# How to block DC of triangle waveform from 555 powered by single supply?

I'm using a 555 timer to generate a triangle waveform with a DC offset. I would like to feed the waveform into a comparator along with an AC signal with zero offset to get a PWM signal, so I need the average voltage of both waveforms to be equal. I'm trying to do this with an DC coupling capacitor to make the triangle waveform centered around 0V, but I'm only getting negative voltage after the capacitor. Why is this happening? How can I get a triangle waveform from the 555 with zero DC offset?

Here is the schematic:

Here is the voltage after C7. It has a ~-3.7v DC offset for some reason:

• If need a low impedance then a biploar supply is an alternative – Tony Stewart Sunnyskyguy EE75 Apr 4 '18 at 15:58
• That looks like a simulator, not a real steady state trace. Try again 200KHz with a 0.16 Hz HPF and a simulation after 10us is still got a DC offset. Think! (steady state) – Tony Stewart Sunnyskyguy EE75 Apr 4 '18 at 16:19

C7 and R5 make an RC high pass filter with a cutoff frequency of .16 Hz and an RC time constant of ~1s. In order to have no DC offset, the capacitor must charge which takes $5\tau$ to reach 99% charge, or 5 seconds in your case.