Voltage regulators
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Posts: 46,084
try 78L05, low dropout version of 7805
Original Message
From: Pat Matthews <patmat2350@a...> [noparse]/noparse]mailto:[url=http://forums.parallaxinc.com/group/basicstamps/post?postID=vW_4Sm-DTScqywAqFiAxXpebW7x5Zbbw5dQaFQSQPbf_HnFN4WfFRnEGqm743ItuG8GeGaZ5nvNA4w]patmat2350@a...[/url
Sent: Saturday, December 21, 2002 12:19 PM
To: basicstamps@yahoogroups.com
Subject: [noparse][[/noparse]basicstamps] Voltage regulators
I think I need a primer on v reg's.
Just discovered that my generic 1A 7805's need >7.5v to output 5v.
I'm dealing with 7.2 and 6.0 v battteries, but need clean 5v for
certain components... and > 0.1A capacity.
What should I be looking for?
Thx, Pat M
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Original Message
From: Pat Matthews <patmat2350@a...> [noparse]/noparse]mailto:[url=http://forums.parallaxinc.com/group/basicstamps/post?postID=vW_4Sm-DTScqywAqFiAxXpebW7x5Zbbw5dQaFQSQPbf_HnFN4WfFRnEGqm743ItuG8GeGaZ5nvNA4w]patmat2350@a...[/url
Sent: Saturday, December 21, 2002 12:19 PM
To: basicstamps@yahoogroups.com
Subject: [noparse][[/noparse]basicstamps] Voltage regulators
I think I need a primer on v reg's.
Just discovered that my generic 1A 7805's need >7.5v to output 5v.
I'm dealing with 7.2 and 6.0 v battteries, but need clean 5v for
certain components... and > 0.1A capacity.
What should I be looking for?
Thx, Pat M
To UNSUBSCRIBE, just send mail to:
basicstamps-unsubscribe@yahoogroups.com
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Comments
Just discovered that my generic 1A 7805's need >7.5v to output 5v.
I'm dealing with 7.2 and 6.0 v battteries, but need clean 5v for
certain components... and > 0.1A capacity.
What should I be looking for?
Thx, Pat M
trick - 1A, with a 6V input. Less if you are taking less than 1A.
http://www.national.com/ds/LM/LM2940.pdf
Al Williams
AWC
* NEW: PAK-VIa - Read PS/2 keyboards or mice -- double the buffer, lower
current consumption.
http://www.al-williams.com/awce/pak6.htm
>
Original Message
> From: Pat Matthews <patmat2350@a...> [noparse]/noparse]mailto:[url=http://forums.parallaxinc.com/group/basicstamps/post?postID=7TLUCHcCI-ceD2On5sDJ17oq0OPYpREf2QDdWgO4hVeWwGFVTQNwcUqlBigyaH_Fg1Fu_noyYGQ]patmat2350@a...[/url
> Sent: Friday, December 20, 2002 10:19 PM
> To: basicstamps@yahoogroups.com
> Subject: [noparse][[/noparse]JUNK] [noparse][[/noparse]basicstamps] Voltage regulators
>
>
> I think I need a primer on v reg's.
> Just discovered that my generic 1A 7805's need >7.5v to output 5v.
> I'm dealing with 7.2 and 6.0 v battteries, but need clean 5v for
> certain components... and > 0.1A capacity.
> What should I be looking for?
> Thx, Pat M
>
>
> To UNSUBSCRIBE, just send mail to:
> basicstamps-unsubscribe@yahoogroups.com
> from the same email address that you subscribed. Text in the
> Subject and Body of the message will be ignored.
>
>
> Your use of Yahoo! Groups is subject to
> http://docs.yahoo.com/info/terms/
>
>
>
>
the same package as a 7805. You do need to use the filtering suggested in
the documentation as these things can go to lala land and drop out of
regulation and get hot with out it. There are lower current versions of this
as well which are a bit cheaper. I believe both Mouser and Digi-Key sell
them. Here is the spec sheet.
http://www.national.com/ds/LM/LM2940.pdf
Original Message
> I think I need a primer on v reg's.
> Just discovered that my generic 1A 7805's need >7.5v to output 5v.
> I'm dealing with 7.2 and 6.0 v battteries, but need clean 5v for
> certain components... and > 0.1A capacity.
> What should I be looking for?
Original Message
> What you want is a low drop out regulator. The LM2940 should do the
> trick - 1A, with a 6V input. Less if you are taking less than 1A.
>
> http://www.national.com/ds/LM/LM2940.pdf
the capacitors still confuse me. I've looked at some 5 volt circuits with
the 7805 and they all seem to use different values for the capacitors.
What does the capacitor do in these circuits and what is the impact of higher
and lower values?
thanks
bob
[noparse][[/noparse]Non-text portions of this message have been removed]
side, and the output side.
