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'[OT] PSU for slotcars'
2000\03\22@112655 by Quentin

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A friend of mine asked me about making a PSU for the slotcar track for
his club. Seems like these things needs lotsa power. After he gave me
the specs I went UHHMMM......
They are using 16 tracks. Each track use 13V and drawing between 4 to 8A
which can max to about 15A on startup EACH!
They were using an electric motor which drives a car generator
(altinator) that charges a 12V car battery on their old 8 lane track. My
friend is now thinking of using 250W PC PSU's for each track, but I
don't think it will last due to the abuse the little motors will give
it.

Anybody got comments or other ideas?

Quentin

2000\03\22@120224 by Chris Eddy

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Quentin;

I find it super hard to believe that these slot cars take 104 watts each!  I
strongly suggest you get out there and measure some currents your self.  I
would never accept laypersons figures in such a matter.  I don't doubt there
are some serious surge currents, but 832 watts is bolderdash!

I would also prepare to measure the current on your scope, with an IA across
a sense resistor (or a hall effect module) to see what the currents really
look like.  A handheld meter may give you a fantasy.

Let's assume you are now in posession of figures such as 4A surge for 800mS
and 1A continuous.  And the 13V is at full speed, and applies at 1A, not
4A.  I would make a constant current source in order to get smooth operation
over dirty tracks.  Assume that the surges all happen at one time, when
everyone starts out.  The supply needs a beefy hold up cap.  Further assume
that the supply will sag some under the starting conditions.  A switcher
circuit for each controller would allow the user to have constant
performance through the power supply sags and allow you to implement current
mode control without too much trouble.  Further assume that 1A/13V is driven
by a 24V supply with a buck converter in current feedback mode (not to be
confused with the current mode implemented in the DC/DC swithch controller
chip).  13W times 8 is 104W continuous, so use a 200W 24V supply with an
added 10,000uF cap.

Be careful in the choice of DC/DC controller, you need a variable output,
where many DC/DC siwtchers are fixed reference.  I solved this once using a
good old LM78S40 (check discontinued) modified the circuit to drive a FET
and made it a variable output switcher.  A crafty individual might use a
simple op amp in a feedback loop, driven to instability by an RC in the
feedback, to make a free oscillating switcher.  Others make suitable chips,
maybe Unitrode, Linear tech, or National.  You could skip the current
feedback mode if it makes life simpler.  Maybe just put an inductor in
series with the track line to smooth out the motor operation.

If the switcher is cool enough (.15 * 13 is 2 watts) you might sneak it into
the hand held controller.  Probably depends on the size of the inductor and
capacitors.

Sounds like fun!  I'll be right over.  Africa, right?

Chris Eddy
Pioneer Mcirosystems, inc.

Quentin wrote:

> A friend of mine asked me about making a PSU for the slotcar track for
> his club. Seems like these things needs lotsa power. After he gave me
> the specs I went UHHMMM......

2000\03\22@121642 by Alan Pearce

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>
>I find it super hard to believe that these slot cars take 104 watts each!  I
>strongly suggest you get out there and measure some currents your self.  I
>would never accept laypersons figures in such a matter.  I don't doubt there
>are some serious surge currents, but 832 watts is bolderdash!

I would not be surprised at a high current draw. When I was at secondary school
I remember guys rewinding the motors with about 8 turns of 16 gauge wire, and
then encasing it all in epoxy so it did not fly apart with centrifugal force. I
do not know how long the brushes lasted, but I could well believe some of the
figures quoted in the original request, although they would be taken under ideal
conditions ( wires fixed to car on rolling test bed) and would be reduced
somewhat by the wiring to the hand controls. I would still work on the basis of
several amps per track.

2000\03\22@123734 by Quitt, Walter

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Our little RC cars take several hundred amps at start up.
That's peak current for some amount milliseconds.
Accelarating fast at around 10 - 20 A.
Cruisng at couple of Amps or so.
A 7.2 A hour battery is done in under 10 minutes.
So that's 7.2 V x 1 A hour or for 10 minutes that's 6 A hours.
Now double the voltage for the slot car and it could well be up to:
a few hundred Amps for milliseconds
10A for accelarating
and a few Amps cruising.

Those sparks on slot car tracks don't come with low current.
Also those sliding contacts have ressistance and hence add to the issue.

Therefore:
No problem, dude seeing that those little cars can draw lotsa Amps

Amazing, eh?

I'd try:
http://www.fairradio.com

for one of their surplus high current supplies for old radios.

