Low Cost (8-Channel) Thermocouple Signal Conditioning With Analog Multiplexer and Analog Optocoupler.

In the current project will be created a signal conditioning for thermocouple equipped with an analog multiplexer, the total input for this circuit is 8-channel thermocouple input.

Some of the problems in a thermocouple signal conditioning is required strengthening gain and also giving a pretty good filter so the noise do not stay in the thermocouple which has a weak signal.

Some of the main components in this project are:
1. Multiplexer Analog Input For Thermocouple (ADG507A)
2. Thermocouple Amplifier (AD595)
3. Op-Amp (INA126)
4. Analog Optocoupler (HCNR201)



ADG507A



The ADG507A are CMOS monolithic analog multiplexers with 16 channels and dual 8 channels, respectively. The ADG507A switches one of eight differential inputs to a common differential output, depending on the state of three binary addresses and an enable input. Both devices have TTL and 5 V CMOS logic compatible digital inputs. The ADG507A are designed on an enhanced LC2MOS process, which gives an increased signal capability of VSS to VDD and enables operation over a wide range of supply voltages. The devices can operate comfortably anywhere in the 10.8 V to 16.5 V single or dual supply range. These multiplexers also feature high switching speeds.


AD595



The AD595 is a complete instrumentation amplifier and thermocouple cold junction compensator on a monolithic chip. It combines an ice point reference with a precalibrated amplifier to produce a high level (10 mV/°C) output directly from a thermocouple signal. Pin-strapping options allow it to be used as a linear amplifier-compensator or as a switched output setpoint controller using either fixed or remote setpoint control. It can be used to amplify its compensation voltage directly, thereby converting it to a stand-alone Celsius transducer with a low impedance voltage output.

You can see the AD595 output voltage as a representation of the temperature thermocuople complete in datasheet. The Appropriate thermocouple type for the AD595 is J and K-type thermocouple.


INA126



The INA126 is precision instrumentation amplifiers for accurate, low noise differential signal acquisition. Their two-op-amp design provides excellent performance with very low quiescent current (175μA/channel). This, combined with a wide operating voltage range of ±1.35V to ±18V, makes them ideal for portable instrumentation and data acquisition systems. Gain can be set from 5V/V to 10000V/V with a single external resistor. Laser trimmed input circuitry provides low offset voltage (250μV max), low offset voltage drift (3μV/°C max) and excellent common-mode rejection.

You can see the basic connection and how to setting the gain for INA126 in the picture below.




How The Circuit Work

The complete picture thermocouple signal conditioning circuit as shown below.



Instal the thermocouple sensor in the channel that you want on the input terminals. For example if you want to put on channel 1 you can plug thermocouple in the terminal J1. Do addressing for ADG507A multiplexer in accordance with the input channel that you want. For reference ADG507A addressing you can use the reference to the ADG507A datasheet which you can download at the bottom part.

Thermocouple output on this ADG507A will be given to the AD595 as an information signal from the thermocouple. ADG595 will process this data and then converted and amplified into a voltage that represents the signal from the thermocouple. The magnitude of this voltage can you see on the AD595 datasheet.

The output of the AD595 is still too small so need to be strengthened again using the INA126 op-amp, Set INA126 Gain by adjusting the value of VR1 to get the gain that you want. Do not forget to set the offset of the INA126 with setting VR2, adjust VR2 until the voltage at pin5 of ICINA126 = +/- 0 Volt.


HCNR201



Actually the output of the INA126 is good enough to be used, but sometimes there are problems when the tip of this thermocouple we attach to the body of the equipment that containing the ground, therefore in this series are equipped with analog isolatar HCNR201 to isolate the output voltage of the INA126 to not blend with another ground. In the picture you can see, there are two power supply that is VCC1 and VCC2, VCC1 is devoted only to the Thermocouple where ground VCC1 (GND) should not be fused with the another ground, GND is ground only for a series of ADG507A, AD595 and INA126. While VCC2 with ground GNDA is ground that can be associated with other equipment such as ground on data acquisition board.

HCNR201 is high-linearity analog optocoupler consists of a high-performance AlGaAs LED that illuminates two closely matched photodiodes PD1 and PD2, as shown in Figure 1. The input photodiode PD1 can be used to monitor, and therefore stabilize, the light output of the LED. As a result, the on linearity and drift characteristics of the LED can be virtually eliminated. The output photodiode PD2 produces a photocurrent that is linearly related to the light output of the LED. The close matching of the photodiodes and advanced design of the package ensure the high linearity and stable gain characteristics of the optocoupler.

