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What are OHMs?

3090 Views 27 Replies 16 Participants Last post by  Phil Kalbfell
With reference to Top Banana's post could somebody slow it down a little and explain what ohm really is I never thought it could affect the handleing of a car, and what diffrence would it make say if I am useing a 45 ohm and the guy next to me is useing 30 ohm in what way would i see a diffrence.

Just now I use two type of controllers the stock Ninco and the Ninco Vario 16 Sure I can see the diffrence in the two but would that no be because of all the diffrent settings that are available on the Vario 16. Now when you set the vario 16 are you changeing the ohm in any way.

Go easy on me guy's this may be a no brainer subject but it's not my kind of thing.

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Ohms is the measurement for resistance values in electrical circuits & engineering

using ohms law you can work out the resistance value (measured in ohms) as long as you know the voltage & current of a circuit

also if you know the resistance value and voltage you could find out the current, or resistance value and current would give voltage

so : resistance = voltage value divided by current value

for example if you have 12v and 500mA flowing through a component then its resistance is 12 divided by .500 = 24ohms

or, if you had 24ohms with 12v through it you get the current
12 divided by 24 = 0.500A (500mA)

in a controller application the resistance value will affect how sensitive the controller is depending on the current drawn by the car. On a strong magnet type car a lower resistance value will give control over a wider range of trigger stroke

hope thats not been to technical

regards TB
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O.K. - here goes "Electricity and Magnetism 101"

Ohm is a unit of resistance named for Georg Ohm, a german physicist who promulgated "Ohm's Law" which connects the variables of voltage, current and resistance in an electrical circuit. It is conventionally written as:

E = I/R where E = voltage, I = current and R = resistance

A useful analogy is a garden hose where the water pressure is analogous to voltage, the current is compared to the volume of water flowing at a given pressure and resistance is the resistance that the hose/nozzle combination offers to the flow. (e.g. - tighten down the nozzle andf not only will you go from a stream to a spray, but the volume of water flowing through will diminish)

Now, why is this important for slot car controllers? It all has to do with the degree of control you want to achieve. With few exceptions, any controller will modulate the speed of a slot car from dead stop to flat out. The issue is how much of the physical range of the controller, i.e. the arc of the trigger, is available to make those adjustments?

Now why is this the case? We must first look at the motor: The current drawn by an electric motor is a function of the voltage applied. Let's simplify this, to be most useful for our purposes, and say that it is proportional to the voltage (although this is not really the case, it doesn't affect the explanation) Thus a motor might draw 0.05 amps at 6 volts and 0.1 amps at 12 volts - again, these are higher than one would predict for a typical can but it doesn't matter. If you load the motor by making it drive a car, the current drawn will increase and if you add to the load by putting magnets on the car, there will be a further increase.

Now it starts to get a little more complicated: In order for current to flow in an electrical circuit, the circuit must be "complete" i.e. there must be a connection from one side of the power supply through all of the elements, in this case the controller, the track and the car and back through the track to the other side of the power supply. Now, the voltage drop across this circuit must equal the voltage of the supply - let's say 12 volts and, further, the voltage drop across each individual element of the circuit will be proportional to the resistance of that element. The resistance of motors is low (O.K., I'm keeping back EMF and all the other consdierations out of this) - say about 0.1 ohm and, if we have a good track and good connections, that resistance will be low, too - perhaps 1 ohm. Now we insert a controller - and that represents the third element. If we want the car to barely move when we move the controller just off the "stop" or "brake" position - allowing perhaps 3 volts to appear at the motor, the resistance of the controller at this first position + the resistance of the track must be sufficient so that there is a 9 volt drop across these two elements - Well, how much resistance do we need? That depends - on the current draw of the motor at 3 volts! Lets assume that the motor will draw 0.05 amps @ 3 volts, then. by Ohm's law, the resistance of the track + controller must be R = E/I or 9 (the desired voltage drop) / 0.05 or 180 ohms. Now, from practical experience this is too high (but if you were bench testing a bare motor it would be about right) and, given the physical load on the motor and the need to generate the needed "break-away" torque to get a stationary object moving, you need more like 4 volts and the current will be closer to 0.1 amp so you need about 80 ohms. Take a Ninco Classic with an NC-1 motor and no magnet on 12 volts with a 60 ohm controller and you will find that it just starts to move on the first band. If you try to use a 30 ohm controller, you will immediately apply about 8 volts to the motor as soon as you move from stop and it will take off at nearly full speed!

