| Electronics > Beginners |
| Safest way to measure AC mains 220-250 voltage? |
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| soldar:
--- Quote from: paulca on July 19, 2019, 06:14:45 pm --- If logged and graphed this can show me an estimate of the base load and see where and how hard the spikes hit when the "big hitters" are used, shower, washing machine, cooker, space heaters etc. --- End quote --- In this case you can just measure current, you do not need to measure voltage. If you are going to assume constant voltage then measuring only the current is a directly related magnitude. But you need to measure RMS current because measuring peak current is going to be very misleading. A diode charging a capacitor is going to give you a big peak. So you can put the 50 mA of the current transformer through a bridge and get DC pulses. A current transformer should never be left open so I put a Zener diode to prevent that. The Zener is just a protection and has no use in normal operation. I guess you could follow with a low pass filter to get rid of current spikes. ETA: I guess I am a bit confused because the title of the thread is how to measure voltage but then in the discussion the OP says he wants to measure power and without need for precision and power is correlated with current since we can assume voltage is stable. So I am not really sure what the purpose of all this is. |
| paulca:
--- Quote from: soldar on July 20, 2019, 08:12:06 am ---ETA: I guess I am a bit confused because the title of the thread is how to measure voltage but then in the discussion the OP says he wants to measure power and without need for precision and power is correlated with current since we can assume voltage is stable. So I am not really sure what the purpose of all this is. --- End quote --- I wanted to find the easiest and safest way to measure the voltage, but then started to question if I needed to or not. With the current clamp, through a burden resistor and into a voltage divider to size it for my MCU there is very little that can go wrong, there is no mains connection. When I come to measuring voltage, I need to actually connect something to the mains and I don't like mains. I'm still not 100% decided on whether to try and measure the voltage or not and if I do, how. Option 1: Chinese module: https://uk.banggood.com/Single-phase-AC-Active-Output-Voltage-Transformer-Voltage-Sensor-Module-p-1228141.html?gmcCountry=GB¤cy=GBP&createTmp=1&utm_source=googleshopping&utm_medium=cpc_bgcs&utm_content=garman&utm_campaign=pla-gbg-ele-diy2-pc&ad_id=339489662626&gclid=CjwKCAjw98rpBRAuEiwALmo-yi4ig3-CtTFTSgPBmLTfxfxN_iy3w0Mn14YECcKBKPcyJBi3743RDhoCZcUQAvD_BwE&cur_warehouse=CN Option 2: Ac-Ac power supply, but finding the right one that isn't regulated does not appear to be easy, although I could modify it. Then I need to start considering the transformer phase shift, voltage dividers, offsetting the voltage etc. etc. |
| MrAl:
Hello, When measuring AC voltage and current usually the AC voltage is converted into DC and then the DC level is calibrated against the measured AC level. This way you get good readings. For voltage, one way to do it is to rectify the AC into peak AC which is DC, then calibrate it using known AC voltages. The drawback to this method is that if the peak is loaded down (often the case with softer AC lines) then you dont get an accurate reading unless of course you are just interested in the peak AC. You could look into averaging methods for better results. For AC current, it is usually to use a current transformer. The current transformer output is rectified into DC. The DC is then calibrated against a known AC current. The reason for the calibration in these methods is because each method is somewhat non linear and thus the scale factor changes depending on actual signal level. It's not too hard to do though just input some known levels and note the output then include that in your measurement algorithm. Often a microcontroller is used to do the measuring. Also note that playing with live mains is not always a good idea. To measure voltage an AC transformer can be used that steps the voltage down to a more manageable level and also isolates your measurement equipment from the live mains line. |
| richnormand:
--- Quote from: MrAl on July 20, 2019, 12:16:09 pm ---Hello, When measuring AC voltage and current usually the AC voltage is converted into DC and then the DC level is calibrated against the measured AC level. This way you get good readings. --- End quote --- Indeed for voltage using a wall-wart with an AC transformer is most likely the safest way to have isolation and low voltages for hobby measurements. Calibration is easy like pix 3 of post 27 in this thread....... Current measurement will require a loop/ring to keep a non-contact setup an can be calibrated using various loads on the circuit. |
| paulca:
--- Quote from: MrAl on July 20, 2019, 12:16:09 pm ---When measuring AC voltage and current usually the AC voltage is converted into DC and then the DC level is calibrated against the measured AC level. This way you get good readings. For voltage, one way to do it is to rectify the AC into peak AC which is DC, then calibrate it using known AC voltages. --- End quote --- Now I'm even more confused. Why do I need to rectify them? If I offset them so that I end up with, say, 0-3V with 1.5V being the mid point, when I do the RMS calculation it will (by it's squaring nature) end up positive. Although I can see that a rectified waveform would give more accuracy as I could use the full 0-3V for both positive and negative giving twice the accuracy. Also will rectifying the voltage not change it requiring I calibrate out the diode drops? I realise rectifying the current transformer will have little to no effect as it's a current source. So it's looking like I do need to measure the voltage and I might as well go the full RMS voltage RMS current calculation. I already have the current transformer. Gives me 50mA per 100A, a correctly calculated burden resistor and an offset divider. For voltage I'm going to need a 12Vac adapter and a voltage divider to get it down to 3Vac offset by 1.5V. What about this transformer phase issue? Do I need to worry about it? How much will it shift the phase by? Is it determinable/measurable/negligible? |
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