Using a transistor to switch on/off -5V?

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gregmcc

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I'm busy with a Arduino project where I need to switch -5V to a semiconductor. I'm going to use a ICL7660 to generate a negative voltage but now I only want the -5V to be present when the output of the pin on the Arduino goes high.

I've been using the below circuit to switch +5V on and off. For the -5V I'm guessing I can't just replace the +5 with -5?
 

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That's a VERY poor way to do it, and you lose at least 0.7V - you should use a PNP transistor to switch the +ve rail (output from collector) and an NPN to switch the -ve rail. Depending on exact requirements, you may need two transistors to do it.
 
Why not switch the 5V supply to the ICL7660 with a PNP transistor or get a -5V converter with an enable pin?

Mike.
 
MikeMl said:
Would make a very ragged edge...

View attachment 117562
Click to expand...

I've no idea what your circuit is all about?, Pommie's idea of simply switching the +5V to the converter chip with a PNP transistor seems a good one.

However, is a fast response time even required? - but I don't see as your simulation shows anything?, why (and how?) are you feeding it a very slow analogue input, rather than a fast digital one?.
 
As we don't know what the -5V is used for the rise/fall times are irrelevant.

Mike.
 
Nigel Goodwin said:
...
However, is a fast response time even required? - but I don't see as your simulation shows anything?, why (and how?) are you feeding it a very slow analogue input, rather than a fast digital one?.
Click to expand...
To prove that the switching point is at about the mid point of the Arduino Port Pin Swing.


Why the antagonistic attitude?
 
Below is another way to switch -5V using two PNP's:
Note that the logic is inverted -- 0V in is -5V out.

 
MikeMl said:
Why the antagonistic attitude?
Click to expand...

Not antagonistic, just that I didn't (and still don't) see the point?.

And as I also mentioned, Pommie's suggestion of switching the +5V to the converter seems by far the best option, and requires considerably less parts than either your or crutshow's examples.
 
Pommie said:
As we don't know what the -5V is used for the rise/fall times are irrelevant.
Click to expand...
My experience says otherwise. The OP doesn't know the questions to ask, or even that it might matter...
 
MikeMl said:
My experience says otherwise. The OP doesn't know the questions to ask, or even that it might matter...
Click to expand...

I think it's EXTREMELY likely that it doesn't matter, it would be pretty difficult to design something where it DID matter.
 
Nigel Goodwin said:
I think it's EXTREMELY likely that it doesn't matter, it would be pretty difficult to design something where it DID matter.
Click to expand...
The OP specified he was switching -5V to an IC. Suppose this is some sort of programming pulse.
 
I assumed the -5 was a power rail and the op wanted to save power or reset the chip. Maybe we shouldn't assume.

Mike.
 
Difference of opinions is how we learn.... Nigel can be a little harsh sometimes, but antagonistic is a little strong... All the senior members above have extremely good input to these threads... All we can do is give the TS ( OP ) our views, even if they are a little controversial... The TS may think that MikeMl's solution will work for him..
 
Almost forgot about this thread. Thanks for all the replies guys. I'm using the -5V on a old RAM Chip (4116) and the Arduino will run a memory test (and turn the -5V on and off)

Pommie is spot on, the solution is right there. Why didn't I see that I'm using a ICL7660 anyway to generate the -5V. I'll use a PNP to turn the voltage on/off to the ICL and then just add a delay on the Arduino code so the -5V can setting before I start testing.
 
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