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 Post subject: Negative voltage regulator
PostPosted: Jun Fri 28, 2019 2:41 am 
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When looking at the data for a negative voltage regulator, how do I interpret the voltages listed?

http://www.ti.com/lit/ds/symlink/lm2990.pdf

I'd normally use a 7912, but the B- is only a max of 16Vdc and the regulated voltage is -12Vdc so I need one that is a low drop out.


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 Post subject: Re: Negative voltage regulator
PostPosted: Jun Fri 28, 2019 2:49 am 
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What do you need help on?
Looks like it will need 12.6V minimum.

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 Post subject: Re: Negative voltage regulator
PostPosted: Jun Fri 28, 2019 3:00 am 
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Yes the way they list the voltages in that datasheet is quite confusing at least to me given I'm used to mostly working with positive regulators.


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 Post subject: Re: Negative voltage regulator
PostPosted: Jun Fri 28, 2019 11:05 am 
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negative regulators are the same as positive regulators, only negative. :). Not sure what the confusion is all about. The spec sheet is pretty clear on the 7912.......

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Last edited by Barry H Bennett on Jun Fri 28, 2019 11:48 am, edited 1 time in total.

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 Post subject: Re: Negative voltage regulator
PostPosted: Jun Fri 28, 2019 11:22 am 
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I just got confused when I saw for instance the max voltage listed as more positive when at least to me it would have made more sense for the max voltage to be listed as more negative.


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 Post subject: Re: Negative voltage regulator
PostPosted: Jun Fri 28, 2019 11:50 am 
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It's a matter of perspective. When you have, say -10V, and you change it to -5V, the -5 is more positive than -10, so you'd call that a voltage increase. If itr went to -15, that's less voltage (referred to zero) so it's more negative.

Not to be negative, but I'm not positive about all this. It's all a matter of which way you are facing :-D

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 Post subject: Re: Negative voltage regulator
PostPosted: Jun Fri 28, 2019 11:51 am 
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Even an old, plain Vanilla 7912 only needs about a Volt and a half of head room to stay in regulation, so 16 Volts is plenty.

Quote:
...Not to be negative, but I'm not positive about all this. It's all a matter of which way you are facing :-D..


Into the wind, I hope.......so you can at least see "it" coming. :wink:

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 Post subject: Re: Negative voltage regulator
PostPosted: Jun Fri 28, 2019 2:21 pm 
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Barry H Bennett wrote:
It's a matter of perspective. When you have, say -10V, and you change it to -5V, the -5 is more positive than -10, so you'd call that a voltage increase. If itr went to -15, that's less voltage (referred to zero) so it's more negative.


Yes.

I think it is just how that one datasheet put it as I looked at a different datasheet and it was a bit easier to understand.


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 Post subject: Re: Negative voltage regulator
PostPosted: Jun Fri 28, 2019 7:30 pm 
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If you look at the 7912 datasheet:

http://www.ti.com/lit/ds/symlink/lm79.pdf

Specifications are made with input voltages at -14.5V as one of the range limits. So only 1.5V of headroom. I've always used 2V, as a rule of thumb, but I recall the recommended headroom, being larger in the past.

Make sure that you're using a part made after 2000. Don't use old parts with 70's date codes.

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 Post subject: Re: Negative voltage regulator
PostPosted: Jun Fri 28, 2019 8:48 pm 
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Concerning the one I linked to what would be the closest to -12Vdc the power supply could be in order to properly regulate the voltage?


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 Post subject: Re: Negative voltage regulator
PostPosted: Jun Fri 28, 2019 9:27 pm 
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Tube Radio wrote:
Concerning the one I linked to what would be the closest to -12Vdc the power supply could be in order to properly regulate the voltage?

At 100ma -12.3V
At 1A -13V

Specified as dropout voltage in table.

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 Post subject: Re: Negative voltage regulator
PostPosted: Jun Fri 28, 2019 9:37 pm 
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Some regulators also need a tiny amount of load otherwise they won't work properly. 5ma or so? I just built a circuit with lm7815, lm7915 and after reading the datasheets i swore the neg one needed a bit more input voltage to function.


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 Post subject: Re: Negative voltage regulator
PostPosted: Jun Sat 29, 2019 4:27 am 
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Oh ok thanks.

I'll order that one then.


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 Post subject: Re: Negative voltage regulator
PostPosted: Jun Sat 29, 2019 3:39 pm 
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Mikeinkcmo wrote:
Even an old, plain Vanilla 7912 only needs about a Volt and a half of head room to stay in regulation, so 16 Volts is plenty.

Some manufacturers specify 2 volts dropout voltage. but, yes, a 7912 will do fine with a 16 volt supply.

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