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Selecting MOSFET

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permal:

--- Quote from: spec on January 17, 2019, 02:03:56 pm ---+ permal

I meant to say that you can cater for different voltages just by adding another resistor. But it does mean that you would have to change the resistor to suit the voltage range that you are using, as shown in the attached schematic below:

--- End quote ---
Yeah, but in my case I have 5V and 12V available and I don't know before hand what will be connected so I'll have to go for the zener-variant.


--- Quote from: spec on January 17, 2019, 09:46:44 am ---Sure :)

-- great explanation by Spec here --

--- End quote ---
Thank you  :-+

Ok, so I'm now at the following schematic.

Questions:

1. I've noticed that you don't add the pull-down resistor in your example schematics, @Spec - is there a reason for that? The MCP23017 do not have built-in pull downs so I see no way of excluding them (R1605).

2. R1603 and D1601; the current limiter and LED, how should I reason about those? I'm sure they affect the circuit somehow (a part from drawing current).

3. If the wire between CTRL and R1601 is "long", would a decoupler cap do any good? Sprinkle a 10nF on it?

4. The inrush resistors - how do I calculate appropriate values for them? If they can be reduced to 10R, then they feel somewhat arbitrary to me (though I assume they are not)?

5. Am I reading the data sheet for CZRQR52C6V8-HF correctly, in that its foot print is SOD-923F, equal to 0402 (1005 metric)?

spec:

--- Quote from: permal on January 17, 2019, 07:31:09 pm --- in my case I have 5V and 12V available and I don't know before hand what will be connected so I'll have to go for the zener-variant.
--- End quote ---
That is probably the best move all round. You then have a universal switch.


--- Quote from: permal on January 17, 2019, 07:31:09 pm ---1. I've noticed that you don't add the pull-down resistor in your example schematics, @Spec - is there a reason for that? The MCP23017 do not have built-in pull downs so I see no way of excluding them (R1605).
--- End quote ---
Nothing to do with me mate- you are the systems engineer on this. :)


--- Quote from: permal on January 17, 2019, 07:31:09 pm ---2. R1603 and D1601; the current limiter and LED, how should I reason about those? I'm sure they affect the circuit somehow (a part from drawing current).
--- End quote ---
Not sure what your intention is with the resistor and LED.


--- Quote from: permal on January 17, 2019, 07:31:09 pm ---3. If the wire between CTRL and R1601 is "long", would a decoupler cap do any good? Sprinkle a 10nF on it?
--- End quote ---
Yes, decoupling is always a good thing. 100nF X7R are the standard decouplers. But as long as the gate stopper is wired OK, there should not be a problem with long wires.


--- Quote from: permal on January 17, 2019, 07:31:09 pm ---4. The inrush resistors - how do I calculate appropriate values for them? If they can be reduced to 10R, then they feel somewhat arbitrary to me (though I assume they are not)?
--- End quote ---
They are not inrush current limiters, they are gate stoppers. 50R to 100R is a good value for medium speed work and lower values for high speed work. The gate stoppers play no part in the basic function of the circuit. They are normally only there to inhibit oscillation. But with the zener diode limiters they also limit the current when the zener diodes are conducting. So on one hand you would like to make them 100R say, but on the other hand you want to make them as large as possible to limit the current. So 470R is a compromise.


--- Quote from: permal on January 17, 2019, 07:31:09 pm ---5. Am I reading the data sheet for CZRQR52C6V8-HF correctly, in that its foot print is SOD-923F, equal to 0402 (1005 metric)?

--- End quote ---
Afraid you are asking the wrong person. Perhaps another member can advise.

permal:

--- Quote from: spec on January 17, 2019, 09:01:25 pm ---They are not inrush current limiters, they are gate stoppers. 50R to 100R is a good value for medium speed work and lower values for high speed work. The gate stoppers play no part in the basic function of the circuit. They are normally only there to inhibit oscillation. But with the zener diode limiters they also limit the current when the zener diodes are conducting. So on one hand you would like to make them 100R say, but on the other hand you want to make them as large as possible to limit the current. So 470R is a compromise.

--- End quote ---
Hm, I'm sure I read somewhere they were called inrush resistors, but based on your explanation stop resistors makes sense so we'll go with that :)


--- Quote from: spec on January 17, 2019, 09:01:25 pm ---Nothing to do with me mate- you are the systems engineer on this.

--- End quote ---
Just thought I'd ask, since you provide such a complete circuit otherwise but let the gate float on that mosfet.


--- Quote from: spec on January 17, 2019, 09:01:25 pm ---Not sure what your intention is with the resistor and LED.

--- End quote ---
Just as an indicator showing the state of the switch.


--- Quote from: spec on January 17, 2019, 09:01:25 pm ---Yes, decoupling is always a good thing. 100nF X7R are the standard decouplers. But as long as the gate stopper is wired OK, there should not be a problem with long wires.

--- End quote ---
So, "sprinkling caps" is really a thing then? As long as it's not A/C, put them everywhere?

permal:
Finally got the parts and PCB in hand. Success! :)

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