Power Latching Circuit

[@BersekViking]

I think you should have a resistor in series with the push button. As the circuit is now, you are connecting a 22uF capacitor, charged to 9V, directly over the B-E junction of the transistor. Just a matter of time before it fails.

[@paulpkae]

Thanks for your comment. It's given me an idea for an interesting experiment. Watch this space...... 😁

[@EdevaldoPereira]

And as you increase input voltage, the energy of the discharge will increase with the square of the voltage, making the device more likely to fail. A few kOhm in series should help. But when the transistor on the bottom right turns on it may also be taking the energy discharge. So having it in series with the capacitor may work better.@@paulpkae

[@paulpkae]

@BersekViking many thanks again for your comment. I have now put this through some testing and have published a follow up video. A link has been added to my comments above (click "...more" if you cannot see it). Would be very interested to hear your thoughts. Thanks again.

[@onit996]

I agree. In the datasheet, it looks like 500mA are still okay for the base current, so a 22ohm resistor would adequately limit the current in a 9V circuit, and shouldn't harm the functioning.

[@onit996]

But maybe put it in series with the capacitor to also limit the current through the collector.

[@davethedaemon9024]

Few seem to acknowledge high-side switching with P-Channel MOSFETs. Thanks! I build my own multi-rail power supplies for my projects and need high side switching due to a common ground. A problem with P-Channel mosfets is a relatively high VGS(th). The IRF4905 may not work well with 3.3v because the VGS(th) is 2.0v to 4.0v. It's hard to find ones with lower VGS(th). I have some very real NDP6020Ps with a very very low VGS(th). I also have dozens of counterfeit ones. Fair warning - These aren't made anymore and you can only buy counterfeit rejects that are unusable. FYI - There's an interesting IC for N-Channel high-side switching. The LTC1157 is expensive at $6 US. An internal charge pump will boost the gate drive voltage 5.4V _above_ the _positive_ rail to drive an N-Channel mosfet on the high side. Charts seem to suggest a normal operating range between 2.75v and 5.5v. I bought 10 for my next set of projects.

[@paulpkae]

Thanks for your comment. Some interesting aspects for people to consider.

[@pault6533]

I like your schematic animation. Is any switch debounce warranted, or is there sufficient switching delay? What voltage range applies? How much current to operate?

[@paulpkae]

The capacitor is already wired inline with the switch, there is no additional debounce circuitry required. I have tested at 5V, 9V, and 12V but you're really only constrained by the ratings of the components you utilise. The 2N2222 has an absolute max Collector to Emitter voltage of 40V, but you can always substitute the transistors with those of your own choice. With differing transistors however you may need to also play around with the resistor values slightly. The power overhead is negligible. In its OFF state there is no current drain once the capacitor is charged; there will be some minor leakage current but

[@S_e_r_g_ei]

I'm watching from Russia. Well done author! Respect! Made simply and reliably! But, there should always be a scheme! Preferably posted separately or in the video itself, at least for 4-5 seconds. Then it will be considered a full-time job, and this is already professional and high-quality! And you will want to watch something else on this channel! //// Смотрю из России. Автор молодец! Респект! Сделано просто и надёжно! Но, всегда должна быть схема! Желательно отдельно выложенная или в самом ролике, хотя бы на 4-5 секунд. Тогда это будет считаться полной работой, а это уже профессионально и качественно! И захочется ещё что-то смотреть на этом канале!

[@paulpkae]

Thanks for your comments. It's interesting to know where in the world my videos are reaching. When you say "scheme", do you mean schematic? If so, you can always pause the video at say 1:00 exactly.

[@S_e_r_g_ei]

