A 7805A mystery
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Posts: 46,084
OK, I'm puzzled. My goal is to take a center tapped 12.6v transformer, and
get two DC outputs: 5v, regulated; and 8-14v, regulation unnecessary, with a
common ground. I thought I had a simple solution, but...
I took the left and center taps from the transformer (6.3v), put them
through the usual: through a bridge rectifier, across a cap and into a
7805A. The output of the 7805A is 5v, as expected.
I took the right tap of the transformer, through a diode (anode) -
(half-wave rectification is ok for my purposes, I just need to run one
transistor and a relay) - and across a cap. The other end of the cap is tied
to the negative output pin of the bridge rectifier, my common ground.
I should see (thinks I) somewhere near 12 v across that cap, at no load.
What I see there is 5v. Not 12, 6, not 9. 5. How is the 7805 getting
involved?
But when I put a full wave recitifer across the left and right taps, and put
the output of that across a cap, I get over 12v, as expected.
I can use the full wave rectifier, so I have a working solution. But why
didn't the other work?
get two DC outputs: 5v, regulated; and 8-14v, regulation unnecessary, with a
common ground. I thought I had a simple solution, but...
I took the left and center taps from the transformer (6.3v), put them
through the usual: through a bridge rectifier, across a cap and into a
7805A. The output of the 7805A is 5v, as expected.
I took the right tap of the transformer, through a diode (anode) -
(half-wave rectification is ok for my purposes, I just need to run one
transistor and a relay) - and across a cap. The other end of the cap is tied
to the negative output pin of the bridge rectifier, my common ground.
I should see (thinks I) somewhere near 12 v across that cap, at no load.
What I see there is 5v. Not 12, 6, not 9. 5. How is the 7805 getting
involved?
But when I put a full wave recitifer across the left and right taps, and put
the output of that across a cap, I get over 12v, as expected.
I can use the full wave rectifier, so I have a working solution. But why
didn't the other work?
Comments
You can get two different voltages (Edc and 2xEdc)if you use a
voltage doubler.
If interested, contact me off the group and will send you the
connections and some graphs that show the wave forms using a SPICE
program.
across the two outer taps and 6.3 vac from the center to either outer tap
i.e.
I 6.3 I 6.3I
I 12.6 I (I= Taps)
And you have A full wave bridge Across 6.3 and the 1/2 wave across 12.6
If this is the case ,with a 1/2 wave you will only get one half (less than)
of the ac wave if you want the full 12.6 you will have to go with a full
wave bridge .
I think I'm right in my thinking
Denny
Original Message
From: "Scott" <scott@m...>
To: <basicstamps@yahoogroups.com>
Sent: Sunday, December 14, 2003 2:54 PM
Subject: [noparse][[/noparse]basicstamps] A 7805A mystery
> OK, I'm puzzled. My goal is to take a center tapped 12.6v transformer, and
> get two DC outputs: 5v, regulated; and 8-14v, regulation unnecessary, with
a
> common ground. I thought I had a simple solution, but...
>
> I took the left and center taps from the transformer (6.3v), put them
> through the usual: through a bridge rectifier, across a cap and into a
> 7805A. The output of the 7805A is 5v, as expected.
>
> I took the right tap of the transformer, through a diode (anode) -
> (half-wave rectification is ok for my purposes, I just need to run one
> transistor and a relay) - and across a cap. The other end of the cap is
tied
> to the negative output pin of the bridge rectifier, my common ground.
>
> I should see (thinks I) somewhere near 12 v across that cap, at no load.
>
> What I see there is 5v. Not 12, 6, not 9. 5. How is the 7805 getting
> involved?
>
> But when I put a full wave recitifer across the left and right taps, and
put
> the output of that across a cap, I get over 12v, as expected.
>
> I can use the full wave rectifier, so I have a working solution. But why
> didn't the other work?
>
>
>
>
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>
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>
>
> across the two outer taps and 6.3 vac from the center to either outer tap
> i.e.
> I 6.3 I 6.3I
> I 12.6 I (I= Taps)
Yup.
> And you have A full wave bridge Across 6.3 and the 1/2 wave across 12.6
> If this is the case ,with a 1/2 wave you will only get one half (less
than)
> of the ac wave if you want the full 12.6 you will have to go with a full
> wave bridge .
