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power supply 2
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Ken O
electronics forum Guru Wannabe


Joined: 27 Oct 2005
Posts: 116

PostPosted: Mon Jul 17, 2006 11:40 pm    Post subject: Re: power supply 2 Reply with quote

"Ken O" <lera@yahoo.com> wrote in message news:e9h5rm$jh4$1@nntp.aioe.org...
Quote:
HI,

I am working on a power supply. The transformer is getting hot after 30
minutes of use. I can touch it , but not more then a few seconds in a row.
I guess this is because of too much current.??
I use a 120v to 25v AC transformer, to a Bridge rectifier, then to a
2000uF 110 v capacitor . This capacitor is then discharge through a
transistor, with a timer at 2% duty cycle to its base. The transistor too
get very hot, hotter then the tranformer.
The following is the rest of the circuit.
Should I be adding a resistor to decrease the current ?



|
o------o||o--------o
| | |
o |
.------------. | o
| | |/ ---
| Timer |-| ---
| | |> o
| | o |
'------------' | |
| |
| |
o---------o---------o
|
o
ground
(created by AACircuit v1.28.6 beta 04/19/05 www.tech-chat.de)


By the way, if the output is 25v, and the resistance of the coil in the
secondary is 2 Ohm, does that mean I am pulling 12 amps on the secondary ??
if so no wander its getting hot, but the transformer is rated 2 Amp.

ken
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Alan B
electronics forum Guru Wannabe


Joined: 05 May 2006
Posts: 137

PostPosted: Tue Jul 18, 2006 12:39 am    Post subject: Re: power supply 2 Reply with quote

On Mon, 17 Jul 2006 19:40:52 -0400, in message
<e9h75u$r0b$1@nntp.aioe.org>, "Ken O" <lera@yahoo.com> scribed:

Quote:

"Ken O" <lera@yahoo.com> wrote in message news:e9h5rm$jh4$1@nntp.aioe.org...
HI,

I am working on a power supply. The transformer is getting hot after 30
minutes of use. I can touch it , but not more then a few seconds in a row.
I guess this is because of too much current.??
I use a 120v to 25v AC transformer, to a Bridge rectifier, then to a
2000uF 110 v capacitor . This capacitor is then discharge through a
transistor, with a timer at 2% duty cycle to its base. The transistor too
get very hot, hotter then the tranformer.
The following is the rest of the circuit.
Should I be adding a resistor to decrease the current ?



|
o------o||o--------o
| | |
o |
.------------. | o
| | |/ ---
| Timer |-| ---
| | |> o
| | o |
'------------' | |
| |
| |
o---------o---------o
|
o
ground
(created by AACircuit v1.28.6 beta 04/19/05 www.tech-chat.de)


By the way, if the output is 25v, and the resistance of the coil in the
secondary is 2 Ohm, does that mean I am pulling 12 amps on the secondary ??
if so no wander its getting hot, but the transformer is rated 2 Amp.

The coil resistance is irrelevant, or at least it should be, because the
transformer should be seeing only AC current. The current must be deduced
by calculation of the parameters of the circuit after the rectifier. The
schematic you've included does not show what is going on; I can't make any
sense of it. Could you try to be a little more specific about your circuit
description?

--
Love is like a dying ember
And only memories remain
And through the ages I'll remember
Blue eyes cryin' in the rain
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John Popelish
electronics forum Guru


Joined: 29 Apr 2005
Posts: 1601

PostPosted: Tue Jul 18, 2006 1:14 am    Post subject: Re: power supply 2 Reply with quote

Ken O wrote:
Quote:
"Ken O" <lera@yahoo.com> wrote in message news:e9h5rm$jh4$1@nntp.aioe.org...

