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Super capacitor to make 24v

raman00084

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I have 5.5v 1 farad super capacitors
I must get a 5sec back up time after power cut.
I am using a 24v smps my load is a HMI device it operates from 12 to 24v dc operating current is 500ma max at 12v
How can I connect the 5.5v 1farad capacitor to my smps how much capacitors and what connection I must use to get 24v from 5.5v capacitor.
Kindly help me out
 

wwfeldman

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questions:
1) the 24V smps - i assume that is the output - what is the input?
2) what do you mean by a power cut? - does the input (to the smps) become open ? or short?
or so you mean the smps stops working, regardless of the stae of the input power?

first try at a simple solution:
4 capacitors at 5.5 V in series is 22V (at best)
if you use 5 capacitors in series, you get 27.5V - a bare minimum for a 24V system

5 second backup approximation:
500 mA at 12 V for 5 seconds, requires a capacitance (C = I * dt /dv or 0.5 A * 5 sec /12 V) = 0.208 F
the 12 V is from the 24V start to the 12V end, the voltage change of the capacitor
i assumed a constant 500 mA, the current at minimum votage
what current do you draw at 24V?

so you need about 0.2 farads worth of capacitance
that's 5 one farads caps in series

what is the series resistance of your supercap?
that forces an RC time constant into the mix which I have not considered.

I have never used a supercap. In fact the last time I looked at them, about 30 years ago,
they were only good for holding up memory because of the high series resistance and the
resulting long charge times. (100 ohm for a 0.5 F cap, if memory serves, for a 50 second time constant)

you might want to add some large resistors in parallel with the caps to force voltage balancing,
but not lose too much energy

these caps would be wired in series and placed across the output of the smps
 

KlausST

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Hi,

The super capacitors in series need some kind of "voltage balancer" else they may drift over 5.5V in time...

Also the "cheap" ones are made for very low load current...with high internal series resistance. They are not made to drive 500mA.
You urgently need to read it's datasheet.

Klaus
 

thunderdantheman

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Slightly different approach to wwfeldman.... but same questions... and agree the question isn't quite clear.

questions:
1) the 24V smps - i assume that is the output - what is the input?
2) what do you mean by a power cut? - does the input (to the smps) become open ? or short?
or so you mean the smps stops working, regardless of the stae of the input power?
How I interpret it.. You have a HMI device powered from a SMPS. The HMI device can run from a supply of 12 to 24V. At 12V the HMI device will draw a max of 500mA.

It's more than likely the HMI device will have it's own SMPS internally that allows such a wide operating voltage range. A SMPS is a constant power device provided the load remains constant which we'll assume it does.

The HMI in that case consumes 6W. Can use the capacitor equation for constant power...

C = 2 ×P ×t /(V0^2ーV1^2)

V0 -> is charge voltage V1 -> discharge voltage

C = 2 ×6W × 5s /(22^2ー12^2)
C = 0.18F

So 4 in series should do the trick nicely...

In fact... your 4x1F super caps in series will give you a run time of approx.

t = 0.5×C×(V0^2ーV1^2)/P

t = 0.5×0.25F×(22^2ー12^2)/6W

t = 7 seconds

However... it's not quite as easy as it sounds... you will need something that can charge them.. you can't just string them across your output.. it will short it out when the power returns.. the caps will be flat and want to draw as much current as they can. Even with some tricky diode ORing and current limiting resistor you can only charge 4 in series to 22V.

You could potentially do somthing like this using 5 super caps in series... They won't be fully charged.. but will still give you about 7sec as they will be charged to 24V.

scap.PNG

D1 may not be needed... just stops backdrive into the unpowered SMPS.
D2 prevents the SMPS from dumping all it's got into flat super-caps and likely tripping it out..
You can play around with R1... it basically will charge them up from dead flat in 11sec use a max of 400mA... All diodes can be 1N4001 rectifier...

One point to note.. my calculations above don't take into account the 0.7V diode drop... but it should still be OK.

EDIT - I did neglect to mention balancing... A 1k resistor across each cap should give reasonable balancing if the caps are all the same and of reasonable quality.


I have never used a supercap. In fact the last time I looked at them, about 30 years ago,
they were only good for holding up memory because of the high series resistance and the
resulting long charge times. (100 ohm for a 0.5 F cap, if memory serves, for a 50 second time constant)
They're pretty amazing now... you can pull 10s of amps out of even basic ones and charge them even quicker! As you move up you're talking 100s of amps... their biggest pitfall is getting them in high temp.
 
Last edited:

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