This cannot be right? - Page 1
Forum section: electronics / beginners

author: king.oslo
author_rank: Frequent Contributor
timestamp: June 12, 2012, 08:49:08 pm
content: Hello there,Look at these simulations. The gate of the PMOS is grounded. In the other, I have 5V on the gate. How can I get 5V on the output either way?Thanks.M

author: 8086
author_rank: Super Contributor
timestamp: June 12, 2012, 08:54:57 pm
content: Interesting. I had similar result today in multisim (when simulatinghttps://www.eevblog.com/forum/projects-designs-and-technical-stuff/mosfet-power-switch-with-auto-off/msg120110/#msg120110), but since I found many designs online that say this shouldn't happen, in the end I just put it down to multisim being shit.Maybe it's not multisim after all...[for some reason, I misread your initial post, your edit is what I experienced with multisim]

author: MikeK
author_rank: Super Contributor
timestamp: June 12, 2012, 08:57:31 pm
content: I thought that's how P-channels worked. You have to appy a voltage at the gate to restrict the flow, otherwise it's fully open?

author: king.oslo
author_rank: Frequent Contributor
timestamp: June 12, 2012, 08:59:22 pm
content: MikeK, after you started typing your reply, I noticed that I had made the error of showing you the wrong simulation. I have updated the thread. ThanksM

author: king.oslo
author_rank: Frequent Contributor
timestamp: June 12, 2012, 09:13:11 pm
content: Here is another one. Watch how the output from the PMOS is higher than the input voltage.

author: johnmx
author_rank: Frequent Contributor
timestamp: June 12, 2012, 10:04:49 pm
content: The simulations are right. Because the drain is floating (hi-z), even the smallest leakage current of the MOSFET will put the drain at 5V. Remember that the voltmeter used to measure that voltage is an ideal one and has infinite impedance.

author: king.oslo
author_rank: Frequent Contributor
timestamp: June 12, 2012, 10:07:02 pm
content: John, which of the three simulations are you referring to? Thanks!M

author: johnmx
author_rank: Frequent Contributor
timestamp: June 12, 2012, 10:10:19 pm
content: Quote from: king.oslo on June 12, 2012, 10:07:02 pmThe 2nd one, when the gate & source are at 5V level. That's the topic main question, right?

author: king.oslo
author_rank: Frequent Contributor
timestamp: June 12, 2012, 10:12:54 pm
content: Yes you are right.Let me see if I understand what you are saying.You are saying that the transistor is in its non conducting state, but the voltage is at 5V because the transistor leaks current through because the impedance is not infinite. Is that correct? Thanks.M

author: jimmc
author_rank: Frequent Contributor
timestamp: June 12, 2012, 10:15:04 pm
content: I suspect that your post #1 results may be caused by leakage, try adding a high value resistor (100k) from output to ground and see if that makes a difference.For your last circuit I suspect that is a function of gate-drain capacity which is charged to 5v when the gate is at 0v and then hits the parasitic source-drain diode when the gate is driven to +5v (It would pulse the drain to +10v if the diode were not present). Again you have no DC load so very small currents are needed.Slow typing I see others have suggested leakage as well.Jim

author: johnmx
author_rank: Frequent Contributor
timestamp: June 12, 2012, 10:23:16 pm
content: Quote from: king.oslo on June 12, 2012, 10:12:54 pmThe transistor is in non-conductive state, but there is always a leakage current passing through it. This value is in the data sheet of the device.The drain pin is acting like a small capacitor with almost zero capacitance that keeps charged at the source voltage level. In the real world you cannot verify this because the voltmeter input impedance will discharge this ‘capacitor’.

author: king.oslo
author_rank: Frequent Contributor
timestamp: June 12, 2012, 10:28:25 pm
content: Thanks for that johnmx.Jimmc, how can this happen? Thanks.M