The circuit shown in the figure uses stored energy in the multiplexer relay coil to boost the power-supply voltage from 3.3 to 5.1 V. This extra voltage enhances the amplifier's ability to produce a larger output voltage suitable for interfacing to a 5-V analog-to-digital converter. It also extends the measurement range of the thermocouple signal.
This technique requires the faster relays to be used in continuous switching mode for signal scanning at regular intervals. A switching frequency between 50 and 1000 Hz is needed so that continuous energy is pumped into the voltage-enhancer reservoir capacitor (C1) from the relay coil. Obviously, the circuit should not be used with very low scanning rates.
An OPA277 (www.ti.com) low-offset, low-drift op amp amplifies the signals from the type-K thermocouples. The 3.3-V relay is a 9852-3.3 from Coto Relay (www.cotorelay.com), and it's rated for about 100 million operations. An amplifier gain of 251 was used to get about 10-mV/8C output. The cold junction is to be read separately.
For comments by Mr. Mike W. Wade, I want to point out that if one designs the relay multiplexer, then one also knows its life. For faster data rates, it does not mean you will use the circuit all the time-- 24-hours a day. Also, relays are available for 1000 million operations. The circuit will work even for 1 Hz to 100 Hz as long as the op amp uses the stored energy in the capacitor in a limited way. Perhaps new low-power op amps will work much better. Low switching rate will also have greater ripple voltage in power for sure. Relays are used for high isolation in contact points and can switch high current with minimum resistance and yet not out from market. The purpose of the designs looks to only be to show that the stored energy can be used to raise the voltage to power the op amp. Hence, all other things we may talk about will be out of topic.
Shyam -February 04, 2005 (Article Rating: )
Although the idea here is ingenious, I wonder if the device is meant to operate for a long time? Using a clock rate of 1000 Hz and continuous operation, you’ll be at the design lifetime of the relay in about 28 hours. There are a number of single chip switchers in SO and SOT form factor that could replace the NPN, diode and Zener in less space. I have to assume that the clock rate is also synchronized with the A/D sample and hold so that the device on the output pin can tell if it is the hot junction or the cold junction that its reading.
Miles W Wade -January 10, 2005
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