Power Supply Battery or SuperCap Backup

[cfernandez]

Hi,

I need to implement in my circuit a Battery o SuperCap for continue working for around 2 seconds (my circuit have 200mA) when the power supply is down.

Somebody can send me a schematic for this?

Thank you very much!

[Ttelmah]

We need to know a lot more.
What voltage does your circuit run on?. What voltage does it 'need' (for instance, if you are using a PIC that supports operation down to 2v, but are running at 3.3v, then the supply rail can fall by 1.3v, without endangering operation of this part of the circuit)?. What is the 'supply' voltage (if you are running from a AC source for example, which is rectified, then regulated, different solutions may exist)?. Is '200mA', the average consumption or the 'peak'?. What happens if the 'break', is longer than 2seconds?. Do you want 'powerfail imminent' signalling to the PIC, to allow a clean shutdown?. How frequent are the breaks likely to be (affects how fast the circuitry must recover)?. How much surplus current can be drawn when the power is 'on'?.

Best Wishes

[cfernandez]

1) We have 5v on circuit
2) PIC is 5v
3) Supply is external DC 5v regulated
4) 200mA is the normal consumption
5) With 2 seconds is ok for me.
6) I want to know in the PIC when I have not external Power Supply
7) We work with information that we need save before, the frequent is too small but I need have this.

[Ttelmah]

What is the minimum voltage the circuit can run on?.
A'5v PIC', can run below 5v, depending on the clock frequency being used.
What surplus power is available?.
You need an idea of the likely repetition frequency of failures.

If you absolutely have to maintain 5v, the circuit is going to be relatively complex. You would need an internal 5v inverter, generating a higher charging voltage, to feed the battery, or a inverter from a lower voltage battery to develop the 5v. To change over, you would also need to use FET switches, end even then there will be some slight change in voltage as they switch.
Funnily enough, the easiest way, would probably be to build a charging circuit to feed perhaps a 3v rechargeable battery, and then an inverter circuit, to permanently run the main circuit from this. Your monitoring, then becomes a simple case of testing for the presence of the incoming 5v, and there is no 'switchover' as such.
Supercapacitors, are really only suited to circuits that can accept a wide range of input voltages. Their characteristic, is simply to have the output voltage droop linearly with time/load, so if the target circuit can handle a wide range of operating voltage, they can be ideal, but otherwise they are not very suitable.

Best Wishes

[Humberto]

Without knowing your project I can infer that 200mA of consumption dissipates mainly in peripherals.
While the real necessity is to maintain the voltage stable by a short period - after a fault in the power - to protect data only in the related components . In order to get this, the most suitable solution it is to separate the consumptions and to maintain the voltage solely in the sensitive components, is to say microcontroller and memory.


saludos,

Humberto

[Ttelmah]

A good point.
I must admit from the original post, I originally felt he wanted to keep the system fully 'alive' for 2 seconds, rather than just save the data. The latter posts help a little in understanding the 'target', but still do not give enough data to really work out a route.
As an example, it'd be far easier if the sytem had access to the unregulated DC in, and had it's own regulator. It'd then just be a matter of switching, when the incoming DC supply approched the point where the regulator could not maintain operation. Combined with a circuit/software to turn off the peripherals, it becomes 'easy'.

Best Wishes

[kender]

It might be benefitial to use the LF series pic, because it can work at a lower voltage. Also, it might be useful to power the PIC only with a supercap. The follwong topology might work for you.