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Polarized Capacitors and 555



This circuit of yours is EXACTLY what I was looking for. TOTALLY cool! However, I have a few questions about it…mostly because I’m a software guy. Astable 555 Timer Flasher

So please bear with me if these questions sound stupid…

1) I want a 50us on / 280 us off (3000 khz ish with at 15% duty cycle) circuit. THis means that Ct needs to be about 7000pf. Unfortunately, you can’t get 7000pf caps polarized. Does Ct REALLY need to be polarized?

2) Also, what does a polarized capacitor do that an unpolarized one can’t? Could I just stick a diode on it to make it polarized?


3) THere’s a bubble on pin 4 (the reset pin) on the 555. Does that mean I need an inverter? Would a 74HC04 do the trick, or is there an easier way to do that (I need the HC because I have 6v Vcc)

4) I just want the output signal…can I drop all the LED and battery stuff to the right without affecting the circuit?

Mail from GK

1) A cap being polarized is due to manufacturing. Only Electrolytic Caps including tantalum caps and others that are polarized give you high capacitance. If you do it with just a plastic, mica or paper separator (dielectric) then you get very low capacitance. So they have to wrap or coil many meters of two-metal-films and a insulator like plastic sandwiched to get upto 10uF or even more. These are non-polar and will become very big for say 100uF. Hence we have the electrolytic caps with high uF for the PCB real-estate they occupy.

UnPolarized are ideal and can be used anywhere. Use Polarized caps where the footprint area you need is less for a certain value of cap.

So you choose caps based on space, cost, AC-DC, purpose. Thumb-Rule 1uF and above only el-caps. Unless you need it in a AC circuit or high voltage like 1 Kilovolts where polyester caps are used. 7000pf or 7nF is available as ceramic, mica, plastic, paper. use what you get right now. Making 7nF el-caps was never required. 1uF and Above el-caps.

2) Being a polarized cap is an handicap. So you dont need to polarize a unpolarized cap, so just leave it alone, it is fine as it is.

3) The bubble says “I pin 4 of 555 have a inverter inside the 555”.

4) You can drop the LEDs, but you have to power the circuit with a batt or mains-DC-9V-adapter.


Thanks for the response! So I don’t need a 74×04 chip to go into the reset pin? Cool. I’ll stick to the non-polarized cap and and a direct input into the 555 pin 4. The board I’m using is powered by 6v, I hope this will do the trick, don’t want to go the rectifier route.

Reply from GK

“When you build projects Software and Hardware, design products and services that are useful to society” – Solderman 1709



Product Design - Industrial Automation and Instrumentation.

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