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Topic: sg90 servo deflection (Read 4447 times) previous topic - next topic

Robin2

#30
Feb 26, 2017, 10:21 am Last Edit: Feb 26, 2017, 10:22 am by Robin2
If you switch before the servo the grounds are the same.
Maybe I misunderstand but that sounds like asking the Arduino to saw through the branch it is sitting on :)
Also, the whole point of the exercise is to switch the servo on a little later than everything else.


@colubridae, if this was my project I would just put the switch on the servo V+ line. And, sorry, I am not proficient enough to offer advice on transistor or mosfet selection. I just work my limited analog electronics on the suck-it-and-see principle.

...R
Two or three hours spent thinking and reading documentation solves most programming problems.

colubridae

Maybe I misunderstand but that sounds like asking the Arduino to saw through the branch it is sitting on :)
Also, the whole point of the exercise is to switch the servo on a little later than everything else.


@colubridae, if this was my project I would just put the switch on the servo V+ line. And, sorry, I am not proficient enough to offer advice on transistor or mosfet selection. I just work my limited analog electronics on the suck-it-and-see principle.

...R
ALKA
putting a mosfet/bjt switch below the servo still mismatches the grounds.

Thanks for your advice anyway.



ROBIN"
I'm pretty much flying by the seat of my pants and trying out various options.

ALKA suggested a mosfet for its low resistance which seems like a good idea.

Also points the fact that by using the ULN2003 as a switch for the servo I'm the putting the servo below its quoted operational voltage.



Saddo railway nerd (65yrs)

alka

Sorry, switching before the load might not be the best way to word it.

I mean switching high side rather than low side, ie with a PNP transistor or p-channel MOSFET.

That puts the ground of the load at the same potential as the Arduino ground.

alka

#33
Feb 26, 2017, 01:57 pm Last Edit: Feb 26, 2017, 01:58 pm by alka
This is what I am trying to say...


Robin2

I mean switching high side rather than low side,
Yes. That is what I have had in mind.

...R
Two or three hours spent thinking and reading documentation solves most programming problems.

alka

#35
Feb 26, 2017, 05:59 pm Last Edit: Feb 26, 2017, 06:02 pm by alka
Keep in mind you cannot use a NPN bjt or N-FET if you switch high side. Must be P-Channel unless you find some way of keeping your gate voltage well above threshold (charge pump) when the MOSFET is conducting.

 If your source voltage is the same or less as your switching voltage (and under the 5v arduino limit) you can get by without the second NPN transistor driving the gate of the P-FET.

colubridae

Keep in mind you cannot use a NPN bjt or N-FET if you switch high side. Must be P-Channel unless you find some way of keeping your gate voltage well above threshold (charge pump) when the MOSFET is conducting.

 If your source voltage is the same or less as your switching voltage (and under the 5v arduino limit) you can get by without the second NPN transistor driving the gate of the P-FET.
Bugger! I did not know that.

Thanks for the circuit diagram. I will need a few days to figure them out.

Sorry I misunderstood your meaning.
Saddo railway nerd (65yrs)

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