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Discussion Starter · #1 ·
To the scientist...an all

When the slot car does dynamic breaking in what direction does the power attempt to travel? Does the power go the same direction as the current which came from the power supply OR does the power attempt to go the reverse direction? I would guess the reverse direction since there are brake light system which only light based upon the direction power travels through a diode?



Thanks VERY much!!

-Maltese
 

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Ooh, I'd love a decent explanation of what happens when the anchors go out. Decent as in, in simple terms a simpleton might comprehend.


I mean, three wires to the track? What's that all about then? Eenie and meanie I understand, I think, but miney?


Sorry if this is taking your thread to a more basic level, Maltese. I'm just the forum spokesperson for dullards.
 

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DT
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When you release the trigger of the controller (with brakes), instead of a reverse current being fed into the track (Like Graham's setup), or nothing happening at all (no brakes), the track is basically shorted and thus a load (resistance) is applied to the motor of the car that turns it into a dynamo or generator. This is known as EMF, or electromotive force. This in turn slows down the car. Highter torque motors brake better as they make better dynamos.

If you add a potentiometer to the brake line, you can alter the amount of load (or resistance) applied and thereby control the amount of braking.
 

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Mark Wain
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I dont want to sound really dumb here (but i probably will!!)
i thought a potentiometer had 3 connections? do you just wire it in series the same as a variable resistor?
 

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DT
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A potentiometer has 2 connections. Remember it's a variable resistor - between two points.

Those things with 3 connections allow you to select the polarity of the resistance - 0 to full on way and full to 0 the other. Does that sound clear or is it more
 

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Mark Wain
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Oh ok.
Nuro, i dont suppose you know where i can get a wiring diagram to make a box with variable resistance control from on the net?
 

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DT
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Not me, but there are quite a few racers here that use such devices.

Anybody got any plans to build a "black box"?
 

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My understanding is that the potentiometers have 4 possible connections.

1 - one end of resistor, connected to positive input voltage (+12V say)

2 - the wiper, connected to the power rail of the track slot (in a no-brakes track, these are the only ones connected)

3 - the other end of the resistor - 45 ohms (or whatever) away from +12 volts. If you pull the trigger a little, you reach this point, and if you hold the trigger here a long time, the controller will heat up at its fastest rate, since the max voltage is being converted to heat)

4 - the dead stop. It is needed so that the wiper is not resting on point 3 all the time! This is the brake connection. I guess it is connected to the other rail because as Nuro says, the result is to connect the two ends of the motor together so its back-emf will act as a power brake.

I think I have something wrong here, although a lot is right. The bit I cant figure is that the non power rail ought to be earth, but if so, in the braking position, the eartth would be connected to both ends of the motor and not allow back emf to brake. O well... nearly
 

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Discussion Starter · #10 ·
I sure appreciate the responses to the question. I suppose it is correct that power has no direction, but current does. So the question would be "In what direction would the current want to flow in the dynamic braking situation?"

The folks who talked about hooking together a light that only gets brighter during the braking instance said that one could put a diode on the motor with an LED behind it so that when current (though small it may be) flows from the motor in its "dynamo" mode the LED would light up making brake lites glow bright.

I am trying to figure out which side of the motor would the current attempt to flow out of if it were an actual dynamo and had to power a light bulb (like the kind of dynamo on a bicycle). Would the output side be the side where the motor normally receives power from the rails or the side that usually goes to the ground of the power source. Every dynamo/generator has an output side. I am just wondering which side.

-Maltese
 

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Hallo.

Yes, while breaking, the current is flowing in the revers direction.

This current could be used for breaklights, but:
The voltage belitlet with the rotation speed (and then also the current goes down). I think a breaklight led will only flash on for a short time and than its off.

Every load between the motor connections will make the breaking action smaller, because through a higher load flows a smaller current = smaller breaking action (same effect like a potentiometer in the break wire - a adjustable break current).

Tschau, Roland.
 

