Fuel Injector Board Circuit. Post 2

The task that had to be done was to wired up components on a PCB board that made LED's light up when a voltage is supplied to the base of the transistor, this causes the LED's to light up and indicates when the fuel injectors are firing.

COMPONENTS

The components that are needed are 4 resistors which can handle a maximum of 0.25 amps at 25 degrees celcius, the values of which have to be calculated. 2 NPN BC547 transistors which can handle a maximum of 45 volts from the collector to the emitter. The maximum voltage through the base is 6 volts and the maximum current through the collector is 200mW, and a maximum reverse voltage of 50volts from the emitter to collector, and a maximum reverse voltage of 50 volts from the emitter to base. All these ratings are for when the transistor is at 25 degrees Celsius, if the transistor is hotter than 25 degrees then the transistors specs are less. Two LED's are needed which can handle a maximum of 180mA when wired up in forward bias, the power dissipation is 684mA and the maximum voltage is can handle is 2.5volts, all these specs are when the LED is at 25 degrees Celsius if the temperature of the component exceeds this then the specs reduce. Then the components are wired up on a bread board to make sure that the circuit works and that we have an idea on how to wire up the components on the PCB board. The calculations for the resistors where used to make sure that the each component was getting the correct amperage to make the transistors and LED's work efficiently. Using a data sheet the amperage that the transistor can operate in was found out so that the correct sized resistor could be used for the pulse signal circuit. The amperage through the base that let the transistor be in the saturated region was 0.5mA so since the amperage is known, and the voltage supply is 5 volts minus the voltage required to turn on the transistors which 0.6 volts. The resistance can be worked out using ohm's law, and the triangle can be seen below.

CALCULATIONS

So to find out resistance the voltage which is 4.4 volts as 0.6 must be minused as that is required to turn on the base side of the transistor, 4.4 is then divided by the amperage (R=V/I) which is 0.005A and this gave an answer of 880 which is the size of resistor required 880 ohm's. But since there was no 880 ohm resistor a 820 ohm resistor was used in the circuit, this is better as it allows a bit more current to flow through the base and lets the transistor be more saturated or turned on so there will be less of a voltage drop through the collector. Which means there is more voltage and current available for the LED so that will be brighter. Then the resistor size was worked for the power side of the circuit that lights up the LED's, the voltage supply is 12 volts but minus the voltage to pass through the LED which is 1.8 volts then minus another roughly 0.2 volts to pass through the collector due to some resistance which is always in the transistor. And the amperage required to turn on the LED is 20mA, so R=V/I, R=10/0.02A so the resistor required was 500 ohm's but since there was no 500 ohm resistor a 560 ohm resistor was used for the bread board circuit. The image below shows the fuel injector circuit wired up on the bread board. I then calculated the current flowing through both the pulse control side of the circuit and the LED side of the circuit. The pulse control current was worked out using ohm's law I=V/R,   I=5volts/820ohm's,    I=0.0061amps, this is how many amps are flowing through the base of the transistor. Then using the same equation the LED circuits current was worked out, I=V/R,     I=12volts/470 ohm's,      I=0.025amps, this is how much current is flowing through the LED

BREADBOARD CIRCUIT

One small problem that was encountered was the 560 ohm resistor was to large for the LED and the LED was not very bright so for the circuit that was going to wired up on the PCB board 470 ohm resistors where going to be used. This will allow for more current to flow through the LED which will make it brighter so that it is more visible when the LED actually lights up.

PCB CIRCUIT

Once the circuit has been wired up on the bread board the circuit then had to be wired up on a program called LochMaster, which allows you to place components and check where the current is flowing and the voltage supply is known so that components are not loaded with to much voltage, this is stopped by putting cuts in the board so that voltage is shared instead of every component getting its own 12 volt supply. The cuts are put on the copper strips between in the middle of components e.g in the middle of a resistor, this puts a break in the circuit and means that the other component can only get the voltage from the first component that has had the voltage drop for example an LED wired up after the resistor can only 1.8 volts power supply as since the resistor consumes the rest. The cuts are simulated on LochMaster, so that these cuts can be placed in the same place on the PCB board, and the other components can be place in the same position as well. Now everything can be placed in the exact same place to make it easy to wire up the circuit, now you just need to look at the LochMaster program to know where to place the components. The injector board circuit is seen wired up on LochMaster below showing where the components have been placed and where the soldering points are on the circuit which can be seen on the board that has been turned upside down.

^Component Position^                                                 ^Soldering points (Upside down)^

The three positve terminals on the left hand side of the left image are the inputs the top positive terminal is the 12 volt power supply for the LED's. The two lower positive terminals are the 5 volt inputs that switch on the transistor and the entire circuit by allowing current to flow through the base to the emitter. This is the pulse signal which makes the circuit switch on and off, which is what makes the LED's switch on and off. The negative terminal at the bottom of the board is the ground for the entire circuit. The image on the right, above shows the soldering points for the circuit and also shows the cuts made in the board to allow the circuit to work properly.


^injector board wired up^                  ^soldering points on board^        ^circuit layout design and working^

The circuits layout and description can be seen below.

