Understanding values chosen for EEVblog #262 - World's Soft Latching Switch - Page 1
Forum section: electronics / beginners

author: luxfx
author_rank: Contributor
timestamp: July 13, 2016, 12:54:23 am
content: I've finally successfully built the soft latch switch circuit from EEVBlog #262 (I can never get drain and source right with MOSFETs).I'm trying to understand why the resistors needed to be so large (100Ks and a 1M). I think I've mostly got it, but would just like some confirmation in case I'm learning something the wrong way.1M in the middle: To slow the RC down enough to allow for human switch speeds100K in the middle: Is it because the internal RDSONof the MOSFET and the 100K in the middle form a voltage divider, and the idea is to balance it so far in one direction that the dropped voltage is negligible?100K on left and right:I'm less sure about these. Maybe they are all the same values just to make it easy. But do they need to be high values? Is it just because the transistor bases and MOSFET gate just don't need as much voltage/current?thanks!

author: rstofer
author_rank: Super Contributor
timestamp: July 13, 2016, 02:10:01 am
content: The resistor values are high because they don't have to be low. That's the fairly simple answer, MOSFET gate currents just aren't very high.The left 100k resistor is there to turn OFF the MOSFET when the left transistor is off. The transistor pulls the MOSFET gate to ground to turn it on.The center 100k resistor keeps the left transistor turned on as long as the MOSFET is on. We'll turn it off in a second.The 1M resistor slowly turns on the right transistor when the MOSFET is on. That puts the right contact of the switch at ground.The button is pressed and the right transistor is on. This pulls the base of the left transistor to ground and the MOSFET shuts off.When the MOSFET is off, the right transistor turns off and the right side of the push button goes high.The next time the button is pressed, theright transistor is off and the collector is high putting a high voltage on the switch. This turns on the left transistor which latches through the center resistor.The RC circuit guarantees a certain amount of debouncing and also makes certain the MOSFET turns on before the button is capable of turning it off again. This is a long time constantI think this is about right...

author: luxfx
author_rank: Contributor
timestamp: July 13, 2016, 05:09:11 pm
content: Thank you, that's very helpful. I keep forgetting that P-channel MOSFETs conduct when the gate is grounded. I'll get this straight one of these days!

author: will900
author_rank: Newbie
timestamp: August 28, 2016, 11:38:51 am
content: Hi all,I'm trying to make this circuit work but having to use a different (hopefully equivalent) MOSFET, I've used an IRF9160 (datasheet attached) as from googleing this is the replacement as can't find IRF9110.What I can't understand about the circuit is why it would turn on when power is supplied first. I would have thought that as the gate is tied high at first it would be off then when the switch is pressed and the gate of the left transistor is brought high and it makes a connection to ground it would switch the MOSFET on and latch?The problem I am having is that when I first apply power to the circuit the output is off, when I press the button it switches on and latches, then that's it, it stays latched on even when the button is pressed. I have to make a direct connection to ground, not through the transistor to turn it off.The only difference between my circuit and the one in the video is that I'm using the IRF9160, my power source is 4x 1.5V AAA batteries, I used two initially but this did nothing.Any help would be very much appreciated.

author: hugo
author_rank: Regular Contributor
timestamp: August 29, 2016, 02:05:49 am
content: Quote from: will900 on August 28, 2016, 11:38:51 amMake sure you have the source of the mosfet connected to the power supply and the drain to the load not the other way around .

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 29, 2016, 01:23:36 pm
content: Quote from: luxfx on July 13, 2016, 05:09:11 pmThink of it this way:To a PMOS, "ground" is the source terminal. The gate is given negative voltage!In many circuits, source is connected to VCC, and gate is connected to VCC or -V to turn off or on, which is convenient.Tim