7812 and 7815 voltage regulators
mark krawczuk
i have some 7812 and 7815 voltage regulators.
would it be best to increase the voltage out of the 7812
or
decrease the voltage from the 7815 ?
how can i do this ?
thanks,
mark k
D from BC
Don't use either..
Get an adjustable regulator..
Or perhaps see Fig. 13 to bring the 7812 up to 13.8V
http://www.fairchildsemi.com/ds/LM/LM7805.pdf
D from BC
myrealaddress(at)comic(dot)com
British Columbia
Canada
Spehro Pefhany
You add (to the 7805) an 8.2V zener (eg. BZX79-C8V2) between the GND
terminal and ground to give you around 13.2V. Or use 9.1V to get a bit
higher.
Best regards,
Spehro Pefhany
--
"it's the network..." "The Journey is the reward"
sp...@interlog.com Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog Info for designers: http://www.speff.com
MooseFET
-------
--+--! 7812 !--------+--- 13.8V
! ------- !
--- ! R1 !
---C1 +---+-/\/\---+
! ! ! !
GND \ --- ---
/ ---C2 ---C3
R2 \ ! !
! GND GND
GND
C1 from data sheet
C2 0.1uF nominal
C3 from data sheet
R1 5mA at 12V see Ohms law
R2 is a bit more complex
R2 has 5mA plus the idle current of the 7812 in it see the data sheet
for the amount more. It has 1.8V on it see Ohms law.
Adrian Tuddenham
> hi, i want to make up a 13.8 volt power supply.
>
> i have some 7812 and 7815 voltage regulators.
If you are intending to charge a lead-acid battery with this, you will
probably need a diode between the regulator output and the battery
positive terminal so as to prevent reverse current, which would
discharge the battery if the mains supply were switched off during
charging. If your charging current doesn't exceed 1 amp, a 1N4001 will
suffice.
This will mean that you need an extra 0.6v (or thereabouts) from the
regulator to overcome the voltage drop of the diode. To get this, put
an identical diode in the earth lead of the regulator (in series with
anything else you may need to put there). If the two diodes are in
thermal contact, they will have approximately the same temperature
coefficient of voltage.
Ready-made crib sheet for a (not very efficient) double charger with
semi-logarithmic metering at:
http://www.poppyrecords.co.uk/other/images/ChargerLA.gif
--
~ Adrian Tuddenham ~
(Remove the ".invalid"s and add ".co.uk" to reply)
www.poppyrecords.co.uk
Tam
"Adrian Tuddenham" <adr...@poppyrecords.invalid.invalid> wrote in message
news:1iqclgb.1jl3qn91dbm94yN%adr...@poppyrecords.invalid.invalid...
13.8V is too low to charge a lead acid battery; I tried it, took at least
14.3 V. He probably want to power a radio, but a 78XX won't power much of a
radio.
Tam
mark krawczuk
supply for testing etc..
thanks,
mark k
"Tam" <t-ta...@comcast.net> wrote in message
news:wKudnSpq9JzHboHU...@comcast.com...
Adrian Tuddenham
> HI, my fault , i should have told you i wanted it for a hobby power
> supply for testing etc..
You need to make sure that it is short-circuit proof so that it cannot
damage itself. Even better, some form of pre-settable current limit
would reduce the possibility that you might damage something else with
it.
The blocking diode might still be a good idea, just in case it ever
finds itself across a battery.
John Fields
(Adrian Tuddenham) wrote:
>You need to make sure that it is short-circuit proof so that it cannot
>damage itself.
---
from:
http://www.fairchildsemi.com/ds/LM%2FLM7812.pdf
"The LM78XX series of three terminal positive regulators
are available in the TO-220 package and with several
fixed output voltages, making them useful in a wide
range of applications. Each type employs internal current
limiting, thermal shut down and safe operating area protection,
making it essentially indestructible."
JF
mpm
How much current do you need?
You might look at a simple switcher type power supply.
For example,
http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail?name=LM2575T-ADJGOS-ND
This version is adjustable from 1.23 VDC to 37 VDC out, by using a
couple small programming resistors. It's not that difficult to select
the external parts required to build it. The datasheet is pretty
straightforward. Sounds like a good opportunity to learn about
switching power supplies....
Also, I think Sharp used to have a 4-terminal (linear) voltage
regulator that you could similarly program with external resistors.
Don't know if that part is still around however...
-mpm
Adrian Tuddenham
The current limit isn't particularly well specified (on the data sheets
I have seen) and I have had at least a couple of these blow up under
short-circuit conditions.
