Battery Backup Integration

[madhippiescientist]

I have a circuit that draws 600ma from a 10VDC wall adapter, and a wall
adapter (12VDC 500ma) that drives a battery charger. I need to keep them
isolated (for the most part the positive, but I would like to keep the ground
isolated as well to avoid a possibility of other problems). But I also need to,
when the A/C coming in fails, to switch the circuit to draw from the battery.
Obviously, the charger won't be charging at this point, and I know I'll need to
route the battery into a 7810 to regulate the circuit's supply, but I was just
wondering, how can I "switch" the supply to the circuit w/o a relay. Again, I
wish to switch both the negative and the positive, and I was thinking that I
could simply do it with a relay that will come on (from battery power) when
the circuit's wall adapter cuts out (before a reverse diode). I know I'll need
to place a big capacitor in there to keep my circuit running (cannot have any
cutout), but was wondering, is there a way to do this (DPDT the circuit's
supply from the wall adapter to the battery/regulator), electronically (w/o a
relay), and have no failure of voltage supply to the circuit? Is it going to be
a TIP41c and a TIP42c, as I've recently learned about? Any ideas...?

 

[MikeMl]

I would just power a DC coil, DPDT relay with either wall-wart (which ever has some reserve ampacity). If the AC goes off, the relay will drop out. Wire the relay contacts to transfer both the negative and positive poles of the battery to the appropriated place. Put the appropriate snubber diode in parallel with the relay coil.

 

[madhippiescientist]

Kinda of exactly what I was going to do, but have the relay off and it come
on (circuit powered by battery) when a gate senses the dropout of the wall
adapter. That way, the relay won't always be on, and will last longer. I just
would like it to be electronically switched, I'm not a fan of relays when it
comes to them coming on once in a blue moon...

Any other suggestions, anyone?

 

[MikeMl]

I dont like AC relays on all the time, but DC relays left on seem to do just fine. Powering the relay off the battery will run down the battery faster.

 

[madhippiescientist]

I planned on using a DC relay (after a wall adapter), but I believe most of
them (low voltage DC "box" relays) only last about 20K hours, which if I leave
it on, the coil would fail in a couple of years. Since it only draws 100ma, and
the battery is 10AH, the main circuit about 600ma, I would likely have more than
enough power to last for hours anyways. How long, if you know, do these
type of relay coils last??? I'm just soooo worried about not being able to find a
replacement that will match my pinout, at a later date (even if it is YEARS away).
Which is another reason I would like to do it with semiconductors. What do you
think about a couple of sets of TIP41 and TIP42 transistors, one set for the float
charger's connection to the battery when "A/C" is present, which will cut out and
the other set will connect the battery to the circuit (more or less immediately), until
the wall adapter's DC signal becomes live again?? This way, with a LARGE capacitor,
I will also be sure the other circuits won't cut out but will continue uninterrupted
(which is a necessity), because I believe the relay may likely cause a LONG cut out,
when going off and/or coming on, and or spikes/surges which are also of concern.

 

[MikeMl]

I have never seen a spec that says a relay will last only X hours if left on! I have seen specs that say a relay is good for n switching cycles. How many power failures are you planning on having?

 

[madhippiescientist]

Maybe a couple of power outages a year. I understand leaving it on and
trying to find a replacement may not be a big deal, but my most significant
piece of all of this, is a stepper motor that MUST keep moving, and whenever
I apply power to it, I FREQUENTLY have to reset the direction selection transistor(s) base, (even though there is a start up delay integrated into that
circuit, it for some reason, about 1/3 of the time, will not start on it's own).
The motor drives a helicorder for a seismometer, so if the power goes out and
the motor doesn't start, the graph will be off time for the rest of the week.

Sorry if I seem to be a pain, but I do know how I could do it with
a relay. My question from all of this thread was,
how to do it w/o one...

...and I do appreciate your responses and input !

 

[Gary B]

First, I assume you are transformer isolated from the wall. Then the DC from the charger can just float across the battery. When power fails, there is no drop because the battery is acting like a UPS the whole time. Also, the charger can be diode isolated if the draw of the filter safety drain resistor is too much load.

