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Last time I asked about changing a sensor (which is a potentiometer) output in various ways. I've decided to go with an opamp and circuit like this:

Snip1

Now I'm looking for some device that will disconnect the opamp from the potentiometer output based on an ON/OFF (3.3V) signal.

Right now the options I see are:

  1. Relay - does exactly that, but I don't want any mechanical parts, also the dimensions are an issue.
  2. MOSFET or MOSFET relay - the opamp output must be always higher than Vgs of the particular MOSFET.
  3. Bipolar analog switch like the MAX4541 - it has low I/O current and I need higher current capability because of the resistor and potentiometer values.

What else can be used?

Also can anyone tell me about a good alternative to AD8244 unity gain quad buffer? I need only one buffer circuit with a smaller and easier to hand-solder footprint (SOT23-5 maybe.)

@EDIT

I will use MOSFET SSR as suggested in comments. Question regarding AD8244 alternative remains.

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  • \$\begingroup\$ A solid-state relay is the easiest option. If you get the right type, there's no requirements placed on the op amp's output at all. \$\endgroup\$
    – Hearth
    Jan 3, 2021 at 15:16
  • \$\begingroup\$ @Hearth I know that is the easiest option, but there are several problems: moving part, dimmension, switching speed, reliability. I placed that there just so You know that I've already considered that. \$\endgroup\$
    – xkam555
    Jan 3, 2021 at 15:19
  • \$\begingroup\$ @Anrzej, Hearth said solid state relay. There are no moving parts. Also, you say you need "higher current capacity". Can you quantify what you mean by "higher" please? \$\endgroup\$
    – Math Keeps Me Busy
    Jan 3, 2021 at 15:55
  • \$\begingroup\$ SSRs have pretty good switching speed and reliability in my experience, and they have no moving parts and while they're not available in the smallest packages, you can get them in SOIC packages. \$\endgroup\$
    – Hearth
    Jan 3, 2021 at 16:05
  • \$\begingroup\$ By higher current I mean something like 200mA. If You can find something that can handle more than 1,5A then I'd be more than happy (I'll remove one circuit). @Hearth Can You give me any particular part number regarding SSR, because I only find "MOSFET" ones, where I see problem with driving potentiometer output LOW. What if potentiometer will be set to 5%, and opamp out voltage to 0.1V? I guess then MOSFET SSR won't work. \$\endgroup\$
    – xkam555
    Jan 3, 2021 at 17:33

1 Answer 1

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I would use a PhotoMOS relay, which uses an LED and photocell to control back-to-back MOSFETs for purely resistive output with no strings attached (unlike conventional SSRs which steal power from the load). It's the closest you can get to a mechanical relay, but much smaller and more reliable.

enter image description here

In your application there is a large resistance in series and low voltage, so a device like the AQY282SX would be fine. It has max. 2.5 Ω 'on' resistance and only needs 5 mA of LED drive current.

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  • \$\begingroup\$ What conventional SSRs are you talking about, just optocouplers? This is the main type of SSR that I use for situations where I need one, and it'd be the first thing I think of if an SSR is mentioned. Though I usually go for one of IXYS's options, personally. \$\endgroup\$
    – Hearth
    Jan 4, 2021 at 16:25
  • \$\begingroup\$ By 'conventional' I mean AC SSRs like this:- sparkfun.com/products/13015 \$\endgroup\$
    – Bruce Abbott
    Jan 4, 2021 at 19:55
  • \$\begingroup\$ Ah, triac-based ones? Yeah, I've never used those; haven't had much need to. I imagine they can handle more current and voltage than MOSFET based ones. I wonder if there are any SiC-based MOSFET relays yet? \$\endgroup\$
    – Hearth
    Jan 4, 2021 at 20:18

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