The input side just needs to hold enough steady
voltage to stay above the regulator's 'drop-out'
voltage -- 10 uF cap typical -- even
as the unregulated input voltage has lots of
'ripple'. There are rule-of-thumb books for
what the size of the input cap should be,
based on ripple and amount of current
to be drawn through the regulator.
Output Side:
First, you need to realize that the regulator is acting
like a variable resistor -- any voltage over the rated
voltage out is dropped in the regulator.
Next, you need to realize that digital circuits have
a very 'noisy' use of VCC (+5V). When you switch lots
of gates simultaneously, there is a rush of current to
support this, so for a short while (a 'transient')
a larger amount of current is needed than average.
Next, large capacitors are good for holding lots of
charge (to support the current transient) BUT they
don't have very good high frequency response, which
means the beginning of the transient may not be
well supported.
Small capacitors have very good high frequency
response, but then they run out of charge for longer
transients.
The best balance (on the regulator output side)
is maintained when a large electrolytic
(1 to 10 to 100 uF) capacitor is put in
parallel with a small (.1 to .01 uf) capacitor.
The actual values depend on what the designer expected
in terms of transient current draw, and what
rule-of-thumb book he read last, and what other
'decoupling capacitors' (.1 uF for each TTL chip) are
on the board.
So: On the input, large capacitors support lots of
current (with the limit being how much heat the
regulator can dissipate before it goes into thermal
shutdown). On the output, a small cap handles the
start of transients, while the large cap allows
larger/longer transients.
Also: Large value capacitors tend to be physically
large -- and above a certain value they aren't getting
you anything, so you want the smallest value that will
work.
Too small of an input capacitor, and your regulator
can't regulate -- input noise shows up on your output.
Too small of an output capacitor, and when you have
a sudden current demand (switch gates, motor starts)
your voltage drops, and you get a reset.
The 7805 literature should have the 'rules-of-thumb'
mentioned above.
--- In basicstamps@yahoogroups.com, eclecticrr@A... wrote:
> I'm trying to understand voltage regulators and think I may be
close. But
> the capacitors still confuse me. I've looked at some 5 volt
circuits with
> the 7805 and they all seem to use different values for the
capacitors.
>
> What does the capacitor do in these circuits and what is the impact
of higher
> and lower values?
>
> thanks
>
> bob
>
>
> [noparse][[/noparse]Non-text portions of this message have been removed]
about 2 volts to operate, so you must supply it with at least 7 volts.
The 2940 requires only 0.3 volts to operate, so it can supply 5
volts from a 5.3 volt (6 volt) supply. The 2940 is called a low
dropout voltgae regulator. Both the 7805 and 2940 come in a
TO220 form factor.
Paul
The rectifier needs to be big enough to handle the inrush current while the
cap charges up, from the power down state.
I run Stamp II's from 7805 regulators in very noisy industrial environments.
I use a 2.5 VA 8 volt transformer, a 1 amp Dip Bridge Rectifier, a 3,300 uf
input cap and a 300 uf output cap. The data sheets on the 7805 suggests the
min capacitance on the output be 300 uf to prevent the regulator from
oscillating.
This has worked well with a Stamp II, a few digital chips and 16 Solid State
I/O modules.
Alan Bradford
Plasma Technologies
[noparse][[/noparse]Non-text portions of this message have been removed]
earlier -- but linear regulators (7805, 2940, etc)
will only work if they are dropping a certain minimal
amount of voltage -- and the more voltage they drop,
the more heat they generate and the more power they
waste (as heat). Below the minimum drop-out
(7 volts for 7805) they cease regulating
(it keeps at least 2 volts across itself).
So, 'low dropout regulators' (like the 2940) were
made which don't require such large drop-out
voltage. 0.3 is VERY low. This solves several
problems:
1. You can use a 6-volt stack of batteries.
2. Very little of your battery current is lost
in the regulator.
3. You can use more of the range of your
batteries as they discharge (and lose
voltage).
Back to Capacitors: a battery source needs NO
input capacitor, as a battery looks like a
really, really good capacitor already.
--- In basicstamps@yahoogroups.com, <verhap@o...> wrote:
> Instead of a LM7805, consider a LM2940T-5. The 7805 requires
> about 2 volts to operate, so you must supply it with at least 7
volts.
> The 2940 requires only 0.3 volts to operate, so it can supply 5
> volts from a 5.3 volt (6 volt) supply. The 2940 is called a low
> dropout voltgae regulator. Both the 7805 and 2940 come in a
> TO220 form factor.
>
> Paul
frequencies as well as handle the load from current draw.
also, when thinking of a voltage regulator as similar to a variable
resistor, you get the same property and that is power. if you are
feeding it with a voltage 5 times higher than the output (ie : 24 vdc
supply with 5vdc regulated) you will be dumping a LOT of heat !
Staging 24 > 18 > 12 > 5 will spread the heat out, but you will still
be duming the same watts. that is one downfall of the little guys.