-Walt

{Original Message removed}

2000\03\22@125017 by Andrew Kunz

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>Our little RC cars take several hundred amps at start up.
>That's peak current for some amount milliseconds.

Not really.  Your cells usually peak at about 100A even with a direct short
across them, due it the internal resistance of the cells.

In a model boat, though, we dump RC2000's in under 2 minutes.

Most of the car ESCs are way over-rated (ie, Novak is lying).

Andy
http://www.rc-hydros.com/escproducts

2000\03\23@032548 by Graham North

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I think you'll find the reason ESC's are over-rated if for efficiency.  Try
running
your boat for two minutes with an ESC fitted with a couple of crappy FETs
(for the
actual current).  I would like to see you touch the tabs afterwards.

(That's if it didn't have thermal cut out.  No airflow in a boat.)

       ----------
       From:  Andrew Kunz [SMTP:spam_OUTakunzTakeThisOuTspamTDIPOWER.COM]
       Sent:  22 March 2000 17:47
       To:  .....PICLISTKILLspamspam@spam@MITVMA.MIT.EDU
       Subject:  Re: [OT] PSU for slotcars

       >Our little RC cars take several hundred amps at start up.
       >That's peak current for some amount milliseconds.

       Not really.  Your cells usually peak at about 100A even with a
direct short
       across them, due it the internal resistance of the cells.

       In a model boat, though, we dump RC2000's in under 2 minutes.

       Most of the car ESCs are way over-rated (ie, Novak is lying).

       Andy
       http://www.rc-hydros.com/escproducts

2000\03\23@102059 by Andrew Kunz

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Most Novak and Tekin ESCs won't last 2 minutes at 60A continuous.  Been there,
tried that.

BTW, my ESC (http://www.rc-hydros.com/escproducts.html) is water cooled for high-power
setups.  Right now it is only rated to 60A continuous @ 14 cells, but I run my
own at 75A and 12 cells.  Next run has upgraded FETS (40% less RDSon) and one
more FET with a 20 cell limit.  Can't wait to see how they take the punishment.

I _have_ fried a few, mostly as a result of testing what the limits are.

Andy









Graham North <graham.northspamKILLspamLANDINST.COM> on 03/23/2000 03:25:25 AM

Please respond to pic microcontroller discussion list <.....PICLISTKILLspamspam.....MITVMA.MIT.EDU>








To:      EraseMEPICLISTspam_OUTspamTakeThisOuTMITVMA.MIT.EDU

cc:      (bcc: Andrew Kunz/TDI_NOTES)



Subject: Re: [OT] PSU for slotcars








I think you'll find the reason ESC's are over-rated if for efficiency.  Try
running
your boat for two minutes with an ESC fitted with a couple of crappy FETs
(for the
actual current).  I would like to see you touch the tabs afterwards.

(That's if it didn't have thermal cut out.  No airflow in a boat.)

       ----------
       From:  Andrew Kunz [SMTP:akunzspamspam_OUTTDIPOWER.COM]
       Sent:  22 March 2000 17:47
       To:  @spam@PICLISTKILLspamspamMITVMA.MIT.EDU
       Subject:  Re: [OT] PSU for slotcars

       >Our little RC cars take several hundred amps at start up.
       >That's peak current for some amount milliseconds.

       Not really.  Your cells usually peak at about 100A even with a
direct short
       across them, due it the internal resistance of the cells.

       In a model boat, though, we dump RC2000's in under 2 minutes.

       Most of the car ESCs are way over-rated (ie, Novak is lying).

       Andy
       http://www.rc-hydros.com/escproducts

2000\03\26@091350 by John De Villiers

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1 AMP doesnt even turn my car's motor.

These little things do a 35 meter figure 8 track in under 4 seconds

Also, you Brake by shorting its power leads. A PSU for these will have to
have some serious overload protection built-in

We use several heavyduty 12V batteries wired in parrallel ( for those enduro
envents ).

> {Original Message removed}

2000\03\27@012342 by Gennette Bruce

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At last !  An answer from someone who has a slot car power system.

Seriously though, this has always been the traditional answer - use some car
batteries to run the slot cars, one (or more) per slot car.  The better
systems would use continuous, monitoring chargers for each battery.