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QUOTE (astro @ 21 Jun 2004, 23:32)... If you pull the trigger a little, you reach this point, and if you hold the trigger here a long time, the controller will heat up at its fastest rate, since the max voltage is being converted to heat)...
...not exactly, Astro.

First, voltage does not become heat. Only electric power becomes heat (another way of power). Starting from this point, maximum heat is dissipated in the controller when voltage and current reach maximum level. Ok, now let's suppose that our controller starts at the value of Ohms necessary to the motor just starts spinning and, logarithmically decreases its value up to zero as long as we press the trigger.
-Slower value means all the voltage in the controller but almost no current, so the power is almost zero.
-Fastesr position menas all the possible current but no voltage at all, so again the power is zero.
¿Where's the maximum dissipation? just in the middle position.

Coming back to reality, our controller does not control motor at starting spinning, but at a higher level (i.e., if we need 1,5v to start the motor, controller minimum position is 2,5v); and since we have a linear resistive controller(*), maximum power dissipation is around half the controller position.

Apologies for my english and this biblical text and for Astro, no aim to controversy.

(*) That's the answer to why you feel "very smooth" electronic controllers: they control voltage or current, resistive controllers are affected by load (motor).
 

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QUOTE (backMARKer @ 21 Jun 2004, 23:03)Oh ok.
Nuro, i dont suppose you know where i can get a wiring diagram to make a box with variable resistance control from on the net?

backMARKer,

I don't have one at hand, but I'll search it tonight. Anyway it's very easy: just cut the brake wire and conect a 10 Ohm 2 Watt potentiometer in between. That's all.

Well, job is quite easy. Problem is the pot itself, it is hard to find, if you want it let me know.
 

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Mark Wain
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Cheers guys, I get the brake works now (well I had a guess anyway) what was throwing me was the terminology between variable resistor and potentiometer.But if you take the word litteraly it just means potential meter, should have got that myself really!


Now im just trying to figure out how the variable ohms for the controller works in the black box! but i guess that may take longer to explain!

anyway sorry maltese for taking over this thread a bit!
 

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Graham Windle
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Theres a diagram on my web site , a lot of the preston guys have used this it works well.
For Scalex I have now advised the use of 50 ohm x3 watt cermet pots costing about £4 each available through Farnell components in the UK instead of the heavy duty ones on the diag as the original was designed for use with more powerfull motors .
 

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Al Schwartz
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QUOTE A potentiometer has 2 connections. Remember it's a variable resistor - between two points.

Those things with 3 connections allow you to select the polarity of the resistance - 0 to full on way and full to 0 the other. Does that sound clear or is it more

In general, the variable resistors with two connections are called rheostats - they function simply as a variable resistor - one connection goes to one end of the resistance, the other to the wiper and thus on can very a resistance between zero and the full value of the resistor - this is typically what would be used in a variable braking circuit. In use they will be installed in series with the (+) side of the circuit

A potentiometer has three connections - one at each end of the resistance winding and one to the wiper. These devices are typically used as voltage dividers - they are connected across the circuit from the (+) to the (-) sides and then the wiper connection can be used to "pick off" any voltage between zero - when it is all the way to the (-) side to the full voltage of the circuit when it is at the opposite end - (volume controls in audio circuits are a typical example)

variable resistor circuits

offers a simple explanation and diagrams.

EM
 

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On my website you can find a article about the topic: "What´s happen when You build a second resistor parallel to the controller".

It´s in german, but it shows how the restistor rates are shared at several trigger positions.

Also, you can download a resistor-calculator that calculates the complete resistor of a parallel circuit. And it shows you the resistor-curve against the trigger position!(its a windows excel file). It´s a little toy.
But what you will see is, that it is not suggestive, to make he parallel restistor smaller then the Controller!

You can find it under:
Projekte --> Handreglerwiderstand

If you have questions about it...please ask me.

Tschau, Roland.
 
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