Low Volts High Current!

On 13 Nov 2004 16:04:23 GMT, Roger Johansson <no-email@home.se> wroth:

I have to convert voltage from a 12 volt car battery to 7.2 volts,
BUT be able to draw 150 amps for a short time (approximately 2 or 3
seconds). The voltage output must be accurate, can anyone help
regarding design and recommended components please?


The car battery you have there consists of a number of cells, which are
connected in series. The voltage between the two outermost placed
contacts is 12 Volt.

In between them there are other connections which carry different
voltages. If you can remove the plastiv cover on the battery, or drill
through it, you can connect to any number of cells you like, in series.
Click to expand...
I think you have the most satisfactory solution.

The actual voltage isn't extremely important as long as it doesn't cause
the motor to explode immediately upon application. Since a computer is
involved, the power, amps times voltage, is available as one of the parameters
and that should be good enough even if the voltage is a little bit off from the
desired 7.2 volt figure or if it sags a little during the test.

Why throw a lot of electronics at the problem when a little software can
do the job?

Jim
 
Winfield Hill wrote...
"NumanR" <numanracing@tiscali.co.uk> wrote in message
news:49f8a664.0411061231.3b0a1d8e@posting.google.com...

I have to convert voltage from a 12 volt car battery to 7.2 volts,
BUT be able to draw 150 amps for a short time (approximately 2 or
3 seconds). The voltage output must be accurate, can anyone help
regarding design and recommended components please?

NumanR later wrote:

Ok the reason I need 7.2 volts is because this is for a power supply
for a racing motor dynometer. The voltage could be around 7.5 volts
but needs to be stable as possible. The motor is placed in a machine
and has a flywheel fitted and then is accelerated to full speed. The
motor accelerates for approximately 3 seconds depending on power and
then stays at full speed for another two. These results are then
processed and then sent to my computer where they are graphed for
comparison. It is the comparison that makes the voltage supply
accuracy important.
Click to expand...
.. 7.2V 150A Linear Regulator
.. BATT 250A fuse by Winfield Hill
.. POS __or breaker
.. =====|__|==(O)===+===+===+===+===+===+===+===+===+===+===+===+===+===+===+
.. +12.5-14.5V | | | | | | | | | | | | | | |
.. ,---+----+------+ | | | | | | | | | | | | | |
.. | | | | | | | | | | | | | | | | | |
.. | | 2R7 | | | | | | | | | | | | | | |
.. | 470 5W | | | | | |
.. | | | | | | Q3 to Q17 = BDW83C, TIP42 or 2n6284 | | |
.. | | |/ Q2 | | | bank of 15 Darlington transistors | | |
.. | +--| ZTX851 | | | | | |
.. | | |\ Q3 | | | | | | | | | | | | | Q17
.. | | V | | | | | | | | | | | | | | |
.. | | | |/ |/ |/ |/ |/ |/ |/ |/ |/ |/ |/ |/ |/ |/ |/
.. | | +====|===|===|===|===|===|===|===|===|===|===|===|===|===|===|
.. | | | |\ |\ |\ |\ |\ |\ |\ |\ |\ |\ |\ |\ |\ |\ |\
.. | | | V V V V V V V V V V V V V V V
.. | | 220 | | | | | | | | | | | | | | |
.. | | 1W 20m 20m 20m 20m 20m 20m 20m 20m 20m 20m 20m 20m 20m 20m 20m
.. | | | 3W 3W 3W 3W 3W 3W 3W 3W 3W 3W 3W 3W 3W 3W 3W
.. 2k7 | | | | | | | | | | | | | | | | |
.. | | | '===+===+===+===+===+===++==+++=+===+===+===+===+===+==='
.. | | '---------------------, | ||
.. | | | | || +7.20V 0 - 150A
.. | +----100--, | | ++====(O)=============++
.. | | | 0.01 | | ||
.. | '---, +--||--, ,----|---------|---------(O)--------, ||
.. | | | | | | | + sense \ ||
.. | Q1 | __|__ 2k7 4k75 | ,-----+ '--++
.. | |/ | | | | | | | 1000uF ||
.. +-1k--| | |---+----+ | 22 |+ 16V MOTOR
.. | |\ |_____| 2.50V | | 5W === ||
.. | V | IC1 2k49 | | | ,--++
.. | | | TL431 | | | | - sense / ||
.. o +-----+-----------+----+---+-----+---------(o)--------' ||
.. o---33--' ||
.. close = ON ||
.. optional 50mV ||
.. ===========================================+= meter shunt =+=======++
.. BATT NEG

