Re: [EE]: Synchronising 4 Linear Actuators with 4 x Op Amps and 6 MOSFETS

Thanks for the input.  I completely missed the issues raised.

Jason, when you state " halfway on and halfway off ?" is that in the
reverse polarity mode or in both modes you perceive a problem.  I am
thinking I am missing something.  Simulating in LTspice shows that the
relevant FET is fully on when +Ve is directly connected to the motor and in
the reverse mode it is only partially on (~2.5V across the relevant FET)

I simulated moving the load between the FETs.  But they are not driven
fully on in either polarity.  Worst of both worlds.

Powering the LM324 from something greater than Vbatt as Bob suggests solves
two problems.  The data sheet seems to imply  that Non-inverting input
should always be at least 1.5v less than V+. On re-reading it states
"...The upper end of the common-mode voltage range is V + − 1.5 V (at
25°C), but either or both inputs can go to 32 V without damage..." . I
think that implies the opamp will not behave as I expect when the inputs
approach the +ve rail which would happen in my design as it currently
stands.  I also assume it would behave as expected when the
inputs approach 0V (V-).

I had considered using 8PDT switch (and relay) but a quick search found
easily avail switches would suit switching signals not power.

Time to breadboard powering the LM324 with 18v and see how it goes.

Thanks again.

Cheers justin


On Mon, Apr 21, 2025 at 2:35 AM Jason White <
[email protected]> wrote:

> Those FETs are neat - really easy to design around. I wonder if the 5ms max
> turn on time and a 1ms max turn off time might affect drive speed.
>
> Side question: if those SSRs contain back to back FETs, wouldn't the
> current have to flow through a body diode? (have a forward voltage drop in
> addition to resistive losses) Could they be doing some trickery with having
> both P and N type FETs in the same package?
>
> I guess I really don't know how a FET would get activated by light - I
> suppose it might contain some logic circuitry or something.
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