Is this standard NE555P behaviour?

Thread Starter

cspwcspw

Joined Nov 8, 2016
72
I bought some made-in-China NE555P timers. With the simple monostable circuit from a Philips datasheet, the circuits work, but what I see on the oscilloscope doesn't make sense. So I've misunderstood something, or perhaps these clones are behaving badly.

Here's my circuit.

NE555P_Circuit.png

Here's my confusing trace of what I see on the capacitor.

ne555p_trace.jpg

My understanding is that the discharge line should be "open"/High impedance, allowing the capacitor to charge. When the flip-flop sets (output goes low), discharge should be pulled low. So I don't understand the "camel-hump" that tops up the capacitor after my output goes low: could the discharge pin (or the threshold pin) be sourcing the current that is topping up my capacitor before it switches to "drain"?

With faster timeouts the camel-hump is less pronounced.

Thanks for any insights, or even the advice that this should definitely not be happening, and I should buy some better quality components instead.

Peter
 

GopherT

Joined Nov 23, 2012
8,012
@cspwcspw

If your friends in China are going to counterfeit at DIP-8 component, I hope they would be smart enough to NOT pick a $0.35 LM555.

As DodgyDave says, post your full schematic and a photo of your circuitboard.
 

Thread Starter

cspwcspw

Joined Nov 8, 2016
72
Aaargh! Please withdraw / cancel this request. There is nothing wrong with the chip or my circuit. My measuring equipment has some "zoom" X region active in the top half of its screen. The camel hump is not a voltage change, it is something else. On the Tektronix scope the waveform makes perfect sense.

Apologies.

Peter
 
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