On 7/30/05, Groups wrote: > Vasile, > > > > The problem with not isolating the mains voltage is that I would then need > to isolate ALL of the 8 inputs, 8 outputs > Temperature inputs and RS485 instead of just isolating the 3 x 230VAC ok, understood looks the cheapest way in your situation > inputs. > > I expect the additional circuitry to add opto couplers to all inputs and > outputs and DC-DC converters for Analogue inputs There is no need for any DC-DC converter. The OA is on the microcontroller side, on the mains side is just one optocoupler input and one resistor. The RMS is converted in current. So it's a current to voltage optoisolation. > etc will soon add up, not to mention cost. One cheap dual optocoupler (2x photodiode+phototransistor) + 1x op amplifier like LM358 + 2 resistor. Also one resistor on the main input. Multiply with 3 if you need a full three phase arangement. It's the same price like a serious transformer or less. > > It also means I need to remember to use an isolation transformer during > development so the scope probe earth does not > cause problems. The other issue is what happens if the user wires the input > incorrectly and places Phase on the > logic earth ? A transformer does not have better isolation like an HV optocoupler. 5KV isolation for a transformer means special isolation and is not a comercial one. 5KV isolation in an optocoupler is usual, 15KV is an extended range. The input is fully isolated by your ground so it's floated. Switching the phase with the protective earth will blew up some fuses, there is no solution for this, it's called stupid wiring. Use some conectors with positioning key and double check before connecting. > > There was a circuit floating around written by Stephen Woodward that > described using 2 optos which had their outputs > wired together. One opto was driven via the Anode and the other by the > cathode. Have you seen an arrangement similar to this ? Yes, I know at least three different topologies based on the same negative reaction on the OA. The problem is the voltage swing, on the output there is a DC offset of 1/2 Vcc and an AC rectified swing from 1/2 to Vcc or from 0 to 1/2 Vcc. Because on the input is just one photodiode you'll see the optoisolated mains rectified. **** do you see any tag on this topic ? cheers, VAsile > > Kind Regards > > David > > -----Original Message----- > From: piclist-bounces@mit.edu [mailto:piclist-bounces@mit.edu]On Behalf > Of Vasile Surducan > Sent: Saturday, 30 July 2005 1:23 PM > To: Microcontroller discussion list - Public. > Subject: [EE] Re: Circuit for measuring mains voltage > > > On 7/30/05, Russell McMahon wrote: > > > How large is the transformer ? > > > > A transformer is a very eledgant solution, technology notwithstanding. > > You can get very small main transformers - certainly under 25mm^2 > > footprint. > > Jinx may be able to comment due to recent similar experience. > > Using a transformer it's still an old methode for our days having > only the disadvantage of PCB space consuming and a magnetic field > generator. > > > > > > An alternative is a purpose build double tx, single rx opto made for > > this sort of task. There are a number available. The sender uses a > > feedback loop to contioj the sending signal and the receiver slaves > > this. Opto tx drift not a problem. > > Why double tx ? A PIC running floated on to 230V mains with an > optoisolated full duplex communication (half duplex on 2 wire EIA485 > or whatever). I have a few projects running this way. > > > > > > Infineon HCNR200, HCNR201 > > "High-Linearity Analog Optocouplers" > > 0.01% non linearity > > The light side of the force at work. > > > > Clare AN107 Linear Opto Couplers (Google knows) > > LOC110. LOC111. LOC112 > > Techniques apply to other manufacturer's products. > > > > I have often thought that you could probably do an OK version of this > > using a std dual opto IC. > > And at least one operational amplifier. It works very good, is > temperature compensated. On the nonisolated section (mains) the > voltage is converted into current using a simple resistor. On the > isolated section a middle supply voltage is negative feedback > controlled by an OA and two photo-transistors. Two more resistors > required to fix the middle voltage of the supply. It works very nice > and is cheaper than any optoisolated amplifier factory made. > > cheers, > VAsile > > -- > http://www.piclist.com PIC/SX FAQ & list archive > View/change your membership options at > http://mailman.mit.edu/mailman/listinfo/piclist > > > -- > http://www.piclist.com PIC/SX FAQ & list archive > View/change your membership options at > http://mailman.mit.edu/mailman/listinfo/piclist > -- http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist