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multiple DC ouptut PS question

T

tempus fugit

Jan 1, 1970
0
Hi all;

I need a power supply that will give me 12v, 9v, and 5v. I have a 15v
transformer already, with 7812, 7809, and 7805 regulators. Now, should I
hook the regulators up in series (i.e., with the output of the 7812 feeding
the input of the 7809 which would feed the input of the 7805), or in
parallel (i.e., inputs of all 3 regulators connected to the secondary of the
transformer)?

I can't see that one way or the other would make much difference, but I
thought I should check to see if there are advantages/disadvantages to
either method.

Thanks
 
J

Jan Panteltje

Jan 1, 1970
0
Hi all;

I need a power supply that will give me 12v, 9v, and 5v. I have a 15v
transformer already, with 7812, 7809, and 7805 regulators. Now, should I
hook the regulators up in series (i.e., with the output of the 7812 feeding
the input of the 7809 which would feed the input of the 7805), or in
parallel (i.e., inputs of all 3 regulators connected to the secondary of the
transformer)?

I can't see that one way or the other would make much difference, but I
thought I should check to see if there are advantages/disadvantages to
either method.

Thanks

Putting them in series will have the first one dissipate more, as now
all current goes to that one, then the next one.
Depending on the current this may not be a good idea.
Safest is in parallel.
 
T

tempus fugit

Jan 1, 1970
0
Thanks Jan.

So would this also give better isolation of the 3 supplies? I was wondering
if I should use a resistor from secondary to each input to help with
isolation, or if this is even necessary.
 
B

budgie

Jan 1, 1970
0
Hi all;

I need a power supply that will give me 12v, 9v, and 5v. I have a 15v
transformer already, with 7812, 7809, and 7805 regulators. Now, should I
hook the regulators up in series (i.e., with the output of the 7812 feeding
the input of the 7809 which would feed the input of the 7805), or in
parallel (i.e., inputs of all 3 regulators connected to the secondary of the
transformer)?

I can't see that one way or the other would make much difference, but I
thought I should check to see if there are advantages/disadvantages to
either method.

The difference - and the key to selecting the better method - is the current
draw on each output voltage, and the thermal consideration for each regulator
that flows from that information.
 
T

tempus fugit

Jan 1, 1970
0
The 12v will draw a maximum of 200mA, the 9v maybe 200 also, and the 5v is
powering CMOS logic, so in the order of a few mA, maybe less. My transformer
is rated at 500mA, and I am estimating on the high side for the 12 and 9v
supplies, and these will be maximums if everything is on at once (which it
virtually never is).

Given this information, which method would be preferable?

Thanks
 
B

budgie

Jan 1, 1970
0
The 12v will draw a maximum of 200mA, the 9v maybe 200 also, and the 5v is
powering CMOS logic, so in the order of a few mA, maybe less. My transformer
is rated at 500mA, and I am estimating on the high side for the 12 and 9v
supplies, and these will be maximums if everything is on at once (which it
virtually never is).

Given this information, which method would be preferable?

Firstly you need to get a handle on the trough voltage on the output of the
rectifier/filter system you intend. That will need to be high enough to avoid
dropout in the 7812 at load. But for thermal reasons, you don't want it any
higher than it has to be, and dissipation in a series element(reg) will depend
on the amount of ripple above that trough value. More filter cap = less ripple
= less heat in follwing linear regs.

For the sake of the following comparison, assume we have a 15vDC input - no
ripple.

Thermally, running the 7805 off either the raw DC input or either other reg
would make no difference to them or the 7805. Direct.

If you run the 7809 off the input, it will run the hottest and may require a
heatsink depending on the "on" time.

If you run it off the 7812, THAT will become the hotspot and will require a
sink.

Assume steady 200mA on 12v and 9v lines then:

7809 fed by 7182: 7812 dissipation 1200mW 7809 dissipation 600mW
7809 off DC in: 7812 dissipation 600mW 7809 dissipation 1200mW.

Six and two threes.

BUT, as the effective DCin voltage increases above 15, the first scheme becomes
the worse. I'd go with the second scheme and put a small sink on the 7809.

My 2c worth.
 
R

Rich Grise

Jan 1, 1970
0
The 12v will draw a maximum of 200mA, the 9v maybe 200 also, and the 5v is
powering CMOS logic, so in the order of a few mA, maybe less. My transformer
is rated at 500mA, and I am estimating on the high side for the 12 and 9v
supplies, and these will be maximums if everything is on at once (which it
virtually never is).

Given this information, which method would be preferable?

OK, for the purpose of argument, let's allow 50 mA for the +5.

It depends on which regulator you want to dissipate the most power. In
cascade, given a 15V supply, the 12V will have 450 mA plus the idling
current of the 7809 & 7805; so, maybe 475 mA (on the high side), at 3V,
so you know how many watts that is, and so on.

The other way around, the 7812 only dissipates 200 mA * 3V, but then
the 7809 dissipates its 200mA while dropping 6 V; in cascade it would
be 200 mA * 3V, and so on.

Of course, in either case, you need all of the recommended capacitors.

Good Luck!
Rich
 
E

ehsjr

Jan 1, 1970
0
tempus said:
Hi all;

I need a power supply that will give me 12v, 9v, and 5v. I have a 15v
transformer already, with 7812, 7809, and 7805 regulators. Now, should I
hook the regulators up in series (i.e., with the output of the 7812 feeding
the input of the 7809 which would feed the input of the 7805), or in
parallel (i.e., inputs of all 3 regulators connected to the secondary of the
transformer)?

I can't see that one way or the other would make much difference, but I
thought I should check to see if there are advantages/disadvantages to
either method.

