3.1 Generalized Block Diagram Figure 3.
1: Generalize Block Diagram for Blood Warmer3.1.1 Description of Block Diagram• Power supply block to give DC supply to ICs and other devices.
• Temperature sensor with bridge to senses the temperature for blood bag temperature (B) and for patient body temperature (P). • Instrumentation amplifier for channel-1 (Blood bag-channel) • Instrumentation amplifier for channel-2 (Patient-channel)• Differential amplifier block to obtain the temperature difference. • Voltage to Current converter block to convert the voltage into compatible electrical signal for the heating coil. • The heating element block i.e. a fabricated steel tube which is coil coated with mica wound.3.2 HardwareThe working blocks that constitute this project include • Power Supply Block• Temperature Sensing Circuit (blood bag and patient)• Instrumentation Amplifier (B and P Channel)• Differential Amplifier Circuit• Voltage to Current Converter Circuit• Heating Element3.
2.1 Power Supply Block Figure 3.2: Power Supply BlockIC 7805, 7905 (+5V,-5V) are used to get 5 V output. The bridge rectifier is used to convert AC voltage into DC voltage. Center tapped transformer (6-0-6) is used to supply 12 V. This circuit provides power to the further circuit. Mains supply is given to the center-tapped transformer. Then output of secondary winding is given to the bridge rectifier IC.
This bridge rectifier converts the ac current to dc current. Then to get peak-to-peak output voltage signal, capacitors are used.3.
2.2 Temperature Sensing Circuit (Blood Bag) Figure 3.3: Temperature Sensing Circuit (Blood Bag)This block senses the temperature of the blood bag by thermistor as a sensor. Thermistor are most commonly used for moderate temperature range.
They have negative temperature co-efficient. Thermistors are temperature sensitive resistors. Thermistors are constructed of semiconductor material with a resistivity that is especially sensitive to temperature. Thermistor used here is in form of bridge configuration so as to provide temperature compensation and the circuit for it is given below. Figure 3.
4: Temperature Compensation CircuitIn this bridge circuit, three resistors are constant, Ra, Rb, and Rc, while the resistive sensor, Rs, varies depending upon some physical variable – like temperature, light level, etc. The thermistor can be placed anywhere in the bridge with three constant resistors, but different placements can produce different behaviour in the bridge. e.g.
, different placements might cause the output voltage to go in different directions as there are changes in temperature. This is temperature sensing bridge. Firstly, temperature of interest is sensed and manually bridge balance condition which is null output is obtained by varying the pot. Now as the ambient temperature is changed, according to the change in the output.
This output is further given as input to the instrumentation amplifier.3.2.3 Temperature Sensing Circuit (of Patient)This block senses the temperature of the patient body by thermistor as a sensor. So all the details regarding to sensor will be same to that given in the description i.e. circuit diagram, components used etc. on the previous block.
3.2.4 Thermistor SpecificationsNTC-Negative Temperature Co-efficientValue of Co-efficient: a = 1.
40*10-3, b = 2.37*10-4, c = 9.90*10-8 3.2.
5 Instrumentation Amplifier 1 Circuit (B Channel)This block provides sensor output signal the sufficient amplification so as to drive further circuits properly and without loading. “B channel” refers to the blood-bag channel. As thermistor senses in range of micro volts, we have to amplify it in 2 stages of 1000 gain.Instrumentation amplifiers are actually made up of 2 parts• A buffered amplifier OP1, OP2 and• A basic differential amplifier OP3. The differential amplifier part is often essential when measuring sensors. Because a sensor produces a signal between its terminals.
The buffered amplifier OP1 and OP2 provides gain. They also prevents the sensor resistance from affecting the resistors in the op amp circuit.IC OP07 is used as they offers excellent performance in applications requiring low offset voltage, low drift with time and temperature and very low noise. Power supply of 12 V DC is given as input power supply.The instrumentation amplifier offers two useful functions• Amplify the difference between inputs and reject the signal that is common to the inputs (Common Mode Rejection).
• OP1 and OP2 are the two input amp’s and connected in the non-inverting configuration. It provides high gain and high input impedance.3.2.6 Instrumentation Amplifier 2 Circuit (P Channel)The circuit diagram, working of the circuit, component specifications and all other details will be remaining same as above block except the change in channel. Here “P Channel” refers to Patient-channel.This block provides error signal by detecting temperature difference of both channels.There are 2 inputs in each terminal of op-amp.
1) From B-Channel instrumentation amplifier.2) From P-channel instrumentation amplifier.Now as name suggests, it amplifies the difference between input 1&2 by gain 1000. So, finally the thermistor initial output is converted into volts.
3.2.8 Voltage to Current Converter CircuitThis block provides conversion of the output of differential amplifier to the corresponding value of current that a heating coil is capable to handle. Figure 3.8: Voltage to Current Converter Circuit.The main working is based on the well-known Ohm’s law.
According to this law, in the given conditions, voltage (V) and current (I) are directly proportional. It is given by V=IR (1) WhereR = Constant of proportionality called Resistance which is a property of material to oppose the current flow. I= (V/R) (2)And here I=5/1000=5 mA.3.3 Heating ElementThis block performs heating operation on the cold blood of the blood bag. A very thin coil of mica is wound on a stainless steel tube.
This typical design requires the fabrication. As the no. of turns increases the heating efficiency also may increase. So it has to take care about heating path i.e. there should be no overlapping of coil. If so, it will lead to the damage of the coil material due to excessive heating.
The schematic design of this block is given below. 7