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EEVblog #262 – World’s Simplest Soft Latching Power Switch Circuit
Posted on March 30th, 2012 32 comments
Want to use a single cheap momentary action push button switch to toggle your circuit power on and off? Try this circuit on for size.
EEVblog #262 - World's Simplest Soft Latching Power Switch Circuit [ 17:43 | 197.92 MB ] Download (89)29 responses to “EEVblog #262 – World’s Simplest Soft Latching Power Switch Circuit”
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I watch, the whole mobie.You had some really great ideas with ‘Soft Latching Power Switch’, I will try them out my myself, thanks!
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Ed Voncken March 31st, 2012 at 01:14
Heh – and here I was, thinking that Jellybean was an Aussie supplier of electronic parts
Love the explanation, thanks!Would it be better to replace both bipolars with mosfets?
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Kukinjos March 31st, 2012 at 04:10
Hi Dave,
Great idea. I made this, and it works well, no oscillating at all, and it is simpler.http://img36.imageshack.us/img36/420/davidlw.png
Greetings,
Kukinjos-
Wow that’s a whole lot more elegant, thank you for sharing!
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Looks cool but I can’t figure out how Q1 gets turned off. Also, when it is not powered on how do you ensure that the gate of Q1 is not high?
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Kukinjos March 31st, 2012 at 06:45
Q1 discharge C1 thru R3 when is turned ON, so when you push switch, discharged capacitor pulls gate to 0V. When is not powered on, gate of Q1 is low, why? because it is not powerd on and Q2 is allso not powered on, and on output is 0V.
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Jorge Garcia March 31st, 2012 at 06:58
Here’s how it works, initially the unit is off. The capacitor is charged through R1 and R3, the voltage across the CAP will be the same as Vin. When you push the button a Voltage of Vin shows up at Q1′s gate turning it on which in turn activates Q2. Now we have a voltage at the output which will latch Q1 on.
Here’s where the magic happens if you notice the RC circuit formed by the Cap and R3 is connected to the drain of Q1, so when Q1 get turned on it will actually discharge the CAP through R3. With enough time the voltage across the cap will fall to zero and be ground potential. The next time you press the button now ground will be presented to the gate of the Q1 which will turn it off as well as Q2.
The only problem I see with this circuit is that technically (due to capacitor leakage) it does consume some power when off to refresh the capacitor but it so small that it wouldn’t make a difference in the grand scheme of things. Sorry for splitting hairs.
hth,
Jorge Garcia
Cadsoft Support-
a problem i see with both solutions, yours and daves, is that you will have a problem switching it off when you have a capacitive load connected to it. this might especially be an issue when applied in a lab power supply. if you have current flowing back from the device connected to the power supply, it might keep the power supply on.
i haven’t tried it though on the breadbord or pspice…
regards,
Bart M-
f4eru April 3rd, 2012 at 05:57
Bart, there’s an easy solution to that one also !
Just add a C2 capacitor between the gate of Q1 and GND.
C2 =1/10 * C1R2/C2 has to have a bigger time constant than the falldown of the power supply.
Another solution for long times : a second mosfet in “parallel” of Q2 (one for the output, one for R2. That needs another resistor as a load, but works nicely.
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Looks like it might struggle to switch off if your load has any significant capacitance. If the output voltage is still high enough to keep the pass mosfet on by the time C1 is charged by the 1M+100K it will fail to turn off.
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I really like the elegant solution posted by Kukinjos.
I have a Li-ion battery as Vin on the latching circuit. It works fine lighting up an LED – I can click the switch on and off and the latch circuit behaves like I would expect it to.
But, when I try to attach it to a boost voltage regulator, it won’t turn on. My voltage regulator is boosting the 3.7V Li-ion up to 5V.
Voltage regulator: LM2623
P-MOSFET: IRF9510
N-MOSFET: ZVNL110AI notice people have posted concerns about the switch not being able to turn the supply off. I am having issues just turning the latch on.
Any thoughts?
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Antonio March 31st, 2012 at 06:01
Loved this one. simply brilliant. Is this a design that could be easily ported to SMDs for a low power design?
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Kukinjos March 31st, 2012 at 06:31
Of course, I tested it down to 3.2V. It depends on mosfets Vgs you use.
