Power Regulation Advice
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Posts: 46,084
Hello,
I've got a question about power supplies. I've got a project that
needs to be able to run off of both 12 and 24 volts and I'm afraid
that the 7805 regulator might burn out after time if run from 24
volts continuously.
I'd like to limit the voltage that enters the 7805 to 12 volts and
I've heard that you can put a 12volt zener diode, reverse biased,
across the power terminals. This is supposed to siphon off extra
voltage (over 12v) so that the voltage output would be limited to 12
volts. However, it seems to me that that approach would effectively
short circuit the extra voltage and could destroy the battery.
If anyone could explain to me if this works or not, or offer a better
solution, I'd appreciate it. I had just thought that the zener diode
approach would be very simple and easy to implement.
Thanks,
Stewart
I've got a question about power supplies. I've got a project that
needs to be able to run off of both 12 and 24 volts and I'm afraid
that the 7805 regulator might burn out after time if run from 24
volts continuously.
I'd like to limit the voltage that enters the 7805 to 12 volts and
I've heard that you can put a 12volt zener diode, reverse biased,
across the power terminals. This is supposed to siphon off extra
voltage (over 12v) so that the voltage output would be limited to 12
volts. However, it seems to me that that approach would effectively
short circuit the extra voltage and could destroy the battery.
If anyone could explain to me if this works or not, or offer a better
solution, I'd appreciate it. I had just thought that the zener diode
approach would be very simple and easy to implement.
Thanks,
Stewart
Comments
voltages you really need 24,12,and 5 not just 24,12 this is one
unit.
Original Message
From: "Stewart Mayer" <stewlist@k...>
To: <basicstamps@yahoogroups.com>
Sent: January 26, 2002 11:22 AM
Subject: [noparse][[/noparse]basicstamps] Power Regulation Advice
Hello,
I've got a question about power supplies. I've got a project
that
needs to be able to run off of both 12 and 24 volts and I'm
afraid
that the 7805 regulator might burn out after time if run from 24
volts continuously.
I'd like to limit the voltage that enters the 7805 to 12 volts
and
I've heard that you can put a 12volt zener diode, reverse biased,
across the power terminals. This is supposed to siphon off extra
voltage (over 12v) so that the voltage output would be limited to
12
volts. However, it seems to me that that approach would
effectively
short circuit the extra voltage and could destroy the battery.
If anyone could explain to me if this works or not, or offer a
better
solution, I'd appreciate it. I had just thought that the zener
diode
approach would be very simple and easy to implement.
Thanks,
Stewart
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resistors will transform the zener into an LED with a very short life
span. Zeners are usually used as voltage referances, not brute force
regulators. If you scrimp and cheap out on your power supply,
everything downstream from that will suffer the consequences caused
by poor regulation and design. There are many vendors selling well
made power supplies at very reasonable prices; AllElectronics,
Mouser, Nuts and Volts all carry numerous ads for all kinds.
Happy hunting
Hal
--- In basicstamps@y..., Stewart Mayer <stewlist@k...> wrote:
> Hello,
>
> I've got a question about power supplies. I've got a project that
> needs to be able to run off of both 12 and 24 volts and I'm afraid
> that the 7805 regulator might burn out after time if run from 24
> volts continuously.
>
> I'd like to limit the voltage that enters the 7805 to 12 volts and
> I've heard that you can put a 12volt zener diode, reverse biased,
> across the power terminals. This is supposed to siphon off extra
> voltage (over 12v) so that the voltage output would be limited to
12
> volts. However, it seems to me that that approach would
effectively
> short circuit the extra voltage and could destroy the battery.
>
> If anyone could explain to me if this works or not, or offer a
better
> solution, I'd appreciate it. I had just thought that the zener
diode
> approach would be very simple and easy to implement.
>
> Thanks,
> Stewart
voltage can be anywhere from 12 to 24 volts, depending on what
batteries are being used and their charge. All I want to do is
protect the 7805 from too much input voltage so it doesn't burn up.
I've seen the specs on some 5volt regulators that will accept up to
35v input, but I'm not sure if they will last long at those high
voltages. Most devices with 7805s, like the stamp BOE, recommend
that the input voltage be between 9v and 12v. Will the lifespan of
of the regulator be the same at higher voltages if it is still
"within spec"?
Stewart
shorten the life of the part. To prevent premature failue from overheating,
provide an adequate heatsink. The input voltage should never exceed the
rated maximum.
