how to increase the current?

[raff5184]

Hi I have a simple circuit. Basically a storage component (such as a battery) powering an evaluation board containing an IC. When the battery (a 3V or a 3.7V) is connected I measure a current of about 50mA flowing into the board.
I tried to substitute the battery with a capacitor/supercapacitor (charged at a voltage of about 4V), because I need powering for only 1sec, but the current is much lower (about 10mA). Does it exist a way/circuit to boost the current? And I mean a small solution like 1cm x 1cm component

Thank you

[Zero999]

It sounds like you used a supercapacitor designed for SRAM backup with a high series resistance.

You need to use a capacitor with a low ESR: Effective, Series, Resistance.

How long does it need to last for? What's the minimum operating voltage?

Batteries have a much higher energy density than supercapacitors.

[raff5184]

only 1sec but I can recharge it. The problem with a battery is that I need a rechargeable battery.  Min voltage is 3V

[danadak]

Here are some pretty low ESR super caps you can consider -


http://www.murata.com/~/media/webrenewal/products/capacitor/edlc/techguide/electrical/c2m1cxs-053.ashx?la=en


Regards. Dana.

[Zero999]

I measure a current of about 50mA flowing into the board.
I tried to substitute the battery with a capacitor/supercapacitor (charged at a voltage of about 4V)

only 1sec but I can recharge it. The problem with a battery is that I need a rechargeable battery.  Min voltage is 3V

So you need starting voltage of 4V, 50mA discharge and cut-off voltage if 3V.

The capacitance is actually equal to the number of Amps which can be put in, or taken out of a capacitor for a second, before the voltage changes by 1V. For example, if you connect a 1 Farad capacitor to a constant current supply for 1 second, it will charge up by 1V. If you then connected it  to a 1A constant current load, it will discharge by 1V. Doubling the capacitance will double the time it takes to for the voltage to change by 1V.

In your case a 50mF (0.05F or 50 000µF) capacitor will be able to supply 50mA for a second before the voltage drops by 1V. To allow for the ESR, tolerance and ageing, you need a slightly larger capacitance than 50mF.

The ESR needs to be as low as possible. The voltage will immediately drop by ESR*V as soon as the load is connected. For example, an ESR of 10 Ohm means the cut-off voltage (open circuit) needs to be 3.5V, rather than 3V, so the voltage drop over discharge is now 0.5V and you need C = 0.5/0.05 = 0.1F

Keep the ESR below 2 Ohm and its effect will be negligible.

You say it needs to be 1cmx1cm but how high can it be?

[raff5184]

For example, an ESR of 10 Ohm means the cut-off voltage (open circuit) needs to be 3.5V, rather than 3V, so the voltage drop over discharge is now 0.5V and you need C = 0.5/0.05 = 0.1F

Do you have an analytical formula (or a rule of thumb) that, given the ESR tells me what is the increase in the voltage that I need?

You say it needs to be 1cmx1cm but how high can it be?

2 or 3 mm

[Zero999]

For example, an ESR of 10 Ohm means the cut-off voltage (open circuit) needs to be 3.5V, rather than 3V, so the voltage drop over discharge is now 0.5V and you need C = 0.5/0.05 = 0.1F

Do you have an analytical formula (or a rule of thumb) that, given the ESR tells me what is the increase in the voltage that I need?

Ohm's law.

The capacitor can be modelled as a perfect capacitor with a parasitic internal series resistance (ESR). When a load is applied to the capacitor, then the voltage will immediately drop, even before it does due to discharging. Vdrop = I*R.

You say it needs to be 1cmx1cm but how high can it be?

2 or 3 mm

I've had a look and I couldn't find anything that small with a low enough ESR. Most small supercapacitors are designed for memory backup applications, very low current draw and have a relatively high ESR: often 50 Ohms or more which won't to. A 50 Ohm ESR would drop 2.5V at 50mA. When it's charged to 4V, the voltage will drop to 1.5V, as soon as the load is connected.

[raff5184]

thank you everybody, very helpful and clear.

I've had a look and I couldn't find anything that small with a low enough ESR. Most small supercapacitors are designed for memory backup applications, very low current draw and have a relatively high ESR: often 50 Ohms or more which won't to. A 50 Ohm ESR would drop 2.5V at 50mA. When it's charged to 4V, the voltage will drop to 1.5V, as soon as the load is connected.

Last thing, please could you post the links to a couple of components so I can have an idea of the possibilities and dimensions. Thank you

[Zero999]

Here are a few datasheets. You can use lower voltage capacitors in series but the capacitance will be halved, the ESR double and you'll need to add a voltage balancing resistor (100k to 1M will do) in parallel with each capacitor to ensure they both get half the voltage.
http://www.farnell.com/datasheets/481832.pdf
http://www.farnell.com/datasheets/1640985.pdf
http://docs-europe.electrocomponents.com/webdocs/10eb/0900766b810eb39c.pdf
http://docs-europe.electrocomponents.com/webdocs/11de/0900766b811dedfb.pdf