Fifteen places NPN power Darlington, mounted on large aluminum heat sink
with thin heat-sink grease, and no insulating pads. Insulate heat sink from
the chassis and add plastic to protect it from exposure to metal tools, etc.
---

To analyze the circuit we'll start with the 10-milli-ohm emitter resistors,
which are required to insure equal current sharing among the transistors.
These are Ohmite 630HR020 metal-element types, at DigiKey for $0.42 each,
and they'll drop 0.2V at the nominal 10A current each transistor carries at
full load. The Darlington power transistors will have a Vbe drop of up to
2V at 10A, severely eating into the voltage difference available between the
battery and the motor at 150A, so we'll have to be careful with what's left.
(The high Vbe drop is one reason we're limiting each pass transistor to 10A,
which then requires us to use 15 parallel transistors in the pass bank.)

Although we have used high-gain Darlington transistors in an attempt to get
the base drive current down to reasonable levels, assuming a minimum gain of
500 at 10A means Q2 will have to provide over 300mA at full output. Q2 must
have high beta at 300mA, and dissipate up to 1W for high 14.5V battery (i.e.
under charge). A Zetex ZTX851, etc. (available at DigiKey), should work OK.

We cannot use a Darlington transistor at this spot because too much voltage
has been used up by the 15 power Darlington transistor's base-emitter drop
at full current. We have to save some overhead to allow the battery to sag.

In the event of an output short, Q2's 2.7-ohm collector resistor limits its
current, but the power transistors must fend for themselves until the fuse
blows (you can get 250A fuses at auto parts stores). Alternately a ~ 200A
current limit function could be added by amplifying the 50mV shunt voltage,
comparing it to a fixed voltage, and pulling down Q2's base.

Another valuable feature would be a comparator to tell whenever battery
sag means the circuit is running out of headroom, which can be seen by
looking at the control-loop voltage on the TL431's output terminal.

Some folks will say this linear power regulator design illustrates why a
buck switching converter should be used instead. I will not contest their
point, except to point out that I've learned from making SMPS in the 100
to 1000A region that there are many non-trivial issues one will encounter,
dictating knowledge, experience, and good instruments on your workbench.
On the other hand, this circuit can be made and tested by a hobbyist.


--
Thanks,
- Win
 
On Sat, 13 Nov 2004 05:29:11 -0800, Winfield Hill wrote:
I have taken the liberty of cross-posting this to s.e.d. because it
can be nicely considered as a nontrivial electronics design problem.
"NumanR" <numanracing@tiscali.co.uk> wrote in message
I have to convert voltage from a 12 volt car battery to 7.2 volts,
BUT be able to draw 150 amps for a short time (approximately 2 or 3
seconds). The voltage output must be accurate, can anyone help
regarding design and recommended components please?

I suggested a commercial 1 or 2kW power supply, such as a Xantrex
XHR 7.5-130, spec'd at 7.5V 130A (but can be externally programmed
to operate to 150A for a short time, and I gave a $350 eBay link,
http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=3850054721
That one finished, but the seller still has it, plus a few more...
Click to expand...
I was thinking, "Battery charger/boost starter" (among other ideas,
most of which involve huge banks of very pricey batteries), but
then realized:

Welder!

I've seen welders that can hold a very stiff voltage, at arbitrarily
high currents up to hundreds of amps. Search on constant voltage
MIG or TIG or FCAW, and you could lurk in news:sci.engr.joining.welding .

