Decoupling caps for battery powered PICAXE work...

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Grogster

Senior Member
Hello. :)

Generally speaking, with battery power being so clean and free from any PSU noise or ripple, do you need the same decoupling caps in a given design?

Currently, I still use 470n SMD caps on the 1703 SMD regulators I use for PICAXE supply, and 100n caps on the PICAXE chip itself, but not much else.

I had an issue today, with a little battery operated circuit which was working fine, but then not.
Voltages fine on battery and on PICAXE, but unit not working.

Hmmmmmm.

Isolate power, replace power, and away it went - it was a controller crash.

I added a 100uF 6.3v electro across the PICAXE thinking that perhaps there was some kind of nasty on the supply, but I don't really see how, as a battery is about as smooth a supply as you are ever going to get. :confused:

What is the general consensus here with respect to decoupling caps on battery powered designs?
Is it advisable to just use the same decoupling caps as I would have on a PSU fed design?

All information gratefully assimilated.
 
I

inglewoodpete

Senior Member
Grogster said:
What is the general consensus here with respect to decoupling caps on battery powered designs?
Is it advisable to just use the same decoupling caps as I would have on a PSU fed design?
Click to expand...
Noise in a microcontroller system comes from 2 places. Inside the chip and outside the chip. If the peripherals are not creating the noise, then are there other electromagnetic or electrostatic sources?

Be aware that one of the biggest sources of noise is the chip itself. That's why a 100nF capacitor mounted close to the PIC can often fix the problem.
 
G

Goeytex

Senior Member
I added a 100uF 6.3v electro across the PICAXE thinking that perhaps there was some kind of nasty on the supply, but I don't really see how, as a battery is about as smooth a supply as you are ever going to get.
Click to expand...
Batteries are smooth as long as the current draw is steady. However when current is pulsed some nasties can happen. The battery itself can act as an LRC circuit and create significant and relatively high amplitude spikes and oscillations. This is particularly true of Lead Acid Batteries but battery related oscillations can also be seen in other types as well.

Long leads from the battery to the circuit can also cause problems. Many regulator datasheets recommend a capacitor on the input to the regulator based upon the length of the battery leads. Likewise it is generally a good idea to place an input capacitor on a circuit board that is battery powered when the battery leads are more than a few inches long.
 
B

BeanieBots

Moderator
This may sound obvious but it's often overlooked. You need decoupling caps whenever you need decoupling!
Decoupling is when you don't want what one device is putting on the power rail to effect another.
Think of a micro as the electronic equivalent of a pnumatic drill. It makes a lot of noise and puts large pulses on the air line just like a micro puts noise and pulses on a supply line. If it's ONLY the micro, then you can often get away with no decoupling but ironically, the use of batteries is the WORST time to have no decoupling. That's because batteries have no built in decoupling whilst all power supplies do have some output capacitance.
As for the caps around the regulator, you need to read the regulator datasheet. Many regulators simply won't work or go unstable without the correct capacitors around them.
 
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MFB

Senior Member
Mounting a power supply decoupling capacitor near the electronics can improve 'battery life'. As a battery is drained its internal resistance increases. This can result in troublesome noise, when the load demands spikes of current, whilst there is still a useful amount of charge left in the battery.

Another reason to add local decoupling is that the inductance and resistance of wire -between the battery and digital circuitry- can cause switching noise problems.
 
G

Grogster

Senior Member
Very interesting read.
I will read up more on this.
Until now, I have not, as a rule, bothered with much decoupling on battery operated circuits, safe, dumb and happy that batteries are such a smooth power source.

The datasheet for the 1703(well, MCP1703-33 to be exact) regulator specifies that 1u caps(ceramic, tantalum or aluminium electro) should be used on input and output. As I am using 470n, this is a little low, so I think I will change these to 1uF, if I can find some 1uF ceramic in 1210 SMD footprint. Should not be that hard - I will check Element-14...

EDIT: Found some...
http://nz.element14.com/taiyo-yuden/tmk325b7105kd-t/capacitor-ceramic-1uf-25v-x7r-10/dp/1683705
 
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W

westaust55

Moderator
The 100 nF decoupling caps should be used to help filter out the high frequency switching noise from IC's which occurs each time an internal signal changes state between high and low.

