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

[gregmcc]

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?

 

[Nigel Goodwin]

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.

 

[gregmcc]

Thanks for the tip. Makes sense. I've been doing it wrong for ages.

 

[Nigel Goodwin]

There's some examples of +ve switching in my tutorials:

 

[Pommie]

Why not switch the 5V supply to the ICL7660 with a PNP transistor or get a -5V converter with an enable pin?

Mike.

 

[MikeMl]

Why not switch the 5V supply to the ICL7660 with a PNP transistor or get a -5V converter with an enable pin?

Mike.

Would make a very ragged edge...

 

[Nigel Goodwin]

Would make a very ragged edge...

View attachment 117562

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?.

 

[Pommie]

As we don't know what the -5V is used for the rise/fall times are irrelevant.

Mike.

 

[MikeMl]

...
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?.

To prove that the switching point is at about the mid point of the Arduino Port Pin Swing.

Why the antagonistic attitude?

 

[crutschow]

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

 

[Nigel Goodwin]

Why the antagonistic attitude?

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.

 

[MikeMl]

As we don't know what the -5V is used for the rise/fall times are irrelevant.

My experience says otherwise. The OP doesn't know the questions to ask, or even that it might matter...

 

[Nigel Goodwin]

My experience says otherwise. The OP doesn't know the questions to ask, or even that it might matter...

I think it's EXTREMELY likely that it doesn't matter, it would be pretty difficult to design something where it DID matter.

 

[MikeMl]

I think it's EXTREMELY likely that it doesn't matter, it would be pretty difficult to design something where it DID matter.

The OP specified he was switching -5V to an IC. Suppose this is some sort of programming pulse.

 

[crutschow]

Not antagonistic

Well you fooled me also.

 

[Pommie]

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.

 

[DerStrom8]

Why the antagonistic attitude?

Well you fooled me also.

I see nothing antagonistic about Nigel's posts. Perhaps you're looking for a negative tone, so that's how you're reading them?

 

[crutschow]

I see nothing antagonistic about Nigel's posts. Perhaps you're looking for a negative tone, so that's how you're reading them?

Don't think so.
Mike saw it and I saw it, but if you don't, that's fine.

 

[Ian Rogers]

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..

 

[gregmcc]

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.