Just for the sake of contrariness, I do want to mention that it
sounds very much like they're spec'ing starter motors. :)

<half-anecdote>
Once, at a science fair, some kid asked how he'd get 50 amps at
about 12 volts, and being into ham radio at the time, and since
I practically carried the "Allied" catalog around with me, the
first thing I thought of was stud diodes. Nobody at the time
thought to use a car battery, like was suggested a couple of
months later in Scientific American "Amateur Scientist". It
was for a plasma jet. Nowadays, you just go buy or rent a plasma
cutter. :)
I don't know what the kid finally settled on. But I won a blue
ribbon that year for my two-transistor AM radio. :)
I think it was a winner because the kit came with one transistor,
and I bought another and added it as an audio amp, and kinda
made up the circuit, even though I really didn't have much
Clue at the time. :) The Radio Amateur's Handbook hadn't
heard of transistors yet. :)

Now that I think about it, I think I might have accidentally
invented negative feedback - I had the base bias resistor
going right to that transistor's own collector. Hey, it was
a Lucky WAG, and sound came out!
</half-anecdote>

Good Luck!
Rich
 
Winfield Hill wrote:
Winfield Hill wrote...

"NumanR" <numanracing@tiscali.co.uk> wrote in message
news:49f8a664.0411061231.3b0a1d8e@posting.google.com...

I have to convert voltage from a 12 volt car battery to 7.2 volts,
BUT be able to draw 150 amps for a short time (approximately 2 or
3 seconds). The voltage output must be accurate, can anyone help
regarding design and recommended components please?

NumanR later wrote:

Ok the reason I need 7.2 volts is because this is for a power supply
for a racing motor dynometer. The voltage could be around 7.5 volts
but needs to be stable as possible. The motor is placed in a machine
and has a flywheel fitted and then is accelerated to full speed. The
motor accelerates for approximately 3 seconds depending on power and
then stays at full speed for another two. These results are then
processed and then sent to my computer where they are graphed for
comparison. It is the comparison that makes the voltage supply
accuracy important.


. 7.2V 150A Linear Regulator
. BATT 250A fuse by Winfield Hill
. POS __or breaker
. =====|__|==(O)===+===+===+===+===+===+===+===+===+===+===+===+===+===+===+
. +12.5-14.5V | | | | | | | | | | | | | | |
. ,---+----+------+ | | | | | | | | | | | | | |
. | | | | | | | | | | | | | | | | | |
. | | 2R7 | | | | | | | | | | | | | | |
. | 470 5W | | | | | |
. | | | | | | Q3 to Q17 = BDW83C, TIP42 or 2n6284 | | |
. | | |/ Q2 | | | bank of 15 Darlington transistors | | |
. | +--| ZTX851 | | | | | |
. | | |\ Q3 | | | | | | | | | | | | | Q17
. | | V | | | | | | | | | | | | | | |
. | | | |/ |/ |/ |/ |/ |/ |/ |/ |/ |/ |/ |/ |/ |/ |/
. | | +====|===|===|===|===|===|===|===|===|===|===|===|===|===|===|
. | | | |\ |\ |\ |\ |\ |\ |\ |\ |\ |\ |\ |\ |\ |\ |\
. | | | V V V V V V V V V V V V V V V
. | | 220 | | | | | | | | | | | | | | |
. | | 1W 20m 20m 20m 20m 20m 20m 20m 20m 20m 20m 20m 20m 20m 20m 20m
. | | | 3W 3W 3W 3W 3W 3W 3W 3W 3W 3W 3W 3W 3W 3W 3W
. 2k7 | | | | | | | | | | | | | | | | |
. | | | '===+===+===+===+===+===++==+++=+===+===+===+===+===+==='
. | | '---------------------, | ||
. | | | | || +7.20V 0 - 150A
. | +----100--, | | ++====(O)=============++
. | | | 0.01 | | ||
. | '---, +--||--, ,----|---------|---------(O)--------, ||
. | | | | | | | + sense \ ||
. | Q1 | __|__ 2k7 4k75 | ,-----+ '--++
. | |/ | | | | | | | 1000uF ||
. +-1k--| | |---+----+ | 22 |+ 16V MOTOR
. | |\ |_____| 2.50V | | 5W === ||
. | V | IC1 2k49 | | | ,--++
. | | | TL431 | | | | - sense / ||
. o +-----+-----------+----+---+-----+---------(o)--------' ||
. o---33--' ||
. close = ON ||
. optional 50mV ||
. ===========================================+= meter shunt =+=======++
. BATT NEG

