min NiMH charge voltage and extra parts

[xtal01]

OK ... I was given the job of duplicating a "simple" box that monitors air flow in a duct.

Seemed simple ... I dabble in electronics (I design and build industrial control panels .. starts ...plc's ... HMI ....)

I knew the guy who designed and built the originals about 25 or 30 years ago ... long since retired and passed

SO ... I found an old drawing we had ... hmmmm ... does not match the circuit.

I traced the board ....

1) R7 (drawn in red) is a 1.2K resistor not on the drawing but on the board. It is not something added in later as it even has artwork on the board for it. I don't understand what it is for ... just adds load (maybe needed as the relay only draws 30mA)?

2) Don't understand D6 ... D7 is for voltage foldback(flyback diode) but why D6 ?

3) R2 says optional but is installed on all boards. I don't see what it does as it bypasses D5 ... which is a problem as it backfeeds the yellow led (glows on even when power fails).

4) "see note 2" .... no note 2

And now my big question ....

Reading the data sheets, I see the LM317 (REG 2) is fed from the 12 volt regulator REG 1. The LM317 requires 3 volt differential to operate properly.

This means I can get 9 volts at best out of REG 2. After going through the diode D5 ... maybe 8.5 volts.

The battery is a 9 volt NiMH ... 8.4 rated voltage... 7 drop off ...10.5 max charging

I checked the units .... 7.24 volts at the regulator! After sitting a while (put in a new battery) the same voltage at the battery!

OK, so I know these are working ... and I know they have been used for years ... but how is the battery changing at only 7.24 volts?

The beeper is a sonalarm ....6-28 volts at 6-26 mA ... maybe if you charge the battery at only 7.24 volts it will stall have say 10% of it's capacity. So a 280 mA battery will still have maybe 28mA ... enough to keep the buzzer going for an hour (which is probably fine) ?

Just never heard of "under charging" .... the boss just wants me to duplicate this ... am I missing something?

 

[EnzoAus]

Xtal01,
Although only a past electronics enthusiast, in the absence of more learned responses, I think your review of the circuit is legitimate. Before replying to your questions I offer the following suggestions.
1. By all reports the "circuit is working" as "they have been used for years". Given that it is an alarm circuit which by definition should only be needed rarely, do you actually know that the battery is there when needed to drive the beeper? Your factual observations suggest there might be issues. If the many examples in service are just illuminating the "flow ok" leds, they might not actually be available to advise of an alarm if one occurs.
2. It would be useful to check the transformer secondary voltage, LM340-T output as well as the LM317 output voltages. Your concerns about inadequate LM317 output are possibly applicable to the LM340T as well. The datasheet I have suggests min (Vi-Vo) is about 2.5V. A full wave rectifier on 10Vac has 14.4V p-p. With two rectifier diodes in series, this is reduced even further before the input of the LM340T. Possibly OK if the transformer is only lightly loaded and the output is higher than 10V.

To respond to your specific questions.
1. R7 could be there to provide minimum current through the pressure switch if the jumper is removed. Won't do any harm if left. If jumper is always in, probably not needed.
2. D6 and D7 are about protecting the pressure switch contacts from inductive spikes (V = L * di/dt) from the relay when it is switched. D7 clamps the voltage across the relay to no less than Vd below the -ve rail and D6 clamps it to no higher than Vd above the positive rail. May or may not be necessary depending on switch sensitivity. Again, no harm if left in.
3. Agree with your comment on R2 - seems unnecessary if Vo of LM317 was correct and battery is charged properly and just compromises the illumination of yellow led as you suggest.
4. Maybe Note 2 was to say there might be a problem with the voltage on LM317

Final suggestion is that you might be able to improve things by connecting the input of the LM317 directly to the bridge rectifier filtered output (from C1), where the higher voltage will give the LM317 some more headroom. Check the Lm317 datasheet to see if you should add an output cap.

Hope this comment is not too late and is useful.