Help for simulating soft power latch - Page 1
Forum section: electronics / beginners

author: polarKaung
author_rank: Contributor
timestamp: February 05, 2022, 11:18:10 am
content: Hello, i have been trying to simulate a soft power latch Dave build like many years ago, but in LTSpice. You can see the current schematic and simulation results in the attachments below. I can't fine IRF9110 in LTSpice so, i used IRFL9110 model. The simulation result is not what i expected it to be. Can anyone take a look at it?

author: Capernicus
author_rank: Frequent Contributor
timestamp: February 05, 2022, 05:15:26 pm
content: I dont know the answer to your problem, but mosfets latch by themselves so all I would do is power the mosfet and be done with it.Transistors are mostly all or nothing so I wouldnt think the transistor mismatch would count. (but I could be wrong...)

author: polarKaung
author_rank: Contributor
timestamp: February 05, 2022, 05:16:50 pm
content: oh, I was trying to simulate the circuit in this Dave Video:

author: ledtester
author_rank: Super Contributor
timestamp: February 05, 2022, 05:31:32 pm
content: Can you supply the .asc file? - it would help a lot.

author: LaryPant
author_rank: Contributor
timestamp: February 06, 2022, 01:49:39 am
content: Hi,It appears that the output is oscillating during the 0,2 seconds where S1 is ON, which is probably due to the time constant given by R5 and C1 being to low. Dave also mentions and explains this towards the end of the video, which I would definitely check out even if I am wrong and this isn't the root cause.Try and increase R5 to 1 megaohm for example and see if it helps. If this was a real circuit then you would probably have to tweak the time constant very carefully depending on the user and the switch, but I saw some comments on the video with suggesstions on how to improve the circuit in order to avoid the oscillation.Quote from: Capernicus on February 05, 2022, 05:15:26 pmNo! A MOSFET or any kind of transistor for that matter does not latch. A transistor has no memory and will be either ON or OFF depending on the control signal (base or gate) currently applied.The only semiconductor with a latching property, that I am aware of, is the thyristor.

author: polarKaung
author_rank: Contributor
timestamp: February 06, 2022, 03:51:40 am
content: thank you, it kinda works. But it gives me a new problem. I added a load of 10k but it is switching on when it is supposed to be off.

author: ledtester
author_rank: Super Contributor
timestamp: February 06, 2022, 04:09:17 am
content: fwiw, here is a simulation which seems to work:dave soft power switch.asc(2.15 kB - downloaded 45 times.).asc file attached at end.Note that if the Ton time of V2 is increase to 0.05s then it stops working.

author: polarKaung
author_rank: Contributor
timestamp: February 06, 2022, 04:32:38 am
content: Hello, it works great but if i set the source to flat DC volt the operation seems to be reversed. Why? (first attachment)Also, why does the power goes up when switched off in the correct simulation. Is there something i am missing? (second attachment)

author: ledtester
author_rank: Super Contributor
timestamp: February 06, 2022, 05:47:22 am
content: Quote from: polarKaung on February 06, 2022, 04:32:38 amSee if I(V1) is negative.Btw, if you are interested in a design that is less finicky, I'd check out this post and the follow-up:https://www.eevblog.com/forum/beginners/tactile-mom-switch-to-enable-ic/msg3962609/#msg3962609The idea is that the inverters will always be powered but will consume microamps so it can be used for battery-powered applications.