Which Cap Type

Thread Starter

DC_Kid

Joined Feb 25, 2008
1,072
for long accurate life of a astable 555 in the 1-200Hz range, which type of caps are best suited or the job? electrolytic, tantalum, other ?? my values will be 1u and 68u. operating temps may be -10 to 110F, but would normally be in that 70-80F "room temp" range.
 

Wendy

Joined Mar 24, 2008
23,415
Tantalum are better than electrolytic, but with timers and oscillators both tend to drift over the long term. It may not matter for your application, but then again, it might. Ceramic types (as in 1uF) are a bit more stable. Any RC circuit can have this problem.

By drift I'm figuring 10% give or take.

Oh, BTW, when tantalum are used as power supply filters they can go up like match heads if they fail. They are basically better caps all around than electrolytic, so they get used anyhow.
 

MrChips

Joined Oct 2, 2009
30,714
For stable timing on a 555 circuit you want to stay away from all electrolytics, aluminum and tantalum.

You want to choose mica or plastic (poly---). Since you will have difficulty finding the capacitor in large values ( > 1μF) go with what is available and use a divider circuit to divide down from a high frequency to a low frequency.
 

THE_RB

Joined Feb 11, 2008
5,438
For stable timing on a 555 circuit you want to stay away from all electrolytics, aluminum and tantalum.
...
And polyester!

They are badly temperature dependent and as the 555 has been running for a minute the freq will change as the cap warms up.

Actually it's hard to get a stable freq out of a 555 even on a good day. :)
 

Veracohr

Joined Jan 3, 2011
772
Yes, they are electrolytic capacitors, they just use a different kind of electrolyte than the ones called "aluminum electrolytic".
 

#12

Joined Nov 30, 2010
18,224
so, these AL Polymer caps also show up under the AL Electrolytic category. are these AL Polymers a type of AL Electrolytic? and "polymer" in general is attached to the names of many types of caps.
I don't know. MrChips said, "poly" so I got prices for anything that started with, "poly". Poly-propolene, poly-ethylene, poly-mer, whatever is available.
 

Thread Starter

DC_Kid

Joined Feb 25, 2008
1,072
I don't know. MrChips said, "poly" so I got prices for anything that started with, "poly". Poly-propolene, poly-ethylene, poly-mer, whatever is available.
just the insulator, like mica. ceramic in 1u is fine, the 68u is the hard one. thre is a 68u ceramic but its like $3 ! i am looking for the 10c/ea at qty 100, etc.
my application has adjustable frequency, so accuracy for freq is not so important, but looking for the parts that will last a "lifetime", etc.
 

#12

Joined Nov 30, 2010
18,224
I guess that puts you in the ceramic department, but you still haven't said what voltage you need.
 

Thread Starter

DC_Kid

Joined Feb 25, 2008
1,072
I guess that puts you in the ceramic department, but you still haven't said what voltage you need.
oh, 5V dc is the operating voltage from a 7805 reg.
a 6.3v cap is a tad close. 10 or higher would make me feel better, etc.
 

Wendy

Joined Mar 24, 2008
23,415
And polyester!

They are badly temperature dependent and as the 555 has been running for a minute the freq will change as the cap warms up.

Actually it's hard to get a stable freq out of a 555 even on a good day. :)
For an RC oscillator, the 555 is the best of breed. It tries to compensate for power supply variation and several other things. The problem is the other components, mostly the capacitors.

RC oscillators are used for everything from triggering SCRs on ovens to old oscopes and old TV sets. They work well, so it is a mistake to reject them because of drift of their external components. The trick is to be aware of their limitations, and know if it really matters. Most cases it doesn't.

10% drift (which is a worst case number) for 200Hz is 180Hz to 220Hz.

You have to keep things in perspective.
 

Thread Starter

DC_Kid

Joined Feb 25, 2008
1,072
10% drift (which is a worst case number) for 200Hz is 180Hz to 220Hz.

You have to keep things in perspective.
10% is acceptable..... tolerance is one metric, longevity of the item is the other for me. good electrolytics should last a long time, cheap ones not so long, etc. depending on cost i may need to use the forbidden types, etc.
 

ErnieM

Joined Apr 24, 2011
8,377
Smaller value caps such as the NPO dielectric are best for long term and temperature stability. But using a '555 limits you in in cap value selections, with long gimes demanding long caps.

If you skip over the '555 and look at chips such as the CD4536 you get an RC oscilator driver coupled to a long chain divider. That's the best of both worlds, you get to use small accurate caps and divide down the output to what you need.

That particular chip is not the easiest to understand but is capable of doing lots of tricks.
 

Thread Starter

DC_Kid

Joined Feb 25, 2008
1,072
Smaller value caps such as the NPO dielectric are best for long term and temperature stability. But using a '555 limits you in in cap value selections, with long gimes demanding long caps.

If you skip over the '555 and look at chips such as the CD4536 you get an RC oscilator driver coupled to a long chain divider. That's the best of both worlds, you get to use small accurate caps and divide down the output to what you need.

That particular chip is not the easiest to understand but is capable of doing lots of tricks.
hmmm, 4536 is ~50cent/1pc vs ~150cent/1pc 7556

well, i already have my schematic done w/ 7556 and proto parts should arrive today.
 

#12

Joined Nov 30, 2010
18,224
I've been using the CD4017and CD4020.
They can't count as far as the CD4536, but they worked for what I needed at that moment.
 

R!f@@

Joined Apr 2, 2009
9,918
Not a good reply but a PIC will be far easier, accurate and no cap issues that is if you can program.
Am I wrong guys.
I got the urge to post due to the state I am in.
 

Thread Starter

DC_Kid

Joined Feb 25, 2008
1,072
Not a good reply but a PIC will be far easier, accurate and no cap issues that is if you can program.
Am I wrong guys.
I got the urge to post due to the state I am in.
sure, an option. not so sure its "easier". i have done some picaxe stuff in the past.
 

#12

Joined Nov 30, 2010
18,224
When analog design is the right thing to do, it's excellent, but when a microprocessor can do the job, it's usually a lot smaller and cheaper. Choose your poison.
 
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