Post by thread:[EE]: Tantalums cap for power supply regulation

Hi, SMD tantalums seem to offer a great advantage in terms of size over normal electrolytic caps in stabilising a power supply. However I'm concerned with voltage spikes killing the cap. Basically my application is a small robot with 2 motors (Minimotor 1724 6volts version) with a current draw of around 300mA each, through a MOSFET H-bridge. Power is provided by a LiPo pack providing 7.4V nominal. What I'm concerned about is whether a 47uf 20V SMD tantalum will help stabilise the supply adequately for my 5V LM2940 reg as well as whether motor spikes might kill the cap. Comments anyone? Thanks a lot in advance, ------------------------------------------------------------------------ Daniel Chia "Genius is one percent inspiration and ninety-nine percent perspiration." - Thomas Edison E-mail: spam_OUTdanielcjhTakeThisOuTyahoo.com.sg MSN: .....danstryder01KILLspam@spam@yahoo.com.sg ICQ: 37878331 ------------------------------------------------------------------------ Send instant messages to your online friends http://asia.messenger.yahoo.com

> SMD tantalums seem to offer a great advantage in terms of size > over normal electrolytic caps in stabilising a power supply. However I'm > concerned with voltage spikes killing the cap. Tants hate large inrush currents, and this sounds like an app that would have a fair bit of them. Check the specs, and look at what the real inrush current could be, figuring real impedances and lead inductances.

Hi, Sorry for my ignorance, but could you please elaborate on what you mean by inrush currents and how would one go about figuring them out? ------------------------------------------------------------------------ Daniel Chia "Genius is one percent inspiration and ninety-nine percent perspiration." - Thomas Edison E-mail: danielcjhKILLspamyahoo.com.sg MSN: .....danstryder01KILLspam.....yahoo.com.sg ICQ: 37878331 ------------------------------------------------------------------------ > {Original Message removed}

At 02:52 AM 9/5/2005 +0800, you wrote: >Hi, > Sorry for my ignorance, but could you please elaborate on what >you mean by inrush currents and how would one go about figuring them >out? What's the peak current that can flow through the capacitor (eg. suppose it supplies *all* the peak motor current), or what is the peak current that will charge the capacitor. Is it limited, and if so, to what maximum current? Think about all possible conditions. Generally, tantalums are most *un* suitable for power supply applications. They are quite prone to failure, sometimes in spectacular fashion. A low-Z aluminum is a much safer bet. If you *must* use them, then heavily derate the voltage (use a capacitor rated for 3 times as high as the working voltage), and follow all data book recommendations to the letter. Best regards, Spehro Pefhany --"it's the network..." "The Journey is the reward" EraseMEspeffspam_OUTTakeThisOuTinterlog.com Info for manufacturers: http://www.trexon.com Embedded software/hardware/analog Info for designers: http://www.speff.com ->> Inexpensive test equipment & parts http://search.ebay.com/_W0QQsassZspeff

> Generally, tantalums are most *un* suitable for power supply > applications. > They are quite prone to failure, sometimes in spectacular fashion. A > low-Z aluminum is a much safer bet. > > If you *must* use them, then heavily derate the voltage (use a > capacitor > rated for 3 times as high as the working voltage), and follow all > data > book recommendations to the letter. Yes. This is a warning which must be noted. Tantalum capacitors are superb when they work. They offer very high capacitance per size and excellent ESR. But in an amateur or experimental situation where conditions are not well behaved or well known they can fail catastrophically. A tantalum cap that experiences a voltage spike higher than its rated voltage, even a very brief one, will often fail very spectacularly. They often smell bad, make interesting sounds, smoke, catch fire or explode in various combinations. I've seen one do all of these in that order :-). Manufacturers spec sheets often specify minimum supply impedances they should be used with and even suggest adding some ohms of resistance in series. A **solid** aluminium capacitor has the advantages of tantalum without the catastrophic failure mode. Russell McMahon

Hi, First off, think I wasn't very clear in my post, but my motors run off the unregulated side of the power supply. The regulated side is for digital circuitry and such. Right now what I'm gathering is that the tant prolly isn't up to the task of regulating the battery supply. Instead I probably could consider a solid aluminium electrolyte. However I think that for the 5V regulated side a 16V tant should do the job pretty well. Thoughts anyone? ------------------------------------------------------------------------ Daniel Chia "Genius is one percent inspiration and ninety-nine percent perspiration." - Thomas Edison E-mail: danielcjhspam_OUTyahoo.com.sg MSN: @spam@danstryder01KILLspamyahoo.com.sg ICQ: 37878331 ------------------------------------------------------------------------ > {Original Message removed}

Juan Cubillo wrote: >I have always used Ceramic Disc 0.1uF caps as much as I can all over my >projects and they seem to work great. Adding an extra 47uF electrolitic on >the ouput of voltage regulators is also a big help. >Juan Cubillo > > > > That's been my experience, too. Using ceramic caps then adding tantalum electrolytics in parallel seems to utilize the best of both technologies, i.e. no problems with failures of tantalums due to spiking. --Bob -- Note: To protect our network, attachments must be sent to KILLspamattachKILLspamengineer.cotse.net . 1-866-263-5745 USA/Canada http://beam.to/azengineer

Might also look at using Os-Con type caps....better than electrolytics at least Russell McMahon <RemoveMEapptechTakeThisOuTparadise.net.nz> wrote:> Generally, tantalums are most *un* suitable for power supply > applications. > They are quite prone to failure, sometimes in spectacular fashion. A > low-Z aluminum is a much safer bet. > > If you *must* use them, then heavily derate the voltage (use a > capacitor > rated for 3 times as high as the working voltage), and follow all > data > book recommendations to the letter. Yes. This is a warning which must be noted. Tantalum capacitors are superb when they work. They offer very high capacitance per size and excellent ESR. But in an amateur or experimental situation where conditions are not well behaved or well known they can fail catastrophically. A tantalum cap that experiences a voltage spike higher than its rated voltage, even a very brief one, will often fail very spectacularly. They often smell bad, make interesting sounds, smoke, catch fire or explode in various combinations. I've seen one do all of these in that order :-). Manufacturers spec sheets often specify minimum supply impedances they should be used with and even suggest adding some ohms of resistance in series. A **solid** aluminium capacitor has the advantages of tantalum without the catastrophic failure mode. Russell McMahon