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Hewlett Packard 740B DC Standard Digital Voltmeter (and 740A)

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Dave Wise:
Thank you!

You have it exactly right, I'm using a pair of H11F1's on the demodulator side - where offset can be ignored - while keeping the modulator photocells and driving them with LEDs.

With a 3V LED, my observation did not agree with the datasheet; Cree JB2835 needed 60mA to excite the photocell the same as my 24V LED at 3mA.
Or maybe the spectrum changes when you drive it hard, to something the photocell doesn't like.
Anyway, I abandoned that design path.

The LEDs I tested are supposed to emit 120-150 degrees which covers the whole photocell.

During "cruise", the demodulator only carries a few uA, but when slewing, the shunt switch has to charge 1uF to 20V in a couple milliseconds.  The meter loop series switch peaks around 200uA.

Are we looking at the same datasheet?  OnSemi Rev 2 (2021) Figure 2 Output Characteristics shows almost equal-size increases in saturated current for drive at 2-6-10-14-18mA, with 16mA yielding about 600uA.  I gravitated to 16mA because it's the test current for several specs.

Using the -42V rail, current transients are not an issue since the instrument's PWM output pulls amps not milliamps.
That said, if 9V LEDs work, I want to ensure the board can be used on the 419A, so I am looking at the lower voltage supplies, like +16.5 or -16.5 .  And since the board has moved from the main chassis to the (grounded) chopper housing, there's an exciting possibility of running it off one of the in-guard rails like +22 or -22 which would simplify the insulation.  Like -42V, these power high-level signals that don't need to be isolated from transients.  The 419A expects a quiet battery so I will see if I can squeeze an RC filter onto the board.

UPDATE.
9V LEDs are usable!

My blind photocell that needs 3mA on Lumileds 24V and 60mA on 3V Cree JB2835BWT-G-H30GA0000-N0000001, needs
* 12mA with 9V 2700K 80CRI Lumileds L128-2780EC35000B1
* 20mA with 9V 3000K 80CRI Cree JK2835BWT-W-H40EC0000-N0000001



Kleinstein:
For the datasheet i have lloked at the 2002 Fairchild version. The main difference is whether looking at the on resistance or the the maximum current (e.g. some 2 V). For the current a higher drive seems to help, for the on resistance mot very much beyond some 6 mA. I don't expect the slew rate to be that relevant. The analog meter and input filter are slow anyway. I don't see a need to be faster than some 100 ms to come out of saturation.

If the H11F1 really needs some 10 mA, I would looks for a different solution that does not get as hot. Photomos may be option, though a litte on the slow side (e.g. 0.2 to 2 ms).

Even it the circuit may tolerate some ripple in the supply, it is still worth having supply filtering. The LED current part (especially with unequal current) is in sync with the chopper signal and can thus effect the result. Mains ripple would be far less critical. Filtering is easy (just a resistor (e.g. some 100 Ohms)  and a electrolytic cap (e.g. 470 µF). There is anyway pelnty of voltage to loose on the series resistor.

The spectrum can change a little with intensity, but not very much. The color temperature of the white LEDs can make a much biger difference. From looking at LDR datasheets it looks like they are most sensitive for green to orange light, just the part of the spectrum where most white LED are rather week. So both a green and an orange LED could be an alternative even though the brightness to the human eye may be lower. The extra red and blue has mainly a negative effect in heating up the LDR and possibly causing thermal EMF problems (more warm up drift). The other point with high power is heating up the circuit and this way indirectly causing possible drift from the LED intensity changing with temperature.

Dave Wise:
Looks like I could fit in a 6.3mm through-hole cap; that's either 47uF/50V or 100uF/35V.   There's not enough room for 470uF.  I don't think 100uF is big enough to make a difference.

In the 740B, +/-22V don't have enough filter cap; we're stuck with out-guard supplies.  Insulate the board to 500V.  Use -42V or string it across +/-16.5V; these tolerate noise.  Either connection can use 24V or 9V LEDs.

The 741B doesn't have Guard.  Use -34V (24V LED) or +25V (9V LED).  These tolerate noise.

In the 419A, disable the neon driver (disconnect XA2 pins 9 and 11) to save on battery.  Use 9V LEDs, and string the new board across +/-13V behind an external RC filter.  Insulate the board to 500V.

In all cases, select the zener series resistor and the LED dropping resistors to get the appropriate current with your supply voltage.  In the 419A, pick a supply voltage, tune pulse widths on an external power supply, then adjust the R in the RC filter to get that voltage.

I tested two eBay H11F3's, one marked GE, the other Harris.  Both far exceeded H11F1 datasheet Figure 2 Output Characteristics current, and U3 would not need 16mA drive.  Put them on a curve tracer, with LED on Base and FET on Collector, and see how much you need to make the switch conduct 0.5mA .

I'll put all this in an Installation Guide, if there are enough people in the world interested enough to use it.

Dave Wise:
Boards ordered from OSHPark.

Dave Wise:
Assembling one to see if I need to fine-tune any mechanical interference issues.  The A16 meter-loop chopper assembly is near some HV points on the A11 board.

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