A bulk energy capacitor as typically used on the output of linear voltage regulators of the order of 10 to say 470 uF of the Tantalum or Electro type can still be useful on battery supplied circuits to over some brief voltage dips when there is a momentary increase in current draw and battery interal resistance limits available current.
When used with a voltage regulator, typically a 100nF ceramic capacitor is added in parallel to amongst other things prevent any possibility of the regulator going into high frequency oscillation.
 
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marzan

marzan

Senior Member
I have always wondered. Does it matter if the decoupling caps are tantalum, ceramic disk, polyester ? is there any advantage of one over another?
 
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MFB

Senior Member
The characteristics of the capacitors you mention are very different and you would really need to read-up on them. For example, low cost electrolytics normally need a smaller ceramic capacitor in parallel to deal with high frequencies but tantalum electrolytics don't.
 
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Grogster

Senior Member
Agreed, and as MFB mentions, you need to check the datasheet.
In my case, I am using the MCP1703-33 and MCP1703-50 SMD regulators, and the datasheet for these says you can use either ceramic, tantalum or aluminum on the input and output caps, so in the case of these regulators, it would seem it does not matter, but other devices may require something more specific.
 
fernando_g

fernando_g

Senior Member
marzan said:
I have always wondered. Does it matter if the decoupling caps are tantalum, ceramic disk, polyester ? is there any advantage of one over another?
Click to expand...

Esentially there are three characteristics: Volt-Farad product vs size (volumetric efficiency), equivalent series resistance (ESR) and self resonating frequency (SRF). Ah!...almost forgot. Cost.

For volumetric efficiency Tantalum has been king for many years, but comes at a very steep price, which has become worse as the world's largest Tant deposits are what is called blood-Tantalum...located in Africa's most repressive countries. Tantalum has a very low ESR, which is good.

Ceramic capacitors, if they are MLCCs (chips) right now offer the best ESR, highest SRF and are getting close to Tant's volumetric efficiency at somewhat lower cost. You can see in modern electronic equipment that MLCCs have essentially replaced most of the Tantalum applications. MLCCs do suffer from substantial bias and temperature drifts, and the larger sizes flex-crack easily which leads them to fail short-circuit.

Aluminum capacitors have an advantage: its very low cost. They have low SRF and mid to low ESR. Unless you are building a high frequency switchmode PSU, they are your best overall choice.

Film capacitors, for which there are polyester, polypropylene and other polys, are used where drift and stability are essential, like analog filter circuits. They tend to be expensive, large, and its self resonating frequency is low. Many films do have self-healing characteristcs, so they are used across-the-mains applications.


So what should you do? For simple Picaxe duty, a 100uF aluminum paralleled with a pair of 0.1uF MLCC provide the best price/performance ratio.

This description is not all inclusive....capacitor technology is a science and an art on itself.
 
D

Dippy

Moderator
Same here John .
The information given has been nicely condensed. I think I've seen it before about 50 times :)

This is yet another example of where a Tutorial section would be good - surely that can't be against rev-Ed policy?
After all, it IS education.
 
marzan

marzan

Senior Member
Dippy i agree. There are probably plenty people on this forum like me who have many questions, but dont want to be asking what seem to be obvious questions, and so spend a lot tof time trolling through the thousands of threads trying to get as much information as we can before asking in the forums. You guys obviously get asked the same questions over and over again, so why not a FAQ section? it could be categorised : i2C, OLEDs, Picaxe chips, servos, etc. I have tried looking up commands like port masking in the tutorials and find very limited info on the subject, so you end up wading through threads for any mention of the subject. Most of us newer picaxe users dont mind doing a little digging to educate ourselves, but taking port masking foe example I would like to know what it does, does it apply to inputs and outputs etc. but I dont think we should be asking you guys better educated in the use of picaxes the same questions ad nauseum. Don`t get me wrong though. Like others have said, this is the most respectful and helpful forum I have had experienced.

Thanks.
Marz.
 
marzan

marzan

Senior Member
Sorry, forgot to add. Fernando, that is exactly the information that is handy to new users. enough information to explain the differences without getting too technical. I paricularly liked the bit about blood tantalum. I never knew it was a problem.

Thanks again.
Marz.
 
