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'[EE]: Voltage Regulators'
2002\06\29@121901 by Richard Mellina

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I am new to electronics and have some questions about the 7800 series
voltage regulators. What happens when the supply voltage goes below the
regulated output voltage? Does the regulator stop giving an output or does
it just output the supply voltage? Any help would be greatly appreciated.

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2002\06\29@143144 by Olin Lathrop

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> I am new to electronics and have some questions about the 7800 series
> voltage regulators. What happens when the supply voltage goes below the
> regulated output voltage?

It starts a runaway nuclear reaction that obliterates the regulator, the
circuit board, and everything else within 20Km in a large fireball.  This is
generally considered "bad".

That failing, the output voltage drops below spec.

> Does the regulator stop giving an output or does
> it just output the supply voltage?

The regulator will continue trying to regulate.  In normal operation it
drives its pass element as hard as it has to in order to maintain the
desired output voltage.  When that output voltage goes low, the pass element
is turned on harder.  If the input voltage is below the output voltage set
point, it obviously can't maintain the desired input voltage, so that pass
element will be driven as hard as it can manage.  However, there will always
be some voltage drop accross the regulator, so the output voltage will be a
bit less than the input voltage.

Note also that the input voltage must be kept at or above the output
setpoint plus some margin for the regulator to work properly.  This margin
is often called the "dropout voltage", which is a rather high 2V for the
LM7805.  Therefore, the input voltage for a LM7805 must be at least 7V for
it to maintain 5V output over the full current range.


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2002\06\29@152630 by Dale Botkin

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face
On Sat, 29 Jun 2002, Olin Lathrop wrote:

> It starts a runaway nuclear reaction that obliterates the regulator, the
> circuit board, and everything else within 20Km in a large fireball.  This is
> generally considered "bad".
>
> That failing, the output voltage drops below spec.

Could I be seeing things?  I'd swear Olin just made a joke...  8-)

Dale

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2002\06\29@180156 by Joe Farr

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Thank god for that - I was just about to rip out all my voltage regulators and bury then in the back garden...

Seemed like a good reason why I was loosing all my hair. Maybe it's old age after all - doh !



{Original Message removed}

2002\06\30@054438 by Roman Black

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face
Richard Mellina wrote:
>
> I am new to electronics and have some questions about the 7800 series
> voltage regulators. What happens when the supply voltage goes below the
> regulated output voltage? Does the regulator stop giving an output or does
> it just output the supply voltage? Any help would be greatly appreciated.

Some of the low-dropout 5v regulators i've tried
are ok with this. At least under 100mA or so.
When the supply volts are less than 5v the output
is fixed at the supply volts minus about 50mV.
-Roman

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2002\06\30@130458 by Shawn Mulligan

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Since your're new to electronics it might be fun to answer this question
experimentally. You can build a simple adjustable power supply and test the
behaviour of the 7800 series regulators directly.

The power supply can be built using the LM317 adjustable voltage regulator
and 4 external components. The actual circuit is available in the datasheet
(and you should start becoming familiar with datasheets) or through an
online search (try Google, for example).

Connect the power supply to the 7800 to be tested and the 7800 to the load
to be driven. If you don't have the actual load/device built yet, mock it
with a resistor or similiar device drawing the expected amount of current.
This is important because the voltage regulator may 'drop out' or loose
regulation at different points depending on current draw.

Then you're ready to experiment. Adjust the input voltage while measuring
the output voltage and observe the behaviour of the regulator when it
approaches the critical point where Vin < Vout.

Just as an aside, you don't want Vin to be too high either (e.g. 12V into a
7805, supplying 5V) because voltage regulators are very inefficient and
you'll simply waste power, which will be apparent by the heating of the
voltage regulator.

Hope this was helpful.

Shawn


>I am new to electronics and have some questions about the 7800 series
>voltage regulators. What happens when the supply voltage goes below the
>regulated output voltage? Does the regulator stop giving an output or does
>it just output the supply voltage? Any help would be greatly appreciated.


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'[EE]: Voltage Regulators'
2002\07\01@123158 by Harold M Hallikainen
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On Sat, 29 Jun 2002 11:14:41 -0500 Richard Mellina <rsillemspamspam_OUTFLASH.NET>
writes:
> I am new to electronics and have some questions about the 7800 series
> voltage regulators. What happens when the supply voltage goes below
> the
> regulated output voltage? Does the regulator stop giving an output
> or does
> it just output the supply voltage? Any help would be greatly
> appreciated.

       Linear regulators have a "drop out voltage." For the 7800 series, it's
about 2V. If the input voltage is less than the rated output voltage +
2V, the output falls out of regulation and the output voltage starts to
decrease. Once out of regulation, the output voltage is about Vin - the
dropout voltage. If you are using a line powered supply, AC line ripple
shows up in the regulated output when the "valley" of the incoming DC +
ripple falls below the Vout+Vdo . I typically look at the regulator Vin
on a DC coupled scope to make sure the bottom of the ripple is still
above the input dropout voltage under low line and high load. You can
also by low drop out regulators ("LDO's") that reduce this dropout
voltage. These are good for battery operated products since the equipment
can operate longer before the battery voltage is low enough for the
regulator to drop out.


Harold


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