Now lets look at the other side - a heavy car with magnets and an uprated motor that may draw as much as 0.3 amps at start-up and really needs 5 volts to get moving. It won't start to move until the controller resistance is down to about 22 ohms. You can. of course, get to that with a 60 ohm controller but you will have used up about 2/3 of your trigger travel before the car starts to move and you have only 1/3 left to control the speed.

There are a lot of other things going on that make this explanation only an approximation but the principle holds. This is why people use different controllers for different cars - but wait - there is another way - diode and electronic controllers operate on a different principle and are not bound by Ohm's law. To a reasonable level of approximation, the voltage drop across these devices is independent of the current in the circuit so you sort of a "one size fits all" situation - again, a simplification but generally correct.

Hope this helps

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Those were excellent but highly technical explanations.

Please, don't anyone take this to heart, but I am going to have a shot at a simpler man's guide. I am going to leave out all references to volts and amps and also refer as little as possible to ohms - purists may shudder suitably now and then stamp off in high dudgeon!

The motor and the car
Obvious starting point - motors need electrical power.
In theory, you could run a car under the control of just a simple press on/release off switch and this is how the earliest slot cars WERE operated. But, in practice, this is too violent an extreme between full power and no power, so we really need variable power to provide variable speed on different parts of the track.

The variable speed is provided by a controller.
To keep this simple, we will leave out electronic controllers and brake circuits.

Essentially, the controller works by passing the full electrical current from the power supply through a tightly wound coil of high resistance wire called a resistor, which dissipates some of the excess power as heat.
A contact on the end of the trigger or plunger slides along this resistor coil and the amount of useful current that actually reaches the car depends on just where the contact is along the length of the coil. The more of the resistor wire that the current must traverse, the less of it can reach the car on the track.

When at maximum, no power passes through the resistor at all - it all goes straight to the car and, in this situation, it makes NO difference what the resistance of the controller is as no current goes through the resistor. The car will get ALL the power, completely regardless of the resistor rating.

As soon as the trigger is backed off a little from maximum, the current is diverted through just a few coils of the resistor, loses a little of its power and the car slows down

As the trigger or plunger is moved further and further back from the maximum position, the power is diverted through more and more coils of the resistor and so less is available to run the car, so the car slows down more. Eventually, the contact reaches the other end of the resistor where almost all of the power is dissipated as heat through the full length of the resistor coils and almost nothing reaches the car. A fraction past that, and the contact on the trigger/plunger reaches a point where all power is cut off from both car and resistor.
Hopefully, that was fairly easy to follow.

Now, let's look at the slightly trickier bit - the maximum amount of resistance provided by that resistor - the point that was asked to be explained

It is perhaps not entirely obvious that cars need a bit of power just to overcome inertia and to start moving at all. Well, obvious or not, they do!

After accepting that, I think it's probably easy to accept that a HEAVY car will need more power to get moving than a light one would and also that one with heavy magnetic assistance will act similarly, because of the drag of the magnet. The same applies to a car with high friction or low numeric ratio gears. All need more power just to get them moving.

A nice, average, 'normal' car might need, say, 10% of the total power just to get it moving, but a high drag car could easily need 50% of the power to do the same thing and an exceptionally high consumption motor might need as much as 90% of the available power to get off its ass. This translates to 10%, 50% and 90% of the controller travel, just to start the car moving!

Let's take the 50% case.
If it takes 50% of the controller travel to move the car, then that only leaves 50% of the travel to control its speed between a dead crawl and max speed. This is heading back towards that on/off, all or nothing, switch scenario - not very satisfactory. The answer is to use a lower resistance coil for these cars. The principle here is that, if the resistor is changed for one with half the resistance, then the trigger will not need to be pulled as far in order to let the same amount of current through as before. Therefore the user regains much of the 'lost' travel for finer speed control.

In the 90% case, an even lower resistance would be needed to regain a good amount of fine control.