Fifteen places NPN power Darlington, mounted on large aluminum heat sink
with thin heat-sink grease, and no insulating pads. Insulate heat sink from
the chassis and add plastic to protect it from exposure to metal tools, etc.
---

To analyze the circuit we'll start with the 10-milli-ohm emitter resistors,
which are required to insure equal current sharing among the transistors.
These are Ohmite 630HR020 metal-element types, at DigiKey for $0.42 each,
and they'll drop 0.2V at the nominal 10A current each transistor carries at
full load. The Darlington power transistors will have a Vbe drop of up to
2V at 10A, severely eating into the voltage difference available between the
battery and the motor at 150A, so we'll have to be careful with what's left.
(The high Vbe drop is one reason we're limiting each pass transistor to 10A,
which then requires us to use 15 parallel transistors in the pass bank.)

Although we have used high-gain Darlington transistors in an attempt to get
the base drive current down to reasonable levels, assuming a minimum gain of
500 at 10A means Q2 will have to provide over 300mA at full output. Q2 must
have high beta at 300mA, and dissipate up to 1W for high 14.5V battery (i.e.
under charge). A Zetex ZTX851, etc. (available at DigiKey), should work OK.

We cannot use a Darlington transistor at this spot because too much voltage
has been used up by the 15 power Darlington transistor's base-emitter drop
at full current. We have to save some overhead to allow the battery to sag.

In the event of an output short, Q2's 2.7-ohm collector resistor limits its
current, but the power transistors must fend for themselves until the fuse
blows (you can get 250A fuses at auto parts stores). Alternately a ~ 200A
current limit function could be added by amplifying the 50mV shunt voltage,
comparing it to a fixed voltage, and pulling down Q2's base.

Another valuable feature would be a comparator to tell whenever battery
sag means the circuit is running out of headroom, which can be seen by
looking at the control-loop voltage on the TL431's output terminal.

Some folks will say this linear power regulator design illustrates why a
buck switching converter should be used instead. I will not contest their
point, except to point out that I've learned from making SMPS in the 100
to 1000A region that there are many non-trivial issues one will encounter,
dictating knowledge, experience, and good instruments on your workbench.
On the other hand, this circuit can be made and tested by a hobbyist.
Click to expand...
Eh- you have approximately doubled the required AH of the battery.
 
Winfield Hill wrote:
Winfield Hill wrote...

"NumanR" <numanracing@tiscali.co.uk> wrote in message
news:49f8a664.0411061231.3b0a1d8e@posting.google.com...

I have to convert voltage from a 12 volt car battery to 7.2 volts,
BUT be able to draw 150 amps for a short time (approximately 2 or
3 seconds). The voltage output must be accurate, can anyone help
regarding design and recommended components please?

NumanR later wrote:

Ok the reason I need 7.2 volts is because this is for a power supply
for a racing motor dynometer. The voltage could be around 7.5 volts
but needs to be stable as possible. The motor is placed in a machine
and has a flywheel fitted and then is accelerated to full speed. The
motor accelerates for approximately 3 seconds depending on power and
then stays at full speed for another two. These results are then
processed and then sent to my computer where they are graphed for
comparison. It is the comparison that makes the voltage supply
accuracy important.


