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12VDC to 18.5VDC

I have an old laptop (366MHz) which takes in 18.5V @ 3A by external
(switching?) power supply. (It didn't have a battery with it when I
bought it - got it 2 years ago for about $200.)

I recently built an external battery pack, powered by 16 Energizer 1.2V
2500mAh NiMH AA cells, to power my laptop in lieu of the external power
supply, and it does work. So far I have been able to run the laptop
for about an hour and a half, and still going strong. (Decided to stop
stress-testing at 1:15am, since I had work the next morning.) A far
cry from paying $150 for a "new" laptop battery, when the laptop only
cost me $200. Although, my 16 NiMH AA battery pack screams "GEEK!" at
anyone who glances at it... ;-)

For my next project, a DC-DC converter.

So... I'd like to convert 12VDC from a jump-start car battery to
18.5VDC, at 3A.

After reading a recent thread about avoiding Maxim like the plague, I
went over to TI's website, entered my parameters (Input V: 12VDC;
output V: 18.5 VDC; current: 3A.)

I found this from the search result:

http://focus.ti.com/lit/ds/symlink/tps5430.pdf

but I was a bit confused why this would have been suggested, since it
seems to be a step-DOWN regulator.

Any suggestions...?

Thanks,

Michael
 
I have an old laptop (366MHz) which takes in 18.5V @ 3A by external
(switching?) power supply. (It didn't have a battery with it when I
bought it - got it 2 years ago for about $200.)

I recently built an external battery pack, powered by 16 Energizer 1.2V
2500mAh NiMH AA cells, to power my laptop in lieu of the external power
supply, and it does work. So far I have been able to run the laptop
for about an hour and a half, and still going strong. (Decided to stop
stress-testing at 1:15am, since I had work the next morning.) A far
cry from paying $150 for a "new" laptop battery, when the laptop only
cost me $200. Although, my 16 NiMH AA battery pack screams "GEEK!" at
anyone who glances at it... ;-)

For my next project, a DC-DC converter.

So... I'd like to convert 12VDC from a jump-start car battery to
18.5VDC, at 3A.

After reading a recent thread about avoiding Maxim like the plague, I
went over to TI's website, entered my parameters (Input V: 12VDC;
output V: 18.5 VDC; current: 3A.)

I found this from the search result:

http://focus.ti.com/lit/ds/symlink/tps5430.pdf

but I was a bit confused why this would have been suggested, since it
seems to be a step-DOWN regulator.

Any suggestions...?

Thanks,

Michael


Eh, no response from sci.electronics.basics; belatedly cross-posting to
SED. Better than multiposting... ;-)

Michael
 
R

Rich Grise

Jan 1, 1970
0
Eh, no response from sci.electronics.basics; belatedly cross-posting to
SED. Better than multiposting... ;-)

Well, for one thing, don't power it from a "car" battery, unless the
battery is on a continuous float charge. And _especially_ don't use a
jump-starting battery. They're designed to provide hundreds of amps,
for a minute or so. Use a golf-cart battery, wheelchair battery, boat
battery, whatever - the operative term would be "deep-cycle" or "deep
discharge".

I can't adivse you on a boost regulator, but I do think it seems simpler
to just use 24V of batteries, and regulate it down with a linear regulator
on a mongo heat sink.

Or did you want this thing to be portable? ;-)

Good Luck!
Rich
 
Rich said:
Well, for one thing, don't power it from a "car" battery, unless the
battery is on a continuous float charge. And _especially_ don't use a
jump-starting battery. They're designed to provide hundreds of amps,
for a minute or so. Use a golf-cart battery, wheelchair battery, boat
battery, whatever - the operative term would be "deep-cycle" or "deep
discharge".

I can't adivse you on a boost regulator, but I do think it seems simpler
to just use 24V of batteries, and regulate it down with a linear regulator
on a mongo heat sink.

Or did you want this thing to be portable? ;-)

Good Luck!
Rich


Well, portability was kind of the idea, since I already have the 110
AC-to-DC switching power supply... ;-)

I was planning on using the jump start battery, without using my
square-wave inverter (Vector 75W I think is what it is) to power the
switching power supply, and exchange this extra bulk with a homebuilt
DC-DC converter.

So a jump-start battery isn't a deep-cycle, eh? Thanks for the info...

