2) The 0.2V VCESat is acceptable. The transistor will not create 0 Ohms.
I'm not too worried about whatever small amount of resistance the 'zero' transistor will have. So long as it's no more than a couple-hundred ohms, it should be fine. The existing potentiometer has a 13K resistor in series before it to set the base resistance anyway, so plus/minus a few ohms is no big deal.
Your circuit should work. You could replace the transistor and resistor with a mosfet such as the common 2N7002 with the gate directly to the inhibit input.
Yes, thanks, I think a MOSFET might be a better choice here, for reasons I have discovered and will describe further below.
Well... how long are the delays? There's a difference between picoseconds and hours.
Now I have examined the DG408 datasheet in more detail, I find it says total transition time is specified as 160-250ns, and break-before-make interval is 10ns. I guess this is not a big deal after all, and will have sod-all effect?
If this was a real concern you could instead of switching resistors in/out, recalculate the values so you could put them all in parallel. With all 8 in parallel, the total resistance is 3.9K. Then you remove 1 resistor, you're left with 10K. And so on. Of course you couldn't use a 4051. And trying to formulate the equation for that is making my heard hurt.
Ah, yes, I see. And just thinking about attempting to work out all the necessary values to arrange the resistors in parallel is making my head hurt too.
Have you already considered bypassing the RC timing circuit, altogether, and switching it out for your own, controlled via micro? Or is the pot the part that is easily physically accessible?
Yes, the pot is the only part that is easily modifiable/substituted. The rest (well, except an start/stop switch) is all on one PCB, and would require butchering the circuitry to modify it in any way.
I was examining the existing circuitry a bit closer yesterday, as there was one aspect of it I still didn't understand, but while doing so I discovered something that throws a slight spanner in the works.
Firstly, the part I wasn't sure about was how the RC timing circuitry that switched the relay managed to reset itself. I wasn't immediately seeing how that happened. What I figured out was that the mechanism that the relay activates has a 'home' switch that works as follows:
1. The home switch, which is SPDT, is by default connected to ground.
2. When the relay activates the mechanism, after a very short period, the home switch is toggled and takes over the power feed to the mechanism, ensuring that the mechanism always completes its cycle, regardless of what the user does with the controls.
3. When the cycle completes, the mechanism returns to the home position and toggles the home switch back to ground.
This home switch's output is also actually what is connected to the other side of the electrolytic capacitor that is in the RC timer (which appears to be a non-polarised one, as it doesn't have a stripe).
I put together a simulation to better understand what's going on, and discovered that when the home switch goes back from +12V to ground, the other side of the timing capacitor goes negative! Of course, this is what causes the relay to turn off, and the delay period to begin again as the cap re-charges. But this has ramifications for my planned additions, does it not? It needs to handle the input terminal being +12V while the output terminal potentially is -12V!
That's not all. The start/stop switch is actually between the cap and the transistor/relay part of the circuitry. This means that after the device has been on for a little while, but not started, the cap will be fully charged with +12V. When started, the cap will start to discharge, but not by much before the home switch kicks in and transfers +12V to the other side of the cap, which means the first side will jump to potentially +24V!
I screen-capped the simulation running that shows both these things:
Now, I think I can handle the over-voltage by inserting a diode to +12V right after the pot (or indeed my substitute circuitry). According to the simulation, this prevents the over-voltage surge. However, I'm not sure how to handle the negative. Can't simply put a diode to ground, as that stops the whole thing working - it
needs to be able to go negative.
I don't think the negative voltage will have any effect on the MOSFET, as a 2N7000 is rated for 60V V
DS. But what about the analog mux? I'm not sure what to do about that.