. 7.2V 150A Linear Regulator
. BATT 250A fuse by Winfield Hill
. POS __or breaker
. =====|__|==(O)===+===+===+===+===+===+===+===+===+===+===+===+===+===+===+
. +12.5-14.5V | | | | | | | | | | | | | | |
. ,---+----+------+ | | | | | | | | | | | | | |
. | | | | | | | | | | | | | | | | | |
. | | 2R7 | | | | | | | | | | | | | | |
. | 470 5W | | | | | |
. | | | | | | Q3 to Q17 = BDW83C, TIP42 or 2n6284 | | |
. | | |/ Q2 | | | bank of 15 Darlington transistors | | |
. | +--| ZTX851 | | | | | |
. | | |\ Q3 | | | | | | | | | | | | | Q17
. | | V | | | | | | | | | | | | | | |
. | | | |/ |/ |/ |/ |/ |/ |/ |/ |/ |/ |/ |/ |/ |/ |/
. | | +====|===|===|===|===|===|===|===|===|===|===|===|===|===|===|
. | | | |\ |\ |\ |\ |\ |\ |\ |\ |\ |\ |\ |\ |\ |\ |\
. | | | V V V V V V V V V V V V V V V
. | | 220 | | | | | | | | | | | | | | |
. | | 1W 20m 20m 20m 20m 20m 20m 20m 20m 20m 20m 20m 20m 20m 20m 20m
. | | | 3W 3W 3W 3W 3W 3W 3W 3W 3W 3W 3W 3W 3W 3W 3W
. 2k7 | | | | | | | | | | | | | | | | |
. | | | '===+===+===+===+===+===++==+++=+===+===+===+===+===+==='
. | | '---------------------, | ||
. | | | | || +7.20V 0 - 150A
. | +----100--, | | ++====(O)=============++
. | | | 0.01 | | ||
. | '---, +--||--, ,----|---------|---------(O)--------, ||
. | | | | | | | + sense \ ||
. | Q1 | __|__ 2k7 4k75 | ,-----+ '--++
. | |/ | | | | | | | 1000uF ||
. +-1k--| | |---+----+ | 22 |+ 16V MOTOR
. | |\ |_____| 2.50V | | 5W === ||
. | V | IC1 2k49 | | | ,--++
. | | | TL431 | | | | - sense / ||
. o +-----+-----------+----+---+-----+---------(o)--------' ||
. o---33--' ||
. close = ON ||
. optional 50mV ||
. ===========================================+= meter shunt =+=======++
. BATT NEG

Fifteen places NPN power Darlington, mounted on large aluminum heat sink
with thin heat-sink grease, and no insulating pads. Insulate heat sink from
the chassis and add plastic to protect it from exposure to metal tools, etc.
---

To analyze the circuit we'll start with the 10-milli-ohm emitter resistors,
which are required to insure equal current sharing among the transistors.
These are Ohmite 630HR020 metal-element types, at DigiKey for $0.42 each,
and they'll drop 0.2V at the nominal 10A current each transistor carries at
full load. The Darlington power transistors will have a Vbe drop of up to
2V at 10A, severely eating into the voltage difference available between the
battery and the motor at 150A, so we'll have to be careful with what's left.
(The high Vbe drop is one reason we're limiting each pass transistor to 10A,
which then requires us to use 15 parallel transistors in the pass bank.)

Although we have used high-gain Darlington transistors in an attempt to get
the base drive current down to reasonable levels, assuming a minimum gain of
500 at 10A means Q2 will have to provide over 300mA at full output. Q2 must
have high beta at 300mA, and dissipate up to 1W for high 14.5V battery (i.e.
under charge). A Zetex ZTX851, etc. (available at DigiKey), should work OK.

We cannot use a Darlington transistor at this spot because too much voltage
has been used up by the 15 power Darlington transistor's base-emitter drop
at full current. We have to save some overhead to allow the battery to sag.

In the event of an output short, Q2's 2.7-ohm collector resistor limits its
current, but the power transistors must fend for themselves until the fuse
blows (you can get 250A fuses at auto parts stores). Alternately a ~ 200A
current limit function could be added by amplifying the 50mV shunt voltage,
comparing it to a fixed voltage, and pulling down Q2's base.

Another valuable feature would be a comparator to tell whenever battery
sag means the circuit is running out of headroom, which can be seen by
looking at the control-loop voltage on the TL431's output terminal.

Some folks will say this linear power regulator design illustrates why a
buck switching converter should be used instead. I will not contest their
point, except to point out that I've learned from making SMPS in the 100
to 1000A region that there are many non-trivial issues one will encounter,
dictating knowledge, experience, and good instruments on your workbench.
On the other hand, this circuit can be made and tested by a hobbyist.
Click to expand...
The answer to this may be a hobbyized version of a multiphase buck
converter- maybe summing currents at the load destination - to break the
problem into manageable pieces:
http://powerelectronics.com/mag/power_capacitor_ripple_current/
 
Ken Smith wrote...
Winfield Hill wrote:
[.....]