Michael
 
J

Joerg

Jan 1, 1970
0
Hello Michael,


Now don't show up at airport security with that concoction :)


Look at the LM3478 from National. Nice chip, have used it myself but
AFAIR it only comes in the teeny MSOP package. Besides some mundane
parts around it you'd need a stiff N-channel FET, a beefy inductor and a
fast diode that can handle the amps. Since it is usually operated at
several hundred kHz your inductor won't have to be huge.
 
J

Joerg

Jan 1, 1970
0
Hello Michael,
Correct.


So a jump-start battery isn't a deep-cycle, eh? Thanks for the info...

At Walmart I saw a deep-cycle 12V last week for around $65, about the
size and capacity of a car battery.
 
Eh, no response from sci.electronics.basics; belatedly cross-posting to
SED. Better than multiposting... ;-)

Michael

You could use an isolated 12 V to 6 V DC-DC converter and add the
output to your 12 volts from the battery. You could probably run it
with a full square wave without regulation to simplify feedback. Just a
high frequency transformer driven by something like a UC3526 and a
couple MOSFETs, and a simple rectifier and filter on the isolated
output in series with the battery voltage to get what you need. You
could use the PWM to make it regulated if you want.

Paul
 
Joerg said:
Hello Michael,



Now don't show up at airport security with that concoction :)

Oh, man! It's a pretty straightforward concoction - two 8-AA battery
holders, each with a 9V clip on top. I could disassemble it, but I
wouldn't want the clip on top to get shorted by any bare metal - could
cause a fire. The two 9V clips are soldered to a "C"-style Adaptaplug
from Radioshack.

http://www.radioshack.com/product/i...daptaplug+c&kw=adaptaplug+c&parentPage=search

I guess I could take apart the 16 AA's... I'd feel like a guerilla
soldier, though.

Then again, while I would be happy as a clam for maybe an hour or so on
the plane, once the batteries died, recharging would be problematic...

Look at the LM3478 from National. Nice chip, have used it myself but
AFAIR it only comes in the teeny MSOP package. Besides some mundane
parts around it you'd need a stiff N-channel FET, a beefy inductor and a
fast diode that can handle the amps. Since it is usually operated at
several hundred kHz your inductor won't have to be huge.

Hmm... would operating at that frequency cause bad things to happen to
the pilot's collision avoidance system, etc.? Encase it in aluminum
foil...?

Thanks,

Michael
 
You could use an isolated 12 V to 6 V DC-DC converter and add the
output to your 12 volts from the battery. You could probably run it
with a full square wave without regulation to simplify feedback. Just a
high frequency transformer driven by something like a UC3526 and a
couple MOSFETs, and a simple rectifier and filter on the isolated
output in series with the battery voltage to get what you need. You
could use the PWM to make it regulated if you want.

Paul


Oh, that's elegant! Thanks!

Michael
 
J

Joerg

Jan 1, 1970
0
Hello Michael,
Oh, man! It's a pretty straightforward concoction - two 8-AA battery
holders, each with a 9V clip on top. I could disassemble it, but I
wouldn't want the clip on top to get shorted by any bare metal - could
cause a fire. The two 9V clips are soldered to a "C"-style Adaptaplug
from Radioshack.

http://www.radioshack.com/product/i...daptaplug+c&kw=adaptaplug+c&parentPage=search

I guess I could take apart the 16 AA's... I'd feel like a guerilla
soldier, though.

.... and then carry them in a leather ammo belt with an NRA sticker :)
Hmm... would operating at that frequency cause bad things to happen to
the pilot's collision avoidance system, etc.? Encase it in aluminum
foil...?

You aren't supposed to let any more leak out than the FCC alloweth.

But the same goes for use elsewhere. If you build something and then it
interferes more than permitted it could lead to problems.
 
You could use an isolated 12 V to 6 V DC-DC converter and add the
output to your 12 volts from the battery. You could probably run it
with a full square wave without regulation to simplify feedback. Just a
high frequency transformer driven by something like a UC3526 and a
couple MOSFETs, and a simple rectifier and filter on the isolated
output in series with the battery voltage to get what you need. You
could use the PWM to make it regulated if you want.

