Analog switch - Page 1
Forum section: electronics / projects

author: jfigge
author_rank: Contributor
timestamp: September 30, 2022, 08:33:17 pm
content: Hi gurus,I'm working on an electronic load project and I'd like to have both constant current and constant voltage options. My design is pretty basic and uses an opAmp to drive a Mosfet gate etc... You can see the basis of my design here:https://www.altoo.dk/files/keysight/Electronic_Load_Fundamentals.pdfThe problem I need to solve is that the opAmp needs to be wired for positive or negative feedback depending on where I want CC or CV mode. I can think of several solution, but lack the understanding to decide which is best and how to implement the solution the right way. I have 0v and 12v rails on the opAmp, and am driving the inputs between 0v-5v.Option 1) Configure two opAmps and switch which output goes to the mosfet gate (SPDT)Option 2) Configure one opAmp and switch the inputs (DPDT)Option 3) Use a virtual ground (~6v), invert one input and offset by 11v. I've no idea how i'd do this, but it sounds coolI think option #1 is simplest, then #2, and I'd leave #3 to Dave. So my question is really...Does anyone know how to create an analog 2:1 mux? Or should I give up know and buy an IC / latching relay for the job? I think I can tolerate some signal voltage loss because after all this is an active circuit and will compensate for the loss.

author: Zero999
author_rank: Super Contributor
timestamp: September 30, 2022, 09:58:25 pm
content: The normal way to do this is with two op-amps connected to a diode OR gate.https://www.eevblog.com/forum/projects/how-to-design-fast-bench-supply-with-cc-and-cv/

author: Kleinstein
author_rank: Super Contributor
timestamp: October 01, 2022, 06:47:27 am
content: The easier and more convenient way is the diode OR. So like with a lab supply there is automatic cross over between voltage and current regulation.The normally not used limit would act as a kind of protection.Analog switches are a bit difficult to build from discrete parts. It would with JFETs but the gate control is a bit tricky. The CMOS swichtes use MOSFETs with separate substrate connection, that are rare as separate parts, especially the p-channel. Ready made CMOS chips are readily available (at least in theory - some are effected by the chip shortage) and the CD4053 or 74HC4053 are not even expensive.

author: jfigge
author_rank: Contributor
timestamp: October 02, 2022, 03:11:25 pm
content: Thank you for the replies. I’ll check out the video as that’s always a good way to learn. I’ve also looked at some chips and may even have found the one you recommended, but I’m concerned that there’s a voltage drop out. I think it has to be between two and 6 V in some cases, which seems like that would interfere with the signal?

author: Kleinstein
author_rank: Super Contributor
timestamp: October 02, 2022, 04:33:54 pm
content: The CMOS switch chips require the signal to be within its supply range (and only a few 100 mV beyound). The switch itself can have quite some on_resistance (e.g. in the 100 ohms range, depending on the typ). There would be a voltage drop about proportional to the current. Normally these switches are used with low currents and low voltage drop.AFAIR the 74HC... are good for a supply up to some 6 V and the HC405x can also have a negative supply that could reach some -5 V. So they are good for signals upt to some +-5 V (depending on the supply), but not much beyond that.