / 12 = 5.
We need a transistor with an
hfe >5.
It’s good practise to choose a transistor hfe
of 5x this value i.e 25.
2N2222A chosen – hfe=100, IC(max)=800mA
Calculate value for base
resistor:
Rb = (Vc . hfe) / (5 . Ic)
Rb = (12 . 100) / (5 . 0.060) = 4000
Rb = 4k7 chosen.
Voltage Reducer Circuit.
The AtoD
converter can only accept voltage inputs of up to +4v. As the max voltage of a battery could be
>12V the voltage needs to be scaled down.
A simple potential divider can be used to achieve this.
Vout =
(R1 / (R1 + VR1)) . Vin.
Vin is measured from the positive terminal of the
battery under test.
A test voltage of 13.5V
should be inputted and VR1 adjusted to give make Vout
4v.
Complete Circuit Diagram, excluding
PSU.
The Battery
Voltage from the Voltage Reducer circuit and the Load Current from the Voltage
Amplifier are connected to the S1 & S2 inputs of the multiplexer
respectively. Address line A0 to A3 on
the multiplexer are used to select the desired input (S1 or S2). The multiplexer can take up to 8 inputs, but
for this project I am only using two.
Stripboard Layout
Constructed unit showing PSU board
and interface board mounted in box.
Testing the circuit using a simple
program written in QuickBasic
Test Load Box
The test
loads are housed within an aluminium casing.
Each load is brought outside the box using banana terminals.
The test
box contains the following loads:
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Load
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Suitable Use
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2 x 15R (50W) Resistors in Parallel = 7R5
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6V & 12V batteries at
about 1.7Amps
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1.2R (10W) ( Plus 0.1R shunt resistor )
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1.2 – 1.5V lower
capacity batteries
Typical discharge current
is 1Amp.
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0.22R (25W) ( Plus 0.1R shunt resistor )
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1.2 – 1.5V higher
capacity batteries
Typical discharge current
is 3.8Amps
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47R (25W)
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9V batteries. Typical discharge current is 0.2Amps
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Note. If used as above, heat dissipation won’t be a problem. Only the two 15R resistors in parallel will
get hot. With a fully charged 12V
battery, the maximum temperature of the resistors was measured at a maximum of
60°C. By placing a 12V fan across the
two resistors, and drilling ventilation holes around
the resistors the temperature dropped to less than 50°C. The resistors are rated at 200°C.
Circuit Diagram
Examples of Batteries Box
Lead Acid BATTERIES
LOAD – 2 x 15R (50W) Resistors in Parallel = 7R5
Used for 6 – 12V batteries
Typical discharge current is
(12/7.5) 1.6Amps for a 12V battery and 0.8Amps for a 6V battery.
Power dissipated through the
resistors will be: (12 * 1.6) 19W
Due to the internal shunt
resistor the circuit is limited to 2Amps maximum.
Capacities range from
1200mAh to >10Ah
A 10Ah capacity battery will
take >6 hours to test.
NiCd & NimH
BATTERIES
LOAD – 1.2R (10W) ( Plus
0.1R shunt resistor )
Used for 1.2 – 1.5V lower
capacity batteries
Typical
discharge current is 1Amp for a NiCd or NimH
Power dissipated through the resistor will be: (1.2 * 1) 1.2W
AAA, AA capacities range from 600mAh to 2600mAh
Discharge time at 600mAh = 0.6 hours
Discharge time at 2600mAh = 2.6 hours
LOAD – 0.22R (25W) (
Plus 0.1R shunt resistor )
Used for 1.2 – 1.5V higher capacity batteries
Typical discharge current is 3.8Amps for a NiCd or NimH
Power dissipated through the resistor will be: (1.2 * 3.8) 4.6W
D cells have ratings up to 11,000mAh
Discharge time at 11,000mAh = 2.9 hours
LOAD – 47R (25W)
Used for 9V batteries
Typical discharge current is 0.19Amps for a NiCd or NimH
Power dissipated through the resistor will be: (9 * 0.19) 1.7W
PP3 capacities range from 100mAh to 300mAh
Discharge time at 100mAh = 0.5 hours
Discharge time at 300mAh = 1.5 hours
Calibration
The analyser was powered up with a 12v, 20W lamp as a
load.
A 12V, 1300mAh Lead
Acid battery.
Voltage
The analyser was set to measure
battery voltage and the output of the AtoD converter recorded. The AtoD converter output will be a value
between 0 and 4096 (12 bit AtoD). To
calibrate the output, we need to measure the actual battery voltage using a DMM.
AtoD Output Actual Battery Voltage
3620 12.17V
If we now divide the AtoD Output by
the battery voltage we get a calibration factor of : 3620 / 12.17 = 297.5
For the software to display the
actual battery voltage all it needs to do is divide the AtoD output by the
calibration factor.
Current
The analyser was set to measure
current and as above, the AtoD converter output recorded. The actual current was measured using a DMM
in series with the battery and load.
AtoD Output Actual Battery Current
1860
1546mA
If we now divide the AtoD Output by
the current we get a calibration factor of : 1860 / 1546 = 1.203
For the software to display the
actual current all it needs to do is divide the AtoD output by the calibration
factor.
