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| Hewlett Packard 740B DC Standard Digital Voltmeter (and 740A) |
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| Dave Wise:
UPDATE. My fastest photocell is also nearly blind; using a 24V Luxeon 3535HV LED, 3mA is required to get the cell down to 20K. Using a 3V Cree JB2835, I expected 24mA, but it actually requires 60mA, a factor of 20 instead of 8. A less-blind cell that needs 1mA on 24V, needs 10mA at 3V, a factor of 10 instead of 8. This points to a steep drop in LED efficiency as current goes up. This means I cannot run 3V LEDs on 5V, I must use 24V LEDs driven by transistor switches. The extra space taken precludes through-hole hand-assembly of a prototype, plus it's not usable in a 419 Null Detector with its 24V supply. You can probably manage with 9V LEDs, which should find themselves on the left side of the efficiency knee. (12V-18V parts are expensive for some reason. The cheap ones are 3/6/9/24/48.) I'm working on a circuit board layout with surface-mount parts including in-circuit-programmed uC. There's enough space for everything. The board will mount on the demodulator side of the old chopper housing, sticking LEDs down the V3/V4 wells. (Remove the old photocells first.) The back side of the board is copper fill connected to Guard. Cover it with a few layers of Kapton or polypropylene tape to insulate from the grounded chopper housing. The H11F1's mount on edge, with the LED pins in through-hole pads above the V3/V4 wells. Air-wire the signal pins to the old terminals. I'm a total noob with KiCad and OSHPark. Maybe someone can look over my shoulder and stop me from making dumb mistakes I can't back out of. HP-Chopper-Photocell-Driver-first-draft.pdf (17.29 kB - downloaded 54 times.) |
| Dave Wise:
I think we have a winner. Wonder what OSHPark will say. Now I get to lay out the LED discs that go into the photocell wells. I tried to make it friendly to hand-solder, with 0805 and 1206 parts. |
| Kleinstein:
For the driver circuit it would help to also show the schematics. For the layout the design rule checks and PCB tools usually do a good job ckecking for major errors. From what it looks like the optocouplers ouput side is not connected, not even to some connectors, which is odd. The origianl neon bulb circuit has one hidden function, that may not be obvious: it alternates between the 2 neons and has a little pause in between. It is kind of normal that the fast LDRs are less sensitive, but there is also a point when the sensitivity gets too low. With very high power from the LEDs one gets increasing levels of thermal effects that can cause thermal EMF. This would not be real drift, but cause a rather slow settling for the meter to warm up. |
| Dave Wise:
Thanks, Kleinstein. I added the schematic to post number 61. Everything on the board is at or near Guard potential, which may be up to 500V away from the signal pins. I will solder in only the LED side of the optocouplers. I'll connect the signal pins to the module terminals with point-to-point wire, to avoid leakage. I wasn't willing to put in the work of creating a custom footprint for half of an H11F1, so in KiCad I just pretended they were hanging off the edge, with the signal pins set as no-connect. (The only DRC warnings were "Silkscreen outside board outline".) The microcontroller program will include dead time. The original 740B driver has dead time on the Main chopper but not the Meter chopper. Testing showed that it's of critical benefit to both. I listened to my blind, brightly-lit photocell with my General Radio 1232-A Tuned Amplifier, and didn't hear a thing. I worried about those four clearance holes. The teflon insulators the holes are meant to clear extend right to the edge. A drill bit would want to bend. I made them recesses in the outline, so OSHPark can mill them instead of drilling. I figured out how to make KiCad do it. It's tedious but I did it. String together lines and arcs. It's finicky about the outline making a closed shape, you have to drag line ends together at extreme zoom. Back copper is a fill zone, connected to Guard. Have to insulate from the chopper housing. Kapton tape, polypropylene tape (Scotch "Magic" tape)... how about a phone screen protector? See any show-stoppers? |
| Kleinstein:
Partially mounting of the OK makes absolute sense. The drive level is quite high and this can cause some thermal offsets, that may increse the warm up drift (would not make is slower but more amplitude). I would assume the LEDs to drive LDRs are for the input modulator and the H11F1 is for the demodulator. The demodulator part is after amplification and thus way less critical. I don't hink the H11F1 would need that much current for the demodulator - the signal current is still relatively low. From the datasheet there is rather little gain from using more than some 5 mA and even 2 mA should be enough. Anyway the drive current could come from the 5 V supply - no need to start from 24 or 40 V to drive a 1.2 V IR LED. Of cause this does not help with the 5 V derived from the 40 V with a linear regulator. some 10 mA for the H11F1 would be quite some power from the 40 V rail ! For the 5 V regulation it may with adding more filter capacitance - for just the µC and low power the simple zener may be enough. Similar the 40 V supply for the LEDs should have a filter capacitor (e.g. , so the extrernal supply would not see much of the modulated current. For the LEDs the focussing of the LEDs can also have quite some effect. The LEDs for illumination use tend to be wide angle. The viewing angle can also be different for the 3 V LED. The Cree LED is spedied for 150 mA, so at 60 mA the efficiency should still be good. Another point could be the color: the white LEDs have quite some red and blue. The red light part may not have any effect and the blue part could be already less effective. The color temperature make quite some difference to the spectrum. The question would be if a high efficiency green or even orange LEDs could work better. |
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