Cascade two voltage regs
Largo
into a LM317T. I other words, I need 9 volts and then feed that to a
5 Volt regulator for digital circuitry. Any pitfalls here?
What bypass capacitors are uses for the input and output of each
regulator?
I see the data sheet says .33uF on the input and .1uF on the output
for these regulators. I use tantilum dipped electrolytics on both
input and output. Would this mean I'd have a .33 and .1 in parallel
between the 7809 and LM317?
Do I need electrolytic caps on the inputs?
Thanks
Largo
Eeyore
Largo wrote:
> I use tantilum dipped electrolytics on both input and output.
For any good reason ? Or just because National IIRC said to do so ? I can tell
you that tantalums are certainly not needed.
And yes, capacitance adds. The importance for regulators is *where* the cap is
for the most part though.
Graham
Phil Allison
"Largo"
> I wish to cascade two 3-terminal voltage regulators. One MC7809CT
> into a LM317T. I other words, I need 9 volts and then feed that to a
> 5 Volt regulator for digital circuitry. Any pitfalls here?
>
> What bypass capacitors are uses for the input and output of each
> regulator?
>
> I see the data sheet says .33uF on the input and .1uF on the output
> for these regulators.
** The data sheets ASSUME that a large value electro (or a battery) is
connected in parallel with and close to the input of linear regulator ICs.
> I use tantilum dipped electrolytics on both
> input and output.
** LOL - chocolate dipped is better.
> Would this mean I'd have a .33 and .1 in parallel
> between the 7809 and LM317?
>
> Do I need electrolytic caps on the inputs?
** The LM317 needs a low impedance at its input at frequencies between 100
kHz and 10 MHz.
A 100 uF, 25 volt electro should do that fine.
Best avoid tantalum bead caps completely.
....... Phil
Largo
So it's okay to use unpolerized multilayer ceramics at the output?
>
>
>
>
>....... Phil
>
>
John Larkin
>I wish to cascade two 3-terminal voltage regulators. One MC7809CT
>into a LM317T. I other words, I need 9 volts and then feed that to a
>5 Volt regulator for digital circuitry. Any pitfalls here?
>
>What bypass capacitors are uses for the input and output of each
>regulator?
>
>I see the data sheet says .33uF on the input and .1uF on the output
>for these regulators. I use tantilum dipped electrolytics on both
>input and output. Would this mean I'd have a .33 and .1 in parallel
>between the 7809 and LM317?
>
Tantalums on power rails are bad news; they tend to blow up. Aluminums
are much safer.
>Do I need electrolytic caps on the inputs?
I'd use 'lytics at the input and output of both regs, which would be
three total, with the middle one shared. 22uF 16v or something.
John
ian field
"Largo" <la...@fullBS.cox.net> wrote in message
news:iisp92p6rmk22djpl...@4ax.com...
Multilayer ceramics should work fine so long as you have decently large
values to hand - equipment manufacturers tend to be too stingy to use these!
Largo
wrote:
What does this mean? Please suggest values.
Thank you.
Largo
>
ian field
At least 0.47uF better yet 2.2uF - I personally would still use alu-electro
about 22 or 47uF as well just to make sure!
Largo
wrote:
So what I gain from this relative discussion is the more capacitance
the better but put something in. Small is okay, only because
"manufacturers tend to be too stingy". Larger is better--"just to
make sure".
i.e. I'd be well off to use aluminum electrolytic [on the output], up
to 47uF. What about the input? Use .33uF electrolytic as the
datasheet says? or 22 or 47uF? Or does this apply on both input and
output?
Thank you for your help.
Largo
DaveM
>>>>>>> into a LM317T. I other words, I need 9 volts and then feed that to a
>>>>>>> 5 Volt regulator for digital circuitry. Any pitfalls here?
>>>>>>>
>>>>>>> What bypass capacitors are uses for the input and output of each
>>>>>>> regulator?
>>>>>>>
>>>>>>> I see the data sheet says .33uF on the input and .1uF on the output
>>>>>>> for these regulators.
Of course, you realize that the 7809 will have to pass both the 9V load current
and the 5 volt load current. Make sure that the heat sink is adequate for the
total power dissipated.
