how to increase the current? - Page 1
Forum section: electronics / beginners

author: raff5184
author_rank: Regular Contributor
timestamp: February 20, 2017, 11:36:48 pm
content: 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 componentThank you

author: Zero999
author_rank: Super Contributor
timestamp: February 20, 2017, 11:42:40 pm
content: 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.

author: raff5184
author_rank: Regular Contributor
timestamp: February 21, 2017, 12:12:30 am
content: only 1sec but I can recharge it. The problem with a battery is that I need a rechargeable battery. Min voltage is 3V

author: danadak
author_rank: Super Contributor
timestamp: February 21, 2017, 02:02:20 am
content: 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=enRegards. Dana.

author: Zero999
author_rank: Super Contributor
timestamp: February 21, 2017, 09:12:22 am
content: Quote from: raff5184 on February 21, 2017, 12:12:30 amSo 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 = 0.1FKeep the ESR below 2 Ohm and its effect will be negligible.You say it needs to be 1cmx1cm but how high can it be?

author: raff5184
author_rank: Regular Contributor
timestamp: February 21, 2017, 05:24:06 pm
content: Quote from: Hero999 on February 21, 2017, 09:12:22 amDo 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?Quote from: Hero999 on February 21, 2017, 09:12:22 am2 or 3 mm

author: Zero999
author_rank: Super Contributor
timestamp: February 21, 2017, 06:36:12 pm
content: Quote from: raff5184 on February 21, 2017, 05:24:06 pmOhm'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.QuoteI'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.

author: raff5184
author_rank: Regular Contributor
timestamp: February 21, 2017, 07:27:43 pm
content: thank you everybody, very helpful and clear.Quote from: Hero999 on February 21, 2017, 06:36:12 pmLast 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

author: Zero999
author_rank: Super Contributor
timestamp: February 21, 2017, 09:14:58 pm
content: 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.pdfhttp://www.farnell.com/datasheets/1640985.pdfhttp://docs-europe.electrocomponents.com/webdocs/10eb/0900766b810eb39c.pdfhttp://docs-europe.electrocomponents.com/webdocs/11de/0900766b811dedfb.pdf