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555 question

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daviddoria

New Member
can i do without a resistor between 7 and 8?

http://www.bandtank.com/~calculusap/555.wmf

can i just have 8 to +v and 7 through R2 to C1?

this would be easier because the time constant would only be determined by 2 components (C1 and R2) instead of 3. let me know any reprocussions of this? do i need to replace R1 with a jumper, or can i just take it out?

david
 

kinjalgp

Active Member
No you can't do without it. It will blow off the internal resistors of 555. For safe operation, resistor of atleast 5k is required between pin 7 & 8.
 

sam_h

New Member
You can use 555 without having to use resistors in series with 7 and 8 but it depends on the chip itself but you'll have to check the ratings of your IC
 

kinjalgp

Active Member
This thing has been mentioned in the datasheet and use of resistor is logical. If you see internal block diagram of 555, pin 7 and 8 has a resistor of 5K which provides a 2/3 Vcc voltage to the upper comparator of 555. Now if you connect a lower value rsistor(<5k), the potential divider network will be disturbed and thus this reference voltage will also be disturbed and you won't get desired operation.
 
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