Latch circuit and Mosfet question? - Page 1
Forum section: electronics / projects

author: kgavionics
author_rank: Regular Contributor
timestamp: March 10, 2023, 03:06:46 pm
content: Hello guysI came up with the bellow latch circuit, as I have all these components handy, but the problem is the Mosfet which is not a logic level one and as a result I have a 1.6 V drop between the drain and source. Is there any way to modify the circuit to make it work before I order some new Mosfets?Thanks in advance!

author: ledtester
author_rank: Super Contributor
timestamp: March 10, 2023, 03:29:26 pm
content: Where is the power for the logic chips? And what voltage are they running at?What's the voltage at the drain of the MOSFET and what voltage do you want to supply to the MCU?Note that the IRFZ44N is an N-channel MOSFET so the gate has to be higher than the source which is the MCU positive supply in this case. You might have better luck using that MOSFET as a low-side switch -- i.e. use it to disconnect ground instead of the positive supply.

author: kgavionics
author_rank: Regular Contributor
timestamp: March 10, 2023, 03:38:12 pm
content: Quote from: ledtester on March 10, 2023, 03:29:26 pmSorry, My schematic was incomplete, I did update it!

author: Ian.M
author_rank: Super Contributor
timestamp: March 10, 2023, 03:41:59 pm
content: Swapping in a logic level MOSFET of the same channel polarity wont help much - it will reduce the voltage drop but not eliminate it.Your problem is you have attempted to use a N-MOSFET as a high side switch without driving the gate to well above the rail it is switching. Substitute a P-MOSFET, source to supply, drain to load, with a max. gate threshold voltage of less than half your supply voltage, and it should 'just work', though note that the gate level for power on will be inverted, i.e. low=On.However, your whole circuit is over-complex. If you've got a spare inverter left on U1, you can eliminate U2. See the two inverter toggle latch circuit presented by eblc1388 here:https://www.eevblog.com/forum/beginners/help-on-how-to-choose-an-ic-for-a-project/

author: Peabody
author_rank: Super Contributor
timestamp: March 10, 2023, 05:13:28 pm
content: +1 on the wrong mosfet type. If you use a P-channel mosfet, with the gate connected to the not-Q output, it should work with no voltage drop. The drain toward the load.

author: MathWizard
author_rank: Super Contributor
timestamp: March 10, 2023, 08:32:42 pm
content: I've seen modern desktop motherboards use generic N-ch mosfets backwards, Source-drain, and switch them with 5 or 12V no problem , in series like the circuit above, to run a few chips, with LM358 circuits for the gates, responding to the logic

author: SiliconWizard
author_rank: Super Contributor
timestamp: March 10, 2023, 08:54:58 pm
content: No good reason not to use a PMOS here.And, a IRFZ44N to power a microcontroller? Maybe it's because you have a handful of them in a drawer?

author: Benta
author_rank: Super Contributor
timestamp: March 10, 2023, 09:11:17 pm
content: Quote from: MathWizard on March 10, 2023, 08:32:42 pmI doubt that. The DS diode would then supply the load directly.

author: kgavionics
author_rank: Regular Contributor
timestamp: March 11, 2023, 09:59:58 pm
content: Quote from: Ian.M on March 10, 2023, 03:41:59 pmYou're right man, my circuit was over-complex, I replaced the JK flip-flop with another inverter and the latch is working except for the MOSFET which I will replace with a P channel one!Thank you all for your inputs, guys!