tantalum cap leakage?

AndrewT · 12th December 2005, 08:58 PM

Hi all,
I want to build a preliminary timer using 555 to get about 20minutes delay. Eventually I will go digital using a microprocessor after I have proved the philosophy of my control system works.

To achieve 20 minutes I will need a large value resistor and an electrolytic to get the time constant. But leakage through the electrolytic will upset the timer. To complicate it there will be about 10 cascaded timers giving an adjustable total delay of upto about 3 to 4 hours.

I tried measuring the leakage of a 47uF 16V tantalum cap but I can't measure it. I tried to measure the voltage across the 1M0 resistor in series with the cap and the 10V supply.
The resistance (10M) of my DMM upsets the voltage readings indicating that the effective leakage was a lot less than 1uA.

Q. is tantalum this good at 10volts? It appears to be a lot better than the data books state.

anatech · 12th December 2005, 09:17 PM

Hi Andrew,
I've used tantalum caps for that. They are better than electrolytics. In my application, the interval was not critical, nor was it necessary to be repeatable. You could use a lower value cap and a cmos counter too.

-Chris

dhaen · 12th December 2005, 11:07 PM

Not what you asked, but:
You might consider the 74HC4060 / CD4060 14 stage ripple counter with built in oscillator. It requires only 4 external components and the cap can be in the order of 20n for your required time interval.
74HC4060

poobah · 12th December 2005, 11:17 PM

Here's is another...

http://www.onsemi.com/pub/Collateral/MC14536B-D.PDF

There are a few more in the same (OLD) HV CMOS family...

These won't be so touchy over temperature.

anatech · 12th December 2005, 11:26 PM

I used the IC that John "pointed" to for more critical applications. 4060 at the time.

-Chris

poobah · 12th December 2005, 11:35 PM

Yep...

And these things are built to be cascaded so you'll only need 2 instead of 10.

14 or 24 bit counter doubled... 2^28 & 24^48 = big number

12th December 2005, 08:58 PM	#1
AndrewT diyAudio Member Join Date: Jul 2004 Location: Scottish Borders	tantalum cap leakage? Hi all, I want to build a preliminary timer using 555 to get about 20minutes delay. Eventually I will go digital using a microprocessor after I have proved the philosophy of my control system works. To achieve 20 minutes I will need a large value resistor and an electrolytic to get the time constant. But leakage through the electrolytic will upset the timer. To complicate it there will be about 10 cascaded timers giving an adjustable total delay of upto about 3 to 4 hours. I tried measuring the leakage of a 47uF 16V tantalum cap but I can't measure it. I tried to measure the voltage across the 1M0 resistor in series with the cap and the 10V supply. The resistance (10M) of my DMM upsets the voltage readings indicating that the effective leakage was a lot less than 1uA. Q. is tantalum this good at 10volts? It appears to be a lot better than the data books state. __________________ regards Andrew T.

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12th December 2005, 09:17 PM	#2
anatech diyAudio Moderator Join Date: Jun 2004 Location: Georgetown, On	Hi Andrew, I've used tantalum caps for that. They are better than electrolytics. In my application, the interval was not critical, nor was it necessary to be repeatable. You could use a lower value cap and a cmos counter too. -Chris

12th December 2005, 11:07 PM	#3
dhaen diyAudio Moderator Emeritus Join Date: Jun 2002 Location: U.K.	Not what you asked, but: You might consider the 74HC4060 / CD4060 14 stage ripple counter with built in oscillator. It requires only 4 external components and the cap can be in the order of 20n for your required time interval. 74HC4060

12th December 2005, 11:17 PM	#4
poobah diyAudio Member Join Date: Nov 2005	Here's is another... http://www.onsemi.com/pub/Collateral/MC14536B-D.PDF There are a few more in the same (OLD) HV CMOS family... These won't be so touchy over temperature.

12th December 2005, 11:26 PM	#5
anatech diyAudio Moderator Join Date: Jun 2004 Location: Georgetown, On	I used the IC that John "pointed" to for more critical applications. 4060 at the time. -Chris

12th December 2005, 11:35 PM	#6
poobah diyAudio Member Join Date: Nov 2005	Yep... And these things are built to be cascaded so you'll only need 2 instead of 10. 14 or 24 bit counter doubled... 2^28 & 24^48 = big number

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