So, these low resistances are good for high consumption cars, but the opposite holds true for low consumption cars. If you use a low resistance controller on a low consumption car, even a small throttle setting will permit too much current for this car and it will hit close to maximum speed on tiny throttle settings. It will be just as undrivable as the high consumption car using a high resistance coil. The difference is only a matter of which end of the controller setting becomes uncontrollable.

Now to mention the ohms word just lightly!
Ohm is a standard unit of electrical resistance.
A high resistance controller suitable for low consumption motors is rated at around 60-70 ohms.
A medium resistance is rated at around 25-45 ohms.
A low resistance for only the very highest powered cars could be anywhere between 5 and 0.5 ohms.
At the risk of being shot at, that third category is far too low for the average home slot car and I would advise that light or no mag cars will generally be happy on 60/70 ohms and the high mag home cars will be happy on 30 ohms, but your mileage may differ a little, as they say.
People may argue about exact figures to their heart's content, but this is just intended as a simple guide to give you a general feel for what it's all about.
I do hope it helps, or I have just wasted a couple of hours of my time!
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Tropi -

thats an excellent way of explaining the way that different resistance value controllers will affect how a car behaves

nice to have a technical subject looked at from a different angle

regards - TB
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very nice explanation. I got lost in the other explanations even with my O level in Physics.

Wish you'd done that prior to my article on controllers on GS. I would have completley plagiarised it (I believe plagiarism is the highest form of compliment - I just wish my exam markers felt the same way!)

Whwew! Pleased it came across all right - the damn thing took ages!

Anyone is welcome to nick it - we all got our knowledge from someone else originally, though I honestly forget who it was that penetrated my mental miasma and made it make sense to ME in the first place!
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Thank you all I was getting a bit worried there I thought I was going to have to go down to the libary and pull a couple of Physics books but Tropi
stopped that, great explanation, thanks to all of you for making the effort to reply to me.

Another question, a stock Ninco controller what ohm would it have .

Thanks again Guys

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A standard Ninco controller is rated at 70ohm, they have also released a 55ohm controller, both controllers will also work ok plugged into a Scalextric Sport powerbase.
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Excellent Reply Tropi !

Stick in an Article format and I will add it to resources. thank you!

Now, that' the mystery of Ohm Controllers sorted but I have something called an Electronic Controller - so.......


who said life was easy!
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I Always enjoy tropis lectures,Even the one on why track should not be black because real pavement has many different shades of black, gray etc..

a very good explanation in easy to understand terms.
QUOTE Even the one on why track should not be black because . . .
I guess I'd better leave it a couple of years before repeating that one then!
Trouble is, I will probably have forgotten why by then . . . now what was my name . . . ?
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Now, that' the mystery of Ohm Controllers sorted but I have something called an Electronic Controller - so.......


Let's see if I can demystify it without using any formulae or forbidden words:

We now all understand how the resistor in a standard controller works to reduce the voltage - but a problem remains: The resistor doesn't reduce the voltage by the same amount for all cars The more power a car draws, the more the voltage will drop across the resistor which, of course, is why we use different controllers for different cars.

Now suppose we had a resistor that dropped (absorbed) the same amount of voltage no matter how much the car drew - wouldn't that be nice? Well, we do, it's called a diode. This little device will drop the voltage by 0.7 volts from one side to the other. Now, we can't wind them in a coil like high resistance wire but we can line them up, end to end and put a little button between each one and then have the wiper of our controller go back and forth across the buttons - going, as we move the trigger form full on to as high a voltage drop as we want in steps of 0.7 volts. That's the Professor Motor controller.

Then there are the "fully electronic" or transistor controllers A transistor is simply a device for controlling the flow of elecrticity. It is the modern substitute for what we in the U.S. call a vacuum tube but in the U.K. is more descriptively termed a "valve"

The principle is simple: it has three connections - two of them are "inline" with the power going from the supply to the track and the third is where we apply a small electrical signal that controls that power - think of a faucet with a supply coming in, water going out (the inline connections) and a handle (the third "signal" connection)

Add some more electronic bits and you can adjust the trigger response to whatever you like - The Ruddock and DiFalco are examples and I think Ninco made (makes?) such a device.

O.K. - I only used "volts" twice, no amps or ohms and no equations.