Marc Reynolds
[noparse][[/noparse]Non-text portions of this message have been removed]
<stewlist@k...> writes:
> Hello,
>
> I've got a question about power supplies. I've got a project that
> needs to be able to run off of both 12 and 24 volts and I'm afraid
> that the 7805 regulator might burn out after time if run from 24
> volts continuously.
>
> I'd like to limit the voltage that enters the 7805 to 12 volts and
> I've heard that you can put a 12volt zener diode, reverse biased,
> across the power terminals. This is supposed to siphon off extra
> voltage (over 12v) so that the voltage output would be limited to 12
>
> volts. However, it seems to me that that approach would effectively
>
> short circuit the extra voltage and could destroy the battery.
>
> If anyone could explain to me if this works or not, or offer a
> better
> solution, I'd appreciate it. I had just thought that the zener
> diode
> approach would be very simple and easy to implement.
>
> Thanks,
> Stewart
>
>
>
>
> 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/
>
>
What is the current drain in the 24 volt mode when your system is connected
to the 7805 regulator under full load?
What is the current drain in the 12 volt mode?
Do you have a heat sink on the 7805 regulator?
If so what size?
Over what temperature range do you want your device to function?
What is driving the requirement to operate at either 12 volts or 24 volts?
If you were to use a zener diode it would be in series with the power in
lead?
You need to know the maximum line current and maximum temperature to
determine the power dissipation in order to select a zener diode suitable
for the proposed task.
Steve
Original Message
From: <agarb@j...>
To: <basicstamps@yahoogroups.com>
Sent: Saturday, January 26, 2002 4:51 PM
Subject: Re: [noparse][[/noparse]basicstamps] Power Regulation Advice
>
>
> On Sat, 26 Jan 2002 13:22:27 -0600 Stewart Mayer
> <stewlist@k...> writes:
> > Hello,
> >
> > I've got a question about power supplies. I've got a project that
> > needs to be able to run off of both 12 and 24 volts and I'm afraid
> > that the 7805 regulator might burn out after time if run from 24
> > volts continuously.
> >
> > I'd like to limit the voltage that enters the 7805 to 12 volts and
> > I've heard that you can put a 12volt zener diode, reverse biased,
> > across the power terminals. This is supposed to siphon off extra
> > voltage (over 12v) so that the voltage output would be limited to 12
> >
> > volts. However, it seems to me that that approach would effectively
> >
> > short circuit the extra voltage and could destroy the battery.
> >
> > If anyone could explain to me if this works or not, or offer a
> > better
> > solution, I'd appreciate it. I had just thought that the zener
> > diode
> > approach would be very simple and easy to implement.
> >
> > Thanks,
> > Stewart
> >
> >
> >
> >
> > 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/
> >
> >
>
> To UNSUBSCRIBE, just send mail to:
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Body of the message will be ignored.
>
>
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>
>
communicate with serially and want to keep it simple by having a
single source of battery power. Normally I run the motor off of 24
volts, but when I need to go slow it is better to run off 12 volts to
increase accuracy. I've got two boards with 7805s on them, each pull
500ma so a total of 1amp is required.
I think I might just get a high-rated 7805 and stick a heat sink on
it and see what happens.
thanks,
Stewart
On Sat, 26 Jan 2002 20:54:47 -0800, Stephen H Chapman wrote:
>Stewart, What is the current drain in the 24 volt mode when your
system is
>connected to the 7805 regulator under full load?
>What is the current drain in the 12 volt mode?
>Do you have a heat sink on the 7805 regulator?
>If so what size?
>Over what temperature range do you want your device to function?
>What is driving the requirement to operate at either 12 volts or 24
volts?
>If you were to use a zener diode it would be in series with the
power in
>lead?
>You need to know the maximum line current and maximum temperature to
determine
>the power dissipation in order to select a zener diode suitable for
the
>proposed task.
--
Stewart Mayer, stewlist@k... on 01/27/2002
like you will be wasting alot of power in the form of heat.
If I remember my math correctly, you are dropping 19 volts at 1 amp -- thats
19 watts. Quite a bit of heat to dissipate.
Are you running that much stuff on 5 volts that you need to supply 1 amp, or
is this a guesstimate?
Original Message
> I'm controlling a motor with a pwm amplifier/controller which I
> communicate with serially and want to keep it simple by having a
> single source of battery power. Normally I run the motor off of 24
> volts, but when I need to go slow it is better to run off 12 volts to
> increase accuracy. I've got two boards with 7805s on them, each pull
> 500ma so a total of 1amp is required.