 

[madhippiescientist]

The charger x-former supplies 500ma DC, & excluding the electronic voltmeter
and other little things (float charger/power failure circuitry), the rest of the
circuit draws over 650ma. That's why I have two seperate AC to DC
adapters, one at a voltage too low (10V1A) for battery charging, to supply
the circuitry that's always running, and the other adapter at a high enough
voltage for the float charger (12VDC500ma), but it
won't supply enough
current (if motor stalls it goes to about 1A). I was worried also about if the
charger is connected ("full" on w/o floating or with utilizing the float charger)
to the battery when the power cuts out, I wouldn't want voltage to go back
into the charger circuit and drain the battery or sacrifice circuit components
(I want it to disconnect when the battery connects to the always on circuit).
I also need the seperate voltage supply to get an
accurate battery reading from the voltmeter, since
the voltage will drop depending upon what the rest
of the circuit (stepper driver, servo controller) is doing.

What is a "UPS" ?

I understand diode isolation, but what is a "filter safety drain resistor" ??

 

[Gary B]

UPS is Uninterrupted Power Supply. Every Power supply that has filter capacitors also has a safety resister that acts as a small load to discharge them when it is turned off. Otherwise, there would be a shock hazard long after being turned off or disconnected.

Another thing to consider is what happens to the output under load. When the battery is dry, the voltage from the charging source will be low. As it charges and draws less current, the voltage will rise.

The way I read the above, I don’t know that it will work as expected. The 12 volt charger will attempt to both charge the battery and supply the load. I suspect that once you turn it on, it will load down until it matches the output from the 10 volt source, then they will load share. That means the battery will never receive a full charge if it is connected to the load. If it is not, there will be a switching delay and consequently a momentary drop upon AC failure as the battery is switched in.

This is pretty much saying you need to use transistor or FET switch and an output filter large enough to carry over the transition.

 

[madhippiescientist]

The 12 volt charger will attempt to both charge the battery and supply the load.

The 12 volt charger would cut out before the battery would supply the load
to the 10 volt circuitry. Here's a basic flowchart schematic of what I want
to do:

1.) A/C comes into the wall adapters. The 10V1A goes into the power
controller, and as long as power exists from this transformer, it passes
through a diode to the "relay i don't want", (and to the buffer capacitor
and then into the "always running circuit", a.k.a. "The Load").
2.) The 12V500ma adapter goes into a DPDT center-off switch, to either
connect directly to the battery (fast charge), be off, or connect to the
float charger (LM2941?). Both on choices are routed through a voltmeter
controller. The voltmeter gets it's power from this adapter, and will not
be powered when the A/C fails.
3.) The voltmeter controller uses two sets of TIP41 & TIP42 high current
transistors, which are selected via a long-term variable pulse astable
timer, which has the chargers connected to the power failure controller
for most of the time. When the astable goes into it's short duty, it turns
off the TIPs that route the charging into the power failure controller, and
turns on the TIPs to "sample" the battery voltage from the power failure
module (when A/C exists, the battery is connected to the charger supply
through the power failure controller). The voltmeter is an LM3914 bar/dot
driver in dot mode, that connects LM3914 outputs through an inverter to
specific resistors bussed into pin7 on a 50/50 astable timer, to flash a
single LED at a rate according to battery voltage.
4.) When A/C power fails, I want to disconnect the load from the adapter
and disconnect the battery from the voltmeter controller, THEN connect
the load to the battery. The voltmeter signal remains open (without A/C
power the 500ma adapter won't supply it power). The A/C power failure
module's circuit (senses the A/C presence from 1A adapter's DC supply),
which gets it's power from the battery, will continue to monitor the DC
adapter until supply returns (though the adapter is disconnected from
everything else). Hence, the "relay i don't want". The charger supply will
also shut down in the event of a power failure (no 12VDC coming in for it).
So when A/C fails, everything shuts down, and the load connects to the
battery, which also supplies the power to the circuit that senses when
A/C returns (so the battery doesn't "hold it" in outage mode). This is the
switching I want w/o a relay (the battery/charger & the adapter/load).