replace a 9v power outage backup sorce

[tuneup]

I have an older water controller for a sprinkler system. It's working voltage is 24vAC that is quite normal and common. The electronics portion has a 9v (DC) battery supply that keeps it's memory and program intact during a power failure.
Most of our power failures are nothing more than a momentary bump whose duration lasts from less than a second to less than a few minuets. Without the 9v supply, the memory is lost.
I usually don't realize that battery is dead until it's too late and the memory is gone.
Since it is older, the programing is more difficult than the newer versions. It's a pain to reprogram.
I would like to build some sort of a replacement supply for the battery that would last for a few minuets of power failure. One that is powered off of the 24 v main supply. Something that would replace the battery for those short intervals of power loss.
I would imagine that the electronics would work with voltages from 6v to 12v. The controller can't be that picky to require a constant 9v.
I'm not an Electronic engineer, but I do understand circuits, especially if they are explained.
If anyone has some simple ideas or circuits, I would really like to give them a try.
Thanks Rex

 

[Bluejets]

9v seems a high level to keep memory however, one can buy a rechargable 9v unit and you would need to build/buy a small trickle charger.
Thing is, you are adding more links to the chain of failure points.
What is the 24v, AC or DC..??

Simple alternative, memory battery could be like smoke alarm batteries, replace on your birthday.

 

[Harald Kapp]

Use a 9 V NiMh rechargeable battery.
As the battery is rarely used and not for a long time, use trickle charging with a low current to keep the battery alive (of course, you'll have to insert it fully charged in the first place).
A typical 9 V rechargeable battery has a capacity of 200 mAh and can be trickle charged with 6 mA ... 10 mA.
You do not need a fancy circuit, a diode and a resistor are good to charge the battery from 24 V (DC). Add azener diode if you wish for added protection:

The 2 k resistor limits the current to ~ 7 mA. Any resistor between 1.5 kΩ and 2.2 kΩ should be practical, whatever you can lay your hands on.
The 24 V you'll have to find somewhere within the controller, usually near the power supply input.

You should still check the battery at least once a year to verify that it is still in good working order.

 

[tuneup]

Use a 9 V NiMh rechargeable battery.
As the battery is rarely used and not for a long time, use trickle charging with a low current to keep the battery alive (of course, you'll have to insert it fully charged in the first place).
A typical 9 V rechargeable battery has a capacity of 200 mAh and can be trickle charged with 6 mA ... 10 mA.
You do not need a fancy circuit, a diode and a resistor are good to charge the battery from 24 V (DC). Add azener diode if you wish for added protection:
View attachment 51661
The 2 k resistor limits the current to ~ 7 mA. Any resistor between 1.5 kΩ and 2.2 kΩ should be practical, whatever you can lay your hands on.
The 24 V you'll have to find somewhere within the controller, usually near the power supply input.

You should still check the battery at least once a year to verify that it is still in good working order.

Thanks for your help. I like this simple circuit, it will fit easily in the box. but I do have one question, it appears that the circuit is doing a half wave rectification. but you have stated that this circuit works well on a 24 (dc), not (ac) supply. I don't know whats inside the box, but I do have easily access to the 24 (ac) connections for the system supply. I could imagine how it could work with AC power, something like my old Yamaha Motorcycle did in the 70's on a 6 v system. It worked with just one diode, but didn't ever have much battery life and the headlights needed to run directly of the magneto.
Will this circuit still function off of the AC supply? Does it need any type of capacitor to smooth the wave out?
Thanks again for your help. This will be just as satisfying to build as well as functional to accomplishing a task!

 

[AnalogKid]

If the incoming 24 Vac is full-wave rectified internally, there could be a "common ground" issue with the half-wave circuit above.

ak

 

[Harald Kapp]

If the incoming 24 Vac is full-wave rectified internally, there could be a "common ground" issue with the half-wave circuit above.

Huh?

Correct if the circuit were attached to AC, but it isn't.
My circuit is not intended to be attached to AC, but to the 24 V DC side (I noted that in my description and also note the "+" and "-" signs on the input). The diode is in place only to prevent current flowing from the 9 V battery back to the 24 V power supply side.
Ground (0 V, "-") needs to be common.

As for connecting the circuit to AC, whether that is possible depends on the circuit within the controller. Possibly not ,as @AnalogKid rightly noted.

