Ratch

Just kind of thinking about something...

Would it be fairly easy to build a circuit that switches between battery power and line power when line voltage anomalies are detected? Like a UPS, but for a 9-volt wall-wart and battery?
Getting more complicated I'm sure, but I was also thinking of putting a 9-volt rechargable battery as the "ups", and have it recharge when not in use and under normal line voltage.
Would the recharging process change the voltage? So once the line comes back to normal and input power is switched back to it, the charging process will throw it back out of whack until it's recharged (creating a catch22).

Here's the background:
I have wireless security cameras around my house and they recently started frequency-drifting sporadically. I *think* I've traced it down to our air conditioners (and the rest of the neighborhoods a/c's) changing the incoming line voltage, causing the camera power to change slightly, resulting in slight frequency shift. They're cheap little cameras, so input power affects frequency. The shift is just enough to cause annoying lines on the screen, which fools the software into thinking it's motion and constantly recording...

Nigel Goodwin

All you generally require is a couple of diodes, and a resistor - it's VERY simple, and 'switches' instantly.

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Ratch

How would the circuit be selective between sources? Through whichever one is higher?
I can see arranging the diodes so as not to feed line voltage to the battery and vice verse, but I can't seem to grasp how to make it selective. Can you elaborate a little?

Hero999

Here it is, it's nothing complex as Nigel said.

If you're worried about the battery discharging back into the wallwart then add another diode on the input.

If you really want a steady voltage then add a voltage regulator on the output.

Attached Images

Battery backup.GIF (1.7 KB, 24 views)

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Ratch

edit: Nevermind, just realized my mistake...

What is the resistor for? It seems like the circuit would also work if the resistor was just an open. Does it smooth out the power or something?

Ratch

What about using Zener's on the line input? So all incoming voltage would simply cease when it drops below the zener value.
Is that practical?

Hero999

The resistor limits the current into the battery, if it was short circuited the battery would overcharge and explode, it it was open circuit it would never charge.

Why would you want to do that?

The circuit starts running from the battery when the input falls 0.7V below the battery voltage.

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Ratch

ah, I see. I didn't realize that circuit was including the rechargable battery, I thought the rechargable idea was ignored for the moment.

I figured a 9-volt zener would prevent both sources from being "on" at the same time, but I guess that really doesn't matter.

on1aag

Hi Ratch,

If you want a more sophisticated solution check this out:

http://www.intersil.com/data/fn/fn3183.pdf

on1aag.

Nigel Goodwin

It's in the wrong place though! - it's better across the bottom diode, that way it doesn't provide a discharge path back through the charger circuit when the power goes off. It may make no difference, as the charger may just be coming from a bridge rectifier? - but it does no harm across the diode, and most likely will be advantageous!. Certainly though, without knowing the exact circuit of the charger, it makes sense to put the resistor across the diode.

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Hero999

I agree.

It seems so obvious I don't know why I didn't think of it.

I've modified the original schematic.

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Gayan Soyza

I don’t know what type of battery you use.

Its better you ignore the recharging unit for the time being.
Because battery charging is a different wide subject. Different circuits needs to charge each battery types (chemistries).
Due to unavailability of NICD now widely use NIMH. It needs complicated charging circuits.

If charging required SLA can use for the circuit in its STANDBY VOLTAGE specifications with continuous charge. But SLA battery voltage comes in 6V & 12V series.

If no charging required the simple diode can do the job to shift to the battery backup.

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Nigel Goodwin

Not really much different to NiCd? - and this same circuit will work perfectly with either - the resistor is only for trickle charging the battery.

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Gayan Soyza

Will see how long he will use his battery even with a trickle charge current through this 50 cent resistor continuousely?

Reducing battery life is a good idea! Otherwise no business for the battery manufactures.

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CheapSlider

In this application the circuit given is quite adequate, the resistor serves as a charging current limiter for the battery.

NiMH cells are very similar to NICd and are supposedly less prone to 'memory' effect.

I'm wondering if you're thinking of Lithium Ion cells where this would be true- Lion cells do need a more complex charging circuit.