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Lots of Far-Eastern scooters are fitted with GY6 engines. These already elderly units are sturdy and economical, but if you want to  “push” the power a bit (so called ‘Racing’  kits, better handling of the advance, etc.), you soon find yourself faced with the problem  of the engine temperature, and it becomes essential to f it a heat sink (of ten wrongly  referred to as a ‘radiator’) on the oil circuit. Even so, in these circumstances, it’s more than reassuring for the user to have a constant clear indication of the oil temperature. Here are the specifications we set for the temperature gauge we wanted to build:

Let’s start by the sensor. This is a type-K thermocouple, as regularly used by multimeter manufacturers. Readily available and fairly cheap, these are robust and have excellent linearity over the measurement range we’re interested in here. The range extends from 2 mV to 5.7 mV for ten measurement points. The positive output from the thermocouple is applied to the non-inverting input of IC3.A,  wired as a non-inverting amplifier. Its gain  of 221 is determined by R1 and R2. IC3 is an LM358, chosen for its favourable characteristics when run from a single-rail supply. IC3.B is wired as a follower, just to avoid leaving it powered with its pins floating.

IC3.B output is connected to pin 5 of IC1, an LM3914. This very common IC is an LED display driver. We can choose ‘point’ or ‘bar’ mode operation, according to how pin 9 is connected. Connected as here to the + rail, the display will be in ‘bar’ mode. Pin 8, connected to ground, sets the full scale to 1.25 V. R3 sets the average LED current. Pin 4, via the potential divider R7/R8+R9, sets the offset  to 0.35 V. Using R8 and R9 in series like this avoids the need for precision resistors.

As per the LM3914 application sheet , R4-R5-R6 and C5 will make the whole display flash as soon as D10 lights (130 °C = 226 °F). Simultaneously, via R10 and T1, the (active) sounder will warn the user of overheating. Capacitor C6 avoids undesirable variations in the reference voltage in ‘flashing’ mode. IC2 is a conventional 7808 regulator and C1– C4 filter the supply rails. Do not leave these out! D1 protects the circuit against reverse polarity.

The author has designed two PCBs to be fit-ted as a ‘sandwich’ (CAD file downloadable  from [1]). In the download you’ll also find  a document with a few photos of the project. You’ll note the ultimate weapon in on-board electronics: hot-melt glue. Better than epoxy (undoable!) and quite effective against vibration.

Author : Georges Treels - Copyright : Elektor

Low-cost semiconductor diodes such as 1N914, 1N4148 and 1N400X can be used as temperature sensors in applications where high accuracy is not required. They can be mounted on transistors, power diodes, transformers, heat sinks, rechargeable batteries, crystals, PCB, etc to monitor their temperature.

It is highly desirable to use temperature sensors with linear temperature characteristics and the mentioned diodes are best suited for that. To use them as temperature sensors, these diodes first need to be sorted according to their temperature coefficient. (Please refer ‘Signal Diode-Based Fire Alarm’ schema idea published in February 2013 issue to understand how these diodes can be used as temperature sensors.) This schema can help quickly sort different diodes based on their temperature coefficient.

The schema diagram of the device for selection of temperature-sensing diodes is shown in Fig. 1. The schema is built around step-down transformer X1, voltage regulator 7818 (IC1), voltage regulator 7809 (IC2) and three diodes 1N4001 (D1 through D3). The mains supply is stepped down to 21V, 250mA using transformer X1. Diode D2 is sufficient to rectify it since the required output current is typically below 20mA. The linear regulator IC1 provides the 18V power supply for the diodes (connected at CON1 through CON12) to be tested. The linear regulator IC2 provides the 9V power supply for the digital voltmeter (DVM) used for measuring the voltage drop across diodes.

Utilising this device, it is possible to test the diodes at around 0.1mA of forward current. The current for their PN junctions is provided through individual resistors R1 through R12. The value of a resistor is much higher that the resistance of the tested diode’s junctions, so changing the diodes will not change the forward current significantly.

 

The number of the connectors for the test diodes can be increased or decreased. The device does not need any adjustment or calibration to operate properly. To calculate the temperature co-efficient (TKU) of each diode and to sort the diodes accordingly, we need to measure the voltage drop across the diodes under test.

To sort the diodes, ensure that all of them are connected in the schema and the voltage drop (say U1) across each diode is measured one by one at the same temperature, say 25°C. The voltages can be read at TP3 through TP14 with respect to TP0 using the DVM as shown in Fig. 1. Now change the temperature to, say, 40°C, and take the measurements (say U2) again. We can use an electrical equipment, such as a heater, to produce the second temperature. So now the temperature coefficient of the PN junction for a diode will be:

The voltage drop change of the PN junction due to temperature change is linear, so we should take measurements at only two temperatures for each diode. The test temperatures can be the same for all the diodes or different. Most of the diodes can be used in the temperature range of -25°C to +150°C without any problem. The temperature coefficient of most of the PN junctions is in the range of -1.3mV/°C to -3mV/°C, so we can use these diodes as sensors.

An actual-size, single-side PCB for the device is shown in Fig. 2 and its component layout in Fig. 3. After assembling the schema on PCB, enclose it in a suitable plastic case.

To test the schema for proper functioning, verify correct power supply for the diodes and DVM at TP1 and TP2 with respect to TP0. Voltage drop across each diode can be verified at TP3 through TP14. 

Sourced By: EFY : Author: Petre Tzv. Petrov