EEVblog Electronics Community Forum
Electronics => Projects, Designs, and Technical Stuff => Topic started by: Jester on April 10, 2019, 09:44:33 pm
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Playing around with a ICM7555 variety of the 555 timer, datasheet advertises delays from us to hours. This version has much lower input currents so in theory, larger resistors can be used compared to the original 555's.
If you use a large value resistor > 1M \$\Omega\$ and a large "leaky" electrolytic capacitor I suppose you might get something to work beyond say 60 minutes. I would guess that the timing would vary a fair bit over time and temperature though because the capacitor leakage current would have a significant impact on the timing.
The datasheet shows R value as high as 100M, I'm not sure how practical that is for real world use with eventual contamination on the board etc? I did a reality check with a 4.7M \$\Omega\$ and, 47uF Tantalum and that yields about 16 minutes in astable mode, however with a 10M \$\Omega\$ and the same capacitor it never timed out.
Assuming using a divider is not an option, has anyone been able to generate long time periods and or really low frequencies without an electrolytic?
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Ceramics may have lower leakage, but it is hard to find large values, and leakage may not be very good by then. The highest C*Rleak capacitors I'm aware of are wet-slug tantalum. But they're quite expensive. I would much rather buy a reel of CD4040s. :-DD
Tim
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Ceramics may have lower leakage, but it is hard to find large values, and leakage may not be very good by then. The highest C*Rleak capacitors I'm aware of are wet-slug tantalum. But they're quite expensive. I would much rather buy a reel of CD4040s. :-DD
Tim
Or use the fancy LT part, small and no capacitor required LTC6995CS6-1#TRMPBF
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The hours is marketing wank.
The 4060 has a built-in oscillator and a counter.
Also look at the 74HC5555 and 4536.
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Well, a 100 M resistor and a 10 uF film cap give a time constant of 1000 seconds so if the chip lives up to its specs it should work. The board would have to be dead clean and a few guard traces to intercept stray currents might be needed. I suppose if you make CMOS device, why not tout its low bias current inputs even if the overall concept may be difficult to use?
Wavetek (and probably others) use capacitance multipliers in their analog function generators to extend the low frequency ranges without using electrolytics. Here's one for the 555: https://forum.allaboutcircuits.com/threads/trying-to-apply-capacitance-multiplier.128372/