# Different Power Tap Locations for Pos/Neg?



## Grandcheapskate (Jan 5, 2006)

I was researching braid and the supplier was recommending how to wire the track. He suggested that the positive and negative taps (one each) be at opposite ends of the track.

For example, on a 60 foot track, the power tap for the positive wire would be at the 0 foot mark and the negative tap would be at the 30 foot mark. This, in his recommendation, created an even distribution of power.

Everything I've read seems to indicate that you tap both the positive and negative wires at the same point. You may tap at more than one point, but when you do create a tap point, you connect both wires.

Assuming continuous braid does not require multiple power taps, if you do tap the positive and negative at opposite ends of the track, where is the point of most resistance and lowest power? In the 60 foot example, would it be at the 0 and 30 foot marks? And would the least resistance point be at 15 and 45 feet?

Thanks...Joe


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## Tycoarm (Jan 7, 2006)

If the connection is bad from either rail before each point it kinda defeats the purpose I would think. I would stick to taps at both rails at each point where a tap will be placed.


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## NTxSlotCars (May 27, 2008)

That sure is an interesting approach. I would like to try that out and see the difference, if any.
That method would seem to even out the resistance, not the power, around the track.

Rich


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## AfxToo (Aug 29, 2003)

The supplier is absolutely correct if you desire a better power distribution over the entire track. 

To verify this you need to consider the fact that each track rail is a continuous loop. This means that for each rail the total resistance between the tap and the car's pickup will be the equivalent of two resistances in parallel, where one resistance represents the shortest path from the tap to the car and the other one being the the longest path from the tap to the car. The total resistance of two resistances in parallel is:

Rt = (R1 x R2) / (R1 + R2)

To figure out the maximum resistance in the rail circuit you need to add the total resistance on the positive rail circuit to the total resistance on the negative rail circuit:

Rt = Rp + Rn

Consider a track made from 8 equal segments. Assume the rail for each segment has a resistance of 1 ohm. For the aligned taps case assume they are both at segment 8. For the split taps case assume one tap is at segment 8 and the other tap is at segment 4. (You can draw this out on paper if you would like).

1) For the aligned taps case, the total circuit resistance seen by the car at segments 1-8 will be as follows: (both taps at segment 8)

Segment - Resistance
S1 = 1.75
S2 = 3.00
S3 = 3.75
S4 = 4.00
S5 = 3.75
S6 = 3.00
S7 = 1.75
S8 = 0

2) Fot the split tap case, the total circuit resistance seen by the car at segments 1-8 will be as follows: (one tap at segment 8, the other at segment 4)

Segment - Resistance
S1 = 2.75
S2 = 3.00
S3 = 2.75
S4 = 2.00
S5 = 2.75
S6 = 3.00
S7 = 2.75
S8 = 2.00

There should be no surprises here. With the aligned taps you get a big surge at the tap location and then you drop off until the halfway point, at which point it picks up again as you get back around closer to the taps. The minimum and maximum resistance variation for this model with aligned taps is 4 ohms. 

For the split tap case you see that the resistance is much less variable around the track. It still varies as you move away from one side of the tap, but the proximity to the other side of the tap picks up the slack. You don't get the big surge at the common tap location either, which is nice. The minimum and maximum resistance variation for this model with split tap case is only 1 ohm. 

For even power delivery, the split tap configuration is definitely worth considering.


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## wm_brant (Nov 21, 2004)

The big commercial 1/24 scale tracks use multiple taps, but they usually don't put the taps side by side -- they are almost always offset. Sometimes the offset by just a few inches (resulting in less crowding of the wiring underneath), sometimes by several feet (the point that AfxToo made).

You want to minimize resistance because more resistance results in losses with both voltage and amps. With only one set of taps, halfway around the track your car is going to have several volts less power that it would at a tap -- just as if you were letting up on the controller trigger. However, the severity of this problem varies depending on how powerful your motor is -- polymod or RO cars will have more problems than T-Jets.

The BSCRA (British Slot Car Racing Association) has a good introduction on the need for additional power taps on their website. While their focus is on copper tape vs braid, the same issues hold true for railed track, with the major source of resistance being the joints between tracks. The article is well worth a read:

Wiring Resistance


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## Bill Hall (Jan 6, 2007)

*When in doubt...choose all of the above!*

Wow! All of that? Takes longer to do the calculations than to trial and error it a few times 'til ya get the results you need.

I just went every seven feet...cuz it divided out into my track length without an annoying remainder that would catapult me into an inescapable OCD vortex.

...er was it every seven sections...cuz it divided evenly into the total number of track sections that I had used?

Perhaps I just read it somewheres...

Then again it may have been that seven is a known lucky number and that's what I secretly wanted it to be; because on top of my OCD I am equally superstitious about such things.

Oh! Now I remember! It was a thread that conveniently ( or debatably) occurred just like this one when I was scabbin' my little track together. After much debate, and a virtual forest of "O-pines" were seeded; I read the answer in the entrails of the dead horse we had ritualistically vaproized. 

I chose to pinch it off short and get off the pot and go with seven. Just because that's about how high I can count without getting nervous about having to take my shoes off. It also divides evenly by three ( a holy and magic number) into the total number of digits I can muster when totally stripped down and stymied in the bitter end.

Regardless of yer method of calculation I submit that any additional power taps are better than none. Not really hard to determine where... the ideal location is where yer cars crap out ....duh...

More amps + more taps = more better!


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## Grandcheapskate (Jan 5, 2006)

AfxToo said:


> The supplier is absolutely correct if you desire a better power distribution over the entire track.
> 
> For even power delivery, the split tap configuration is definitely worth considering.


AFXToo,
Thank you for that explaination. It does behave kinda like I thought it would; the low power point (highest resistance) would be at the (multiple) point(s) furthest from the two taps.

Combining the split tap method with multiple taps would distribute the power more evenly. Just alternate tap points - positive then negative then positive, etc. I think this is the way I will rewire the track when I have to redo my power tap tracks.

Thanks...Joe


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## AfxToo (Aug 29, 2003)

If you have more than one tap you can calculate the resistance seen by the car for each rail using the generalized version of the parallel resistance formula:

Rt = 1 / ( (1 / R1) + (1 / R2) + (1 / R3) ... + (Rn) )

where 'n' is number of parallel resistances per rail. 

The parallel resistance formula also illustrates why multiple taps, jumpers, tiger tails (shunts), etc., are so effective. When you place resistances in parallel, the total resistance will *always* be less than the smallest resistance that's in parallel. Even adding a shunt that is _higher resistance_ than the resistance that's being shunted will result in a decrease in total resistance. 

Say you had a pickup shoe to brush barrel resistance of 10 ohms so you decide to add a shunt. Because you're a lousy solderer your shunt has a resistance of 25 ohms. Sounds bad, and it is, but in fact the total resistance is now 7.1 ohms! That's better than 10 ohms. Of course you would be far happier if your shunt was only 1 ohm, because that would take the total resistance down to 0.91 ohms. Adding the shunt cannot make matters worse - assuming you don't melt a hole in your chassis, but that's another matter altogether.

_Twenty years of schoolin' and they put me on the day shift._


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## Grandcheapskate (Jan 5, 2006)

In order to get the resistance of a given length of track, you have to put the probes of your voltmeter at the opposite ends of the rail, correct? And would you also have to disconnect that section of track from the rest of the layout? I'm assuming you couldn't just measure the resistance from point A to point B if you left that section of track connected to the rest of the layout.

Joe


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## AfxToo (Aug 29, 2003)

Correct.


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