HI,

I am working on a power supply. The transformer is getting hot after 30
minutes of use. I can touch it , but not more then a few seconds in a row.
I guess this is because of too much current.??
I use a 120v to 25v AC transformer, to a Bridge rectifier, then to a
2000uF 110 v capacitor . This capacitor is then discharge through a
transistor, with a timer at 2% duty cycle to its base. The transistor too
get very hot, hotter then the tranformer.
The following is the rest of the circuit.
Should I be adding a resistor to decrease the current ?



|
o------o||o--------o
| | |
o |
.------------. | o
| | |/ ---
| Timer |-| ---
| | |> o
| | o |
'------------' | |
| |
| |
o---------o---------o
|
o
ground
(created by AACircuit v1.28.6 beta 04/19/05 www.tech-chat.de)



By the way, if the output is 25v, and the resistance of the coil in the
secondary is 2 Ohm, does that mean I am pulling 12 amps on the secondary ??
if so no wander its getting hot, but the transformer is rated 2 Amp.

Normally, the coil resistance drops only a few percent of the total
voltage when the transformer is operated within its ratings. Do you
have an amp meter that reads over 2 amps that you could put in series
with the battery being charges, to see what current is charging it?
You also probably don't need that big capacitor between the rectifier
and the battery, and having it makes the transformer hotter and raises
the charging current a bit. I suspect you do need some resistance in
series with the battery (or in series with the primary of the
transformer) to set the charge current to below what the transformer
is rated to supply.
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Abstract Dissonance
electronics forum Guru


Joined: 22 Dec 2005
Posts: 380

PostPosted: Tue Jul 18, 2006 1:35 am    Post subject: Re: power supply 2 Reply with quote

"Ken O" <lera@yahoo.com> wrote in message news:e9h5rm$jh4$1@nntp.aioe.org...
Quote:
HI,

I am working on a power supply. The transformer is getting hot after 30
minutes of use. I can touch it , but not more then a few seconds in a row.
I guess this is because of too much current.??
I use a 120v to 25v AC transformer, to a Bridge rectifier, then to a
2000uF 110 v capacitor . This capacitor is then discharge through a
transistor, with a timer at 2% duty cycle to its base. The transistor too
get very hot, hotter then the tranformer.
The following is the rest of the circuit.
Should I be adding a resistor to decrease the current ?


I'm not sure why its getting hot but remember that if you are dropping the
voltage on then you are increasing the current.

you you have

120/25 ~= 5

Your secondary is drawing 5 times the current then the current on the
primary side. So make sure the transformer is rated to handle this. You can
use a fuse that is ~1/5(not sure exactly how to compute fuse rating cause
many people say different things...) of the maximum current that you will
draw form your secondary side circuit.(fuse, ofcourse, goes on primary
side). You'll want a slow blow fuse because there will be an inrush of
current to charge the cap.

Now, if each cycle you are discharging a large portion of the current then
on recharge you will be pulling a large portion of current through the
transformer. So you shouldn't use any larger of a capacitor than needed.

You really should make some measurements though and see just how much power
is being dissipated in the transistor and transformers.

If your trying to discharge the full value of a large cap very quickly with
a very small time constant then theres a lot of peak inrush of current and
over time the avg current could be larger than what you anticipated.

Maybe your design calls for a larger transformer and transistor?
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John Fields
electronics forum Guru


Joined: 24 Mar 2005
Posts: 3260

PostPosted: Tue Jul 18, 2006 2:07 am    Post subject: Re: power supply 2 Reply with quote

On Mon, 17 Jul 2006 19:18:39 -0400, "Ken O" <lera@yahoo.com> wrote:

Quote:
HI,

I am working on a power supply. The transformer is getting hot after 30
minutes of use. I can touch it , but not more then a few seconds in a row. I
guess this is because of too much current.??

---
So far, who knows?
---


Quote:
I use a 120v to 25v AC transformer, to a Bridge rectifier, then to a 2000uF
110 v capacitor .

---
So the output of the supply, after the filter, will be about 35VDC
---

Quote:
This capacitor is then discharge through a transistor,
with a timer at 2% duty cycle to its base. The transistor too get very hot,
hotter then the tranformer.
The following is the rest of the circuit.
Should I be adding a resistor to decrease the current ?

---
Post the circuit.


