MCU, PT100 and multiple relays problem

Thread Starter


Joined May 18, 2012
I have a problem with my project. It is a simple board with 4 PT100 sensors, 12 relays, 1 Character LCD and 1 4x4 keypad. The relays and PT100 sensors are on different boards. To simplify the schematic I've removed the LCD and the keypad and left just one relay and PT100 sensor.

The sensor circuit consist of constant current source(MCP1700 with a 2.5k resistor), 1 op-amp subtracting the wire resistance and 1 filtering and amplifying op-amp.
The relays are marked as 5V 200mW.

At first glance everything worked fine. The PT100 sensors are reading the correct temperature, the relays are swithing, the lcd and keypad are working as expected.

The problem is when all the relays switch on, the PT100 sensors get confused and display wrong temperature. The PT100 sensors get confused even with one relay switched on but to a lesser degree.

What I've done so far:
1. I've installed a sepperate voltage regulator for the sensors, relays and the MCU (not at the same time) with no success.
2.Installed capacitors everywhere with hope to filter the rippels of the power supply with no success.
3. Replaced the power supply with a laboratory one, with no success.
4. Bypassed the voltage regulator for the relays and hooked them directly to the unregulated power rail with no success.
5. Replaced the L78S05 voltage regulator with a more precision one BA05CC0T.
6. I've dug out the internet with no success.
7. Measured the constant current source of the sensors, but it was very stable.
8. Measured a voltage drop of 120mV on the unregulated power line.
9. There was no voltage drop on the outpud pin of L78S05 regulator.
10. Measured a voltage drop of 115mV on the MCU Vdd pin.

Since I'm just a hobbyist, not a professional in electronics, I've run out of ideas :( . I'll be much grateful if you give me some directions where and what to start to investigate further.