voltage sensing of 450 VAC 60 Hz to galvanically isolated ADC
voltage sensing of 450 VAC 60 Hz to galvanically isolated ADC
(OP)
I'd like some guidance on how to measure 450VAC, 60Hz, and pass this to a low voltage sensing circuit (approx. 3 V going to a high-impedence ADC). The two circuits (450V and sensing) must be galvanically isolated.
1. The brute force approach would be to use a 450:3V transformer. The ADC will simply grab the voltage coming out of the transformer secondary. However, I've found that this transformer is rather large. I'd like the transformer to fit on a PCB, if possible.
2. Another approach is to use voltage divider with a 1:1 transformer. See attachment. We size the voltage divider resistors such that 3V forms across the transformer. However, two problems. 1) The resistors dissipate about 6W of power. I have to measure nine (9) voltages, so this adds up (9x6W=54W). 2) The voltage divider is directly proportional to the DC resistance of the transformer, which is in turn influenced by the ambient temperature. In other words, the hotter the environment, the higher the resistance.
3. I want to explore other alternatives. I've heard about linear optocouples or high-impedence Op-Amps sitting on the 450 V line, and draw virtually no power, but Op-Amps would require a power supply hanging on the 450 V line as well, and linear optocouples may lose linearity near the peaks and zero-crosses.
Basically, the goal is to design a voltage sensing circuit that doesn't vary with temperature (error < 1% from 0 to 80 C), does not dissipate more than, say, 1-2 Watts, and has a footprint small enough to fit on a PCB, say 3x3 in.
What's the best way to do this? :)
1. The brute force approach would be to use a 450:3V transformer. The ADC will simply grab the voltage coming out of the transformer secondary. However, I've found that this transformer is rather large. I'd like the transformer to fit on a PCB, if possible.
2. Another approach is to use voltage divider with a 1:1 transformer. See attachment. We size the voltage divider resistors such that 3V forms across the transformer. However, two problems. 1) The resistors dissipate about 6W of power. I have to measure nine (9) voltages, so this adds up (9x6W=54W). 2) The voltage divider is directly proportional to the DC resistance of the transformer, which is in turn influenced by the ambient temperature. In other words, the hotter the environment, the higher the resistance.
3. I want to explore other alternatives. I've heard about linear optocouples or high-impedence Op-Amps sitting on the 450 V line, and draw virtually no power, but Op-Amps would require a power supply hanging on the 450 V line as well, and linear optocouples may lose linearity near the peaks and zero-crosses.
Basically, the goal is to design a voltage sensing circuit that doesn't vary with temperature (error < 1% from 0 to 80 C), does not dissipate more than, say, 1-2 Watts, and has a footprint small enough to fit on a PCB, say 3x3 in.
What's the best way to do this? :)





RE: voltage sensing of 450 VAC 60 Hz to galvanically isolated ADC
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If we learn from our mistakes I'm getting a great education!
RE: voltage sensing of 450 VAC 60 Hz to galvanically isolated ADC
If this is part of a 'power' system I can't believe that an isolating step-down transformer can be THAT big. I found several rail-mounted transformers that can take 450V line-line inputs and give low voltage secondaries which can be attenuated to 3V if necessary. OK, they may not be PCB-mounted but this is much safer and more controllable than dealing with a 450V 'power' feed directly to the PCB.
RE: voltage sensing of 450 VAC 60 Hz to galvanically isolated ADC
I do agree that it should be possible to find a small transformer capable of 450V operation though.
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If we learn from our mistakes I'm getting a great education!
RE: voltage sensing of 450 VAC 60 Hz to galvanically isolated ADC
You could self power something like a TL494 and transmit duty cycle % through an opto coupler.
RE: voltage sensing of 450 VAC 60 Hz to galvanically isolated ADC
RE: voltage sensing of 450 VAC 60 Hz to galvanically isolated ADC
This worked very well for us. Our main application was measuring DC, but the same could be (and was at one point) used for AC too. For your wide temperature range you might need to do some compensation or maybe you could get lucky and the resisters vary in proportion to each other.
We weren't particularly space confined so the whole unit was about 150mm x 200mm for 3 channels. This gave us loads of room for creepage and clearance but you could easily miniaturise it by a factor of two or three.