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Reducing 555 discharge current
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TwinOak:
Hello forums, first post here!

Just wanted to get some thoughts and opinions on a problem (and solution) I encountered recently. I was designing a circuit with a few 555s in one-shot configuration with variable pulse duration and wanted to get a large duration span. Not thinking about what I was doing, I whacked 100 ohm resistors in series with 100k pots for charging the timer caps. The supply voltage was quite high, around 13V and on first powerup the regulator got a bit toasty. With the pots at 0 ohms, 130 mA* was of course flowing through the discharge pin of each 555 chip.
So this could be fixed by increasing the minimum resistance and lowering the supply voltage, but what if I wanted an even larger span, I could go up to 1 Meg or even 10 Meg pots, but 10 Meg is not something that I've got in my junkbox, nethier is a large assortment of varable caps (I've got exactly zero). So I came up with this solution:


As I haven't found anything obvious on the subject when googling, and I can't stop thinking that this is somewhat of an ugly hack, I can only assume
-There's a much simpler solution while still using the 555 that I've overlooked
-There are other chips better suited for the application
-It's not a common circuit

Any thoughts?

Best regards
Alexander

*EDIT: actually limited to 100mA, at least for the LMC555
malagas_on_fire:
Let's get started with the basics, first the voltage Supply from a TI part can range between 4.5V to 16V, second the High level voltage of the output can be high as 12V so the LED and resistor will not be so happy.

Datasheet:

http://www.ti.com/lit/ds/symlink/ne555.pdf

Why only 100 Ohms on (Correction) 7 , 6 pins? The datasheet sugest a typical application of 9.2K on figure 9.
SeanB:
How a bout a series resistor to pin 7, probably around 33R. Will reduce the discharge current, and aside from increasing the delay time a little, will work. As well the regular 555 does have some rather large current spikes on switching, even with nothing connected, so a supply bypass capacitor ( 470-1000uF 25V electrolytic right near pins 1 and 8 is about right) is needed, even if your supply is capable of supplying 1A or more.
malagas_on_fire:
Yeah the bypass caps are quite needed maybe an extra  100nF directly between the power pins, in parallell with fat capacitor. The temperature seems to be another factor on the accuracy up to 1.5 %, 50ppms as stated in datsheet.
Gyro:
I've never managed to find a discharge pin maximum or peak current specified in a 555 datasheet, I've looked several times when thinking of using it as an open-collector output.

The main output pin maximum current is well specified and figures are also given of Discharge pin off-state leakage and on-state voltage at a nominal current. Given that in most configurations there is a timing capacitor, without value limits, directly connected between Discharge and ground, it's weird that there's no peak current spec (that I can see). The bipolar 555 block diagram invariably shows the Discharge pin as just an open collector transistor.
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