Star grounding and 9v

[Rob Strand]

The principle is; if the wire resistance and the current are both very low (which they will be for a fat copper wire and a stompbox circuit unless it's frying chips) then any volt drops caused are tiny. So don't sweat it too much!

I pretty much never put angry stuff on the same ground as quiet stuff or sensitive stuff.   It always does something bad even if you don't know it yet!

I remember chasing down mV DC drops in a circuit once.  I couldn't see how the layout (which was fine) could not cause it.  It turned out to be totally unrelated.  I had an NE555 with range resistors from 1k to 1M.  The current from the 1k resistor passed through the NE555 via the discharge pin (pin 7) and the power pins. This cause mV DC drops internally so when the output pin was low (pin 3) it was raised above the power pin (pin 4) by a few mV.  After I worked that out I could see that the 10k range resistor produced 1/10th the DC offset of the 1k.  When the output was high the offset was not present.  (There were other circuit offsets which made it just that little bit harder to identify.)

[amptramp]

555's are notorious for having huge "shoot through" currents when the upper and lower totem pole transistors on the output are conducting at the same time.  This may be on the order of several hundred milliamps.  With very rapid current spike, the resistance is not the only thing to consider on a ground line, the inductance comes into play as well because the edge frequency components may go into the megahertz.  The CMOS version of the 555 timer has a shoot through of about 8 mA, so if the circuit can accommodate it, it is recommended that you make the change.

[Rob Strand]

The CMOS version of the 555 timer has a shoot through of about 8 mA, so if the circuit can accommodate it, it is recommended that you make the change.

Actually it was a CMOS device.  IIRC at the time I did the detailed tests it was TLC555.  (At some earlier stage I had an ICL555 or something in there.)

I'm 100% sure it wasn't feed-through because if I held the o/p at 0V I could see the voltage drop on the o/p pin.   Then I tried 1/10th the current through the timing resistors to confirm it was in fact an ohmic drop.

The only reason I even bothered looking into it was because the cause of offset was unexplained. 

[ElectricDruid]

I've given up putting the 555 in anything I have to listen to. It's always a noise problem. If you've got a 555 in it, it ticks. While you can do all sorts of things to mitigate against that, it's easier to not have the problem in the first place. The 555 isn't such a sophisticated chip that we can't find other ways to do what it does.

[marcelomd]

In 2018 it is totally acceptable to use a cheap microcontroller in place of a 555. Just saying.

[duck_arse]

The CMOS version of the 555 timer has a shoot through of about 8 mA, so if the circuit can accommodate it, it is recommended that you make the change.

Actually it was a CMOS device.  IIRC at the time I did the detailed tests it was TLC555.  (At some earlier stage I had an ICL555 or something in there.)

I'm 100% sure it wasn't feed-through because if I held the o/p at 0V I could see the voltage drop on the o/p pin.   Then I tried 1/10th the current through the timing resistors to confirm it was in fact an ohmic drop.

The only reason I even bothered looking into it was because the cause of offset was unexplained.

interesting. just the other week, I was reading in the ETI Circuit Cook Book, No 6, on how to what with the 555 timer. and I read:

Note that the value of R1 greatly affects current consumption, since pin 7 is shorted to ground by the slave transistor during C1's discharge period.

[ElectricDruid]

In 2018 it is totally acceptable to use a cheap microcontroller in place of a 555. Just saying.

Absolutely! After all, you can always find a couple of other things to do with it too, once it's in there...

[Rob Strand]

interesting. just the other week, I was reading in the ETI Circuit Cook Book, No 6, on how to what with the 555 timer. and I read:

Quote

    Note that the value of R1 greatly affects current consumption, since pin 7 is shorted to ground by the slave transistor during C1's discharge period.

Yes, I make a point of avoiding small R1's.  The higher currents can also contribute to more drift.  I think the TI document about the 555 based charge pump had a 1k in there which chews more current than the load!.

My circuit had a set of range resistors initially covering 1k to 10M.  A few years later I re-jigged the circuit to use 1k to 1M.  That's was at the expense of making the circuit more complex but more reliable/accurate.  The range with the 1k wasn't that critical so I was happy to keep it.   I always had the ability to remove the offset.   The main point was initially I couldn't explain where it came from.

I'm not against the 555.   I've seen it used in Hewlett Packard equipment worth $15000.
Like any circuit you have to trade the good and the bad.

If you look at the 555 circuit there's one thing really clever about it.  The NE555 uses only a small number of transistors (and they are BJT's) yet the timing resistors can be up to 10MEG.   The comparator transistors don't require any base-current while the timing cap is charging between the two thresholds.

[ElectricDruid]

Don't get me wrong, I'm not against the 555. It's a great little chip and I've probably had a few around for 30 years now. When I was at school I modded a friend's joystick so that the "auto fire" function (which used a 555) had a variable speed pot. He was dead impressed, but it was simple to do.

It's just that if you put it in a pedal, you get ticks. It's easier not to and to use some other, non-ticky, way of doing whatever it was you needed it for.

[Rob Strand]

When I was at school I modded a friend's joystick so that the "auto fire" function (which used a 555) had a variable speed pot. He was dead impressed, but it was simple to do.

I remember those circuits!

It's just that if you put it in a pedal, you get ticks. It's easier not to and to use some other, non-ticky, way of doing whatever it was you needed it for.

Agreed.   Having sharp edges and/or current pulses doesn't help the cause for audio.

[ElectricDruid]

When I was at school I modded a friend's joystick so that the "auto fire" function (which used a 555) had a variable speed pot. He was dead impressed, but it was simple to do.

I remember those circuits!

Lol! I'm glad it's not *just* me!