Are Tantalum Capacitors Really Needed?

Thread Starter

SamR

Joined Mar 19, 2019
5,031
I'm looking at an audio amplifier to replace the piezo earpiece for a crystal radio. So, I am in the audio frequency range which is good for tantalums. What is the general consensus as to using them? When are they really necessary and what advantage would they give? Here is the circuit.
1670652700770.png
 

Jony130

Joined Feb 17, 2009
5,487
In this application, you do not need to use tantalum capacitors. Just use as everyone does the electrolytic capacitors.
 

Ian0

Joined Aug 7, 2020
9,671
Don't use them for audio - they are noisy. Don't use them for decoupling - they blow up (and smell bad). in fact, don't use them at all!
Also, avoid ceramic capacitors unless they have an NP0 dielectric - they are piezoelectric.
 

Thread Starter

SamR

Joined Mar 19, 2019
5,031
At what point or under what conditions are tantalums needed? They can't stand voltage spikes or RF apparently so that is where they are not needed, but just where are they really needed. I've even heard the argument that they are never really needed.
 

crutschow

Joined Mar 14, 2008
34,285
All the military electronics I designed required the use of tantalum electrolytics, since standard aluminum electrolytics were considered not sufficiently reliable.
As long as precautions are taken to prevent overvoltage and high surge currents, they are very reliable.

Where have you seen data that they are noisy?
 

Ian0

Joined Aug 7, 2020
9,671
All the military electronics I designed required the use of tantalum electrolytics, since standard aluminum electrolytics were considered not sufficiently reliable.
As long as precautions are taken to prevent overvoltage and high surge currents, they are very reliable.
When I was student at Marconi back in 1985, I was told that tantalums were banned from military electronics due to their tendency to blow up and short out the power supply. Probably a different country's military!
Where have you seen data that they are noisy?
I wish I remember, but I think it was in Cyril Bateman's articles in Wireless World.
https://worldradiohistory.com/Wireless_World_Magazine.htm
July 2002 to January 2003.
I'm not going to read through it all this morning to find it!
 

ericgibbs

Joined Jan 29, 2010
18,766
hi,
The only problems I have seen with Tantalum's is when they are reverse connected or well over voltaged, due to a failure elsewhere on the circuit.
I have used them often in shore and marine designs for low noise applications for decoupling etc with no problems.

E
 

Thread Starter

SamR

Joined Mar 19, 2019
5,031
OK, I can see how they may have an edge over electrolytics (in the low uF ranges) as they don't leak and dry out. I've also seen them used along with electrolytics to decouple in power supply designs. Tantalum for the low freq decoupling and electrolytic for the high?
 

Thread Starter

SamR

Joined Mar 19, 2019
5,031
I didn't see a date on the PDF article and it dealt with failure of SMD chips. It did mention higher ESR at higher frequencies, and I've read elsewhere that tantalums are counter indicated at RF (didn't say why but I can see ESR as a problem). Is there a rule of thumb frequency cutoff for them where the ESR becomes a burden in comparison with electrolytics?
 

Ian0

Joined Aug 7, 2020
9,671
"Some audio power amplifier designs have used small Tantalum bead capacitors, with apparent success. Initial measurements of a number of Tantalum capacitors revealed large distortions. Measured at 0.3 volts with and without DC bias, my Tantalum capacitor stocks produced at least ten times more distortion than found with low cost polar Aluminium electrolytics. I decided to exclude Tantalum bead capacitors from further tests." Cyril Bateman
https://www.dadaelectronics.eu/uplo...ctronics-papers/Capacitor-Sound-C-Bateman.pdf

Bob Pease noted:
"Now, that does NOT mean we can't hear the effects of a tantalum capacitor in a poorly-designed circuit, with improper bias. I'll run some tests on those, soon. But it's well-known that tantalum caps can sound pretty weird if they are ever allowed to get biased the wrong way during part of a cycle. I bet even I can hear that kind of distortion."
https://www.electronicdesign.com/te.../21796328/whats-all-this-soakage-stuff-anyhow

So, it would appear that the tantalum is a bad choice where it has no DC bias across it. Cyril Bateman also noted that the bipolar electrolytic is far superior to the standard type when used as a coupling capacitor with no DC bias.
 

dendad

Joined Feb 20, 2016
4,451
In the industrial control equipment I've designed and built over the last 30 odd years, there have been only a couple of Tantalum cap failures and that is usually being reverse installed or over voltaged. The caps that give trouble have been mainly electros. Most boards that come in for servicing have the power supply eectros needing to be replaced. This is also true for a lot of computer screens and TVs. Failed electros!
If I remember correctly, a few years ago, there was a world wide batch of bad electrolytic caps that caused a great many computer motherboards to fail. Now, most have got rid of the electros and use ceramics as they are now available in reasonably high values.
I still use electros for bulk power supply filters but nowhere else. All the bypassing is with Tants and ceramics. Usually lots of them as good bypassing is very important for solid performance and long life.
 

MrChips

Joined Oct 2, 2009
30,712
I have seen a number of shorted tantalum capacitors across 12VDC or 24VDC supply lines in nuclear instrumentation modules.
 

AnalogKid

Joined Aug 1, 2013
10,987
When I was student at Marconi back in 1985, I was told that tantalums were banned from military electronics due to their tendency to blow up and short out the power supply. Probably a different country's military!
Where I was working at the time, the company shifted from industrial/medical/telecom to military after the 2000 tech crash. Back then, tant caps were common and sometimes required. Somewhere in the 2005 timeframe things started shifting away from tant, and by 2010 they were banned (without explicit sign-off) in the RFQs we were seeing.

ak
 

AnalogKid

Joined Aug 1, 2013
10,987
I'm looking at an audio amplifier to replace the piezo earpiece for a crystal radio.
Two small points that have nothing to do with caps.

1. Consider replacing R1 with a fixed resistor and trimpot in series. This lets you adjust the static current through the output transistors to prevent excessive power dissipation.

2. There is no need to switch the battery negative connection. Also, a DPST switch on the back of a pot is very rare. S1 can be SPST.

ak
 
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