--
John Fields
Professional Circuit Designer
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Alan B
electronics forum Guru Wannabe


Joined: 05 May 2006
Posts: 137

PostPosted: Tue Jul 18, 2006 3:06 am    Post subject: Re: power supply 2 Reply with quote

On Mon, 17 Jul 2006 17:39:33 -0700, in message
<l7bob2hsp6lak88f6u8ouo9t2hale9gdpa@4ax.com>, Alan B
<three-eyes@chernobyl.com> scribed:

Quote:
The coil resistance is irrelevant, or at least it should be, because the
transformer should be seeing only AC current. The current must be deduced
by calculation of the parameters of the circuit after the rectifier. The
schematic you've included does not show what is going on; I can't make any
sense of it. Could you try to be a little more specific about your circuit
description?

On re-reading, this sounds confusing. I'll break it down.

1) The resistance of the coil should be negligible compared to the coil's
inductive reactance; that is to say, when pulling AC through the
transformer, the secondary will have an impedance that should be
significantly greater than its simple DC resistance. If you are letting DC
run through the transformer, then you have a circuit design flaw.

2) You must "do the math:" calculate the expected average current in the
post-rectifier circuit, and make sure your transformer, rectifier and
filter are all rated to handle that calculation.

3) Is that a battery in your circuit? Why do you have both a DC power
supply and a battery? Where are your power inputs to the circuit you have
drawn? Is your transistor switch a direct short from Vcc to Vee when on?
What is the duty cycle? Is that capacitor you have drawn part of the
switch circuit, or part of the power supply filter? Is what you have
labeled "ground" actually attached to anything, or was that just a handy
schematic graphics tool?

--
Love is like a dying ember
And only memories remain
And through the ages I'll remember
Blue eyes cryin' in the rain
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Phil Allison
electronics forum Guru


Joined: 20 May 2005
Posts: 1368

PostPosted: Tue Jul 18, 2006 3:09 am    Post subject: Re: power supply 2 Reply with quote

"Alan Bullshit"



Quote:
By the way, if the output is 25v, and the resistance of the coil in the
secondary is 2 Ohm, does that mean I am pulling 12 amps on the secondary
??
if so no wander its getting hot, but the transformer is rated 2 Amp.

The coil resistance is irrelevant, or at least it should be, because the
transformer should be seeing only AC current.


** Oh dear - wot a fuckwit.




.......... Phil
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Phil Allison
electronics forum Guru


Joined: 20 May 2005
Posts: 1368

PostPosted: Tue Jul 18, 2006 3:14 am    Post subject: Re: power supply 2 Reply with quote

"Alan Bullshit"

Quote:

1) The resistance of the coil should be negligible compared to the coil's
inductive reactance; that is to say, when pulling AC through the
transformer, the secondary will have an impedance that should be
significantly greater than its simple DC resistance.


** Nonsense.

The secondary source impedance of a mains frequency transformer is
*dominated* by its resistance.

Only the primary winding acts like an inductor when the tranny is UNLOADED.




........ Phil
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Alan B
electronics forum Guru Wannabe


Joined: 05 May 2006
Posts: 137

PostPosted: Tue Jul 18, 2006 3:21 am    Post subject: Re: power supply 2 Reply with quote

On Tue, 18 Jul 2006 13:14:43 +1000, in message
<4i3210F1st6iU1@individual.net>, "Phil Allison" <philallison@tpg.com.au>
scribed:

Quote:

"Alan Bullshit"


1) The resistance of the coil should be negligible compared to the coil's
inductive reactance; that is to say, when pulling AC through the
transformer, the secondary will have an impedance that should be
significantly greater than its simple DC resistance.


** Nonsense.

The secondary source impedance of a mains frequency transformer is
*dominated* by its resistance.

Only the primary winding acts like an inductor when the tranny is UNLOADED.

Er, how is the example in question an unloaded transformer?

--
Love is like a dying ember
And only memories remain
And through the ages I'll remember
Blue eyes cryin' in the rain
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Phil Allison
electronics forum Guru


Joined: 20 May 2005
Posts: 1368

PostPosted: Tue Jul 18, 2006 3:29 am    Post subject: Re: power supply 2 Reply with quote

"Alan Bullshit Fuckwit"

Quote:

1) The resistance of the coil should be negligible compared to the
coil's
inductive reactance; that is to say, when pulling AC through the
transformer, the secondary will have an impedance that should be
significantly greater than its simple DC resistance.