So what would you need for 8 tracks . . .
8 deep cycle batteries
8 isolating diodes
8 battery charger circuits set to 14.5V (13.8 + diode drop of 0.7V) @ 1.2A
2 rectifiers
2 transformers  (18V @ 6A)

Except for the batteries I would estimate that this could be built for about
AU$300 (US$200).  Then there are the batteries.  Car batteries are
unsuitable - they are designed to give a big push for a short time and be
recharged *IMMEDIATELY*. Deep cycle batteries are designed to provide
moderate output for long periods, but must not be discharged too deeply,
(discharged to only about 5% below max is recommended for maximum life).

6 hours continuous flat-out running of a slot car drawing an average 4A (3.2
from battery & 1.2 from charger) uses 19.2AHr, and can be replaced over 18
hours at 1.2A from the charger (21.6Ahr).  This would drain the battery by
around 3.2A x 6Hr = 19.2Ahr.  Of course actual usage would normally be less
than an average of 4A so the power usage time would be spread over a longer
track usage time, probably around 9 or 10 hours per 24.

So you need to buy batteries with at least 400Ahr capacity to keep the
discharge per cycle drop below 5%.  (Each extra 5% discharge per cycle
halves battery life - that is a battery regularly discharged to 10% below
max will only last half as long as one regularly discharged to 5% below max.
If discharged to 15% below max per cycle it will only last a quarter as long
as a 5% discharged per cycle).

400Ahr deep cycle batteries are about AU$150 each (US$100).  And you need 8
of them.  That makes a total of AU$1500 (US$1000) for 8 tracks *AND* you are
up for another AU$1200 (US$800) every 4 or so years.

No wonder your friend wants an electronic solution.  So can anyone suggest a
proper solution.  What we need is a 14.5V @ 80A (1200VA) low droop power
supply with 8 isolating diodes and 8 capacitor tanks.

Don't be afraid to suggest expensive solutions (see cost of alternative,
above).

Bye.

       {Original Message removed}

2000\03\27@033024 by William Chops Westfield

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   Deep cycle batteries are designed to provide moderate output for long
   periods, but must not be discharged too deeply, (discharged to only
   about 5% below max is recommended for maximum life.)

Um, doesn't that mean VOLTAGE 5% below max, which is MUCH more discharged
than "5% of total AH capacity"?!  Otherwise, it seems pretty pointless to
rate a battery in units that you can only use 5% of...

BillW

2000\03\27@055655 by Russell McMahon

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A realistic approach is probably a supply that would handle the POWER level
required paralleled with a car battery to provide short current peaks.

A microwave oven transformer with the secondary removed and a new one wound
MAY be about right for the level of power desired. These have something
special done to the magnetic path - to make them droop under high load
AFAIK, so they may not suit. These operate at around 1 KVA but not for long
periods. Enthusiastic fan cooling may help although use of more than one
transformer may be advised.

New transformers of 1 KVA rating (80A at 12v or 40A at 24v) sell here for
about $NZ 250 = about $US125 = $A200 so presumably they are similar there as
copper and steel is pretty universal technology.

The battery would not need to be deep cycle - rather something intended for
float use.
Power Schottky diodes are going to save some heat at these current levels
(and add some dollars).
Use of triacs in a synchronous rectifier may be a cheap solution and
simultaneously allow a dgree of voltage control under the immense load
variations. .



     Russell McMahon
_____________________________

- http://www.easttimor.com
      Updated regularly:
      100,000 refugees STILL in  West Timor face starvation!

- http://www.sudan.com
   And you think Kosovo and Chechnya are bad!

What can one man* do?
Help the hungry at no cost to yourself!
at  http://www.thehungersite.com/

(* - or woman, child or internet enabled intelligent entity :-))


{Original Message removed}

2000\03\27@062132 by Alan B Pearce

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>A microwave oven transformer with the secondary removed and a new one wound
>MAY be about right for the level of power desired. These have something
>special done to the magnetic path - to make them droop under high load
>AFAIK, so they may not suit. These operate at around 1 KVA but not for long
>periods. Enthusiastic fan cooling may help although use of more than one
>transformer may be advised.

This sounds like a constant voltage transformer. They are commonly used in
voltage stabilisers, and have a winding on them with a capacitor across it. The
value of the capacitor and the inductance of the winding are critical at the
mains frequency and voltage, and changing either of these parameters means
changing the tap the capacitor is connected to. I used to service a pack type
disc drive that had one of these in its power supply.

I believe it is actually possible to short circuit the output of one of these
transformers without burning things up, but I never actually tried it.