@@paulpkae I think KZbin is watched all over the world. And You are seen all over the world! And it is very valuable that you conduct an explanation of any product. I watch a lot of similar material from Russia, as well as from all over the world. Of course, you can stop the video and look closely at the scheme (diagram), but this is not serious at all. :) And it is always very valuable when a specialist additionally lays out a scheme (diagram). After all, it is not at all difficult to upload and view the archive on Google, specifying it in addition to the video. And it is advisable to lay out such schemes (diagram) not only for specialized electronics programs with the extension (.spl7 ; lay6), but also in a simple format.png ; .jpg ; .bmp So that you can view these diagrams in a simple viewer. It's just my kind advice. Sincerely from Russia. \\\\ Я думаю, что Ютуб смотрят по всему миру. И Вас видят во всём мире! И очень ценно, что вы проводите объяснение какого-либо изделия. Я смотрю очень много подобного материала из России, а также со всего мира. Можно конечно остановить видео и посмотреть внимательно схему, но это уже совсем несерьёзно. :) А всегда очень ценно, когда специалист дополнительно выкладывает схему (диаграмму). Ведь совсем не сложно выложить и дать просмотр на Гугл архив, указав его в дополнении к ролику. И желательно выкладывать такие схемы не только для специализированных программ для электроники с расширением (.spl7 ; lay6), а и в простом формате .png ; .jpg ; .bmp Чтобы можно было посмотреть эти схемы в простом просмотрщике. Это просто мой добрый совет. С уважением из России.

[@Vulpecula607]

For some reason this circuit works for me on a breadboard only. I used off the shelf TO92 components for that. But implemented on a PCB with SMD Parts (BC847CDW1T1G and BUK4D38-20PH) it is just on all the time and trying to switch it only lets the voltage dip down for a second before coming back up. (I’m trying to latch a 7.4V 2S LiPo battery.)

[@paulpkae]

That sounds frustrating. The type of transistors your using will influence the perfect resistor sizing. It sounds to me like you need to get a scope on your board and see what's happening. Monitor the capacitor charge and NPN base voltages as a minimum. It will probably shed some light on what the issue is.

[@user-py7ih9et4p]

Did you find the problem?

[@jensschroder8214]

If there is a large capacitor at the output. Then it can switch the transistor back on via the 100k.

[@yongsu2733]

Block by a diode?

[@paulpkae]

Well spotted, thanks for your comment. Yes, any back feeding current could turn the two BJTs back on and activate the output again. A blocking diode at the output will resolve it. You will have to take into account the voltage drop across the diode (in addition to effect of the MOSFET ).

[@stiiffyrabbit]

It occurs to me that you might obtain additional function from the switch, if you DID time the length it is depressed. I would like a switch that is 'on/off' but, after being pressed for 2 seconds, powers an 'alternative on' function - maybe some other mosfet or circuit?

[@paulpkae]

Hi, thanks for watching and your comment. You could achieve the circuit you suggest by charging a capacitor via a resistor when the button is pressed. Then use a comparator (or 555) to check when the capacitor charge reaches a given threshold. The output of the comparator can then in turn activate another function. I have posted a previous video in which the the single button provided three functions, but that was using a microcontroller. See link below. kzbin.info/www/bejne/aKq3gaSHaLVoprMsi=f8mBrQJYezLPxUF4

[@ConsultingjoeOnline]

Great video. Thanks! Q: Do you think there would be any issue using a 500k ohm resistor instead of the 660k for a 12vdc input and 12v relay on the load?

[@paulpkae]

I would have thought that would be OK. However I did find its a bit of a balancing act between the transistor types and resistor sizing. I may still have this breadboard rigged up at home. In which case I will test it put for you and let you know.

[@js4512]

I am trying to figure out a latching circuit to control a 1 amp draw at 24 volts what component values would I need for each to reliably maintain the overall circuit integrity? I want something bullet proof, I am using it to turn on and off a gas regulator for welding with a tig torch.

[@paulpkae]

Possibly a latching relay but, given the safety nature of your requirements, I would urge you to seek a proprietary, possibly "industrial", product for this.

[@jjoeygold]

Have you considered the limits of Vin ? As I think there may be issues at Vin=24v

[@paulpkae]

Believe I tested it at various voltages, but probably no more than 12V. The circuit is an example arrangement which may need adapting depending upon individual circumstances; load current, voltage supply and component ratings being key factors to consider. Out of interest, why do you specifically think 24V is the limit?

[@jjoeygold]

@paulpkae because I tried similar circuits and I think gate threshold might break down

[@igorzherebiatev5751]

Hi. Thanks for the schematic. I have built this one recently. Based on the comments to the older video with similar one. Didn't work well for me with 4 v power supply. But may I use an n-channel mosfet with p-n-p transistors just to switch polarity? I have a bunch of n - channel, but no p-channell. And they are too expensive to buy.

