Electronics > Beginners
Reducing 555 discharge current
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Gyro:

--- Quote from: Gary350z on September 18, 2018, 02:33:22 pm ---
--- Quote from: Gyro on September 14, 2018, 03:53:57 pm ---I've never managed to find a discharge pin maximum or peak current specified in a 555 datasheet

--- End quote ---
I recently found the answer to this:

--- End quote ---

Thanks Gary, I clearly haven't checked the latest TI datasheet version recently! :D  The LM555 designation implies that it came from their Nat Semi acquisition.

It does look as if it relies no the pulldown transistor coming out of saturation at higher currents - as far as I can see, they are using a common set of output sink figures for the discharge pin and the Output pin. Maximums just referenced to device dissipation.
TwinOak:

--- Quote ---Oh, good!

I didn't see anywhere in this thread that you did.
--- End quote ---

I didn't think that it mattered that much since we're dealing with the current consumption (mostly) dictated by the external components and not the chip itself.


--- Quote ---I once made a triangle wave generator with a hysteresis comparator, FET follower and two current mirrors.  With heavy bias (~10mA) in the comparator (this was all made discrete from 2N4401/3s, except for the 2N4393 JFET), the maximum frequency was some 33MHz (Ct = 100pF I think it was at), while the minimum frequency was about 100Hz, limited by leakage current in the 2N4401/3 current mirror, corresponding to a few nA I think it was?  Which is not at all bad for a relatively large (600mA) general-purpose transistor.  Probably helps that it was cold in the shop at that time, too...

If you're going for low supply current consumption, a 555 of any type is simply not what you need (note LMC555 is 50uA and up!).  There are micro- and nano-power timers on the market that may be of interest, or you can build your own from logic chips and/or low power comparator(s).
--- End quote ---

Now we're getting to the interesting stuff, thanks! I'll probably dig down into building discrete solutions in the spirit of this for later designs. Regarding low power timers, I don't know, I'm not really into building super low power designs for the sake of it at this stage. I just thought it was interesting that you could get the 555 to run much more efficient as a monostable with just a few jellybean parts. Perhaps I should have been a bit more restrictive with my questions in the OP, for the sake of clarity :)

I took the time to do some actual measurements. At 5V supply, the idle current of my particular LMC555 (with no charging circuit connected) is 100uA. With the FET/diode hack and R1 at 100k, the idle consumption rises to 150uA, regardless of the value of R2 (Rcharge), it could be anything from 0 ohms to open.
With the standard 555 monostable circuit, of course I also get a current draw of 150uA when setting Rcharge to 100k, but this increases as the resistance goes down...
0.6 mA at 10k, 4.9 mA at 1k, 42.9 mA at 100 ohms and peaking at around 98mA with 20 ohms. Lower than this and the chip throttles the current to 100 mA.
(reposting the relevant part of the circuit for ease of access)


I also checked to see how the timing would be influenced by the hack, for short pulses the duration increases somewhat, I assume beacause of losses in the FET, at longer durations it goes down because of the capacitor being "pre charged" to the Vf of the diode, and some initial reverse leakage. C3 at 220uF.


--- Quote ---The lowest leakage commodity one I know of is BAS70, which isn't bad, but even 1N4148 outperforms it by 10x, while being rated for over four times more current (so it's ~40x better by die area, presumably?).
--- End quote ---

I did some thinking around this, and realized that for the proposed mod it's not bad at all to use a schottky. The only time it's going to see a reverse bias condition is the first instances after the timer is triggered, when the + of the cap is basically at ground potential and pin 7 goes open (or high in this instance because of R1). It will alter the timing characteristics slightly, but not the idle current consumption.


--- Quote ---It's kind of sad, semantically speaking, to commit thousands of transistors to a task like this, ...
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Exactly what I've always thought! And personally there's another side to it, I've been programming for the best part of my life, it's just no challenge to program a measly timer.

So why do I keep nagging about this quite insignificant mod? Probably because of the 555 being such a classic chip. I'm positive that I won't be the last person to base a design around it, just because there's so much info readily available. Hopefully there's someone else out there who finds it useful to squeeze 5 or 6 decades worth of pulse time from it without pissing away 100 mA (and finds this post :D).
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