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| Soft switch latch help |
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| Peabody:
--- Quote from: Zero999 on December 15, 2019, 11:06:52 pm ---Here's the circuit posted by hugo, changed to high side. One thing to note about this and the circuit I posted is it won't work without a decent minimum load, since the load has to have a low enough impedance to charge C1, without turning on Q1. --- End quote --- As I commented about the hugo circuit when he posted it, I'm still puzzled by how the On to Off transition works. If the bipolar transistor is fully saturated, then pressing the button will simply dump the capacitor without turning off the mosfet. So the 1M base resistor must limit the collector current enough that the transistor acts like a significant resistor, and the capacitor can then switch the gate off when the button is pressed. But if that's the case, then when the mosfet is On, the gate is in the middle of a divider when it's supposed to be at the rail. The link hugo posted has a video showing that circuit working, so I guess it does, but I wonder how well it supplies power to the load with the gate being at a middling voltage. Of course I may be misinterpreting how the hugo circuit works. That video also shows the power oscillating if the button is held down. I guess I still like the edn circuit even though it has one more resistor. |
| SiliconWizard:
True challenge now: designing the simplest possible circuit that: (I think none of the above completely fullfills all of these - I'd say most actually fail most of these) 1. Works properly regardless of the output load; 2. Toggles the switch state when pressing the button, with the following requirements: minimal button press time a few ms (say 10 ms, no need to be accurate, just to avoid spurious toggling), no maximal press time (should not oscillate or toggle back if we keep pressing the button indefinitely); 3. Leaks only minimal current in the OFF state in *any* condition; 4. Works for supply voltages as low as 1V; 5. Has a definite power-on state (either ON or OFF). |
| Zero999:
That's a pretty strict list of requirements there. I don't see why it needs to work down to 1V and if it's part of another circuit, then I don't see why designing it to accommodate a known load current is an issue. I'd probably not bother with these simple circuits and go with a flip-flop IC such as the 74HC74 or similar. |
| Peabody:
Here's the answer. Only 19 cents each: https://www.ebay.com/itm/352483795711 |
| not1xor1:
Here are a couple of other circuits (tested just in LTSPICE) The CMOS version require less components, probably is more reliable (tested within 1-500 ms range of button on-time) and might drive directly small loads. The simulation requires the CD4000_v.lib. Here is an NMOS + PMOS version. The component values depend on the PMOS gate charge and probably require various adjustments in the real world. Anybody willing to experiment? :) |
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