Tim - thanks for the interest,
Wilfred - great article.
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Well I have the first of the 3 integrator chassis' up completed electrically and fully tested and all works absolutely perfectly except for one annoyance. The reed relay model I selected for switching the integration capacitance doesn't seem to be up to the task. I am getting (about 5% of the time) contacts intermittently sticking on after switching in the ~1uF integration capacitor.
The reed relay I am using is HE721A1210, "form A" here:
http://www.farnell.com/datasheets/16797.pdfThe contact ratings are:
Voltage switching : 200Vdc
Current switching : 500mA
Current carrying : 1.2A
Power switching : 10W
When the ~1uF capacitor is switched in by the relay, the potential difference between the contacts can be as high as 210V. The integrator op-amp output V saturates at +/-105V. For example, if the 1uF was switched out while the integrator was railed out at -105V, the 1uF will be floating with a charge of 105V. If the 1uF is then switched back in when the op-amp output is railed out at +105V, then at that instant the relay contacts potential difference will be at that worst case 210V.
The op-amp power output stage is short-circuit proof and current limited to +/- 20mA, so that sets the maximum peak surge current through the capacitor when it is initially switched in. That is well below the relays maximum rated "switching" current of 500mA.
The relay datasheet makes not mention of switching current de-rating with switching voltage, but I figured that 20mA should be pretty safe. Well, unfortunately, not so. I'm getting intermittent contact sticking/fusing with that 20mA peak surge with less that 100V across the contacts. Giving the stuck relay a gentle tap reliably opens the contacts.
I guess that I am going to have to source a replacement relay, quite possibly not of the fragile reed type. I doubt that I'll find one with the same footprint so it will most likely have to be a fudge to the PCB.
Oh well, I guess you can't build something this complicated with out at least one thing going wrong.