The HCNR200/201 can be used to isolate analog signals in a wide variety of applications that require good stability, linearity, bandwidth and low cost. The HCNR200/201 is very flexible and, by appropriate design of the application circuit, is capable of operating in many different modes, including: unipolar/ bipolar, ac/dc and inverting/ noninverting. The HCNR200/201 is an excellent solution for many analog isolation problems.

In the above circuit set the VR3 to obtain your desired voltage according to the thermocouple output gained .

Source :

1. Complete circuit of Low Cost (8-Channel) Thermocouple Signal Conditioning With Analog Multiplexer and Analog Optocoupler.
2. Datasheet ADG507A
3. Datasheet AD595
4. Datasheet INA126
5. Datasheet HCNR201
6. Thermacouple Singnal Conditioning PCB (Bottom Track).zip
7. Thermacouple Singnal Conditioning PCB (Solder Mask).zip
8. Thermacouple Singnal Conditioning PCB (TOP Legend).zip

The author is not responsible for any risk caused by this circuit. Refer all to the datasheet.

Frequency to Voltage Converter Circuit

This circuit good working in the frequency range 0-850 Hz. You can change the value of R8 and C1 in order to get the various frequency range.

Transformerless Power Supply

Electric Shock Hazard. In the UK,the neutral wire is connected to earth at the power station. If you touch the "Live" wire, then depending on how well earthed you are, you form a conductive path between Live and Neutral. DO NOT TOUCH the output of this power supply. Whilst the output of this circuit sits innocently at 12V with respect to (wrt) the other terminal, it is also 12V above earth potential. Should a component fail then either terminal will become a potential shock hazard.




If you are not experienced in dealing with it, then leave this project alone.Although Mains equipment can itself consume a lot of current, the circuits we build to control it, usually only require a few milliamps. Yet the low voltage power supply is frequently the largest part of the construction and a sizeable portion of the cost.

This circuit will supply up to about 20ma at 12 volts. It uses capacitive reactance instead of resistance; and it doesn't generate very much heat.The circuit draws about 30ma AC. Always use a fuse and/or a fusible resistor to be on the safe side. The values given are only a guide. There should be more than enough power available for timers, light operated switches, temperature controllers etc, provided that you use an optical isolator as your circuit's output device. (E.g. MOC 3010/3020) If a relay is unavoidable, use one with a mains voltage coil and switch the coil using the optical isolator.C1 should be of the 'suppressor type'; made to be connected directly across the incoming Mains Supply. They are generally covered with the logos of several different Safety Standards Authorities. If you need more current, use a larger value capacitor; or put two in parallel; but be careful of what you are doing to the Watts. The low voltage 'AC' is supplied by ZD1 and ZD2.

The bridge rectifier can be any of the small 'Round', 'In-line', or 'DIL' types; or you could use four separate diodes. If you want to, you can replace R2 and ZD3 with a 78 Series regulator. The full sized ones will work; but if space is tight, there are some small 100ma versions available in TO 92 type cases. They look like a BC 547. It is also worth noting that many small circuits will work with an unregulated supply. You can, of course, alter any or all of the Zenner diodes in order to produce a different output voltage. As for the mains voltage, the suggestion regarding the 110v version is just that, a suggestion. I haven't built it, so be prepared to experiment a little.

I get a lot of emails asking if this power supply can be modified to provide currents of anything up to 50 amps. It cannot. The circuit was designed to provide a cheap compact power supply for Cmos logic circuits that require only a few milliamps. The logic circuits were then used to control mains equipment (fans, lights, heaters etc.) through an optically isolated triac. If more than 20mA is required it is possible to increase C1 to 0.68uF or 1uF and thus obtain a current of up to about 40mA. But 'suppressor type' capacitors are relatively big and more expensive than regular capacitors; and increasing the current means that higher wattage resistors and zener diodes are required. If you try to produce more than about 40mA the circuit will no longer be cheap and compact, and it simply makes more sense to use a transformer.

RS232 to RS485 Converter

This is a small RS232 to RS485 converter project it use for convert RS232 signal level to RS485 level multidrop.It can use with 32 slave to comunicate with PC and embedded systems board for long distance less than 1.2Km( 4000 feet).

Specification

- 9-12Vdc power supply
- small size only 2.8" X 1.45 "
- use for RS485 multidrop 2 wire connection.
- Direct connect with PC on DB9 connecter.
- Use RTS signal to control direction .
- 32 Slaves (up to 256 slaves with some transceiver i.e. MAX3088)


In the Figure 1 J1 and J2 use for jump R terminate(120) at the
end of communication line or last slave.The 75176 use for
transceiver.
RTS active (logic 0 or -3 to -15V) when we need to send data .

Zipped file consist of :
- Schematic
- PCB