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Large capacitor for power outages?

PizzaCombo

Dec 24, 2013
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Hi all,

So I live in a fairly regional area and most times we get a thunderstorm there's often a short power outage/blackout (usually is quick, half a second at most). As a result, my router often gets shut off and on, and it takes a painfully long time to restart, connect to my ISP, etc.

For small DC appliances (such as my router) can I not just solder a large capacitor in parallel across the power and ground to keep it powered? Is there anything wrong with my idea, as long as the capacitor is above the DC voltage the router uses and has enough capacitance to power it through a very small power outage?
 
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Harald Kapp

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By "power and ground" you refer to the internal supply voltage of the router? Then in theory a very large capacitor could help. There are some possible issues, however:
- The voltage across the capacitor will drop with time because the capacitor is discharged by the router's electronics. One cannot foresee how the router will behave in this case.
- There is almost definitely a reset circuit to ensure proper power-up of the router. One doesn't know how this reset circut may be triggered by a loss of mains (known as primary power fail signal). So even if you keep the operating voltage of the router relatively fixed, the loss of mains may trigger a reset.

Assuming it is a switch mode power supply, a better way to overcome these issues is to increase the main capacitor on the primary side (mains) of the power supply. A change in voltage across this capacitor will be compensated by the controller of the power supply. However, you're dealing with dangerous voltages in this part of the circuit. Unless you're experienced, I recommend hands off.

Regardless of the placement of the capacitor, you will need a fairly big capacitor to bridge an outage of 1/2 second. At 10W, a few thousand microfarad can bridge ~100ms. For 500ms you'll have to use a capacitor in the millifarad range. Look up double-layer capacitors (known under various brand names like supercap, goldcap etc.).
 

Arouse1973

Adam
Dec 18, 2013
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Hi PizzaCombo
If your router has a power adaptor which most do then this can be done. Linear Technology do a wide range of super capacitor chargers and I am sure there will be one for your needs. Basically you need to choose one which will switch in the super capacitor when power is lost from the power adaptor.

In normal operation after the capacitor has been charged it will just sit there being topped up every so often until needed. When the power is lost from the input the super capacitor switches in and delivers power to the load. Some of the devices have a boost convertor built in so it maintains say 5Volts for as long as it can. You just need to choose the correct value of capacitor for the load and duration.

Now a super capacitor will draw a large current to start with hundreds of mA. After an initial period this will fall to virtually nothing. Some of the devices have adjustable charging current so you may not need to change your power adaptor. All you have to make sure is that the power adaptor can supply enough current to charge the capacitors and also the router. Bearing in mind once the capacitor is charge the only appreciable current will be going to the router as before.

Linear Tech do Demo PCBs of most of there circuits, I get mine for free but then I know the Linear reps for the UK and we spend quite a bit of money with them every year. Contact them and ask, just say your doing a project in electronics and would like to use there parts. Otherwise you might be able to buy one, then put it in a nice box and add the connections you need.
Thanks
Adam
 

(*steve*)

¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
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Another (possibly simpler) option is to have a battery (perhaps a rechargeable one) connected so that it takes over from the plugpack when the voltage on the plugpack falls.

Depending on the voltage from the plugpack and the sensitivity of your router to the input voltage, this might be quite a simple option. (To be simple, the plugpack voltage needs to exceed the battery voltage by some small amount, and the router needs to be insensitive to the input voltage falling a couple of volts).

If your power outages are really short, and relatively infrequent, you may even be able to get away with disposable batteries!
 

Arouse1973

Adam
Dec 18, 2013
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Another (possibly simpler) option is to have a battery (perhaps a rechargeable one) connected so that it takes over from the plugpack when the voltage on the plugpack falls.

Depending on the voltage from the plugpack and the sensitivity of your router to the input voltage, this might be quite a simple option. (To be simple, the plugpack voltage needs to exceed the battery voltage by some small amount, and the router needs to be insensitive to the input voltage falling a couple of volts).

If your power outages are really short, and relatively infrequent, you may even be able to get away with disposable batteries!

Good idea Steve. Yes a battery and an ideal diode is your best bet. I must of had capacitors on the brain for not to think of that one.
Adam
 

PizzaCombo

Dec 24, 2013
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Thanks for the suggestions guys. Some disposable batteries sound ideal.

But how would I make it so the batteries only provide power only after the mains power outage? Is there not more to it than just connecting a battery to the routers power along with a diode (and maybe a smoothing capacitor?)?

Otherwise wouldn't the router be constantly drawing current from the batteries, and by the time there is a power outage, they would have run out?

Would I also have to take into account forward voltage drop from the diode when choosing my battery voltage?
 

(*steve*)

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OK, the first thing is: What is the marked output voltage and current on the plugpack? And is itt AC or DC?

Second, what is the exact model router you have (many have 12V power supplies but run from 5V or even 3.3V internally so there is a lot of scope for a slightly reduced input voltage).
 

tryppyr

Oct 22, 2013
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Seems to me the more direct solution (and the one I used) was to plug the router into a surge suppressor with built in battery backup.
 

Arouse1973

Adam
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Seems to me the more direct solution (and the one I used) was to plug the router into a surge suppressor with built in battery backup.

Fair comment but what you have to remember is that people posting on this forum might want to try somthing themselves rather than just buying something.
Thanks Adam
 

tryppyr

Oct 22, 2013
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Understood. My thought was simply based on the assumption that one would pay as much for a large capacitor as for the battery backup unit described.
 

HellasTechn

Apr 14, 2013
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Understood. My thought was simply based on the assumption that one would pay as much for a large capacitor as for the battery backup unit described.

I agree.

A surge protector or even better a small UPS would be alot cheaper and reliable .
 

Arouse1973

Adam
Dec 18, 2013
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I agree.

A surge protector or even better a small UPS would be alot cheaper and reliable .

I doubt it, 2.5F super cap (2*5F 2.7V in series + free demo PCB if you blagg it = 5.92 GBP. A surge protector on its own wont hold the supply up. Can you send me the link for the UPS unit for less than 10 USD that you have seen. :)

Thanks
Adam
 

Thedarkb

Sep 7, 2013
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if i were you I'd get a bunch of ultra caps and run your whole house in the event of a blackout.
 

davenn

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if i were you I'd get a bunch of ultra caps and run your whole house in the event of a blackout.

no, not really practical, they dont work like that


Dave
 
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tryppyr

Oct 22, 2013
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FWIW, I have gotten UPS devices that cheap. Around here we have PC Recycle stores that resell used electronics, and the basic UPS type devices are dirt cheap. I picked up one for about $5 that I used on my cable modem. Can't send you a link, but would simply advise if you are really interested to look around for used units.
 

shumifan50

Jan 16, 2014
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You can buy a 'uninterrupyed power supply' UPS) fairly inexpensively for what you need. This has all the switching, charging circuits and batteries built in.
You could use 'super caps', but they are typically rated at 3.3v and require balancing circuitry if you need a higher voltage - they cannot just be treated as normal caps. They are also quite expemsive.
 
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