I am still stumped on this curcuit (Correction repost)

S

Scott Wiper

Guest
I am trying to make a simple battery float charger for a 12V lead acid
battery into a simple power supply. I have a test design that you can veiw
at;


http://www.travel-net.com/~swiper/test.gif
Lust print this in landscape form

First I would like to thank all you for responding to my problem at the link
listed above. I have reposted this again (Above) with the changes and I hope
I will not fry the transister is this curcuit.

A.

I have made the following changes do to the spacs of the 12 volt 5 amp/hour
battery I will be using for the backup. This battery is sealed and has a
standbye charge current of 1.5A maximum. I am now useing a ballest resistor
39 ohm 10W (R2) The transeformer will be now a 15VAC 3A instead of the 28
VAC one. This should reduce charge current even if the battery is flat to
about .750 I hope well within the 2N3055 I will be putting Q1 on the outside
of my metal case. The hole case should act as a heatsink. I will be fan
cooling.

B.

R2 the bias and D2 the zener will be a 13V 1N4743 this will give me a
output at the emitter of Q1 of 13.7 volts (well with float specs of the
battery). But D4 (There is a mistake in the schematic d4 should be a 1N4743
13v zener) D3 looks like a .6 volt drop. Iahve installed a blue LED (Just
looks cool) on the power line side of the curcuit. I have also put a green
led in this curcuit to show that the battery is holding a charge and stays
on when the 120 volts is lost. I have put a yellow led to show when
cutoff/standbye is hit.

C.

I am useing a 741 for U2 because they are rugged and can opperate up to 36V
for the comparator. R6 and R7 that bias Q2 are to saturate it it because U1
does not go rail to rail ot floats 2.5v above and below rails. R8 is the
hystersis that prevet the relay from chattering when cut is hit bt making
the "No Zone" several volts from trigger point.


Please direct replys to the URL below; (Because my e-mail is munged because
of spammers)

http://www.travel-net.com/~swiper/email.htm


--
My Cat Tigger says every morning...
Before my morning coffee I might as well be a DOG!
Insert ' NO SPAM ' when responding or it wiill be trashed
http://www.travel-net.com/~swiper
 
"Scott Wiper" <nobody@devnull.spamcop.net> wrote in
news:dFb0c.3523$mx4.86288@nnrp1.uunet.ca:

I am trying to make a simple battery float charger for a 12V lead acid
battery into a simple power supply. I have a test design that you can
veiw at;


http://www.travel-net.com/~swiper/test.gif
Lust print this in landscape form

First I would like to thank all you for responding to my problem at
the link listed above. I have reposted this again (Above) with the
changes and I hope I will not fry the transister is this curcuit.

A.

I have made the following changes do to the spacs of the 12 volt 5
amp/hour battery I will be using for the backup. This battery is
sealed and has a standbye charge current of 1.5A maximum. I am now
useing a ballest resistor 39 ohm 10W (R2) The transeformer will be now
a 15VAC 3A instead of the 28 VAC one. This should reduce charge
current even if the battery is flat to about .750 I hope well within
the 2N3055 I will be putting Q1 on the outside of my metal case. The
hole case should act as a heatsink. I will be fan cooling.

B.

R2 the bias and D2 the zener will be a 13V 1N4743 this will give me a
output at the emitter of Q1 of 13.7 volts (well with float specs of
the battery). But D4 (There is a mistake in the schematic d4 should be
a 1N4743 13v zener) D3 looks like a .6 volt drop. Iahve installed a
blue LED (Just looks cool) on the power line side of the curcuit. I
have also put a green led in this curcuit to show that the battery is
holding a charge and stays on when the 120 volts is lost. I have put a
yellow led to show when cutoff/standbye is hit.

C.

I am useing a 741 for U2 because they are rugged and can opperate up
to 36V for the comparator. R6 and R7 that bias Q2 are to saturate it
it because U1 does not go rail to rail ot floats 2.5v above and below
rails. R8 is the hystersis that prevet the relay from chattering when
cut is hit bt making the "No Zone" several volts from trigger point.


Please direct replys to the URL below; (Because my e-mail is munged
because of spammers)

http://www.travel-net.com/~swiper/email.htm


--
My Cat Tigger says every morning...
Before my morning coffee I might as well be a DOG!
Insert ' NO SPAM ' when responding or it wiill be trashed
http://www.travel-net.com/~swiper
Click to expand...
Q1's emitter voltage will be 13V MINUS 0.7V! The base voltage must be
~13.7+0.7 = 14.4V.
I would replace Q1 with a darlington: the mimimum hFE of a 2N3055 is just
20. An emitter current of 2A requires a base current of 100mA. The voltage
across C1 is ~21V, which leaves 21-14.4 = 6.6V. The current through R1 will
be 6.6V/100ohms = 66mA, which is less than the required base current.
Add an extra transistor to make Q1 a darlington. If this extra transistor's
hFE is 100, the base current will drop to 1mA. D2 must be a 13.7 + 2*0.7 =
15.1V zener. 15V will suffice.