--
Dave M
MasonDG44 at comcast dot net (Just substitute the appropriate characters in the
address)
Some days you're the dog, some days the hydrant.
Paul E. Schoen
"John Larkin" <jjla...@highNOTlandTHIStechnologyPART.com> wrote in message
news:50mq92d34ocgudeph...@4ax.com...
It is also a good idea to make sure the capacitance on the upstream side of
the regulator is higher than those downstream, because if the input voltage
of the regulator drops below the output, damage can occur. Another way to
prevent this is by adding a diode from the output to the input.
Most regulators are stable with a 0.1 to 1 uF low ESR capacitor close to
the output, and up to several hundred uF in parallel downstream, but it is
better to have the large capacitance upstream for energy storage, and
smaller capacitors distributed near components which need a low impedance
supply source for switching transients.
Paul
Paul
ian field
There are certain rules to observe, the 3-TRs require small-ish capacitors
from both IP & OP to GND which must have very low values of ESR, this is to
ensure stability as the internal error sensing amplifier that controls the
voltage regulation has a high gain. If you are running from the secondary of
a transformer you have to have a large enough reservoir capacitor (in
addition to the decoupling caps) to smooth any ripple out of the rectified
voltage, but excessively large capacitors on the OP can actually cause
instability!
Paul E. Schoen
"ian field" <dai...@ntlworld.com> wrote in message
news:2ZCng.1537$eQ....@newsfe7-win.ntli.net...[snip]
>> So what I gain from this relative discussion is the more capacitance
>> the better but put something in. Small is okay, only because
>> "manufacturers tend to be too stingy". Larger is better--"just to
>> make sure".
>>
>> i.e. I'd be well off to use aluminum electrolytic [on the output], up
>> to 47uF. What about the input? Use .33uF electrolytic as the
>> datasheet says? or 22 or 47uF? Or does this apply on both input and
>> output?
>>
>> Thank you for your help.
>> Largo
>
> There are certain rules to observe, the 3-TRs require small-ish
> capacitors from both IP & OP to GND which must have very low values of
> ESR, this is to ensure stability as the internal error sensing amplifier
> that controls the voltage regulation has a high gain. If you are running
> from the secondary of a transformer you have to have a large enough
> reservoir capacitor (in addition to the decoupling caps) to smooth any
> ripple out of the rectified voltage, but excessively large capacitors on
> the OP can actually cause instability!
It is also a good idea to make sure the capacitance on the upstream side of
the regulator is higher than those downstream, because if the input voltage
of the regulator drops below the output, damage can occur. Another way to
prevent this is by adding a diode from the output to the input.
Most regulators are stable with a 0.1 to 1 uF low ESR capacitor close to
the output, and up to several hundred uF in parallel downstream, but it is
better to have the large capacitance upstream for energy storage, and
smaller capacitors distributed near components which need a low impedance
supply source for switching transients.
(sorry if this is a repeat. My previous post seems to be lost in
cyberspace)
Paul
Rich Grise
> So what I gain from this relative discussion is the more capacitance
> the better but put something in. Small is okay, only because
> "manufacturers tend to be too stingy". Larger is better--"just to
> make sure".
>
> i.e. I'd be well off to use aluminum electrolytic [on the output], up
> to 47uF. What about the input? Use .33uF electrolytic as the
> datasheet says? or 22 or 47uF? Or does this apply on both input and
> output?
>
> Thank you for your help.
Leave the "reservoir" cap - the BMF filter at the output of the unreg.
supply. Then, as close as possible to the input lead of the 7809, put
a 0.33 uF. As close as possible to its output, put a 0.1 uF. As close
as possible to the input of the 5V reg, put a .33 uF - they will look
like they're in parallel, but you've got that circuit trace between them,
you see. Then, at its output, as close as possible to the chip, put
another 0.1 uF. Or whatever the data sheet recommends, as far as value,
for each.
Yes, multilayer ceramics will be fine - maybe better than
aluminum electros, because I've heard that they do good at high
frequencies, which is what we're trying to bypass here - you don't want
your regulators oscillating. :-)
Good Luck!
Rich