2000\03\27@065700 by Michael Rigby-Jones

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part 0 4402 bytes
<P><FONT SIZE=2 FACE="Arial">Don't be afraid to suggest expensive solutions (see cost of alternative,</FONT>
<BR><FONT SIZE=2 FACE="Arial">above).</FONT>
</P>
<BR>
</UL>
<P><FONT COLOR="#0000FF" SIZE=2 FACE="Arial">You can buy 12 Volt, 16.7 Amp switchers in Farnell for 130 UKP odd.&nbsp; I'm sure they could be bought cheaper elsewhere, Farnell are not noted for super competitive pricing usualy!&nbsp; Having a PSU per track would give increased reliablility.&nbsp; If one psu failed at least you could carry on racing with 7 tracks etc.</FONT></P>

<P><FONT COLOR="#0000FF" SIZE=2 FACE="Arial">Mike</FONT>
</P>
<BR>
<UL>
<P><FONT SIZE=2 FACE="Arial">&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; {Original Message removed}

2000\03\27@193931 by Chris Eddy

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Let's assume you really must build this power supply.  I seem to have been way
off, in that these cars really take that large amount of energy.  I agree with
the others that one should add some form of battery at the output to enable a
high peak current draw.  The call then is simply to make a supply that supplies
most of the power draw, or a fraction, which would have to charge back up with
some delay.  Let us assume you want to supply 13VDC at 40 amps.  That ought to
handle the bulk of the use, barring the high acceleration peaks.  That is a 520W
supply, or thereabouts.  I have gleaned much knowledge from two sources.  One is
the Unitrode (now TI) databook and applications book.  They make discrete PWM
controllers that are very popular in PC switching supplies.  Next, Power
Integrations make a line of lovely off line switchers.  They are nice too.  They
tend to partner with Premier Magnetics (loose use of the term) to make off the
shelf magnetics parts available.  Solves things nicely sometimes.

BUT you need >520W.  No off the shelf magnetics will do that.  Further assume
you want to make this thing work off line, to avoid the use of a 5000 KW linear
supply on the front (changed my mind on that too).  Now we know that the
switcher will convert line voltage down to 13V or so.  The biggest hurdle to
advanced switcher design is the transformer.  Power Integrations actually offers
an Excel spreadsheet for entering parameters and getting design calculations
done for you.  Their databook says so, I assume it is on their web page.  This
could give you the magnetics design parameters.  Then you can find a Power TOPS
big enough, or go to a Unitrode part with an external FET.  There will be a
sizeable 400VDC electrolytic in the rectified line side, and the snubber on the
input side of the transformer will be an apreciable challenge. (leakage on a big
transformer will be relatively high, and so switching waste will be high).  The
output will need some sort of capacitor, but it probably is not critical since
you will have the battery.  The output diode will have to handle some seriious
power.  Remember, 50A ave could be 100A peak or more.  A TO220 part from GI may
be necessary.

The plus is that you can start with the standard feedback circuit for the
switcher in question, and add some circuitry to allow fast charge voltage and
current limits to be custom set to the battery in use.  That way, whe the supply
is not being belted with huge loads, it is properly charging the battery, not
boiling it off.

I would run this thing at 80 to 100KHz.  Anything higher would turn into a NASA
project.

And make certain that you achieve very effective line isolation in the layout
and opto feedback circuits.  It will not do your reputation any good if you fry
a whole troop of Cub Scouts.

Are we going in the right direction yet?  Do you still want to do this?
Chris Eddy

2000\03\28@105640 by Andrew Kunz

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>I would run this thing at 80 to 100KHz.  Anything higher would turn into a NASA
project.

Be prepared to use litzwire at this level of power.  Break out the checkbook!

Andy

2000\03\28@105705 by Michael Wieser

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Hi

I build something similar for an musicinstrument with 30 dc-motors and each
rated to 12V/1,25A which results in a lot of powerwhen "flashing" all
motors.....

I used a normal 15V/40A transformer with an dimmer in front (the basic can
be found on http://www.sts.com, AN 308, I replaced P1 in figure 11  with an
optocoupler)

The dimmer was controled from an opamp-circuit so that the outputvoltage is
held on 13.8V, which is enough to hold a lead acid-battery in the 95% full
state.

I also included a simple current masurement between transformer and battery
so I can see the actual load of the transformer, which is a good visible
feedback.

Also I added a softstart-circuit, which looks for 230V and if primary power
is connected, it starts the transformer with a low current to avoid high
inrushcurrents when connecting transformers with this power to 230V.

The battery I used was 12V 30AH, I took it from a car-mechanican, who gave
it to me without cost.....