EXPLANATION ON HOW CIRCUIT WORKS

The circuit works by having a constant 12 volt power supply which goes to the LED's, this can be seen coming from terminal 1. The LED's will remain off, until a 5 volt power source is given to the base side of the transistors this is from terminal 3 and 4 or PWMO, this then switches on the circuit and allows current to flow from the base to the emitter this then allows current to flow from the collector to the emitter, current can then flow from the 12 volt power source, through the LED's and to earth, this then turns the LED's on. Then when the pulse signal from the 5 volt supply is switched off again the whole circuit is switched off again as current cannot flow through the LED's until the base side of the transistor is switched back on again from the 5 volt pulse supply, then current can flow from the base to the emitter, which in turn allows current to flow from the collector to the emitter which turns the LED's back on again. This pulsing signal comes from the fuel injectors and is meant to represent the fuel injectors on time which is shown when the LED is on and when the fuel injector is off then the LED is off.

TESTING PROCEDURE

Before the circuit was checked and wired up to the 12 volt power supply and the function generator which creates the pulsing 5volt signal which switches the LED's on and off using a duty cycle of 50% on and 50% off. The board was tested to make sure all the components where still working. This was done using a multimeter, first I set the multimeter to ohm's and checked that there was continuity through the input wires from the 12 volt power source and the 5 volt pulse signal wires and through the earth wire, and there was with almost no resistance of 0.02 ohm's which was expected. If there was a high resistance reading this would mean that there could be a bad connection caused by poor soldering, this would cause resistance and voltage would be used up to pass through this resistance this would mean that there is less voltage and current for the LED's to use or less voltage and current for the transistor to switch on properly. This could cause one of the LED's to be duller than the other as it is not getting enough current and voltage to switch on properly. If the resistance was on the control side of the circuit or where voltage flows through the base this could mean that the transistor is not saturated so the collector side is not switched on properly causing a voltage drop across the collector to the emitter and the LED will not switch on properly as some voltage and current is being used up. I tested the wires connections on the solder side of the circuit to make sure that poor soldering was not affecting how the circuit operated.

Then I tested the continuity through the jumper wires with the multimeter still set to ohm's, the jumper wire which supplies 12 volts the LED's and the one that grounds the circuit. If either of these did not work due to high resistance then the LED's may not switch on properly or if the jumper wire to ground did not work then the circuit would not work as current cannot flow and the LED's would not light up. However both jumper wires had minimal resistance with both producing 0.02 ohm's resistance, once again high resistance could be caused by poor soldering. I tested this on the solder side of the circuit to make sure that solder did not affect the circuit operation

I then set the multimeter to diode test mode and checked that the LED's still switched on, which they did switch on with 1.75 volts, this is a good reading, if the Multimeter showed there was open circuit, this could mean that the LED was blown because it was overheated when the solder was being placed as the soldering iron heated the component to long this would mean that the LED would have to be replaced. Or it could mean that poor soldering has caused to much resistance in the circuit and the multimeter cannot switch on the LED, this would mean that the LED would not light up properly and it might not light up at all as not enough current can flow. I tested the LED's on the solder side of the board so that I know if the solder is affecting the operation of the LED's.

Leaving the multimeter set to diode test mode, I then tested the transistors base side of the circuit and the collector side of the circuit, all on the solder side of the circuit so that I know whether solder is having any affect on the operation of the board. Both transistors gave a reading of 0.6 volts on the base side which is a good reading, and both gave a reading of 0.7-0.8 volts on the collector side which is a good reading. A bad reading would be open circuit as this would indicate that the transistor was overheated by the soldering iron when the solder was being placed and this caused the transistor to blow up, and stop working this would mean that the transistor would have to be replaced.

PROBLEMS

The only small problem that was encountered was when the circuit was wired up on the bread board, a 560 ohm resistor was used on the LED side of the circuit and this was to big and did not let enough current through the circuit. This meant that the LED's where dull, so when the circuit was wired up on the PCB board, 470 ohm resistors where used to allow more current to flow through the LED's and make them brighter, so that it was more visible that they where operating.

REFLECTION

The biggest problem I faced in making this circuit was making good soldering connections, so if I could do this  circuit again then I would have more practice at soldering components before I wired up the circuit so that there wouldn't as many bridges made when I was soldering. (Bridging is when solder connects to components that should not be connected together and could even be connected from separate tracks, this would cause major problems with the circuit as this could give voltage supplies to the wrong components and over load the circuit with to much current and voltage.

REFERENCE:

datasheet for transistor http://www.datasheetcatalog.org/datasheet/MicroElectronics/mXuwzwr.pdf

datasheet for LEDhttp://www.lunaraccents.com/educational-LED-bulbs-data-sheets-absolute-ratings.html

(ohm's law chart)http://nz.images.search.yahoo.com/images/view?back=http%3A%2F%2Fnz.images.search.yahoo.com%2Fsearch%2Fimages%3Fp%3Dohms%2Blaw%2Btraingle%26ei%3DUTF-8%26fr%3Dyfp-t-501&w=469&h=182&imgurl=www.electronics-tutorials.ws%2Fdccircuits%2Fdcp23.gif&rurl=http%3A%2F%2Fwww.electronics-tutorials.ws%2Fdccircuits%2Fdcp_2.html&size=9KB&name=Ohms+Law+Triangl...&p=ohms+law+traingle&oid=e7a5feb7731ee5257cfdc33a9a9dcf59&fr2=&spell_query=ohm%27s+law+triangle&no=1&tt=169&sigr=11pp7s5bi&sigi=11h5vl4tj&sigb=12pgie892&type=gif&.crumb=HHVRhjVZ21c