[@paulpkae]

Thanks for the comment. I think many of the circuits we see on KZbin (including my one) have derived from an EEVBlog post he did some time ago. I guess there is only one perfect solution and every variant we see is simply getting closer to that perfect solution. At 4V you may need to play around with the resistor values to obtain suitable current to turn on the BJTs. As for using N-Channel MOSFETs that would of course change the logic of the circuit and would need to be re-arranged. It may be possible to combine two N-Channel MOSFETs (one inverting the other) to achieve the same effect. I bought a pack of P-Channel MOSFETs for just £9.99, See the links in my description.

[@igorzherebiatev5751]

@paulpkae thanks for such a great response. With my first build, I can turn it on, but I can't get it off. About transistors - I suppose to use a p-n-p bjt for a latch and n-channel type as a key

[@paulpkae]

@@igorzherebiatev5751 not quite sure what you intend doing, but bear in mind, depending how you arrange the circuit, a BJT may end up drawing current even when the circuit is in an off state.

[@igorzherebiatev5751]

​@paulpkae Hi again. At first, I wanted to say thank you for your answers. At second. I just got a fully functioning version of your scheme but with a N channel mosfet. The P channel is rare to find from salvaged and desoldered boards and the cheapest ones to buy costs more than pound of white bread here. So I just used a pair of PNP transistors, S8550, and an unknown type of N channel mosfet from an old motherboard. Reversed a polarity of the capacitor and set a "+" as a ground. Tested it in a simulation and built it on board. Had to increase the capacitor to a 1000 micro Farad. And lowered the resistance from 660k to 330 because it will be used in 9 v device. Had an interesting effect. You have to wait for about 15 seconds before you can turn it on again.

[@paulpkae]

@@igorzherebiatev5751glad you got it working. Well done! N-Channel MOSFETs are indeed much more common. As are NPN BJTs.

[@kbiane]

I apologize for the very basic question. I'm new to electronics and am trying to get a handle on the fundamentals. Could someone explain the role a capacitor plays in this context? I understand that it charges and then discharges when the switch is pressed, but I don’t quite understand why this is necessary

[@paulpkae]

Great question thank you. When the circuit is in the ON state, the capacitor provides sufficient delay, for when the switch is pressed again, to allow the MOSFET to turn off. Without it, then when the BJTs turn off, the right-hand side of the switch is immediately pulled high (via the 660K and middle 100K resistor) and subsequently the BJTs turn back on before the MOSFET had a chance to turn OFF.

[@kbiane]

⁠@@paulpkae Thank you!

[@cncdavenz]

Hi, Any chance to modify this circuit to also time off to save battery. I think this would be useful for many people. Good luck with your channel.

[@paulpkae]

Hi thanks for your comment. If you want the circuit to simply timeout and kill power after a given period, that's quite straight forward to implement. If you want the circuit to timeout after a period of inactivity, that need to be implemented within your code. Which option are you interested in?

[@cncdavenz]

just a simple circuit that will alow a battery to last for a long time when off. thanks Dave@@paulpkae

[@paulpkae]

@@cncdavenz the circuit in this video does exactly that. In the off state, the MOSFET prevents current drain from your battery.

[@maximini8427]

Can you explain this in logic gates, i'm having trouble understanding this😅

[@iblesbosuok]

Replace both BJT with _open collector NOT_ gate, base as input and collector as output. Actually, the key is _Thevenin equivalent_ of 680k and 100k resistors vs _Vbe_ threshold.

[@stephenstewart1653]

Is there a way to shut off using Arduino output?

[@paulpkae]

Yes, there certainly is. That's something I have implemented on several of my own projects. The circuit is actually much simpler because you have software control with a microcontroller. I will do a follow up video with the circuit soon. Thanks for the comment.

[@paulpkae]

@stephenstewart1653 I have added a new video in response to you question. kzbin.info/www/bejne/aKq3gaSHaLVoprM

[@harveyellis6758]

Looks a lot like schematic / circuit posted in a KZbin video by Dave Jones from 12 years ago ;-)

[@paulpkae]

There's lots of variations of Dave's circuit. on KZbin This particular iteration is one that I tweaked and found to be the most stable.

[@noslidemais]