What is better for bulk capacitor: tantalum, ceramic or aluminum? - Page 1
Forum section: electronics / beginners

author: alexg
author_rank: Regular Contributor
timestamp: September 16, 2016, 05:56:08 pm
content: Hi All, I have a working circuit powered from 3AA batteries that has voltage drop of 500mV on the battery terminals upon load application, everything works okay and in general no complaint, however I want to reduce voltage drop as much as possible and per my understanding that is done by adding bulk capacitance next to battery terminals, due to space limitation I am limited to few choices below:one tantalum or tantalum polymer of 220uF 6.3Vhttp://www.digikey.com/product-detail/en/avx-corporation/F930J227MBA/478-8168-1-ND/4005703two ceramic caps 220uF each X5R 6.3Vhttp://www.digikey.com/product-detail/en/samsung-electro-mechanics-america-inc/CL32A227MQVNNNE/1276-3375-1-ND/3891461I also have third option that may work if absolutely necessary, this aluminum polymer caphttp://www.digikey.com/product-detail/en/panasonic-electronic-components/6SVPE220MW/P16836CT-ND/4834392Which one technically is more suitable for the task?

author: tggzzz
author_rank: Super Contributor
timestamp: September 16, 2016, 06:14:21 pm
content: How much current drop? How long for? What's the highest frequency in the load current?Remember that Q=I*t=C*V, where I is the current for time t, and V is the voltage drop across a capacitance C.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 16, 2016, 06:17:53 pm
content: If the lead length between battery and load is short, there isn't much you can do that's better than the battery already.Tim

author: alexg
author_rank: Regular Contributor
timestamp: September 16, 2016, 06:24:24 pm
content: Quote from: tggzzz on September 16, 2016, 06:14:21 pmI will have to measure it later tonight, and report back. Thanks for looking into this.Actually, I think I remember values, the current drop is 130mA, for 1 second, frequency is 2500Hz,I will post a waveform later tonight, upon load it drops down, then bouncer up and down within 100mV or so, then comes back up.

author: alexg
author_rank: Regular Contributor
timestamp: September 16, 2016, 06:30:03 pm
content: Quote from: T3sl4co1l on September 16, 2016, 06:17:53 pmThe length between battery and load is about 1 inch, it goes from battery to 10V boost regulator and into the load, no long wires between those points. So if I understand correctly, voltage drop cannot be eliminated completely or reduces down to let's say 100mV? Is it a normal number to have .5V voltage drop for 3AA batteries power supply?

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: September 16, 2016, 06:35:11 pm
content: tant poly or alu. poly will work better than ceramic due to loss of capacitance with DC bias, but will be rather more expensive.

author: tggzzz
author_rank: Super Contributor
timestamp: September 16, 2016, 06:36:39 pm
content: Quote from: alexg on September 16, 2016, 06:24:24 pmI suspected t would be something like that.So with a 220e-6F capacitor, if you draw 0.13A out of it for 1s, the voltage will fall by 590VYou're going to need a much much larger capacitor!

author: Audioguru
author_rank: Super Contributor
timestamp: September 17, 2016, 01:22:07 am
content: Batteries produce DC, not AC. A capacitor filters AC and will have no effect on your DC voltage drop.I think your batteries are cheap and are no good, or the battery holder connections are bad or your battery current is more than 500mA.I hope your batteries are not cheap Chinese "Super Heavy Duty" garbage. Energizer AA alkaline cells are good.

author: alexg
author_rank: Regular Contributor
timestamp: September 17, 2016, 02:44:58 am
content: You were right about bad connections, I connected circuit's input directly to battery terminals and now voltage drop is 140mV, I did keep two 220uF tantalums though, seem to make waveform smoother.I have a silly question, sorry if it sound completely dumb, what causes voltage to drop, batterie's internal resistance or a load?

author: radar_macgyver
author_rank: Frequent Contributor
timestamp: September 17, 2016, 04:22:54 am
content: If it went away when directly connected to the batteries, then most of the voltage drop you saw earlier was due to the series resistance of the interconnecting wire. When directly connected, it would be the internal resistance of the cells.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 17, 2016, 04:30:08 am
content: Quote from: alexg on September 17, 2016, 02:44:58 amYes and yes. V=I*R so the drop across the internal resistance is due to current draw.The time scale matters. Caps are only good for voltage sag in the microseconds range. Batteries are good for voltage sag in the hours...Tim

author: Brumby
author_rank: Unknown
timestamp: September 17, 2016, 05:07:03 am
content: Quote from: T3sl4co1l on September 17, 2016, 04:30:08 amThis ^ ^ ^