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Grogster

Senior Member
@ marzan - Totally agree with you on that. The forum NEEDS a tutorial-type forum thing. I too think it would save a heck of a lot of re-answering of questions. I am guilty of asking questions that have been answered before, so it it NOT just the new users - we are all guilty of that kind of thing from time to time. Not that I think those members who answer a lot of posts really get that upset at posting the same answer again, but I think it would be fair to say that it perhaps irritates them just a little. :D I too have started using the forums search engine a lot more then I used to, but sometimes, to get clarification on something, you just have to post your question or thread - C'est La' Vie...

We already have a Finished Projects type forum, which is VERY useful, explaining how users did various things, but they are by their very nature, specific. Tutorials on commands etc/other users experience with certain commands etc, would be useful I think.

MHOO.(My humble opinion only)
 
W

westaust55

Moderator
marzan said:
Sorry, forgot to add. Fernando, that is exactly the information that is handy to new users. enough information to explain the differences without getting too technical. I paricularly liked the bit about blood tantalum. I never knew it was a problem.

Thanks again.
Marz.
Click to expand...
OT but interesting (for some):

Tantalum was also mined in Western Australia (at Wodgina in the Pilbara region of WA) from around 1900 but the local company is winding down due to the "Blood" Tantalum.
See: http://www.watoday.com.au/business/blood-tantalum-in-your-mobile-phone-20090508-axcs.html
Some very interesting information about the old Wodgina mine is that in 1907 it had a single cylinder diesel engine built to Dr Rudolf Diesel's original design (have some photos I took decades ago of the engine) and for a then very remote and harsh region, in the early years the Manager was a woman.
 
S

srnet

Senior Member
This is a problem I have been wrestling with recently, for my satellite board, should the decoupling capacitors, 0.1uF for PICAXE and two other ICs, plus 1uFs on the input and output of the regulator be ceramic or tantalum.

Tantalum are not subject to the flex cracking that can affect ceramics, but then tants are not exactly the most reliable of capacitors either. I bought an electronics module recently, switched it in and a tant on the board went pop, it was the right way round.
 
B

BeanieBots

Moderator
Personally I NEVER use tants due their unreliability but others may have had different experiences and some of my colleagues simply love them. I guess we could go around in circles for ever on this issue as there are pros and cons with all types.
For satellite use I would be looking more at temperature/radiation characteristics.
 
S

srnet

Senior Member
For the larger values, > 1uF, you dont have a lot of choice for use in a vacuum, normal can electrolytics dont like it.
 
B

BeanieBots

Moderator
Good point!
As with most component choices, there is usually some other factor which determines choice (often cost but not always).
 
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MFB

Senior Member
Is there really a problem with tantalum capacitor reliability? I have never experienced any, other than when connecting them the wrong-way-round. Its always a good idea to use current limited supplies when using electrolytics. I once made the mistake of powering a circuit directly from a small lead acid battery and instantly found that a tantalum capacitor was incorrectly connected. A large bang was followed by a lump of hot metal flying past my head and burning a few-cm hole in the office carpet!

When testing tantalum capacitors for high altitude work I found no problems up to a simulated 35km and temperaature range of -40C to 80C. However, the capacitance reduces with temperature and the voltage rating reduces with increasing temperature. I can also remember an old NASA guide to experimenters that recommended using tants over other types of electrolytics.
 
M

MFB

Senior Member
Thanks JimPerry, Looks interesting but the copy of the NASA Guide to Experimenters that I used (when working on the UK6 satellite) was dated about early 70's!
 
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Grogster

Senior Member
On the tant caps, I am surprised to find that BeanieBots hates them so much.
I thought the idea behind tants was high capacitance in a small package, coupled with high temperature stability and long-life...
All very good aspects.

Perhaps I should go back to foil-type aluminum electros!!! :D
 
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MFB

Senior Member
The high and cold systems testing, that included tants, was conducted 'only' about ten years ago for a NASA Ames project and I don't expect passive component development has progressed much since then (apart from battery technology).
 
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Dippy

Moderator
I've never had problems with Tants, though I've always been very careful not to push them.

I'm afraid, like many things, there are different varities.
And, like even more things, maybe cheap crappoes from you know where.