** Nonsense.

The secondary source impedance of a mains frequency transformer is
*dominated* by its resistance.

Only the primary winding acts like an inductor when the tranny is
UNLOADED.

Er, how is the example in question an unloaded transformer?



** How pathologically asinine.

This PITA is a completely anencephalic public menace.

f*** OFF !!





....... Phil
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Alan B
electronics forum Guru Wannabe


Joined: 05 May 2006
Posts: 137

PostPosted: Tue Jul 18, 2006 3:34 am    Post subject: Re: power supply 2 Reply with quote

On Tue, 18 Jul 2006 13:29:52 +1000, in message
<4i32tcF1th9vU1@individual.net>, "Phil Allison" <philallison@tpg.com.au>
scribed:

Quote:

"Alan Bullshit Fuckwit"


1) The resistance of the coil should be negligible compared to the
coil's
inductive reactance; that is to say, when pulling AC through the
transformer, the secondary will have an impedance that should be
significantly greater than its simple DC resistance.


** Nonsense.

The secondary source impedance of a mains frequency transformer is
*dominated* by its resistance.

Only the primary winding acts like an inductor when the tranny is
UNLOADED.

Er, how is the example in question an unloaded transformer?



** How pathologically asinine.

This PITA is a completely anencephalic public menace.

f*** OFF !!

A simple "I don't know" would have sufficed.
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Phil Allison
electronics forum Guru


Joined: 20 May 2005
Posts: 1368

PostPosted: Tue Jul 18, 2006 3:47 am    Post subject: f*** OFF "Alan B" Reply with quote

"Alan Bullshit " = another cretinous FUCKWIT !!!


** BEWARE :

The autistic cretin calling itself "Alan B" is a know nothing, congenital
troll and a public menace.

IGNORE HIM COMPLETELY !!!


Better still, include him in your killfile now.





....... Phil
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Ken O
electronics forum Guru Wannabe


Joined: 27 Oct 2005
Posts: 116

PostPosted: Tue Jul 18, 2006 3:53 pm    Post subject: Re: power supply 2 Reply with quote

"John Popelish" <jpopelish@rica.net> wrote in message
news:MtCdndlDEb9qqSHZnZ2dnUVZ_v6dnZ2d@adelphia.com...
Quote:
Ken O wrote:
"Ken O" <lera@yahoo.com> wrote in message
news:e9h5rm$jh4$1@nntp.aioe.org...

HI,

I am working on a power supply. The transformer is getting hot after 30
minutes of use. I can touch it , but not more then a few seconds in a
row. I guess this is because of too much current.??
I use a 120v to 25v AC transformer, to a Bridge rectifier, then to a
2000uF 110 v capacitor . This capacitor is then discharge through a
transistor, with a timer at 2% duty cycle to its base. The transistor too
get very hot, hotter then the tranformer.
The following is the rest of the circuit.
Should I be adding a resistor to decrease the current ?



|
o------o||o--------o
| | |
o |
.------------. | o
| | |/ ---
| Timer |-| ---
| | |> o
| | o |
'------------' | |
| |
| |
o---------o---------o
|
o
ground
(created by AACircuit v1.28.6 beta 04/19/05 www.tech-chat.de)



By the way, if the output is 25v, and the resistance of the coil in the
secondary is 2 Ohm, does that mean I am pulling 12 amps on the secondary
?? if so no wander its getting hot, but the transformer is rated 2 Amp.

Normally, the coil resistance drops only a few percent of the total
voltage when the transformer is operated within its ratings. Do you have
an amp meter that reads over 2 amps that you could put in series with the
battery being charges, to see what current is charging it? You also
probably don't need that big capacitor between the rectifier and the
battery, and having it makes the transformer hotter and raises the
charging current a bit. I suspect you do need some resistance in series
with the battery (or in series with the primary of the transformer) to set
the charge current to below what the transformer is rated to supply.

hi I replaced the capacitor with a 680uf 200v. I did not need 200v but that
was the only one over 50v I could find. The transistor is acting normal now,
not too hoe, The transformer is not getting as hot as before but I still
find it too hot. I cant measure the current. I am using the impulses from
the 555 timer to charge it. Unless there is another way beside using an amp
meter, please let me know

ken
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John Popelish
electronics forum Guru


Joined: 29 Apr 2005
Posts: 1601

PostPosted: Tue Jul 18, 2006 5:04 pm    Post subject: Re: power supply 2 Reply with quote

Ken O wrote:
Quote:
"John Popelish" <jpopelish@rica.net> wrote in message
news:MtCdndlDEb9qqSHZnZ2dnUVZ_v6dnZ2d@adelphia.com...