Dave
--- In basicstamps@yahoogroups.com, eclecticrr@A... wrote:
> I'm trying to understand voltage regulators and think I may be
close. But
> the capacitors still confuse me. I've looked at some 5 volt
circuits with
> the 7805 and they all seem to use different values for the
capacitors.
>
> What does the capacitor do in these circuits and what is the impact
of higher
> and lower values?
>
> thanks
>
> bob
>
>
> [noparse][[/noparse]Non-text portions of this message have been removed]
1. They provide current for that very short length of time that it
takes for the regulator to react to changes in load conditions.
2. They provide stability. Many of the regulators will be unstable
unless they have capacitors nearby.
Now, that being said, there is a minimum amount of capacitance necessary
to achieve stability. Unless you are looking at the notes from the
manufacturer, you will likely see lots of different values. I generally
use 10 uF on the input and the output.
Note that in some instances I will use a 10 uF tantalum capacitor AND a
0.1 uF ceramic. The tantalum takes care of the low frequency stuff (and
provides stability) and the ceramic takes care of the higher frequency
stuff.
In general, so long as you have more capacitance than is suggested by
the manufacturer for stability, any amount of capacitance will work<G>.
Note that some of the three terminal regulators (like the 7805) like to
see a minimum current drawn from them. It's hidden in the notes
somewhere. So, I put a led on the output. This gives the minimum
current draw, and is a great troubleshooting aid<G>.
Make sense?
Original Message
From: eclecticrr@A... [noparse]/noparse]mailto:[url=http://forums.parallaxinc.com/group/basicstamps/post?postID=ng_IzeWpK55TiUbnPRsNHaAD_DH8v3bnTJz_DLtrXfENYt3L7VK3tk45rG8c0dFi3h5OmaDGNykCNNMn]eclecticrr@A...[/url
Sent: Friday, May 02, 2003 11:39 AM
To: basicstamps@yahoogroups.com
Subject: [noparse][[/noparse]basicstamps] voltage regulators
I'm trying to understand voltage regulators and think I may be close.
But
the capacitors still confuse me. I've looked at some 5 volt circuits
with
the 7805 and they all seem to use different values for the capacitors.
What does the capacitor do in these circuits and what is the impact of
higher
and lower values?
thanks
bob
[noparse][[/noparse]Non-text portions of this message have been removed]
To UNSUBSCRIBE, just send mail to:
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200 milli-volt external reference. This will give a full 100 mv
bi-polar range. Then for the voltage sources (0 to 5 volts) I will
use the internal reference.
My next question is: Does anyone know of a precision 5 volt regulator
that I can use to power the TLC2543 Vdd and divider chain for the 200
mv reference? Something in the 1% regulation range would be adequate.
Bu using such a regulator I would feel better about the results.
Russ
The National LM2950 (fixed 5v) and LM2951 (adjustable) are almost
reference quality voltage regulators. The LM2951 is very nice, if
you put a 10k pot in series with its internal voltage divider, you
can trim it to make a fine 5.120 volt power supply, which makes the
resolution of the ADC 1.25 millivolts per bit. Or as you suggest,
divide it down to make a 200mv reference.
-- Tracy
>I have decided that for my current measuring application I will use a
>200 milli-volt external reference. This will give a full 100 mv
>bi-polar range. Then for the voltage sources (0 to 5 volts) I will
>use the internal reference.
>
>My next question is: Does anyone know of a precision 5 volt regulator
>that I can use to power the TLC2543 Vdd and divider chain for the 200
>mv reference? Something in the 1% regulation range would be adequate.
> Bu using such a regulator I would feel better about the results.
>
>Russ
UA723 which claims 0.1 mV/V precision for the 5 to 12 volt range. It
too is fine-tuneable.
Russ
--- In basicstamps@yahoogroups.com, Tracy Allen <tracy@e...> wrote:
> Hi Russ,
>
> The National LM2950 (fixed 5v) and LM2951 (adjustable) are almost
> reference quality voltage regulators. The LM2951 is very nice, if
> you put a 10k pot in series with its internal voltage divider, you
> can trim it to make a fine 5.120 volt power supply, which makes the
> resolution of the ADC 1.25 millivolts per bit. Or as you suggest,
> divide it down to make a 200mv reference.
>
> -- Tracy
>
>
>
>
> >I have decided that for my current measuring application I will use a
> >200 milli-volt external reference. This will give a full 100 mv
> >bi-polar range. Then for the voltage sources (0 to 5 volts) I will
> >use the internal reference.
> >
> >My next question is: Does anyone know of a precision 5 volt regulator
> >that I can use to power the TLC2543 Vdd and divider chain for the 200
> >mv reference? Something in the 1% regulation range would be adequate.
> > Bu using such a regulator I would feel better about the results.
> >
> >Russ