Backup battery

[tegwin]

I know this isnt directly related to PICAXE but im not sure where else to ask such a question...

The temperature sensing module im using to feed into my 28X1 (Requires a 1.5V supply) is programable for temperature limits, which is why im using it...

However, should the mains supply that im using to power the circuit fail (power cut etc), it looses the preset temperatures...most annoying!

I want to encorporate a 1.5V battery of some description (AA makes most sence due to the LCD that also needs power).

Is there a simple circuit I can build that effectivley disconnects the battery from the circuit until the main supply cuts out so that the battery is not drained during normal "powered" opperating conditions?

Im limited to about 500 milliamps from the input powersupply max...

I had thought about using a relay that closes the connection to the battery when power is removed but I fear this would be too slow and thus, the memory would be lost..

 

[Michael 2727]

A Diode and a 1 Farad Super Capacitor across
the supply of the thermostat.

The Diode prevents the Super Cap discharging
back into the OFF supply, if that happens.
But the Diode will have a Forward Voltage drop
of around 1V so you will need feed the input
with 2.5V to cover the drop.
You will need to carefully check the voltage
first, as many Doides differ.

The LCD thermostat probably draws very little
power so the cap should last a few mins to
half an hour or so.
Check the current drawn by the Thermostat
first.

PS: Most super caps are rated @ 5.5V and
don't like much above that, they can be
destroyed or even blow up.
Same goes for polarity.

 

[tegwin]

Thanks for that..

Im not convinved half an hour would be enough...There are situations that we are without mains power for up to 12 hours...

Could I use this?

I have increased the normal input voltage to 1.6V from the transformer, so this would be the normal supply...The 1.5V battery would then just sit there because it is lower voltage than the main input..

Only when the main input is less than 1.5 would the battery start to feed the circuit...

 

[leftyretro]

Thanks for that..

Im not convinved half an hour would be enough...There are situations that we are without mains power for up to 12 hours...

Could I use this?

I have increased the normal input voltage to 1.6V from the transformer, so this would be the normal supply...The 1.5V battery would then just sit there because it is lower voltage than the main input..

Only when the main input is less than 1.5 would the battery start to feed the circuit...

Have you considered the voltage drop across the isolation diodes? You may have to see if the actual voltage to the device while drawing from the battery will be enough to support the device. If it draws very little current you might consider the old germanium diodes that have smaller voltage drop then silicon diodes.

Lefty

 

[Michael 2727]

Simple enough to try, measure the current in
the battery, I think it may still go flat.

Post the results

They make Super Capacitors up to 100F FARADs,
prolly cost you an arm and a leg though.
And would take quite a while to fully charge up.

Edit -It's after 5.30am way past my bedtime,
and I didn't see Lefty's post before, and as he has
pointed out, you will lose nearly 1 Volt out of the
battery via the diode. I knew something wasn't right.

 

[MORA99]

You could use a relay for a simple (and big) solution.
When mains is on, you trigger the relay from the ac adapter to NO and connect the same power to one set of pins, then connect battery to the other set, when mains drop out the relay falls to NC and gets power from battery, not sure if the short delay is too long.

You will need a DPDT relay for that solution.

 

[tegwin]

Just tried it, and with a nackered old AA 1.5V cell im getting 1.06V on the LCD side of the diode...and it just about runs... switches faultlessly between main supply and battery...

Not sure how long the battery will survive just sat there not being used though.. But seems to do the job at the moment

 

[lbenson]

Don't know what diode you're using, but others have previously suggested that schottky diode voltage drop (such as from BAT85) can be around 0.2V for very low currents.

 

[demonicpicaxeguy]

a thought that ran through my mind is why bother with a external circuits and a backup battery , if you poked the correct registers you can change the internal oscilator settings and slow the chip right down therefore reducing the power consumption to incredible degrees
i haven't tested it yet as i don't have a 28x1 on hand but going off info from the datasheet there are 8 different oscillator frequecies you can use by setting the appoaproate bits in the osccon register
in essence
01110001 - 8mhz
01100001 - 4mhz
01010001 - 2mhz
01000001 - 1mhz
00110001 - 500khz
00100001 - 250khz
00010001 - 125khz
00000001 - 31khz

the osccon register is at 08f i think - don't quote me

you have to be carefull with it's use though because it you get it stuck on a non picaxe frequency you could have problems getting it back again..... i'd seek advice from technical before trying,

before doing it though

 

[modeng2000]

Why not just have a rechargeable battery. It could be as large as you need to give coverage for mains breaks and is a proven technique.

 

[manuka]

"Mains power down for up to 12 hours"?? Please detail! Is this while en route in the field or your normal supply? If the latter then a UPS could cover not just your need here, but other mission critical applications (such as your beer fridge!) Just teasing...

FWIW Both AA NiCds/NiMH & Schottky diodes may be robbed from the electronics of a solar garden lamp. In fact a lamp booster circuit will fully power a PICAXE OK too. Stan

 

[tegwin]

Chears guys...

The AA cell and 1n4001 diodes just about does the job....I will aquire some schottky diodes and give it another shot....

The power outages dont happen very regularly, but because of where we are they do happen and usualy last a fair while...



Im going to try the relay idea, but I suspect the time delay between power off and emergency power comming on might be too much...

 

[Michael 2727]

A Largish Capacitor 1,000µF should hold the
Voltage on the Picaxe/Thermo side until the
relay kicks in.
Note that any large capacitor on the Relay
side will delay the Relay dropout time also.
And most relays can hold ON with only around
1/2 the supply voltage once they are pulled in.