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Old 21st March 2011, 03:52 AM   #1
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Question ~-1V rail from existing regulated -15V rail - how?

I need to create a third rail at about -1V from an existing dual power supply (+/- 15V for instance). The -1V rail needs to supply up to 50mA but the current demand is not constant.

How do I do this? Can I just use a PNP transistor with the base supplied with the voltage that I would like to achieve, e.g. using a voltage divider between ground and the existing regulated -15V rail as the reference? Also, I should connect the emitter to the -15V rail and the collector to ground?

This seems like a pretty basic valve, but I can't seem to get it to sim correctly.

If this is not the way, then how???? HELP!

-Charlie
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Old 21st March 2011, 04:00 AM   #2
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A diode string is easy enough, though not exactly elegant, it'll do the trick.
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Old 21st March 2011, 04:17 AM   #3
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Quote:
Originally Posted by MJL21193 View Post
A diode string is easy enough, though not exactly elegant, it'll do the trick.
Ummm, not exactly stable as current varies. I need stable voltage independent (more or less) of current. Diode forward voltage is strongly current dependent.

Anything other ideas?

C-
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Old 21st March 2011, 08:51 AM   #4
Elvee is offline Elvee  Belgium
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Quote:
Originally Posted by CharlieLaub View Post
Anything other ideas?

C-
A slightly shifted voltage regulator?
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Old 21st March 2011, 10:09 AM   #5
Bonsai is offline Bonsai  Taiwan
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Try this.

There is no feedback in this circuit, but I am sure it will deliver a resonably accurate, temperature stable -1V. Simply adjust R1 for the correct output voltage. note, the 220 Ohm resostor in the output pass transistor collector is simply a dissipation limiter.
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Old 21st March 2011, 12:39 PM   #6
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Quote:
Originally Posted by CharlieLaub View Post
Anything other ideas?
LM337 will go as low as -1.2V
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Old 21st March 2011, 02:37 PM   #7
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Quote:
Originally Posted by MJL21193 View Post
LM337 will go as low as -1.2V
I noticed this last night after I posted the thread. For some reason I thought the minimum was around -3.5V , but then I read the datasheet.

Since it would be pretty simple to implement, I might just go with an LM337 and live with the -1.2V output. That will probably work just fine for my application.

I am still interested in implementing something like a voltage follower using a transistor, but I have only seen them drawn with a positive source voltage. I can't say that I really know much about transistor circuit design, so I am a bit at a loss. It seems like there should be something using a single transistor that is simpler than the LM337, but I can't come up with it.

-Charlie
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Old 21st March 2011, 02:45 PM   #8
jcx is online now jcx  United States
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which direction is the current going, series Reg chips don't work both directions

a Vbe multiplier shunt is a low parts count option too
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Old 21st March 2011, 03:31 PM   #9
Elvee is offline Elvee  Belgium
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Quote:
Originally Posted by CharlieLaub View Post
It seems like there should be something using a single transistor that is simpler than the LM337, but I can't come up with it.
A standard-red LED and a ballast transistor will give you a crudely compensated ~1V output:
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Old 21st March 2011, 03:56 PM   #10
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Originally Posted by Elvee View Post
A standard-red LED and a ballast transistor will give you a crudely compensated ~1V output:
This is more in line with what I was looking for. I guess I could use one or more diodes, e.g. 1N4148, in place of the LED to get voltages that are even closer to ground...

-Charlie
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