Paul


Say... how would I get the 6V DC isolated? I was thinking of using a
transformer driven by a PWM circuit I built awhile back and just happen
to have in a box, but where could I get a [email protected] (primary) to
6VDC@3A (secondary) transformer? Jameco doesn't seem to carry such a
beast...

Michael
 
T

Tam/WB2TT

Jan 1, 1970
0
Say... how would I get the 6V DC isolated? I was thinking of using a
transformer driven by a PWM circuit I built awhile back and just happen
to have in a box, but where could I get a [email protected] (primary) to
6VDC@3A (secondary) transformer? Jameco doesn't seem to carry such a
beast...

Michael
The thing that TI came up with is a buck regulator. You want a boost, or a
buck/boost. The boost regulator looks almost the same, but the inductor is
connected in a different way. The same IC may work, but you would be better
off with one that shows the boost use on the data sheet. You might try the
TI configurator again, or look at National or Linear. If you Google on buck
boost regulator, you will get gobs of hits.

Tam
 
P

Paul E. Schoen

Jan 1, 1970
0
Say... how would I get the 6V DC isolated? I was thinking of using a
transformer driven by a PWM circuit I built awhile back and just happen
to have in a box, but where could I get a [email protected] (primary) to
6VDC@3A (secondary) transformer? Jameco doesn't seem to carry such a
beast...

Michael

You will probably need to "roll your own", but it should really be nothing
much more than a small toroid about 1" diameter, wound with a couple dozen
turns of wire, if you use about 50 kHz PWM. #22 to #26 AWG wire should be
OK for 3 amps or so. Otherwise, you could probably use parts of an old
computer power supply. Of course, you can just buy a DC-DC converter, but
18 watts will probably cost at least $50.

Paul
 
Paul E. Schoen wrote:

....
You will probably need to "roll your own", but it should really be nothing
much more than a small toroid about 1" diameter, wound with a couple dozen
turns of wire, if you use about 50 kHz PWM. #22 to #26 AWG wire should be
OK for 3 amps or so. Otherwise, you could probably use parts of an old
computer power supply. Of course, you can just buy a DC-DC converter, but
18 watts will probably cost at least $50.

Paul


Oh, thanks! Good idea... roll my own... what kind of torus... powdered
ferrite?

Where could I get more detailed information on the gauge of wire,
versus current... I'd like to build in a safety factor, so, if I want
3A I'd like it to be able to handle 4A without getting too hot.

Thanks again,

Michael
 
Paul E. Schoen wrote:

....
You will probably need to "roll your own", but it should really be nothing
much more than a small toroid about 1" diameter, wound with a couple dozen
turns of wire, if you use about 50 kHz PWM. #22 to #26 AWG wire should be
OK for 3 amps or so. Otherwise, you could probably use parts of an old
computer power supply. Of course, you can just buy a DC-DC converter, but
18 watts will probably cost at least $50.

Paul


My PWM controller is basically this:

Probably a silly question to ask, but just making sure: should I make
C1 equal to 200 (or the more commonly available 220) pF to get 50 kHz?
Any special type of cap, or would a simple ceramic do?

Thanks,

Michael
 
P

Paul E. Schoen

Jan 1, 1970
0
silly, forgot the link:
http://www.cpemma.co.uk/555pwm.html

(sleep deprived - woke up 4:30am this morning to fly out to LA for a
meeting.)

The 555 will handle 50 kHz, but 200 pF is marginal. It would probably be
best to change the pot to 10k, and then use a 2 nF capacitor for C1.
Ceramic or film types are OK.

You cannot just put a transformer in place of the motor, because you will
have DC which will saturate it. The easiest method is a push-pull design
with a center tapped primary, and the best IC is a regulating PWM
controller such as SG3526, UC3526, or TL494, which is designed to drive two
MOSFETs or bipolar transistors. It has provisions for soft start,
overcurrent shutdown, dead time, and voltage regulation. They are used in
many switching power supplies.

For wire size, you can use my spreadsheet at:
http://www.smart.net/~pstech/WireSize.xls

It shows that #22 AWG should handle 3.8 amps. #20 allows 5.3 and #18 will
handle 7.6 amps. These values are for a conductor in free air with
something like 30 C temperature rise. Use the heaviest wire that will
easily fit the core, for best efficiency.

Good luck,

Paul
 
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