Jim Thompson
On Fri, 14 Nov 2008 18:25:48 +0000,
adr...@poppyrecords.invalid.invalid (Adrian Tuddenham) wrote:
>John Fields <jfi...@austininstruments.com> wrote:
>
>> On Fri, 14 Nov 2008 15:58:59 +0000, adr...@poppyrecords.invalid.invalid
>> (Adrian Tuddenham) wrote:
>>
>>
>> >You need to make sure that it is short-circuit proof so that it cannot
>> >damage itself.
>>
>> ---
>> from:
>>
>> http://www.fairchildsemi.com/ds/LM%2FLM7812.pdf
>>
>>
>> "The LM78XX series of three terminal positive regulators
>> are available in the TO-220 package and with several
>> fixed output voltages, making them useful in a wide
>> range of applications. Each type employs internal current
>> limiting, thermal shut down and safe operating area protection,
>> making it essentially indestructible."
>
>The current limit isn't particularly well specified (on the data sheets
>I have seen) and I have had at least a couple of these blow up under
>short-circuit conditions.
I'm not sure that the LM78XX/LM79XX series of regulators have thermal
shutdown... just short-circuit current limiting.
...Jim Thompson
--
| James E.Thompson, P.E. | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| Phoenix, Arizona 85048 Skype: Contacts Only | |
| Voice:(480)460-2350 Fax: Available upon request | Brass Rat |
| E-mail Icon at http://www.analog-innovations.com | 1962 |
Now that the value of our money is all queer they've decided to
inscribe the portrait of Barney Frank onto the One Dollar Bill.
John Fields
(Adrian Tuddenham) wrote:
>John Fields <jfi...@austininstruments.com> wrote:
>
>> On Fri, 14 Nov 2008 15:58:59 +0000, adr...@poppyrecords.invalid.invalid
>> (Adrian Tuddenham) wrote:
>>
>>
>> >You need to make sure that it is short-circuit proof so that it cannot
>> >damage itself.
>>
>> ---
>> from:
>>
>> http://www.fairchildsemi.com/ds/LM%2FLM7812.pdf
>>
>>
>> "The LM78XX series of three terminal positive regulators
>> are available in the TO-220 package and with several
>> fixed output voltages, making them useful in a wide
>> range of applications. Each type employs internal current
>> limiting, thermal shut down and safe operating area protection,
>> making it essentially indestructible."
>
>The current limit isn't particularly well specified (on the data sheets
>I have seen) and I have had at least a couple of these blow up under
>short-circuit conditions.
---
I think that's covered by the "essentially indestructible" part. ;)
JF
Jim Thompson
On Fri, 14 Nov 2008 11:36:27 -0700, Jim Thompson
<To-Email-Use-Th...@My-Web-Site.com> wrote:
>On Fri, 14 Nov 2008 18:25:48 +0000,
>adr...@poppyrecords.invalid.invalid (Adrian Tuddenham) wrote:
>
>>John Fields <jfi...@austininstruments.com> wrote:
>>
>>> On Fri, 14 Nov 2008 15:58:59 +0000, adr...@poppyrecords.invalid.invalid
>>> (Adrian Tuddenham) wrote:
>>>
>>>
>>> >You need to make sure that it is short-circuit proof so that it cannot
>>> >damage itself.
>>>
>>> ---
>>> from:
>>>
>>> http://www.fairchildsemi.com/ds/LM%2FLM7812.pdf
>>>
>>>
>>> "The LM78XX series of three terminal positive regulators
>>> are available in the TO-220 package and with several
>>> fixed output voltages, making them useful in a wide
>>> range of applications. Each type employs internal current
>>> limiting, thermal shut down and safe operating area protection,
>>> making it essentially indestructible."
>>
>>The current limit isn't particularly well specified (on the data sheets
>>I have seen) and I have had at least a couple of these blow up under
>>short-circuit conditions.
>
>I'm not sure that the LM78XX/LM79XX series of regulators have thermal
>shutdown... just short-circuit current limiting.
>
> ...Jim Thompson
Oooops! Looks like thermal shutdown has been added since initial
inception.
Mike V
I don't know about the 78xx parts but our part number for the 317 is
LM317T-not-ST.
They may have fixed the problem but 10+ years ago they were guaranteed
to fail in our app.
Spehro Pefhany
<To-Email-Use-Th...@My-Web-Site.com> wrote:
Yes, but it's typically set around 150°C and the abs. max. operating
Tj is 125°C so allowing it to come on won't do much (good) for the
reliability...
Jim Thompson
Yep. At GenRad I had a part number "LM324-not-Moto" ;-)
JosephKK
Then that would have to be on a three dollar bill.