Certain varieties of tantalum caps are used in military and aerospace including weapons and space products.
As usual good design and sensible use and intelligent selection are crucial.
 
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mrburnette

Senior Member
Dippy said:
<...>
Certain varieties of tantalum caps are used in military and aerospace including weapons and space products.
As usual good design and sensible use and intelligent selection are crucial.
Click to expand...
5 years in the military secure electronics and I've spent the rest of my life trying to forget. I'll not sleep any better tonight having been reminded of the oxymoron around "military intelligence."

- Ray
 
B

BeanieBots

Moderator
Grogster said:
On the tant caps, I am surprised to find that BeanieBots hates them so much.
I thought the idea behind tants was high capacitance in a small package, coupled with high temperature stability and long-life...
All very good aspects.
Click to expand...
It's to do with personal experience. Many of the PCBs I've had to repair over the years have been because of blown tants.
That does NOT mean to say that tants are bad, simply that from experience they fail frequently and that may well be due to poor design but fail they do.
You could compare it to cut-glass wine goblets versus plastic beakers. Which is better?
If you are a night-club owner, then there is no doubt that the plastic beakers will offer higher reliability.
Maybe its because I've spent most of my carear in the semiconductor manufacturing equipment industry where the machines are subjected to high voltage arc-overs, x-rays and other nasties that I have a biased view on tants?
 
fernando_g

fernando_g

Senior Member
This is a very simplified summary of a Tantalum capacitor reliability paper, which I created many moons ago to address a customer concern, and that I developed with assistance from both Kemet and Vishay. Probably also Epcos:

Capacitors are manufactured from pure Tantalum micropowder on which a Tantalum Pentoxide TaO5 dielectric has been grown. Oxide thickness determines the working voltage.
Feature size is 5 ~10 &#956;m.
Unfortunately, there will always be dielectric defects in the form of impurities or microcracks. Those represent weak points where a failure may start.
Fortunately, self healing may be implemeted. A MnO2 layer is deposited in a secondary operation after sintering.
MnO2 is a used as a &#8220;counter electrode&#8221; to implement a self-healing mechanism.

The self-healing reaction is based on thermally inducing oxidization of the conductive MnO2 counter-electrode and converting into Mn2O3 (a higher resistivity form of manganese oxide).
MnO2 + heat ------> Mn2O3 + O*
Thus the conduction site is effectively &#8220;plugged&#8221; and the fault current clears.
The free Oxygen O* harmlessly recombines with lower order Tantalum oxides.

Self Healing Limitations: If the energy initiating the failure is large, there will be more excess Oxygen released that what can be absorbed safely.
Since the TaO5 is also heating, once that it reaches ~480ºC, it will change from an amorphous isolating state to a crystalline conductive state. The fault current then increases.
The additional excess Oxygen combines with the Ta powder, which ignites.
 
J

JPU

Senior Member
This thread is an interesting read as I have just had to have help rebuild a circuit that a 7815 regulator burnt out on. The reg has a maximum input of 35V. We were putting in 29V to 21V from a Lifepo4 battery which ran a brushless motor. There were sprinkles of caps around the board but the opinion was that the reg blew either because of a spike from the motor or oscillation between the 7815 and 7805 reg. The end result was a 5V zener a 1n4002 and two caps. A 470n and a 100uf placed across the positive and negative at the very start of the PCB!

I am a nube to electronics and the guy that helped was on another forum. Thank God for him, he is a star!. I did hint at an explanation for the patch, but by then I think he had had enough of my nube-ness!!! Can someone explain why the 470n and 100uf were placed right next to each other..It might help me understand all this a little better.

Here is the patch he designed for me..

Thanks
 

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JimPerry

JimPerry

Senior Member
The 470uF is a ripple smoothing "bucket" and the 100nF is to prevent the regulator oscillating at high frequency (as is the xxxnf after the regulator) :D
 
J

JPU

Senior Member
JimPerry said:
The 470uF is a ripple smoothing "bucket" and the 100nF is to prevent the regulator oscillating at high frequency (as is the xxxnf after the regulator) :D
Click to expand...
Thanks for your reply,,and decoding of my incorrect post!

What do you mean by ripple smoothing and can you explain what the Zener diode is doing please.

Much appreciated.
 
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