MIAC and temperature sensor

[gizquier2]

Hello Im working with a board based in a LM45 sensor and Im sending the output voltage to MIAC to activate the relays.
Here is the problem, the output of LM45 is in mV and its not enough to make changes in the analog input of the MIAC.
The question is, how can i do more SENSITIVE the input or reduce the range of input (0V<->5V to -500mV <-> +500mV )
Thank you and sorry for my english, is poor.
Gilbert

[Sean]

Hello,

Unfortunately it is not possible to change the analogue resolution of the MIAC, but it should be possible to use a simple op-amp circuit to adjust the signal produced by the LM45 to match the MIAC input.

The main requirement is the addition of an offset voltage to prevent the signal from becomming negative, changing the -500mV to +500mV signal range to a 0V to 1V range. The second (optional) requirement is the addition of a gain stage (up to x12) to match the conversion resolution of the MIAC to the requirements of your application - The MIAC conversion resolution is 20 counts per volt.

The exact op-amp circuit will depend on the power supply voltages available for your application.

[gizquier2]

VS -> 5Volts
-200mV to 1V in Vout.

Im trying to use for control an air acconditioned machine, when its over 25 Β§C put in on cold air, and when its down 15 Β§C put in on the hot air.

[Steve]

Looking at that device's datasheet, you can connect it in a much simpler configuration if you do not need to read -ve temperatures. In this case, simly connect the +V to 5V, ground to 0V and then read the Vout signal directly (see figure 1 on page 2 of the datasheet)

vout = 10mV per degree, so 25 degC = 250mV and 15 degC = 150mV.

The resolution of the MIAC input is probably not good enough to be accurate (without boosting the output signal from the device), so you would need to amplify it with some additional circuitry.

[gizquier2]

wrong

[Steve]

I'm not sure that we can help you to this level of detail - you'll either need to sort this out yourself, or perhaps ask an external contractor to help.

[gizquier2]

hoho ok, my fault, i love miac anyways

[Sean]

You should use a single rail op-amp - like an LM358 - if you only have positive supplies. The op-amp can be powered from the same 12V - 16V supply as the MIAC if you want to generate voltages in the 10V to 12V range. The amplifier output range is 0V to Supply - 1.5V.

Feed the signal voltage to the non-inverting (+) input.

Connect a resistor (R1) from the output to the inverting (-) input; Connect a resistor (R2) from the inverting (-) input to 0V (non inverting amplifier configuration).

The ratio of R1 and R2 sets the gain of the amplifier (formula available from many sources). For a gain of 10, R1 should be 9 x R2.