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| Help with AC ground question |
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| lmagalhaes:
Hi there, I've been working mostly with low voltage stuff, microcontrollers and the like, but now at my new job, I've been asked to design a circuit that needs to switch the hot AC wires. I'm fairly confident in that, but came across something that confused me a bit and want to make sure I'm not doing something stupid. The system in question has a boiler with auto-level detection and the way it detects the water level is by using some probes. Those probes are connected to the boiler chassis, which is connected to AC ground (earth). So, to detect the water, you need to detect when the probe is shorted to the boiler chassis via AC ground. My first instinct was to connect it to a microcontroller pin with a pull-up and when the pin goes low, you know that water has reached that probe. However, for that to work, AC ground needs to be connected to the circuit ground. Since I don't have experience with AC, I ask if this is fine, if it should be protected with something, if it will cause some ground loops or if I'm doing it all wrong. Thanks in advance, Luís |
| Rerouter:
I would recommend an optocoupler for your detection, a string of resistors (higher voltage rating) to limit to e.g. 0.5mA to activate the optocoupler and detect it with your micro never touching mains If you can you should prevent your circuit from being in direct contact with mains where possible, try and use relays and optocouplers to keep things seperate, You use multiple resistors in series for mains as each has a certain voltage it will breakdown at, so by using say 4 that are each rated at 250V, you can have 1 fail and never have any major issues. |
| lmagalhaes:
The digital part of the circuit is being supplied with a transformer and full bridge rectifier, so it's not in direct contact with the mains. Also, the AC switching is being done with TRIACs and optocouplers, as you've said, and it's working well. The problem is with the water detection inside the boiler. Since the probe is floating until water reaches it and, when it does, it shorts to AC ground, how could I use the resistors and optocoupler for the detection ? Are you suggesting to have a resistor divider from the phase and then, when water hits the probe, it will short to AC ground? If not then I didn't understand |
| Ian.M:
Its a bad idea to put DC on the probes as, unless they are made of platinum or other highly corrosion resistant precious metal, thecurrent flow when immersed will cause electrolytic corrosion of the positive electrode. You *MAY* get away with it if the probes are very small with respect to the ground electrode and you bias them negative so the ground electrode is the one that corrodes. Its preferable to energise the probes with low voltage AC, as on the negative going half cycle, metal ions tend to get redeposited close to where they were liberated from the metal surface on the previous positive going half cycle vastly slowing the effective corrosion rate. Its possible to use a MCU PWM module to generate a squarewave probe excitation signal, boost it with something like a MOSFET driver then capacitively couple it to each probe so there is no net DC current. For detection the signal can also be capacitively coupled to a charge pump to get a DC level to indicate whether or not the probe is immersed. Whether you then take that direct to a digital input pin with a schmitt characteristic, use a comparator or use an ADC input pin depends on how fault tolerant you want it to be. Direct to a digital input means you will have to compromise on sensor drive levels etc. to get reliable discrimination between immersed/non-immersed, and any sensor problems wont be noticed until they cause a false indication. Using a comparator you get free choice of threshold, and thus drive level. Using an ADC input, you can monitor the sensor 'health' and if the steady state signal level gets too close to your chosen threshold, you can alert the operator that sensor maintenance is required, *before* it actually fails. Its a lot easier if your MCU board 0V can be coupled to AC ground at the boiler chassis. If your overall system design calls for the MCU board to be mounted remotely this may not be possible, and that may require differential amplifiers for the probe signals to cancel out any noise or other difference between chassis ground and MCU 0V and maybe even transformer coupling for the excitation signal. |
| Rerouter:
Your thoughts are correct, Live - A few voltage rated resistors spaced accordingly - optocoupler - probe Then the micro is just monitoring the other side of the optocoupler for when it shorts you will see pulses |
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