Electrolytic capacitor with less than 10% of rated voltage across it is OK?

[Flyback]

Hello,
C8, in the following schematic , is a 10uF electrolytic capacitor. It is rated 16V.
As you can see, it is across the Base-emitter of a BJT, thus it never gets any more than 0.7V across it.
Do you believe that this should be a ceramic capacitor instead?

 

[crutschow]

Electrolytic is fine for that purpose. You could go to a lower voltage unit if you like.

 

[Flyback]

Thanks, but quoting page 11 of the book "IC555 projects" by E.A.Parr (babani publishers) "An electrolytic capacitor does not become a capacitor until about 0.1 of its voltage rating....."
book first published in 1978
book revised and reprinted n 1981
Book printed last in 2006.

 

[crutschow]

The capacitor is just a bypass cap and does not appear to be a critical component so if it's capacitance is less than stated, it still should not be a problem. But if you are really concerned then use a 6V electrolytic or substitute a ceramic.

 

[Flyback]

Its not a bypass cap, its a timing cap.
After the certain time , the capacitor will switch off the circuitry if the loads don't draw current.
Do you agree it wont look like 10uF, or even half of this?

 

[ronsimpson]

C7 and C8 have this issue, if it is one.
I think placing a capacitor across B-E on a transistor does not make a good timer. The B-E voltage does not change much from on to off. I did not look at the data sheet but: If 0.65=off and 0.68=on that is not much change in voltage. The numbers might not be right. A 555 time uses 1/3 supply voltage change from on to off. Across R8 there is a 4 volt movement. That might be a better place for a timing capacitor.

 

[Mike odom]

I think the purpose of Q5/C8 is to turn Q4 on at startup. Once the system voltage gets to 9V (plus R9/C8 time delay) then Q5 turns on and lets the lockout system (Q4) work (no more current sourced through D5/6. Both capacitors will turn on slowly, and then, when one load opens, the Q4/C7 will delay the turnoff. I think the capacitors are there just in case there is a momentary dip in power so the whole system doesn't shutdown immediately, requires hard failure (loss of power for a certain amount of time). I don't think timing is critical. Q5 only turns off if system power drops below 9V, or at power off. So C8 (the 10uF) is really only used at startup, or 0V - 0.68V. If timing was critical, then they wouldn't use an electrolytic (-80 to +20% tolerance on a good day)

Putting a capacitor on a base/emitter junction is bad form, but I believe I remember reading somewhere in my foggy past that putting the capacitor on the base is like putting a much bigger cap on the collector... it uses the transistor's gain to amplify the capacitance (or looks like it's amplified). You could do the same thing, put a capacitor on the collector to hold the voltage constant (ish) when the transistor turns off/on, to slowly change that node's voltage, but you'd need a much larger one to do the same thing. I think they are just used to avoid spikes upsetting the system).

 

[Flyback]

The book that I linked in #3 states that an electrolytic capacitor doesn't even become a capacitor until it has at least 10% of its rated voltage across it...Is this incorrect?
-It kind of makes sense because capacitors that are in storage tend, every few years, to get powered up to near rated voltage to keep them good.

 

[ronv]

Maybe the effect is similar to storage, where over time leakage will go up unless the cap is "reformed".