>
> I think I might just get a high-rated 7805 and stick a heat sink on
> it and see what happens.
applications, the LM2940, for regulating 12-14V down to 5V. When I powered a
device that consumed some 200mA, the regulator would heat up to abut 85
degrees (celsius). The max junction temperature for these devices is about
120 degrees, so forget running 24V at 1A. I am now using switched power
supplies.
Basically, heat will increase due to two factors, the voltage drop (more
volts dropped = more heat) and the current expected. These type of
regulators are meant to smooth other power supplies, like switched ones,
from say 6 or 6.5V down to 5V. Then you're OK.
Look at the app notes from Micrel regulators, the 4860, it is a
fixed-voltage switcher, when you add an extra smoothing filter (basically a
7805 and a choke) you can get ripple noise down to about 1mV.
Cheers,
Mike
Original Message
From: "Rodent" <daweasel@s...>
To: <basicstamps@yahoogroups.com>
Sent: Sunday, January 27, 2002 8:35 AM
Subject: Re: [noparse][[/noparse]basicstamps] Power Regulation Advice
> If you are pulling 1 amp and regulating down to 5 volts from 24, it seems
> like you will be wasting alot of power in the form of heat.
>
> If I remember my math correctly, you are dropping 19 volts at 1 amp --
thats
> 19 watts. Quite a bit of heat to dissipate.
>
> Are you running that much stuff on 5 volts that you need to supply 1 amp,
or
> is this a guesstimate?
>
>
Original Message
>
> > I'm controlling a motor with a pwm amplifier/controller which I
> > communicate with serially and want to keep it simple by having a
> > single source of battery power. Normally I run the motor off of 24
> > volts, but when I need to go slow it is better to run off 12 volts to
> > increase accuracy. I've got two boards with 7805s on them, each pull
> > 500ma so a total of 1amp is required.
> >
> > I think I might just get a high-rated 7805 and stick a heat sink on
> > it and see what happens.
>
>
>
>
> 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/
>
>
> 35v input, but I'm not sure if they will last long at those high
> voltages. Most devices with 7805s, like the stamp BOE, recommend
> that the input voltage be between 9v and 12v. Will the lifespan of
> of the regulator be the same at higher voltages if it is still
> "within spec"?
You need to observe both the maximum voltage input specified for the
regulator plus the total power dissipation speced for the device. If,
for example, your circuit needs 500mA @ 5 vdc the regulator will have
to dissipate significantly more power with a 24 volt source than it
will at 12 volts. Also, take into account heatsinks, cooling, etc.
They get more and more important as the power dissipation demands on
your regulator go up.
Tim
the amplifier/controller board (www.jrkerr.com). Each board has a
draw of 500ma, and I will only have one to begin with but will add a
second later. 19watts of wasted power if really ugly.
Mike recently posted to look at the Micrel switching power regulators
to use as pre-regulators, and it seems like a simple, cheap, and
elegant solution, almost as if these regulators were made exactly for
this purpose [noparse]:)[/noparse] . Along with the 4680 he mentioned, the LM2576 goes
up to 3amps so I think I'll try this approach.
Thank you everyone for your valuable input.
stewart
>If I remember my math correctly, you are dropping 19 volts at 1 amp
-- thats
>19 watts. Quite a bit of heat to dissipate.
>
>Are you running that much stuff on 5 volts that you need to supply 1
amp, or
>is this a guesstimate?
--
Stewart Mayer, stewlist@k... on 01/27/2002
that can be dissipated as heat before the regulator goes into shutdown. If
for example your circuit is drawing 100ma and the regulator is being fed 30
volts, 30-5=25, 25v*.1a=2.5 watts easily doable with a 7805 and a good
heatsink.
KF4HAZ - Lonnie
Original Message
From: "Stewart Mayer" <stewlist@
> The only regulated output voltage I need is five volts, the input
> voltage can be anywhere from 12 to 24 volts, depending on what
> batteries are being used and their charge. All I want to do is
> protect the 7805 from too much input voltage so it doesn't burn up.
>
> I've seen the specs on some 5volt regulators that will accept up to
> 35v input, but I'm not sure if they will last long at those high
> voltages. Most devices with 7805s, like the stamp BOE, recommend
> that the input voltage be between 9v and 12v. Will the lifespan of
> of the regulator be the same at higher voltages if it is still
> "within spec"?
>
> Stewart