Another soft latch circuit - Page 1
Forum section: electronics / projects

author: superKris
author_rank: Regular Contributor
timestamp: March 07, 2020, 01:03:04 pm
content: Hi! this is my first post on this forum. I decided to post my schematic below as i was looking for a suitable soft latch circuit and a lot info came from EEVblog.So i want to share the latch circuit below and see if you have any feedback. This design meets the following requirements:- No current when off (without LED)- For low sided loads (switches +)- Works without microcontroler- Starts in off state- Does not oscillate- Does not use high value capacitors (great for cheap SMD)- Uses a minimum of FETS/transistors- Has a (optional) shut down input for uC- Can work with slightly higher voltagesSo i looked at many different schematic includingDave's solution. Its nice, but i dint like the large capacitor it needs, and for my purposes its beter if the output is off when powered for the first time. There is alsothis videocomparing multiple circuit. However, all of these need a microcontroller to toggle on or off. For my design i need it to work without the micro controller for maximum reliability of the project. There are also many other circuits out there using IC's like 555 timers, or logic IC's. This i dint like much either.After some browsing i fount thisschematic by Anthony Smith. I think its really nice and smart as it uses very little components, works independently, and seems to meet all the criteria mentioned above. For explanation about how the schematic works, just read the linked article. Its great. One small note: He talks about using a diode in the output to not accidentally toggle the circuit. In my case i use it to drive a relay, and i can confirm the diode is needed for steady operation.So this circuit is almost perfect, but i wanted to add a few additional functions:- Lower Vgs, so its more suitable for higher voltages- Add "power on" LED and standby LED. (i'm usingthis switch)- Make a shutdown function for a GPIO pin of a microcontrollerThis resulted in the circuit below. I kept al part names the same as the original circuit described above. I'm only going to cover the additions i made. The rest is the same.Lowering Vgs (R7)I will be using this in a automotive application. 14-15V is normal, but spikes kan occur. The selected P-channel FET has a Vgs of max 20V, so it will probably be alright but to be on the save side its better to lower Vgs. By adding R7 you create a simple voltage divider with R3 that cuts Vgs in half. Be aware that you should NOT use R7 when powering the circuit with a low voltage source. Most FETs need a Vgs of 4 tot 5 V to work properly so depending on the FET you are using this circuit needs approx 8-9V minimum to work properly (while R7=R3). If you power it below 10V, the is no point in adding R7Power ON, and standby LED (D1,R6)This is actually very simple. The power ON led can get its switching GND from Q1. Only a current limiting resistor (R6) is needed. The standby LED is a little bit more tricky. Either additional transistors are needed, or we can do a simpler trick; In my schematic the red standby LED always gets ground via resistor R5. This can actually never turn off. However, we can stop the current running trough the LED, by using diode D1 to increase the voltage on the cathode to VCC when the circuit is latched in its "on" state. The downside of this is, that in on state, the set LED current will still flow trough R5. This means a few additional mA while the circuit is on. This is no problem for my project.Shutdown by uC trough GPIO (Q3, Q4, R8, R9)So i want my microcontroller to be able to power down the circuit. For example after a certain amount of time, or a a certain voltage. I found the easiest way to do this, is to pull up the gate of Q2 to VCC. This is done by Q3 which can be any P-channel MOSFET. R8 is just a pull up for its gate. To switch R3 on we need to lower its gate, and we can do this with Q4 which can be any NPN transistor. To limit base current to Q4 we use R9. Optionally you can also add a pull down resistor but this will not be needed in most cases or can be provided by the microcontroller. Please note the Vgs of Q4 will be as high as the VCC. I choose a MOSFET that is fine with 20V, and Vgs will only be applied very briefly when shutdown by the microcontroller. However, if you use a MOSFET with a limited Vgs you might want to make a voltage divider by adding a extra resistor between R8 and Q4.So i hope you guys like my solution. I have tested it and everything seems to work great. If you have any feedback or see some room for improvement, i would love to hear it!

author: superKris
author_rank: Regular Contributor
timestamp: March 07, 2020, 01:05:21 pm
content: this looks like shit... How do i get the youtube videos to only appear as link?

author: superKris
author_rank: Regular Contributor
timestamp: March 07, 2020, 01:23:17 pm
content: Quote from: blueskull on March 07, 2020, 01:18:54 pmMuch better, thanks!

author: superKris
author_rank: Regular Contributor
timestamp: March 09, 2020, 10:58:59 am
content: No feedback on what can be improved?