OK ... long story but I have been asked to duplicate a box that monitors air flow in a pipe.

I mainly deal with industrial electrical circuits (PLC's. starters, VFD's ...) not electronics.

Electronics is just kind of a side hobby.

They guy who designed and built these boxes (there is a few around the plant) has long since retired and passed away .. so no asking him for help.

The circuit itself looks simple ... the first regulator powers the entire unit ...12 volts. If air flow is lost, the relay that is always energized drops out and the beeper goes off.

The second regulator maintains 9 volts to keep a 9V NiMH battery charged.

The battery sets off the beeper and LED if power is lost.

Looks like they are using R4 to limit the charging current.

So .... two things I just don't understand ...

First ... there is a resistor (R2) with no value and the note "optional". It jumps around a diode that is preventing the 9V from back feeding he circuit. What is it for? Why is it optional? If I need it, what should the value be?

Second .... the battery (looked up the specs) is listed as "rated voltage 8.4V ... max charge voltage 10.5V"

If they are getting 9 volts out of the second regulator ... and that is dropping by .7 volts going through the diode ... is 8.3volts enough to charge the battery? I see VR1 is a variable resistor so maybe it was meant to set the voltage at the battery to 9V or ???

Looks to me like the "jumper" is a kind of test button on the board ... might be easier to put a NC switch ... maybe even an external one to make it easy for someone to test ... hmmmmm ... and maybe a button to break power to make sure the battery is good?

All I was told is they need a few because the oldest ones were "failing" .... I saw a picture and it does not seem to be the batteries as someone has replaced them over the years ... they through out the failed ones so I don't know what failed!

Any advice would be much appreciated !

Opps .... forgot ... just as a side note, I am sure this has been used and reworked many times over the past 30 or so years. I see a place for a six pin dip socket on the board that is not being used (maybe for testing at one time) ... and three long traces from one end to the other that are not used.

 

The only thing I can think of for R2 is that it provides a trickle-charge current to the battery when it is fully charged.
The value would be determined by the value of trickle-charge current you want.

Yes, VR1 would be adjusted so the voltage would properly charge the battery.
9VDC is likely just a placeholder voltage value.

 

The only thing I can think of for R2 is that it provides a trickle-charge current to the battery when it is fully charged.
The value would be determined by the value of trickle-charge current you want.

Yes, VR1 would be adjusted so the voltage would properly charge the battery.
9VDC is likely just a placeholder voltage value.

Thanks ... I though R4 ... just after the diode in series with the battery ... would set the current (looks like about 9 mA which is about right as "standard" charging is listed at 15mA ... max 150mA for 1.1 hours)

Just not sure why you would have a "bypass" for the diode.

Mike

 

Thanks ... I though R4 ... just after the diode in series with the battery ... would set the current (looks like about 9 mA which is about right as "standard" charging is listed at 15mA ... max 150mA for 1.1 hours)

Just not sure why you would have a "bypass" for the diode.

Because the trickle-charge current is smaller than the charge current and the trickle charge current would then be approximately the diode voltage drop divided by R2 after the battery reaches its charged voltage at the diode output as determined by the voltage setting of REG 2.

 

Because the trickle-charge current is smaller than the charge current and the trickle charge current would then be approximately the diode voltage drop divided by R2 after the battery reaches its charged voltage at the diode output as determined by the voltage setting of REG 2.

Got it ... so say I use a 1K resistor ... and the voltage drop across the diode is .7V ... I would have a trickle charge of .0007 A .... .7 mA.

Since they don't list it on the data sheets I am looking at .... is there a optimum charging voltage? They list drop off voltage as 7 .... rated voltage 8.4 and max charging limit 10.5 volts.

Thanks so much !!!!!!!!

 

What is th battery capacity C in mAh? Trickle charge for NiMH is C/20.

 

What is th battery capacity C in mAh? Trickle charge for NiMH is C/20.

The original batteries were ELB and had a capacity of 280 mAh. I saw the replacement ones are 200 mAh.

So somewhere between 10 and 14 mA?

The 1K resistor in series with the battery already limits the current to 9mA if I am right ... so maybe that is why this resistor is optional ... or am I missing something?

Maybe no need for a trickle charge or ????? They are more less on trickle all the time ... which makes sense at they are only used if the power fails.

 

Yes, the trickle-charge resistor is probably not needed.

 

Yes, the trickle-charge resistor is probably not needed.

WOW ... long way around but at least I know now why they had it listed as "optional".

I didn't want to build anything I didn't understand.

So, time to start layout out a PCB. I did this a few years ago using PBC123 ... probably not even in business anymore. I see now all the vendors have moved to China.

Should be a good little project.

It is funny ... the company finds all kinds of vendors for the big jobs ... everyone wants the ones that pay $100K plus. No one wants the little ones. I do all kinds of things like soldering patch cords with Amphenol connectors on the ends .. or a simple push button station with three buttons ... even something as simple as making labels. Each may not pay a lot ... but money is money ... and when added together it works out.

Thanks!

Just FYI ... this was my first "professional" board. 16 SPDT switches and 12 tri color LED's .. into two DB25's. The guy before me tried to do it with individual wires .... and failed. I then got the job .... figured a PCB was the way to go ... and it worked!