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555 timer output clipped

YASH VERMA

Mar 21, 2018
7
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Mar 21, 2018
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Hi All,

I have a question regarding 555 timer astable multivibrator mode.

I have simulated 555 timer in astable mode for 50% duty cycle, 250Hz wave with supply fixed at 8V, and I am getting it exactly the way I want. However,the same circuit when I am connecting on board with same supply and verifying it, I am getting 555 output as 6.75 V as peak of square wave.

I am using NE555 IC.

Please help me understand this scenario.

Thank you in advance.

Best Regards
 

Bluejets

Oct 5, 2014
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6,901
What loading do you have?
A circuit diagram would also be helpful.
 

kellys_eye

Jun 25, 2010
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The output can drop as much as 2.5V with a 200mA load. Your simulation likely used an 'idealised' component rather than a real-world version.
 

YASH VERMA

Mar 21, 2018
7
Joined
Mar 21, 2018
Messages
7
The output can drop as much as 2.5V with a 200mA load. Your simulation likely used an 'idealised' component rather than a real-world version.

Oh Okay. Thank you.

I actually want to generate a triangular waveform of 0-8V peak, 250Hz.

So i considered generating a square wave using 555 timer for supply of 8V, and 50% duty cycle with frequency of 250Hz. After that, I considered integrating this signal with a RC integrator, then clamping it, then amplifying it.

I am sharing my schematics image. Can you please help me understand why is this happening?

Thank You.

Best Regards,
Yash Verma
2018-05-25_10h23_08.png
 
Last edited:

Audioguru

Sep 24, 2016
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Instead of lowpass filtering a squarewave to make a lousy looking triangular waveform, why don't you make a good triangle waveform with an opamp as a comparator and another opamp as a real integrator?
 

AnalogKid

Jun 10, 2015
2,884
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If I understand things correctly, the problem is that a 555 powered by 8 V doesn't produce an output that peaks at 8 V. This is correct operation for the bipolar NE555, and is shown in the datasheet. Changing to a CMOS 555 (LMC555) will improve the output, but it still will not be perfect.

All transistors have some voltage drop across them no matter what type or what circuit. The only way to get exactly 8.0 V out of any circuit is to power it with more than 8 V so the output stage has some "headroom" to work with.

If you are stuck with the bipolar 555 and an 8 V supply, you can add an improved output stage that will raise the signal to 7.9 V or a little more. This is not nearly as complex as the circuit in post #5. It is a variation of an opamp booster stage, configured to operate as two saturated switches. Do you have access to small signal NPN and PNP transistors such as 2N4401/4403, 2N3904/3906, or 2N2222/2907? Also, 5.1 V zener diodes? Note, R3 does not need to be 1%.

ak
8V-Driver-1-c.gif
 
Last edited:

YASH VERMA

Mar 21, 2018
7
Joined
Mar 21, 2018
Messages
7
If I understand things correctly, the problem is that a 555 powered by 8 V doesn't produce an output that peaks at 8 V. This is correct operation for the bipolar NE555, and is shown in the datasheet. Changing to a CMOS 555 (LMC555) will improve the output, but it still will not be perfect.

All transistors have some voltage drop across them no matter what type or what circuit. The only way to get exactly 8.0 V out of any circuit is to power it with more than 8 V so the output stage has some "headroom" to work with.

If you are stuck with the bipolar 555 and an 8 V supply, you can add an improved output stage that will raise the signal to 7.9 V or a little more. This is not nearly as complex as the circuit in post #5. It is a variation of an opamp booster stage, configured to operate as two saturated switches. Do you have access to small signal NPN and PNP transistors such as 2N4401/4403, 2N3904/3906, or 2N2222/2907? Also, 5.1 V zener diodes? Note, R3 does not need to be 1%.

ak
View attachment 41237
Thank you so much
 
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