27-11-2014, 11:01 AM
Hi Nick,
Good question re. the op-amp. I can't immediately remember as it was last year when I did the work, but I'll see if I can find the old PCB...
About the filter, I did consider using one. I do have good stocks of them, and there is reasonable space in the box. In the end I decided against once I'd decided to add the voltage monitor output. I'm not sure how good the frequency response of my small mains transformer is - probably not great - but given that I was now in a position to view the mains waveform, I thought it best not to filter it. Of course, one approach might be to use a separate filter (as opposed to one integrated into the IEC socket), and connect my monitor to the incoming side of the filter.
I was also considering adding some MOVs for surge protection, but decided against that for the same reason. If I'd stuck with that capacitor-dropper supply, then MOVs would have helped protect the module.
Oh - another factor was that those filters often use the dreaded "Rifa" X2 caps. At a similar time, the one in my frequency counter let go rather dramatically (A Schaffner FN326-1/07 - sadly, I can't find a modern replacement that fits into the space, so I had to use an unfiltered IEC).
As I mentioned in the write-up, I am planning to rig up a large audio amplifier so I can characterise the behaviour of both monitor outputs w.r.t. frequency. One day...
Cheers,
Mark
Good question re. the op-amp. I can't immediately remember as it was last year when I did the work, but I'll see if I can find the old PCB...
About the filter, I did consider using one. I do have good stocks of them, and there is reasonable space in the box. In the end I decided against once I'd decided to add the voltage monitor output. I'm not sure how good the frequency response of my small mains transformer is - probably not great - but given that I was now in a position to view the mains waveform, I thought it best not to filter it. Of course, one approach might be to use a separate filter (as opposed to one integrated into the IEC socket), and connect my monitor to the incoming side of the filter.
I was also considering adding some MOVs for surge protection, but decided against that for the same reason. If I'd stuck with that capacitor-dropper supply, then MOVs would have helped protect the module.
Oh - another factor was that those filters often use the dreaded "Rifa" X2 caps. At a similar time, the one in my frequency counter let go rather dramatically (A Schaffner FN326-1/07 - sadly, I can't find a modern replacement that fits into the space, so I had to use an unfiltered IEC).
As I mentioned in the write-up, I am planning to rig up a large audio amplifier so I can characterise the behaviour of both monitor outputs w.r.t. frequency. One day...
Cheers,
Mark
