Electronics > Beginners

Reducing 555 discharge current

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TwinOak:
Tim, I still don't think you are reading what I'm saying. I AM using the LMC555 already.

I've built the circuit up and it does work. You write that the (internal) transistor already gates the charging resistor, yes it does, but in the opposite state to the external one I've added. When the 555 (in monostable mode) is "off", idle, non-triggered, the internal transistor - be it a CMOS or bjt version - is fully on. To discharge the capacitor, yes, but as a side effect it will also draw full current through the charging resistor. If you like me would need this resistor to be small, much smaller than the recommendations, then that current will be substantial and orders of magnitude larger than any bias or leakage currents.

Adding the external FET only allows current to flow when the 555 is triggered. There will still be current flowing through it's pull-up down through pin 7, but the pull-up could be high value.

I chose the schottky because newer designs are getting quite reasonable in regards to reverse leakage, and I figured the lower forward voltage would give a lower error when calculating the RC circuit using the standard equation.

All the best
Alexander

MK14:

--- Quote from: TwinOak on September 14, 2018, 01:00:59 pm ---
-There are other chips better suited for the application

--- End quote ---

You can get very low power consumption MCUs, and they don't cost much these days. In tiny packages.
It could be put into some kind of "sleep" mode (switching off all the un-needed peripherals, as necessary, to minimise power consumption), until the time is up. Then toggle a port, which could drive a transistor or mosfet, if more output current is needed.
Without looking it up, the MCU current, at low (clock) frequency and using sleep mode etc applicable to the device you are using, is probably around a microamp or so, maybe less. But maybe it is 10 or 20 microamps. Best to look it up.

The program would only need to be quite short and you could easily do any other functionality in the program, depending on what exactly you are trying to do.

Once you have the code written. You could keep it and use it for that type of project, for the next 5 years. Just changing the required time constant(s), at the top of the source file.

But if MCUs are not your thing or you really want it to be all hardware, then carry on.
But all the circuitry you seem to be adding, seems to make a modern, very low power consumption MCU, an attractive option.

T3sl4co1l:

--- Quote from: TwinOak on September 18, 2018, 07:08:05 am ---Tim, I still don't think you are reading what I'm saying. I AM using the LMC555 already.
--- End quote ---

Oh, good!

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


--- Quote ---I've built the circuit up and it does work. You write that the (internal) transistor already gates the charging resistor, yes it does, but in the opposite state to the external one I've added. When the 555 (in monostable mode) is "off", idle, non-triggered, the internal transistor - be it a CMOS or bjt version - is fully on. To discharge the capacitor, yes, but as a side effect it will also draw full current through the charging resistor. If you like me would need this resistor to be small, much smaller than the recommendations, then that current will be substantial and orders of magnitude larger than any bias or leakage currents.
--- End quote ---

Not sure how to parse that part of the sentence  :-[, but anyway -- you aren't locked into the traditional circuit of course: you can put diodes in to speed up charge or discharge, you can tie the timing resistor to the output pin, no pullup needed (only three components required for an astable), you can use a current source or sink to get a linear (and perhaps better controllable?) ramp, etc.  Tons of add-on options. :)

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).



--- Quote ---I chose the schottky because newer designs are getting quite reasonable in regards to reverse leakage, and I figured the lower forward voltage would give a lower error when calculating the RC circuit using the standard equation.

--- End 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?).

Transistors (specifically, B-C junction) can be a lot better, and always perform better than the datasheet (2N4401 is spec'd for 100nA but I saw ~1nA in my example).  Smaller (low Ic(max)) and higher voltage parts typically perform better.  BCX70 is a good small GP transistor, and I've heard tell of some (I forget if it was BCX70 or some RF type) in the pA range.

Another problem with low current or wide range is the hFE reduction at low currents.  MOS may be better, unfortunately very small MOSFETs don't exist anymore (there are plenty of GaAsFETs and PHEMTs out there, but they are definitely not jellybean priced).  If you can't do it with a few CD4007s, you're pretty much SOL outside of getting it done in an ASIC...

...And that's where MK14's note comes in, MCUs can run on less power than LMC555, as long as what they're doing isn't being done very fast.  It's kind of sad, semantically speaking, to commit thousands of transistors to a task like this, but hey, when the incremental cost of a transistor is almost exactly zero as it is today, it doesn't really matter, does it? :P

Tim

Gary350z:

--- 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:

SeanB:
How about then using pin 3 as the voltage source for the timing as then your charge and discharge times can be separated and you have a very wide range available, and the current limitations you have no longer apply.

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