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A little dredging and a bump back into view for our newer members.
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Crudely speaking, the lower your ohms, then the more control you have and the greater ability to enhance high-powered motors and strong magnets.

The higher your ohms, then racing will be sluggish, stuttery, and unresponsive.

If you control your cars with the top end of your controller (ie. nothing happens for the first half inch of pull) then you need lower ohms.
Here's a another little essay on controller resistance, one I posted elsewhere a while back. There is some repetition of material, and it leaves out a lot of EM's excellent technical thoroughness, but perhaps it will help someone.

"Different controller resistances are used because the hotter the motor, the more voltage is required to start the motor turning. For example (and these numbers are just for illustration), a stock 16D might start rotating at 0.3 volt while an X12 motor might need 2 volts before it will start to spin.

The rated resistance of a controller is what the wiper button "sees" when it is on the first band, as it moves from the full-off brake band position. As the wiper moves farther away from the brake band, the controller's resistance drops lower and lower, until the resistance drops to (essentially) zero when the wiper is touching the full-power band.

If the controller resistance is higher than it should be for the motor being controlled, the wiper must move farther from the brake band before the motor begins to turn. In this situation the controller will function (i.e. cause the car to move) on some smaller percentage of its range of movement, say, only on the top 1/2 or top 2/3 of its movement. It should be obvious that this is not the ideal situation.

If the controller resistance is lower than it should be for the motor being used, as soon as the wiper touches the first band, the car will take off at relatively high speed. In other words, it will not be possible to operate the car at slower speeds. This "on-off switch" effect does not make the car any easier to drive.

To match the controller resistance to the motor, select a resistance that results in the car moving slowly when the controller wiper touches the first band on the resistor. This will give the maximum range of control and will result in a car that in almost every case is easier to drive."
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There are at least four different design controllers in production today. Some are very hi tech and some are only medium tech (as in user repairable with high school electronics info).

RESISTOR : uses a wiper arm directly on a variable resistor. The Ohm VALUE of the resistor may be changed or tuned to motor and track conditions. Commercial track motors range from 2 - 7 ohm value. 1/32 homeset range from 7 - 60 ohm. HO tracks range from 25 - 100 ohm. Examples: most oem sets, Parma sebring/turbo...easy to fix and modify.

DIODE : uses diodes in series with the wiper arm moving from all to fewer to control the VOLTAGE directly to the car. Each diode has less than one volt per diode. The more diodes in series, the more volts can range the response. Examples: Professor Motor, Omni, and others. repairable easily.

TRANSISTOR : Uses a big transistor to handle the amps that pass thru the controller with a related resistor network to vary the response curve (TUNABLE). This is the most reliable high end technology in use today. Examples: DiFalco, Ruddock, Third-Eye, Jay-Gee, Ninco Vario, Carrera, etc. repairable and tunable with some skills and experience.

PUlSED BAND WIDTH (PBW) : Uses high tech to do a simple task based on the latest RC car technology. NOT user friendly. Examples: Parma EC and some others.

MAGIC: Uses thought waves to control the car ...
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An excellent overview, Larry.

Regardless of the controller design, matching or adjusting the controller to the car/motor so that the car moves slowly as soon as the trigger is moved from the "off" position is the method to employ in the majority of cases.

BTW, where can I get one of those "Magic" controllers? :cool:
I don't think it has been mentioned that the resistance or ohms (Ω) changes with temperature.

Resistor temperature dependance

Perhaps someone knowledgable in slot car armature resistance values could put some numbers in the calculator and let us know what the difference is between 20deg C and say 90deg C (how hot do slot car motors get - RC cars range betwen 60 C and 120 C for a reasonably geared car)

The hotter the copper gets the higher the resistance - this reduces the current and therefore power generated in the motor. Not sure what effect this has on an old skool wire wound controller - anyone?
Arthur, you're very right that the resistance increases with temperature. But unless the temperature rise is abnormal, the effect is small and things usually balance out.

With a resistive controller, the resistance rise in the hotter armature requires a higher ohm controller which is what you have as the controller resistor also heats up.

Electronic controllers operate without much regard to the resistance of the armature, certainly not the amount of resistance change that we are talking about here.

Generally, by the time an armature or controller resistor heats to the point where the resistance change is truly dramatic, it is far too late to worry about it.
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