# Use of electrolytic capacity as very short term battery/voltage smooting device

#### jgplarnold

Joined May 16, 2020
19
I have a modem/router which occasionally freezes requiring rebooting which I do by disconnecting the power (12v from plugpack) and then reconnecting.

According to something I read this can happen if the supply voltage fluctuates over a short period of time.

I am wondering if this occurs when my solar inverter automatically switches from stored energy from the solar battery to main power (when the solar battery becomes depleted) and if so could connecting a large electrolytic capacitor between the power supply and the modem/router correct this?

#### LowQCab

Joined Nov 6, 2012
2,904
Well, it won't "correct" the Power-problem,
but if it's large enough,
it will probably prevent the very short-term loss of AC-Power
that is causing a glitch in your Routers operation.

However, the best solution is a small UPS.
You can probably get a small UPS for the same cost as a huge Electrolytic-Capacitor,
and, it will have Spike-Protection built-in.
.
.
.

#### DickCappels

Joined Aug 21, 2008
9,615
Just to be clear, the battery would go from the positive (assumed positive) power input terminal to ground (the negative power input terminal), in other words across the power supply input.

Depending upon the design of your power supply (wall wart) you may need a diode between the power supply and the router to keep the power supply from discharging the capacitor when power is interrupted. It is unlikely that the diode will be needed.

#### jgplarnold

Joined May 16, 2020
19
Thanks
I cannot find any (small) UPS for less than $A80 (on Ebay and my local electronics store) so I think that I will just put up with the occasional problem. Unless a say an 8000uf 40V electro at A$15 would be ok or maybe less?

#### jgplarnold

Joined May 16, 2020
19
Thanks Dick. Maybe also a resistor in series to prevent a large load on the plug in power supply on startup?

#### WBahn

Joined Mar 31, 2012
28,172
Thanks
I cannot find any (small) UPS for less than $A80 (on Ebay and my local electronics store) so I think that I will just put up with the occasional problem. Unless a say an 8000uf 40V electro at A$15 would be ok or maybe less?
It comes down to three things:

1) How much can you tolerate the 12 V dropping. If it stays above 10 V, does everything continue to work? Or does it need to be above 11 V? Or what? This is critical.
2) How much current does the router draw? You need to figure out what the peak current requirements are, since you can't assume that the router won't be handling a lot of traffic at the time the power hiccups.
3) How long does the cap have to sustain the power before it's restored.

Let's call the answer to #1 deltaV, the answer to #2 Imax, and the answer to #3 T.

The amount of charge that you cap needs to provide is deltaQ = (Imax*T).

The relationship for a capacitor is Q = CV, so deltaQ = C*deltaV

Hence, the minimum capacitance you need is

C = deltaQ / delta V = (Imax * T) / deltaV

Just throwing some rough numbers at things, if your router/modem uses 12 W, then that's 1 A of current at 12 V. If you can tolerate a 10% drop in supply voltage, that's 1.2 V. If your power needs to be supported 2 s, then the minimum capacitance you would need would be:

C = (1 A * 2 s) / (1.2 V) = 1.3 F

Then you would probably want to consider choosing one that is at least about 50% larger to give yourself some good margin.

As you can see, even if my numbers are overly pessimistic, you are going to be talking about a MUCH larger cap than 8000 µF. To get down close to that size, you'd have only draw 3 W, tolerate a 3 V drop, and only need it to sustain for a tenth of a second.

One thing you might consider would be to use a very small lead-acid battery that can be float charged and then power your router/modem from that and just leave a tiny trickle charger on it.

#### jgplarnold

Joined May 16, 2020
19
Wow. What a great explanation which I thank you for. As the power supply is rated 12V 2A I think I will give up on the idea of trying to use a cap.

#### WBahn

Joined Mar 31, 2012
28,172
Thanks Dick. Maybe also a resistor in series to prevent a large load on the plug in power supply on startup?
Putting resistors in series with power supplies is usually a bad idea. There are a number of undesirable consequences that you are setting yourself up for and it often turns out that the largest resistor you could use and still avoid them is too small to accomplish what you wanted to do in the first place.

#### Ian0

Joined Aug 7, 2020
6,983
Run your router off a small lead-acid battery, and keep it on float charge at 13.6V.

#### Ian0

Joined Aug 7, 2020
6,983
Wow. What a great explanation which I thank you for. As the power supply is rated 12V 2A I think I will give up on the idea of trying to use a cap.
A contactor, even a large one rated at 100A, takes less than the length of a mains cycle to change over from mains to standby supply, so I would expect that 22000uF @16V would do the job.

#### jgplarnold

Joined May 16, 2020
19
That begs another question unrelated. When inverted solar power to fed back into the grid what causes the synchronisation of the alternating current wave form eg what if the inverted solar power is half a wavelength different from the mains?

#### MisterBill2

Joined Jan 23, 2018
14,230
To provide a backup with a capacitor is a very handy scheme because the capacitor will have a much greater lifetime then a battery. And also it will cost much less than a UPS. Capacitors for smoothing power are a rather basic building block and have been for many years. While the calculations say 1.3 FARADs, you may get away with less, depending on the power requirements at the time. A whole amp seems like quite a bit, although that may be the power-on surge current.

And about the question of how power fed into the grid is assured to be the correct phase, the answer is that phasing is a big part of the circuit. The phase angle is adjusted so that the power flow is into the grid, is the basic scheme. And that is straightforward but not very simple.

#### crutschow

Joined Mar 14, 2008
31,506
The actual current drawn by the router may be significantly less than the rating.
You might measure it to better determine what you need.

#### jgplarnold

Joined May 16, 2020
19
I used my plugin energy cost meter and it shows the router drawing between 6.9 and 7.1 watts (12 volts).
Any reassessment of a suitable electro cap?

#### Ian0

Joined Aug 7, 2020
6,983
I used my plugin energy cost meter and it shows the router drawing between 6.9 and 7.1 watts (12 volts).
Any reassessment of a suitable electro cap?
I'd start at 10000uF and if that doesn't work, put another in parallel.

#### jgplarnold

Joined May 16, 2020
19
THANKS

#### WBahn

Joined Mar 31, 2012
28,172
I used my plugin energy cost meter and it shows the router drawing between 6.9 and 7.1 watts (12 volts).
Any reassessment of a suitable electro cap?
The big question is how long it needs to hold the voltage. How sure are you that the issue is when the switch from solar to line power happens? Can you force a switch to determine if that's actually what is causing the problem? If not, there's a possibility that you are spending time solving a problem that doesn't exist and ignoring what the real problem is.

#### crutschow

Joined Mar 14, 2008
31,506
Below is the simulation of a 10,000μF (10mF) capacitor with a 2A load:
As you can see, it provides perhaps only about 20ms of viable power to the load, so you likely will need a larger capacitor, if a voltage drop is the problem.

#### Ian0

Joined Aug 7, 2020
6,983
I also have suspicions as to what is causing the problem.
A 12V 2A switched-mode power supply would have over 1J of stored energy, and is likely to keep operating until the supply sags as low as 90V. That should in itself keep a 7W load going for 200ms, which is longer than it takes a 100A three phase contactor to change over.