If D2 needs 10mA to be stable, R1=(V[C1]-V[D2])/(I[D2]+I[B,Q1])=
(21V-15V)/(10m+1m)=6V/11mA=545ohms. 470ohms is a good choise.

At a 2A output current Q1 will dissipate about 15W. If the max room
temperature is 30C and the max temp of Q1 is 150C, the total thermal
resistance (junction to ambient) should not exceed (150-30)/15 = 8C/W.
The thermal junction-to-case resistance is 1.5C/W, so the resistance of the
heatsink must be 8-1.5 = 6.5C/W or less. You don't need fan cooling.

You also may need to mount the 7805 on a heatsink. If the Project draws
0.5A, the 7805 will dissipate ~4.5W. The R[th,J-C] is 5C/W, so the minimum
resistance of the heatsink is (150-30)/4.5 - 5 = 21.6C/W.

BTW, you've got the LEDs D7 and D11 reversed!


--
Gert van den Heuvel
webmaster www.HobbyElectronics.info
 
"Scott Wiper" <nobody@devnull.spamcop.net> wrote in message
news:dFb0c.3523$mx4.86288@nnrp1.uunet.ca...
I am trying to make a simple battery float charger for a 12V lead acid
battery into a simple power supply. I have a test design that you can veiw
at;


http://www.travel-net.com/~swiper/test.gif
Lust print this in landscape form

First I would like to thank all you for responding to my problem at the
link
listed above. I have reposted this again (Above) with the changes and I
hope
I will not fry the transister is this curcuit.

A.

I have made the following changes do to the spacs of the 12 volt 5
amp/hour
battery I will be using for the backup. This battery is sealed and has a
standbye charge current of 1.5A maximum. I am now useing a ballest
resistor
39 ohm 10W (R2) The transeformer will be now a 15VAC 3A instead of the 28
VAC one. This should reduce charge current even if the battery is flat to
about .750 I hope well within the 2N3055 I will be putting Q1 on the
outside
of my metal case. The hole case should act as a heatsink. I will be fan
cooling.

B.

R2 the bias and D2 the zener will be a 13V 1N4743 this will give me a
output at the emitter of Q1 of 13.7 volts (well with float specs of the
battery). But D4 (There is a mistake in the schematic d4 should be a
1N4743
13v zener) D3 looks like a .6 volt drop. Iahve installed a blue LED (Just
looks cool) on the power line side of the curcuit. I have also put a green
led in this curcuit to show that the battery is holding a charge and stays
on when the 120 volts is lost. I have put a yellow led to show when
cutoff/standbye is hit.
Click to expand...
The 15V Zener for D2 is right; the voltage at the emitter of Q1 will then be
15-1.4 = 13.6V, cause the darlington pair has a Vf of two diode drops.
Another poster suggested using a beefier transister for Q2; However, 100mA
isn't too much for that one. If you are cautious, you could use a 2N2222A
with a metal case, which might dissipate heat a little better.

C.

I am useing a 741 for U2 because they are rugged and can opperate up to
36V
for the comparator. R6 and R7 that bias Q2 are to saturate it it because
U1
does not go rail to rail ot floats 2.5v above and below rails. R8 is the
hystersis that prevet the relay from chattering when cut is hit bt making
the "No Zone" several volts from trigger point.
Click to expand...
You don't need 36V for the comparator, since your rail to rail voltage is
only 12V. Also, I'd say to use a comparator rather than an opamp, just
because they are designed for this application. According to Robert Pease of
Nat Semi, opamps generally don't like to have their inputs very far away
from each other, but its ok for comparators. With your circuit, instead of
the resistive divider feeding Q3, you can use a 3.3k resistor to the 12V
rail. A comparator will pull Q3's base to within about 0.2V of ground when
it the low battery indication happens, and that will open the relay.

Please direct replys to the URL below; (Because my e-mail is munged
because
of spammers)

http://www.travel-net.com/~swiper/email.htm


--
My Cat Tigger says every morning...
Before my morning coffee I might as well be a DOG!
Insert ' NO SPAM ' when responding or it wiill be trashed
http://www.travel-net.com/~swiper
Click to expand...
 
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