 

[AnalogKid]

I'm confused. Posts #1 and #4 state that the external power source is 24 Vac. If that is the only connection point for powering something that then interacts with the internal electronics, then ...

ak

 

[Harald Kapp]

I assumed (I have to admit implicitly), that the "charger" would have to be connected internally of the controller box, where access is possible to the DC supply. I take it from the op's description that the battery is mounted within the controller box (I have a radio that works exactly in that manner).

 

[tuneup]

If the incoming 24 Vac is full-wave rectified internally, there could be a "common ground" issue with the half-wave circuit above.

ak

thanks for the heads up.

 

[tuneup]

Huh?

Correct if the circuit were attached to AC, but it isn't.
My circuit is not intended to be attached to AC, but to the 24 V DC side (I noted that in my description and also note the "+" and "-" signs on the input). The diode is in place only to prevent current flowing from the 9 V battery back to the 24 V power supply side.
Ground (0 V, "-") needs to be common.

As for connecting the circuit to AC, whether that is possible depends on the circuit within the controller. Possibly not ,as @AnalogKid rightly noted.

OK. I think I've got it this time. Open up the electronics and find the full wave rectified ac to dc point to connect to, and hopefully it's also buffered to make a smoother signal. Then use that as the power source for your add on circuit. Connect it in parallel with the existing battery connections. Wala! We now have something that I don't have to think about much and cuss at so often!
If I'm still not correct on this, I welcome the help!

 

[AnalogKid]

Open up the electronics and find the full wave rectified ac to dc point to connect to

If you post a photo of the pc board,we probably can point out where to connect.

ak

 

[Bluejets]

I foresee smoke on the horizon......

 

[tuneup]

If you post a photo of the pc board,we probably can point out where to connect.

ak

wow! I thought we had it, but looking inside I cannot even see anything that resembles a full wave rectifier! there are only 2 diode present on the power board. the 24vac comes in , through a fuse and straight to the black lead of the battery connector. the other side of the ac goes through 2 large 12 ohm, 5 watt resistors in series, a diode, and into a capacitor make up.
I've got photos of both sides of the board, but get an error every time I try to load one of them.
I'll connect up the power and take a few measurements with the battery out and get back with you.
thanks so much

 

[AnalogKid]

The incoming AC might be half-wave rectified. If so, then grounding the charger circuit is not an issue.

ak

 

[Bluejets]

I've got photos of both sides of the board, but get an error every time I try to load one of them.

Photos are likely too large.
Many online facilities available to reduce image size down to that recommended.

 

[Harald Kapp]

Less than 300 kB per image usually works well.

 

[Tha fios agaibh]

I have an older water controller for a sprinkler system. It's working voltage is 24vAC

Yes, 24Vav is available which drives the water solenoids but you also have your line voltage on the primary side of the 24v control transformer.

Why not just use an AC adapter or buy a 9v battery charger? Its likely a cheaper option than buying the parts to make a dedicated charging circuit.

Another idea is a voltage monitor that would beep when the voltage gets low to alert you to replace the battery.

On most controllers, the battery backs up the program for a considerable time unless the battery is completely exhausted.
It's possible that a electrolytic capacitor has gone bad that normally sustains the volatile memory.

 

[Bluejets]

As I said previously, it seems out of the ordinary to use a 9v memory backup.
Usually one find a 3v lithium coin battery doing the job.

 

[Tha fios agaibh]

As I said previously, it seems out of the ordinary to use a 9v memory backup.

Usually the 9v battery controls just the display and its processor.
On my controller (Hunter ICC) it will retain the program if it takes a few minutes to hunt for a replacement 9v battery. If left out longer than that, it will lose its ever loving mind.

I agree, there's likely a lithium battery somewhere.

I don't know if the op is willing to tear it apart to try and replace it.
I would guess it's on the board with the display.

As for myself, changing the battery once a year works fine.

 

[Harald Kapp]

it seems out of the ordinary to use a 9v memory backup.

This is "an older water controller". Before the ubiquitous use of CR2032s the use of 9 V blocks was quite common (I'm old enough to remember ). My radio has one.

I agree, there's likely a lithium battery somewhere.

Why should the controller have 2 backup batteries? a 3 V lithium and a 9 V block. Not logical in my view.

As for myself, changing the battery once a year works fine.

That requires some discipline that @tuneup obviously lacks:

I usually don't realize that battery is dead until it's too late and the memory is gone.

I don't know if the op is willing to tear it apart to try and replace it.

@tuneup : time to go to work and look what's actually inside the controller to stop us from further wild guesswork.