Electronics > Beginners
Latching Circuit
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clevis:
Hi All,

I tried out the simple latching circuit described in EEVBlog #262 (the first circuit diagram discussed, not the one eventually built). This is a two button circuit, first button to latch, second to unlatch. The problem is that current flows immediately after turning on power, without pressing the first button. I suspect this may be down to some leakage current but I have tried several tweaks but no luck. I tried modelling this in LTSpice (schematic and trace attached), and on a breadboard - with same unexpected results.

Any advice?

Thanks.
MK14:
Is the problem with the LTSpice , because of your initial voltage settings, inside the simulator ?
It lets you start up with different methods/voltages, via the settings/options stuff, called something like initial starting conditions.
I think one is called something like, "click here to start at 0V".
clevis:
MK14,

I suspected something like that and had tried with a "pulse" LT Spice supply which turns from 0V to 3.3V after 1 second. The latching button is closed on 4 seconds which should turn things on, but circuit is already on and the button has no effect. I attached that revised simulation and trace, plus the input voltage profile.

I already tried the original circuit (without input delay) on a breadboard and same result as LTSpice.

Thanks.
MK14:
The page I was talking about, looks a bit like this:

"Start external DC supply voltage at 0V" and other settings to play with.
Otherwise the simulator can do funny things, because it automatically sets the initial conditions. Which might not be what you want and/or are expecting.



EDIT:
I thought/assumed you would connect it to the supply voltage, rather than powering it from a pulse.
MK14:
Also R2 100K, will have a tendency, to attempt to turn the unit ON, at first power up. Because of its capacitance (and combined with the various transistor junction capacitances). Also, the various capacitances (stray), of the various transistor junctions.
So I can well believe it powers up, NOT in the way you want (presumably OFF is what you want, I think you said so, earlier).
These circuits (power on/off switch substitution) can be a real pain in the neck, to get them to behave as exactly as you want.

Some of these circuits, are better and much more reliable than other circuits. You are best, going for one of the reliable ones (which that might be, but I'm not sure off-hand. I think the reliable ones that work well, have more complexity to them).
I think there have been threads about it here, which could be a good source of better circuits and/or, later in that video you mentioned.
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