150A regulator. He delivers his kW power level for only a few
seconds, and during that time dissipates 150A * 5V at most (likely
less, due to battery and cabling voltage drops), which amounts to
about 450 * 3 watt-sec = 1350J during that time. Unlike ordinary
linear kW supplies, this won't present a serious heat sink problem.

The TIP35 and TIP36 transistors are fairly cheap. I think I'd start
like this:

10 each TIP35
------------------- ---/\/\---
! \ /e !
! ----- !
! TIP36 ! !
+-/\/\/-- -----------/\/\------+
! e \ / !
! ------ !
! ! --------- !
---+--/\/\/\/--+-----! LM317 !---------+----
---------
!
Click to expand...
Hmm, TIP35 transistors are not Darlington types, which means low
beta, but also lower Vbe. If each transistor conducts 15A with a
beta of 15, then the TIP36 PNP has to handle 10A, which it can do
just fine. If its gain is 25, the LM317 has to handle 0.5A, which
is also fine. At 500mA the LM317's dropout voltage is 1.7V, which
with a Vbe of 2V for a TIP36 at 10A gives us an overall regulator
dropout voltage of 3.7 volts Adding this to the 7.2V output tells
us the battery must stay above 10.9 volts for regulation at 150A.
If the transistor betas are a bit higher, and if the Vbe voltages
are a bit lower, the battery will be able to sag more, and there
may be room left for a cable drop. Now working the TIP35 pathway,
say the emitter resistors drop 0.2V and Vbe = 2V, this leaves 1.5
volts for the TIP36 Vce(sat), assuming a 10.9V battery. Everything
is working out, because 1.5 volts is what the TIP36 datasheet tells
us to expect for Vce(sat) at 10A.

So this is a viable approach, perhaps even better than my drawing.

Here's a small NPN power transistor spec comparison table. All
are big TO-218 style plastic packages, except the 2n6284 is TO-3
(rated Tj = 200C). The TIP35 and TIP36 are NOT Darlington types.

part thermal Hfe @ Ic Vbe @ Ic
number C/W Pd 10A 15A 10A 15A
------ --- --- ---- ---- ---- ----
TIP35 1.0 125W 30 >15 < 2V 2V (also PNP TIP36)
2n6284 1.09 160 750 500 < 2 2.5
BDW83C 0.96 130 750 >100 < 2.5 ?
TIP142 1.0 125 500 ? 2 ~ 2.5

All these transistors are inexpensive and widely available, but
they're comparatively wimpy. For example, a 2n5686 (see below) is
a 50A npn TO-3, rated at 300W, and it's much better suited for the
task, with higher beta and lower Vbe. But it's $11.76 at DigiKey.
The 60A MJ14002 is a bit less at $10.36 qty 10, stocked by DigiKey.

part thermal Hfe @ Ic Vbe @ Ic
number C/W Pd 10A 15A 10A 15A
------ ----- ---- ---- ---- ---- ----
2n5686 0.584 300W 70 50 0.85V 0.9V TO-3 non-Darlington
MJ14002 0.584 300W 100 70 0.8V 0.85V " "

Using one of these transistors would open the possibility of battery
voltages lower than 10.9V, although using a low-dropout LT1085, etc.,
instead of a LM317 (0.9V compared to 1.7V at 0.5A) would be required.
Then a 10V sagging battery + cable voltage could be allowed at 150A.


--
Thanks,
- Win
 
Mark Jones wrote:

Hmm... once I ripped 6 huge electrolytic caps from some old machine
- it might have been a photocopying machine, dunno. It had some kind
of primitive fixed disk in it (and a bubble memory of the magnetic
bead variety, imagine that.) Anyways the caps were at least 4" in
diameter by 10" long and were rated for 16V/250,000uF each. So yep, I
wired them all in parallel and charged them to 12V through a 5kW
autotransformer (used as a potentiometer.)

WOW, instant spot-welder. Jumping wires, permanent magnetization of
nearby metal objects, neat. Today, you could probably do even better
with the 2.0 Farad "supercaps" sold to car audio enthusiasts. (Their
explosion-resistance should be checked though...)
Click to expand...
The Supercaps have such a high ESR that you can't get much current
out of them. You also can't charge them fast or discharge them fast,
which can be a real pain when designing high-speed testers for
products that include them.
 
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