A 1.2V, 2600mAh NiMi battery
Voltage
AtoD Output Actual Battery Voltage Cal
Factor
327 1.140 286.8
Current
AtoD Output Actual Battery Current Cal
Factor
636 523mA 1.216
Now that the Cal Factor has been calculated at two
different extremes, we can work out the linearity of the analyser. At 12V the cal factor is 297.5. At 1.2V the cal factor is 286.8. The percentage difference between the two cal
factors is: 3.6%. So, the analyser can
measure the battery voltage with a linearity of at least 3.6% between 1.2V and
12V
Likewise, for Current:
Cal factor at 523mA = 1.216
Cal Factor at1546mA = 1.203
Linearity between 523mA and 1546mA = 0.25%
NOTE :
This circuit is only suitable for analysing batteries up to 12V. By up-rating the relay there should be no
problems with using higher voltage batteries and higher current loads, although
the shunt resistor may also have to be up-rated or mounted on a heat sink.
Discharge levels for
batteries
Lead Acid
This voltage should be measured after the battery has
been in a state of rest for at least 3 hours.
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State of Charge
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12V battery
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Divide the voltages by twp for a Six Volt battery and multiply them by
two for a 24Volt battery.
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100
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12.7
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90
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12.5
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80
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12.42
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70
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12.32
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60
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12.20
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50
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12.06
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40
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11.90
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30
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11.75
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20
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11.58
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10
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11.31
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0
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10.5
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Battery Recovery after discharge
To determine what the safe discharge level for the
battery under load is the following measurements were made.
Lead Acid
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Battery Type
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Load
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Discharge Current (Amps)
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End Discharge Battery Voltage
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Recovery Voltage after 3 hours
rest.
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State of charge after recovery
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Notes
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12v
1300mAh
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12v,
20W tungsten lamp
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1.7
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9.9v
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12.1
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50%
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Can
go lower than 9.9v during discharge
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6v
2800mAh
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12v,
20W tungsten lamp
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1.1
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4.8v
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NiCd
Battery
voltage drops sharply near the end of the discharge cycle.
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Battery Type
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Size
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Load
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Discharge Current (Amps)
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End Discharge Battery Voltage
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Notes
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AAA
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1.2v, 800mAh
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AA
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1R 10W resistor
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0.6
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0.2v
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C
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D
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PP3
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NiMh
Battery voltage drops sharply near the end
of the discharge cycle.
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Battery Type
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Size
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Load
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Discharge Current (Amps)
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End Discharge Battery Voltage
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Notes
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AAA
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1.2v, 2600mAh
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AA
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1R 10W resistor
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0.9
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0.3v
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C
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D
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PP3
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Interfacing with the Parallel Port.
The Data
Register and the Status Register of the parallel port are used to interface
with the circuit.
Parallel Port Output/Input Lines
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Parallel Port Data Register
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b0
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b1
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b2
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b3
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b4
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b5
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b6
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b7
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Circuit Board
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A0
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A1
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A2
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CS
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SCLK
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RELAY CONTROL
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x
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x
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DG508A Multiplexer Input Select
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MAX 187 AtoD
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Switches Load
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NC
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NC
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Status Register
Bit 5 of
the Status Register is connected to Dout of the Max 187 AtoD.
MAX 187 12bit Serial AtoD Converter
Pin out:
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1
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VDD
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Supply voltage, +5V ±5%
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2
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AIN
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Sampling analogue input, 0V to VREF
range
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 3
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SHDN
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Pulling SHDN high enables the
internal reference
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4
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REF
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Reference voltage - Sets analogue
voltage range to 4.096V.
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5
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GND
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Digital ground
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6
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DOUT
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DOUT Serial data output. Data
changes state at SCLK’s falling edge.
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 7
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CS
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Active-low Chip Select initiates
conversions on the falling edge. When CS is high,
DOUT is high impedance.
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8
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SCLK
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Serial clock input. Clocks data out
with rates up to 5MHz.
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DG508A Multiplexer Pin out:
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1,15,16
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A0,A1,A2
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Address Inputs
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2
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EN
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Enable – High to enable chip
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3 & 13
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V-
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Supply voltage +/-18V max
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4,5,6,7,9,10,11,12
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S1 to S7
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Signal Input Lines
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8
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D
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Switched Signal Output
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14
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GND
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Ground (0v)
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A 9 pin D
type socket was selected for the battery analyser housing. A standard 25 pin D-type printer cable was
used and a 9 pin D type plug placed on the other end wired as below:
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25 Pin D-type connected to PC
Parallel Port
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9 Pin D Type
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Parallel Port Output / Input
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Circuit Connection
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Pin Number
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Wire Colour
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Pin Number
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2
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Red
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5
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data
0
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Multiplexer
A0
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3
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Orange
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4
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data
1
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Multiplexer
A1
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4
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Pink
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3
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data
2
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Multiplexer
A2
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5
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Yellow
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2
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data
3
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A
to D CS
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6
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Green
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1
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data
4
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A
to D SCLK
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7
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Light
Blue
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6
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data
5
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Relay
Control
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11
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White
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7
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Busy
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A
TO D Dout
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25
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Black/Grey
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8
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GND
(0v)
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GND
(0v)
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Software.
The
software has been written using Visual Basic 6 and can be downloaded directly
from this link
The link
contains the full source code ready to be loaded into Visual Basic.