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Half a bee’s dick! Never heard that one before!
Could have used a PIC10F200 and then got all your programmable control and various other functions done in the same part. Only about 10 cents more. The oscillation when holding down is a fail in my book.
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Superb video/tutorial once again, Dave.
I followed along by simulating each of your circuits as you went along – building it up to the final design and I was thrilled to see that my simulation was working just as you had shown in the vid. Magical stuff!
*Although I didn’t really get the simulations working until after you gave the resistor/cap values nearer to the end… but that’s no big deal. I’m not an EE myself so I didn’t have a feel for what the values should have been*
Tremendously good content.
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I haven’t had a chance to watch this video yet, as I am at work. But its funny because I am designing my very own right now as a project: “Ping pong scoreboard”
I have four buttons to the unregulated battery DC power source. I have these isolated by voltage divider/diodes only to the regulated side. This could be better…. Isn’t there some clever capactor trick, to isolate two voltages yet still sense from one side to the other without magnetics?
DJ
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What is this, April Fool’s Day? By logical definition, a power switch that requires power can turn itself off, but can never turn itself back on again.
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cbseattle April 1st, 2012 at 02:51
Absolutely brilliant. I’ve been looking for a way to solve this simply and everything I found is more complicated. But the best part is the presentation and explanation.
If I ever want to do anything, I just hope that you might have done a video about it. This one came just in time. So thanks.
p.s. what I’d really like (if you are taking requests) is an incrementing switch. In the simplest terms, it would act a bit like a manual meter… an LM3914 operated by a switch instead of a voltage.
Again, thanks so much for helping.
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Craig April 1st, 2012 at 05:00
I like how clean and simple this is.. I’ll have to breadboard it up and play around with sometime. Too bad I don’t have any p-channel FETs lying around… maybe I’ll try a PNP BJT.
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A similar approach is used in guitar stomp boxes. Here are some schematics.
http://cdn.tonegeek.com/wp-content/uploads/ts-808.jpeghttp://www.hobby-hour.com/electronics/s/schematics/boss-ce2-chorus-schematic.gif
http://www.guitarelectric.eu/schematy/Efekty/Distortion/boss_ds-1.gif.html
The switch could also be implemented using any D type flip flop arranged as a divider by 2. Most FF chips have multiple independent sections which would allow more switches per single chip.
http://www.discovercircuits.com/H-Corner/on-off-switch.htm
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Some time ago I though up something like this, only coupled with a microcontroller in a battery-operated device.
The idea was that the gate of PFET is pulled low by the button, and then held low by MCU until it’s finished with its task (whatever it happens to be) and then released, turning itself off to save battery until activated again by user.
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Michael K. April 6th, 2012 at 01:53
I love the idea, the solution and your video but…
-> It’s possible to switch it on (again) with an external voltage.
I would prefer the discrete flipflop solution.
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Great circuit. But I wonder how’s the cost of all those parts+ momentary switch compared to the toggle switch solution? Would it still cost less?
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The patented version of this circuit mentioned in the forum discussion is available at:
http://www.pololu.com/catalog/product/750
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Hey Dave I really like this idea but one thing I noticed is the circuit powers up when the supply is turned on. If my project loses power like the lights in my house flicker isnt this going to turn on? I don’t like that any way we could fix this, assuming my project is supplied by a wall wart, ac/dc converter or whatever pluged into mains.
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I may use this for my LCR bridge. The mains have to stay back in their shielded cage in the back. The original push on push off switch died. The EE I bought it from put a mini toggle on the BACK of the bridge. This is too stupid! I saw this used as a way to switch a practical joke into the caps lock key on a computer kbd that would play a musical greeting card you hide in your poor co-worker’s computer keyboard. I was too unamused at the time to even think about it! The bridge does travel with me, but reaching around back kinda sucks! A 5 cent momentary PB sounds great! No matching a part that was likely made special for a $20,000 LCR is very appealing! My friend abandoned being a EE and went to Dentist School! Let’s see them send that job over seas!
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Hi Dave,
I built this circuit on an SMD board – it works great except for that when the board is handled it turns on unexpectedly with no button presses. I assume my body is at another ground potential or my fingers moisture is somehow triggering the FET. Is there some easy component to add to avoid this problem?
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Piotr March 31st, 2012 at 00:39