Normally, the coil resistance drops only a few percent of the total
voltage when the transformer is operated within its ratings. Do you have
an amp meter that reads over 2 amps that you could put in series with the
battery being charges, to see what current is charging it? You also
probably don't need that big capacitor between the rectifier and the
battery, and having it makes the transformer hotter and raises the
charging current a bit. I suspect you do need some resistance in series
with the battery (or in series with the primary of the transformer) to set
the charge current to below what the transformer is rated to supply.


hi I replaced the capacitor with a 680uf 200v. I did not need 200v but that
was the only one over 50v I could find.

Okay.

Quote:
The transistor is acting normal now,
not too hoe, The transformer is not getting as hot as before but I still
find it too hot.

Have you tried with no capacitor? No particular need to have smoothed
DC to charge a battery. After you get the transformer temperature
down, we can get into what you have to do to power the 555 from the
same rectifier.

Quote:
I cant measure the current. I am using the impulses from
the 555 timer to charge it. Unless there is another way beside using an amp
meter, please let me know

If you can find a very low ohm resistor, like .1 ohm, you could put
that in series with the battery, and read the average DC voltage
across it. The average current is 1/R times the voltage you read.

But I think you need something like an automotive head lamp in series
with the battery, to act as a current limit (with a visual indication
of high current).
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Ken O
electronics forum Guru Wannabe


Joined: 27 Oct 2005
Posts: 116

PostPosted: Tue Jul 18, 2006 7:51 pm    Post subject: Re: power supply 2 Reply with quote

"John Popelish" <jpopelish@rica.net> wrote in message
news:k_udnXpcEMLyjiDZnZ2dnUVZ_rmdnZ2d@adelphia.com...
Quote:
Ken O wrote:
"John Popelish" <jpopelish@rica.net> wrote in message
news:MtCdndlDEb9qqSHZnZ2dnUVZ_v6dnZ2d@adelphia.com...

Normally, the coil resistance drops only a few percent of the total
voltage when the transformer is operated within its ratings. Do you have
an amp meter that reads over 2 amps that you could put in series with the
battery being charges, to see what current is charging it? You also
probably don't need that big capacitor between the rectifier and the
battery, and having it makes the transformer hotter and raises the
charging current a bit. I suspect you do need some resistance in series
with the battery (or in series with the primary of the transformer) to
set the charge current to below what the transformer is rated to supply.


hi I replaced the capacitor with a 680uf 200v. I did not need 200v but
that was the only one over 50v I could find.

Okay.

The transistor is acting normal now, not too hoe, The transformer is not
getting as hot as before but I still find it too hot.

Have you tried with no capacitor? No particular need to have smoothed DC
to charge a battery. After you get the transformer temperature down, we
can get into what you have to do to power the 555 from the same rectifier.

I cant measure the current. I am using the impulses from the 555 timer to
charge it. Unless there is another way beside using an amp meter, please
let me know

If you can find a very low ohm resistor, like .1 ohm, you could put that
in series with the battery, and read the average DC voltage across it.
The average current is 1/R times the voltage you read.

But I think you need something like an automotive head lamp in series with
the battery, to act as a current limit (with a visual indication of high
current).

I removed the capacitor:
-The battery is charging a bit slower then before
- The transistor is cool
- The transformer is getting warm just like the small capacitor used before.

the transformer is a 115v to 25.2 v rated 2.0 A
So this some out to putting a 12.6 Ohm resistor @50 watt. But this is AC not
DC.

beside adding a fan next to the transformer , I do not know what to do. and
why it is getting that warm.

Ken
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