When the peak current occurs, the battery supports the transformer and when
it comes back to normal currents, the transformer supports the battery. I
know that the high currents loading the battery are not the optimum for
this kind of battery, but this was not such a problem, because the battery
is a carbattery and it can never become uncharged, the system had to be
connected to 230V after the show at least 45 minutes.... (which was no
problem at all, after the show we had to answer questions, eat and drink
something.....)

This system worked for several years without problems (well, I had to learn
how to work with lead acid-batterys....)

It is very simple to adapt this for much higher loads. Maybe you have to
connect a loadresistance direct on the secondary side (15V) to hold the
"inductive part" of the transformer low (important when the battery is full
and no more current from the transformer required)

One of the great things is, that most parts are not very sensitive
electronic parts, even the dimmers triac, rated with 100A is a very robust
one, so usualy the fuse (F6,3A) blows earlier then the triac. And if 1 lead
acid battery isn`t enough, use a bigger one....


but it is heavy (30kg), and you need some kind of (slow) cooling, the
rectifier and the transformer get warm when running with 80% load.....


Michael Wieser
KILLspamm.k.wKILLspamspamnextra.at

Service and Audiodesign

2000\03\28@125727 by D. Schouten

picon face
> >I would run this thing at 80 to 100KHz.  Anything higher would turn
into a NASA
> project.
>
> Be prepared to use litzwire at this level of power.  Break out the
checkbook!


How about using a sandwiched secondary winding between two half
primary
windings. Just stacking the two windings on top of each other will
definately fry a small snubber circuit away due to the large leakage
inductance spikes.

I've designed several 3kW offline switchers using a two-switch forward
converter, running in series production for five years now without any
problems.
However if you have little or no experience in higher power switch
mode power supply design I wouldn't try to design/build a reliable
500W switching power supply. It's not only the transformer that's
tricky to build, also PCB layout, control loop design, snubber design,
active and passive component selection etc. all are very important
issues in this case.


Daniel...

2000\03\28@130355 by Chris Eddy

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I twas afraid this might come up (because I am unsure of the proper approach).
Controlling phase of the AC mains on the primary side of the transformer is
done, but I do not know the pitfalls.  It will probably cause horrible power
factor figures, and will generate a lot of RF.  Will the transformer have to be
better or higher bandwidth than a simple fillament transformer?  The up side is
that you can put SCR's on the primary side and just rectify the output with
relative ease.  And the frequencies are lower.  And if the phase control method
has poor speed respinse (suspect that is why they are not common in PSU's) who
cares?  The cars will not know the difference.

Chris Eddy

Michael Wieser wrote:

> I used a normal 15V/40A transformer with an dimmer in front (the basic can
> be found on http://www.sts.com, AN 308, I replaced P1 in figure 11  with an
> optocoupler)

2000\03\29@053304 by Michael Wieser

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Am/Um 12:04 28.03.00 -0500 schrieben Sie:
>I twas afraid this might come up (because I am unsure of the proper
approach).
>Controlling phase of the AC mains on the primary side of the transformer is
>done, but I do not know the pitfalls.  It will probably cause horrible power
>factor figures, and will generate a lot of RF.

Who cares about power factor when using a PC-powersupplys??? Of course you
should use some kind of filter in front of the dimmer, but dimming
inductive loads doesn`t make very high di/dt, and du/dt need to be "slow"
also to avoid unwanted triac-fireing (?)....
And when playing with high voltages cabeling should always be "safe", not
only for security but also for stability, so RF shouldn`t make much
headaches when this is dont correct.....

>Will the transformer have to be
>better or higher bandwidth than a simple fillament transformer?

I used this system with several transformers and differnt loads (also for
linear power-supplys with +/-50V 5A) for testing power-amps and I never had
big problems with RF......, no hum and buzz when testing.

For the system I described I used an transformer I found in a powersupply
from a very old computer, all voltages (+/-12V, +5V) were regulated with
uA723 and around 20pcs 2N3055...... I`m sure that nobody who did the
calculations for this transformer 25-30 years ago thought about this if it
could be used for dimming....

>The up side is
>that you can put SCR's on the primary side and just rectify the output with
>relative ease.  And the frequencies are lower.  And if the phase control
method
>has poor speed respinse (suspect that is why they are not common in PSU's)
who
{Quote hidden}

Michael Wieser
RemoveMEm.k.wTakeThisOuTspamnextra.at

Service and Audiodesign

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