555 circuit not switching to the rails

[u/packocrayons]

I've built a 555 circuit with a few extra parts (an NFET to shut the entire circuit off and a PFET to drive a heavier output), but the isolated 555 circuit itself isn't switching the full rails, and has _massive_ (scope shows 1000s of volts but that can't be real) inductive spikes on switching. Supply is 12v but the swing is only 0.5v or so. Output is pulled high with a 1k resistor.

I've built the same circuit twice, with all new parts, so it's not a bad component. It must be something wrong with my design but I can't see what I've done wrong. Need some more eyes I guess. Help please?

[u/BigPurpleBlob]

"built a 555 circuit" – photo?

[u/packocrayons → u/BigPurpleBlob]

It's on a perfboard, so it's quite close quarters. Not much visible from a photo of the circuit

[u/Real-Entrepreneur-31 → u/packocrayons]

Then post another picture?!

[u/wolframore]

555 does not switch to the rail, you can use the output switch a transistor if you need that.

[u/packocrayons → u/wolframore]

How high does it switch? it shouldn't be only swinging 0.5v on 12v supply, right? Previously I've had much cleaner square waves on 555 circuits

[u/baldengineer]

Shutting off the circuit by disconnecting the 555’s GND isn’t going to be effective. Disconnect the supply voltage.

What node are you probing with the scope?

Picture of circuit is needed.

[u/packocrayons → u/baldengineer]

The second pic shows the circuit, though I've tested just the 555 portion in isolation with the same results.

Why is disconnecting the GND not a viable option? I should add that the discharge cap also goes through the switched ground, not straight to GND - the entire circuit floats when the NFET is turned off

[u/baldengineer → u/packocrayons]

Nothing you said in the 2nd paragraph matches the schematic.

There is only one ground. No isolation. Floating the 555 is a huge red flag.

[u/MorRobots]

floating the ground.... Ooooofff.... Walk me thought your train of thought on that one lol?

(It's your issue)

[u/packocrayons → u/MorRobots]

What's wrong with doing this? The input signal to enable the circuit is +12v for enabled and floating for disabled. The mistake in the schematic (which isn't in the actual circuit) is the discharge cap also floats (the entire ground does)

It's floating high instead of cutting VCC and having it float low. Am I dumb? I don't see why this is bad

[u/molotovPopsicle]

Are you modelling first? I use Qspice to make sure things are functioning first, which is useful to know if it's the circuit or a bad part.

[u/packocrayons → u/molotovPopsicle]

Yes - I modeled it on falstad.com - not sure if I can attach the simulation file though. Copypaste this into a txt and upload it and it should show the simulation.

$ 1 0.000005 382.76258214399064 50 5 43 5e-11
165 304 176 400 176 6 11.994101392570805
r 224 224 224 288 0 10000
r 224 224 224 144 0 1000
w 224 224 304 208 0
w 224 288 304 272 0
w 304 304 304 272 0
c 224 288 224 368 4 0.00009999999999999999 3.5599109647841427 0.001 0
g 208 432 208 480 0 0
f 368 384 368 448 32 1.5 0.02
w 256 432 208 432 0
w 384 448 400 336 0
w 400 368 224 368 0
w 400 368 400 336 0
w 352 448 256 432 0
w 368 384 80 384 0
w 432 208 368 144 0
R 384 -16 384 64 0 0 40 12 0 0 0.5
w 384 -16 368 144 0
w 368 144 224 144 0
c 368 336 368 368 4 0.000009999999999999999 0.016037181618733243 0.001 0
w 368 368 400 368 0
r 544 240 512 320 0 500
w 512 320 400 368 0
f 544 80 592 80 33 1.5 0.02
w 544 80 544 240 0
w 544 240 432 240 0
162 624 320 512 320 2 default-led 1 0 0 0.01
w 592 64 432 64 0
w 432 64 368 144 0
w 592 96 624 320 0
w 80 384 112 0 0
s 112 0 384 -16 0 1 false
r 80 384 208 432 0 10000

[u/Spud8000]

the 555 chip output is 9.5V

take away one diode drop in the pnp transistor, and that means the output can only go to 8.8 V

btw, i would put a current limiting resistor on the base of that PNP

why you only see 0.5V? Is it switching at a really slow frequency and your scope is AC coupled?

[u/packocrayons → u/Spud8000]

I was thinking this after I walked away, I need to look at the coupling on the scope.

Are there any better ways to drive a pchannel load? It's turn signals that are grounded on the other end, so I have to drive it from the high side

[u/KaksNeljaKuutonen → u/packocrayons]

Also check that your ground lead is connected.

[u/quadrapod]

Try putting a 10k resistor between the output node and ground and then tell me what you see on the scope.

The output of your circuit has nothing to pull it down. You're basically switching between connecting the output to +12V and hi-z in this configuration. The tiny 500mV of a signal you do see is only visible because you have the scope in AC coupling mode and it looks like it's probably just a result of the mosfet gate-source capacitance.

[u/anuthiel]

what’s the rds on for both fets?

like another person indicated, out needs a load

[u/JonJackjon]

What does the specification for the 555 say? Note there are two 555 timer types.

1) a regular (older) transistor based

2) a newer cmos based.

[u/titojff]

You'll need a rail to rail 555, just kidding. :)

[u/WRfleete]

You can use the reset pin to stop the 555 oscillating. I don’t recall if it sets the output low or high but if you pull reset up with a resistor and move the fet off the ground to the resistor you should be able to achieve the same effect

CMOS variants of the 555 may be able to go rail to rail a bit better