Thanks

_ _ + ----
|X|---|B|------+------+---[22R]---|7812|---> 12V
|F| |R| | | ----
|O| |I| + | | |
|R| |D| [4700uF] | Gnd
|M| |G| | |
|E| |E| | | ----
|R|---|_|------+ +---[33R]---|7809|---> 9V
|_| | | ----
Gnd | |
| Gnd
|
| ----
+---[100R]--|7805|---> 5V
----
|
Gnd




You'll have to dump heat somewhere, regardless of how
you connect. Others have computed based on 15 volts DC,
but you'll actually have about 19.8 volts at the filter
cap. The heatsinkless solution is to use a power resistor
to dump the heat. Use 2 watt resistors. At a few mA 5V load,
the resistor is not needed - but is added anyway, and based
on a 100 mA load. The 12 and 9 volt regulators will dissipate
about 1/2 watt at the 200 mA loads you specified - the 7805
will be far below that at the "few mA" you specified, and
about .4 watt at 100 mA. Add the input and output caps to
the regs that I omitted for simplicity.

Ed
 
E

ehsjr

Jan 1, 1970
0
ehsjr said:
tempus said:
Hi all;

I need a power supply that will give me 12v, 9v, and 5v. I have a 15v
transformer already, with 7812, 7809, and 7805 regulators. Now, should I
hook the regulators up in series (i.e., with the output of the 7812
feeding
the input of the 7809 which would feed the input of the 7805), or in
parallel (i.e., inputs of all 3 regulators connected to the secondary
of the
transformer)?

I can't see that one way or the other would make much difference, but I
thought I should check to see if there are advantages/disadvantages to
either method.

Thanks

_ _ + ----
|X|---|B|------+------+---[22R]---|7812|---> 12V
|F| |R| | | ----
|O| |I| + | | |
|R| |D| [4700uF] | Gnd
|M| |G| | |
|E| |E| | | ----
|R|---|_|------+ +---[33R]---|7809|---> 9V
|_| | | ----
Gnd | |
| Gnd
|
| ----
+---[100R]--|7805|---> 5V
----
|
Gnd




You'll have to dump heat somewhere, regardless of how
you connect. Others have computed based on 15 volts DC,
but you'll actually have about 19.8 volts at the filter
cap. The heatsinkless solution is to use a power resistor
to dump the heat. Use 2 watt resistors. At a few mA 5V load,
the resistor is not needed - but is added anyway, and based
on a 100 mA load. The 12 and 9 volt regulators will dissipate
about 1/2 watt at the 200 mA loads you specified - the 7805
will be far below that at the "few mA" you specified, and
about .4 watt at 100 mA. Add the input and output caps to
the regs that I omitted for simplicity.

Ed

Arrgh! Forgot to check for mains +/- 10% variation.
You would need to change the resistance values to 10
ohms for the 12 volt reg, and 25 ohms for the 9V reg
to accomodate a mains down 10% at 108V. That puts the
9V reg at ~ 1.36 watts & the 12V reg at ~ 1.6 watts
worst case when the mains voltage increases to 132v.
So you'll have to use heatsinks, and the resistors
won't buy you much.

Ed
 
tempus said:
I need a power supply that will give me 12v, 9v, and 5v. I have a 15v
transformer already, with 7812, 7809, and 7805 regulators. Now, should I
hook the regulators up in series ...[or in parallel]

I can't see that one way or the other would make much difference

Others have commented on the power dissipation differences; two
other differences are in the power sequencing (series
connection guarantees the +15 is always higher voltage than +12, and
the
+12 always higher than +5, during power-up and power-off events),
and in the output impedance.

Linear regulators, when lightly loaded, have somewhat higher noise and
output impedance. So, if regulation and noise are critical, either
loading
all three regulators (maybe 20 mA or so) or connecting them in series
and loading the +5 regulator only, is sometimes beneficial. If you
look in the specs for linear regulators, the dynamic/AC characteristics
are always measured with a current of ~500 mA , and load ripple
rejection isn't really guaranteed at all for an unloaded circuit.

Given that I'd not need simultaneous high current from multiple
supplies, I'd
go for series connection. If you go parallel, clamp rectifiers can
deal with
the power sequencing issue.
 
T

tempus fugit

Jan 1, 1970
0
OK guys thanks for all the tips. I already switched it over to parallel by
the time I read your post, whit, so I guess I'll keep it like that for now
and see how it goes. Also, I did a more accurate measurement of the 9v rail,
and the maximum current is more like 100-115 mA (my meter only reads to 2
decimal places, so i don't know if a reading of 0.02 is 20mA or 29 mA or
anywhere in between).

Thanks again.


tempus said:
I need a power supply that will give me 12v, 9v, and 5v. I have a 15v
transformer already, with 7812, 7809, and 7805 regulators. Now, should I
hook the regulators up in series ...[or in parallel]

I can't see that one way or the other would make much difference

Others have commented on the power dissipation differences; two
other differences are in the power sequencing (series
connection guarantees the +15 is always higher voltage than +12, and
the
+12 always higher than +5, during power-up and power-off events),
and in the output impedance.

Linear regulators, when lightly loaded, have somewhat higher noise and
output impedance. So, if regulation and noise are critical, either
loading
all three regulators (maybe 20 mA or so) or connecting them in series
and loading the +5 regulator only, is sometimes beneficial. If you
look in the specs for linear regulators, the dynamic/AC characteristics
are always measured with a current of ~500 mA , and load ripple
rejection isn't really guaranteed at all for an unloaded circuit.

Given that I'd not need simultaneous high current from multiple
supplies, I'd
go for series connection. If you go parallel, clamp rectifiers can
deal with
the power sequencing issue.
 
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