Continue to Site

Welcome to our site!

Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.

  • Welcome to our site! Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.

Capacitor technology selection

Status
Not open for further replies.

mn1247

Member
I have a circuit that uses a 22uF electrolytic capacitor to time a 555.

My understanding is that electrolytics have very poor tolerances (e.g., a factor or two!). Is there a different capacitor technology I should be using? I'd like to keep within 20% of the required time interval.

Thanks
Eric
 
Tantalum capacitors also have a poor tolerance. Ceramic capacitors are generally fairly good, but at 22 uF, the capacitors use dielectrics that are made to give large capacitances rather than accuracy.

Film capacitors are larger but better. ARCOTRONICS|R60DR52205040J|CAPACITOR, 22UF, 63V, 27.5MMP | Farnell United Kingdom would be good.

It is cheaper to use a microcontroller to do the timing. The internal oscillators are trimmed to +/- 2%
 
I have a circuit that uses a 22uF electrolytic capacitor to time a 555.

My understanding is that electrolytics have very poor tolerances (e.g., a factor or two!). Is there a different capacitor technology I should be using? I'd like to keep within 20% of the required time interval.


Thanks
Eric

hi,
Look at this option
Tantalum Capacitors About - Engineers Edge
 
You might also consider adjusting the R in the RC combination so as to get the interval you want.
 
A CMOS version of the 555 such as the **broken link removed** has much lower input bias currents than a standard 555 so you can use higher timing resistor values and lower capacitor values for the same timing interval.
 
Hi there,


One of the important features of a capacitor in this kind of application is long term stability rather than initial capacitance accuracy. It doesnt matter if the cap is 22uf, 20uf, or 25uf, as long as it STAYS that way over a long time period. The problem is, caps change with frequency and run time and even worse, they change with temperature too. The temperature change will probably be a problem if there is any significant change in the ambient temperature unless of course a special oven is constructed to keep the cap (and maybe the whole circuit) maintained at a constant temperature. That's one way to beat this problem.

Polycarbonate caps dont age as much, but i think the market is slowing down with regard to the manufacturing of these kinds of components because of the halt in production of the type of material suitable for use as the dielectric, so it may be difficult to find a replacement in the future. Maybe with a one of a kind circuit build it wont matter though. As Eric was pointing out these are very well behaved, and if you're worried about a replacement you can also consider these:

Polyphenylene sulphide (PPS) [probably first choice]
Polyethylene napthalate (PEN)
Polytetrafluoroethylene (PTFE)
Polyimide (PI)

where PPS has already been used for a direct replacement.

If temperature change isnt an issue, there's also the regular polymer caps, but i have to agree with 'Diver300' in that a microcontroller would be a better way to get the timing right and stable over long periods of time and changes in ambient temperature. You wont have to worry about the long term capacitance changes nor temperature as much and gee if you want to get fancy you can even install a thermistor detector to detect ambient temperature and adjust the timing as needed...would make one heck of a nice timing circuit.
 
Last edited:
Status
Not open for further replies.

Latest threads

New Articles From Microcontroller Tips

Back
Top