Calculate capacitor replacement value?

I have not seen two caps involved. Maybe I missed something.

Try returning the cap to +9 instead of ground. That way the leakage speeds it up instead of slowing it down. (That's a classical trick for using electros with 555 timers.)

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs 
Principal Consultant 
ElectroOptical Innovations LLC / Hobbs ElectroOptics 
Optics, Electro-optics, Photonics, Analog Electronics 
Briarcliff Manor NY 10510 

http://electrooptical.net 
http://hobbs-eo.com

On Saturday, January 26, 2019 at 1:47:51 AM UTC+11, snipped-for-privacy@gmail.com wrot e:

o, I found the culprit was the capacitor, nominally 1500uF but now lifeless . Its actual original value is unknown. There are 8 markings on the knob of the timer?s linear pot (1 to 8 minutes) and 11 on the knob of a 1P x12W rotary switch (from 10 to 150 minutes) . As a first (arguably optimist ic) first step I want to calculate its value, for the replacement. It? ?s not practical to replace the pot and all 11 resistances, some of them with more than one resistor and/or reposition all 19 markings individually. Nor does my temperament allow me to bin it.

s 1.45V. If I stop-watch the time (in seconds) taken to trigger the alarm w ith a known value of C (uF) and R (megohms), what formula will then give m e the new value of C needed to change that time to its original value, as m arked please?

many from the longer duration rotary switch values as patience allows.

ver value is calculated. Will that have a detrimental effect on leakage?

Stick in a tantalum electrolytic of about 330uF.

The nice thing about tantalum electrolytics is that the leakage current kee ps on going down as long as you keep voltage across them (of the correct po larity).

The George Kent analog PID controllers of the 1970s relied on tantalum caps for their long time constant integral terms. Aluminium electrolytics didn' t hack it.

--
Bill Sloman, Sydney

o, I found the culprit was the capacitor, nominally 1500uF but now lifeless

If you want a stable time, a timing capacitor (available with low leakage a nd aging, in the 10 uF range) would be preferable. Film, Al polymer, or somesuch.

Instead of 9V, a resistor, and 1500uF filter capacitor, you'd use 0.060V, t he same resistor, and the 10 uF capacitor, with an op amp (as integrator), which (at the op a mp output) gives the same 9V range and time as the original. As long as the 9V is t here anyhow, an op amp isn't much of an addition....

The drawback, though, is that the ramp is linear, won't match the exponent ial (original) at the endpoint of the long timescale.

Just use a CD4060 instead of a 555 then the 10uF film cap should work ok.

But with the original presumably 555 circuit fit a 2000uF capacitor and put a trimmer on pin5 tweak the control voltage until you get the period you want.

--
  When I tried casting out nines I made a hash of it.

Oops, I misread. That's definitely leakage. Nichicon specialized in low lea kage Al e-caps has an app note on the effect. Turns out the leakage is not constant, it may start out in the uA region, but goes to just a few nA over time (many minutes) as the capacitor holds off an applied bias voltage. It was a test to determine suitability of permanently hanging an e-cap across the battery in an ultra-low power application. You could simulate the same effect by keeping the (-) terminal tied to a negative bias voltage, like -

9V, so that little excursion of 1.45V during charge up won't make much of a difference. Too bad your circuit doesn't have a negative bias floating aro und you can use for that.

An unseen 165k in parallel with your capacitor will cause the problem you are seeing. Or something else that draws 6.5uA as well.

Dumb question: A) Does your timer trip when the capacitor voltage climbs up from zero and reaches 1.45V when charged via R from 9V? or

B) Does the capacitor start out charged to 9V and discharges via R and trips the alarm when cap voltage falls to 1.45V?

piglet

@piglet:

A.

But my further tests today show that John S and others were right about oth er resistance complicating the analysis.

The pot and rotary switch resistances are connected in series from 9V and c harge the cap, the -be side of which is connected to 0V when the timer is s witched on. The caps charging voltage is input to a three-transistor Schmit t,which then triggers a small SCR. That provides a route to ground for a so und generator. So that lowers the trigger voltage reaching the SCR gate. Re sulting in a much longer time than derived from the basic formula.

Terry, East Grinstead, UK

Further work today show that John S and others were right about other resis tance significantly complicating the analysis.

The pot and rotary switch resistances are connected in series from 9V and c harge the cap, the -ve side of which is connected to 0V when the timer is s witched on. The cap?s charging voltage is input to a three-transist or Schmitt, which then triggers a small SCR. That provides a route to groun d for the 9V sound generator. So the extra circuitry lowers the trigger vol tage reaching the SCR gate. Resulting in a much longer time than derived fr om the basic formula.

I was able to get some life back to the original cap of about 1500uF by rep eated charging as recommended. For the range 1 to 10 mins it gave results c lose to my old calibrations, but was hopeless for all above 30 mins. I? ??ve ordered a few new electrolytics.

Terry, East Grinstead, UK

Jason: Another timer I made, of similar vintage to the one under discussion , used a presumably similar circuit to your suggestion. A 555 astable clock s a 4020, whose 8192 divisor output gates a 4011astable (actually a two-ton e output), and after amplification that drives the miniature speaker.

As you imply, such an approach has better inherent accuracy, given the grea ter stability of the much smaller capacitor (mine is 0.1uF). But its seriou s downside IMO is that without greatly increasing the component count it is much harder to design with timings that are of practical ?kitchen convenience?. Unlike the ?capacitor-charging? plus SCR project we?ve been discussing, this 555/4020 version is uncased , and given that design issue I?m not sure it?s worth doing so.

Terry, East Grinstead, UK

electrolytic will have too much leakage for this application.

use a low leakage type of cap, tantalum or a bunch of ceramics.

m