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### Topic: Capacitor choice question. (Read 934 times)previous topic - next topic

#### cjnewbs

##### Feb 26, 2013, 05:00 pmLast Edit: Feb 26, 2013, 05:08 pm by cjnewbs Reason: 1
Hi, hoping for a little help to choose the correct capacitor for my project.

I am modifying a remote control wall switch to be wifi enabled, using an electric imp.

I am trying to use the already present power supply in the wall plug which is 5v @ 25ma. The electric imp needs up to 400mA for a few seconds during startup, genrally max 250ma during transmission burst and during use between 10mA and 6µA.

Edit: I just remembered to add that the relay that controls the switch also will not latch when the imp is trying to start, thats another reason I thought of using a large capacitor.

In order to supply power during the high current draw times, I thought about using a high capacity capacitor just like used in the Adafruit Tweet-a-Watt, where a 10,000µF @6.3v.

The imp needs a 3.3V supply but the power regulator can take in anything up to 17V.

I did do electronics at school but can not remember there being so many equations to pick the right capacitor as I have found online, so am a little confused.

Looking online I see that capacitors have a pretty quick voltage drop curve.

So my questions are:
Can I assume the voltage drop on a large capacitor be ignored when such little current is being drawn, as it will not move far from the filled capacity end of the graph once charged?

I have seen caps have a max working voltage. Does this mean that caps begin to discharge at the voltage they were charged at, and then drop, or will they always begin discharging with a specified discharge voltage.

Based on my assumptions that a high-capacity, low draw circuit (once charged) output should be fairly steady, and voltage is same as charging would a component such as N99CN from http://www.maplin.co.uk/double-layer-backup-capacitors-98185 would do the trick, also as they are described as an alternative to backup batteries I would assume they are suitable.

If there are any good guides to capacitors you could point me to I would be grateful as I am keen to learn as much as possible. Since school I have done nothing electronics related until the last few months where I have been doing a couple of small projects.

Also I have found a capacitor charge/discharge time calculator, but it asks for circuit resistance, when is the best time to measure this, as the current draw changes when its on I would have thought the resistance also does.

I have noticed I have said assume a lot, where in fact I mean more of an educated guess, again any pointers, tutorials or suggestions would be really appreciated.

Many thanks.

#### MarkT

#1
##### Feb 26, 2013, 05:16 pm
Just a capacitor can't simply solve the problem - a beefier power supply is really needed.

Any capacitor big enough to prevent the voltage drooping at 0.4A for "a few seconds" will
overload the 25mA power supply for perhaps a minute while charging, which may or may not
cause damage (can't tell - the manufacturer of the power supply might know).

Furthermore the only capacitors big enough (several farads) are "super" or "ultra" capacitors,
which only handle about 2.7V or thereabouts.  They are tricky to put in series for higher
voltage as you need to ensure the voltage sharing is kept balanced.

You either want a power supply that can handle the peak current, or you want a battery
which can be trickle-charged from your supply (which means you'll no longer be able to run from
5V, but from the battery voltage).

A USB power supply gives 5V at 0.5A - just what you need...
[ I will NOT respond to personal messages, I WILL delete them, use the forum please ]

#### astrofrostbyte

#2
##### Feb 26, 2013, 05:42 pm
just a few idea's:
Those wallsocket RF switches normaly 'steal' power from the mains using a 'transformerless Power supply' and found a bit of background on this in an Application not of Microchip AN954.  But I'm not sure how far you can scale-up such a supply.

I have also been tinkering with a RF-Mainswitch and wanted a tranceiver so i could also read back if a switch is On/Off.  I used a few of those 2.4GHz (nrf24L01) modules here that are extreme low power. made to run on a Coin-Battery.

#### cjnewbs

#3
##### Feb 26, 2013, 05:47 pm
SKU:N99CN on http://www.maplin.co.uk/double-layer-backup-capacitors-98185 is rated for 0.1F at 5.5V. If I were to trickle charge this, would that then work? As you said earlier I need to not overload the built in power supply so if I were to put the cap with a 200? resistor to make sure the max current drawn at 5V is 25mA would this be correct, or are you suggesting a battery due to the dropping voltage during current draw.

#### Grumpy_Mike

#4
##### Feb 26, 2013, 05:54 pm
Quote
I have seen caps have a max working voltage. Does this mean that caps begin to discharge at the voltage they were charged at, and then drop,

maximum working voltage is the maximum you can charge it to. However if you only charge it to half that, that is the voltage it will start discharging from.
Quote
or will they always begin discharging with a specified discharge voltage.

No that would break the second law of thermodynamics and get something for nothing. A capacitor is a jar that holds what you put in. The jar leaks slightly.

Quote
Based on my assumptions that a high-capacity, low draw circuit (once charged) output should be fairly steady,

No. The output voltage is determined by how much charge you have in the capacitor, current is a flow of charge.

Quote
if I were to put the cap with a 200? resistor to make sure the max current drawn at 5V is 25mA would this be correct

Yes but have you any idea of how long it would take to charge it up? Also the current from the supply would be limited even more by the 200R resistor.

You need either a battery or a real power supply.

#### fungus

#5
##### Feb 26, 2013, 08:15 pm

SKU:N99CN on http://www.maplin.co.uk/double-layer-backup-capacitors-98185 is rated for 0.1F at 5.5V. If I were to trickle charge this, would that then work?

Have you check the max. output current of that? A lot of those 'backup capacitors' can only output tiny amounts of current (less than a milliamp). They're designed to keep RAM chips alive and not much more.

No, I don't answer questions sent in private messages (but I do accept thank-you notes...)

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