> I think I'm right in my thinking
With a half rectifier (in English, one diode), you get every other peak of
the AC current, so to speak. But those peaks are still somewhere above 12v,
and run across a capacitor without any other load, the cap will rise to the
peak voltage and stay there. And my test meter doesn't amount to much load.
There's something else going on.
If you have 3 taps. (L)left (C)center (R)right
then L-C is about 6v
L-R is about 12 V.
If you use L as the common ground you should have no problem. Are
you using the C for ground on the 5V ?
Dave
--- In basicstamps@yahoogroups.com, "Scott" <scott@m...> wrote:
> OK, I'm puzzled. My goal is to take a center tapped 12.6v
transformer, and
> get two DC outputs: 5v, regulated; and 8-14v, regulation
unnecessary, with a
> common ground. I thought I had a simple solution, but...
>
> I took the left and center taps from the transformer (6.3v), put
them
> through the usual: through a bridge rectifier, across a cap and
into a
> 7805A. The output of the 7805A is 5v, as expected.
>
> I took the right tap of the transformer, through a diode (anode) -
> (half-wave rectification is ok for my purposes, I just need to run
one
> transistor and a relay) - and across a cap. The other end of the
cap is tied
> to the negative output pin of the bridge rectifier, my common
ground.
>
> I should see (thinks I) somewhere near 12 v across that cap, at no
load.
>
> What I see there is 5v. Not 12, 6, not 9. 5. How is the 7805 getting
> involved?
>
> But when I put a full wave recitifer across the left and right
taps, and put
> the output of that across a cap, I get over 12v, as expected.
>
> I can use the full wave rectifier, so I have a working solution.
But why
> didn't the other work?
establishes an artificial ground reference at one half the applied voltage.
Wire it up like this and you will have the results you want with 3 diodes, 2
capacitors.
transformer output L, Center, R
1. Diode1 anode to R, diode1 cathode to +of cap1, -of cap1 to L (This is
your higher voltage output, half wave rectified)
2. Diode2 anode to Center, diode2 cathode to + of capacitor2,
3. Diode3 anode from L, diode3 cathode to diode2 cathode, -of capacitor2 to
L
4. L to ground (L becomes ground reference for both voltages)
5. Junction of Diode2 cathode and Diode3 cathode and +capacitor2 to input of
7805 (this is full wave rectified low voltage)
6. 7805 center pin to ground (L), 7805 output is 5vdc (bypass as necessary
with capacitor)
Hope this is clear.
jim
http://www.geocities.com/jimforkin2003/
Original Message
From: davemucha@j... [noparse]/noparse]mailto:[url=http://forums.parallaxinc.com/group/basicstamps/post?postID=YfWz9ygEVMOUlaV706C3ew5mA7-jEjzA16qr6eXDTw4NRdayPh7VKhIKmDPkCfhnb7XOdJI4al8]davemucha@j...[/url
Sent: Tuesday, December 16, 2003 8:13 AM
To: basicstamps@yahoogroups.com
Subject: [noparse][[/noparse]basicstamps] Re: A 7805A mystery
I'm a little confused as well.
If you have 3 taps. (L)left (C)center (R)right
then L-C is about 6v
L-R is about 12 V.
If you use L as the common ground you should have no problem. Are
you using the C for ground on the 5V ?
Dave
--- In basicstamps@yahoogroups.com, "Scott" <scott@m...> wrote:
> OK, I'm puzzled. My goal is to take a center tapped 12.6v
transformer, and
> get two DC outputs: 5v, regulated; and 8-14v, regulation
unnecessary, with a
> common ground. I thought I had a simple solution, but...
>
> I took the left and center taps from the transformer (6.3v), put
them
> through the usual: through a bridge rectifier, across a cap and
into a
> 7805A. The output of the 7805A is 5v, as expected.
>
> I took the right tap of the transformer, through a diode (anode) -
> (half-wave rectification is ok for my purposes, I just need to run
one
> transistor and a relay) - and across a cap. The other end of the
cap is tied
> to the negative output pin of the bridge rectifier, my common
ground.
>
> I should see (thinks I) somewhere near 12 v across that cap, at no
load.
>
> What I see there is 5v. Not 12, 6, not 9. 5. How is the 7805 getting
> involved?
>
> But when I put a full wave recitifer across the left and right
taps, and put
> the output of that across a cap, I get over 12v, as expected.
>
> I can use the full wave rectifier, so I have a working solution.
But why
> didn't the other work?
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