# Voltage drop under load



## Fozzie1952 (Jan 1, 2019)

I'm comparing two wall wart power packs, both rated at 1 Amp. The goal is to slow down Tyco/Lifelike slot cars for my young grandkids. When hooked up to a multimeter, one measures 12VDC with no load. The other measures 9.11 VDC with no load. To test each under load, I ran a car around the track, and put the mm probes on the track rails. Not sure if this is the right way to do it. The output of the 12V pack measured 8V, but the 9V pack measured 8.5V. The car ran faster with the "9V" pack. Can anyone explain this to me?


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## Rich Dumas (Sep 3, 2008)

I presume that you read my article on power supplies. Even a very large unregulated power supply will sag when a load is applied. Years ago my track was powered by one that I built myself, the transformer was rated for 12 amps, but it sagged by 2-3 volts with a half amp load. If you want to test power supplies you can load them with a car. You could remove the rear axle or just put something under the back of the car to get the rear wheels off of the track. I like to use a dummy load, big wire wound resistors are the best thing, but a lamp with a 150 watt incandescent floodlight bulb will do the trick. At 10 volts one of those pulls about 0.35 amps, at 12 volts it draws 0.38 amps. That draw would be about the average for a regular inline car on the track. I just checked a Life-Like car with the rear axle out at 10 volts and that drew 0.21 amps. 
You can expect different set type power supplies with similar nameplate ratings to act differently under a load. The 9 volt pack actually had a little more current capacity. Often a power supply will have a watt or VA rating, the ones with a higher watt or VA rating will have more power.


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## alpink (Aug 22, 2010)

Rich gives a very detailed and complete explanation.
my take away from that and your Q is, the 9V pack has more available amperage under load.
as Rich also says, you can find this value on nearly all power supplies listed as WATT or VA (volt amps).
you might want to hunt around a thrift store for laptop power supplies/chargers.
likely you will find some that fit your requirement well and spend a couple dollars.


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## Milton Fox Racing (May 27, 2014)

If you cannot find the printed information on your power supplies; you can depending on your multi meter settings, measure the actual VA output for each. :cheers2:


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## Fozzie1952 (Jan 1, 2019)

Thanks everyone for your responses. I just ordered an inexpensive variable power pack that delivers 12, 9, 7.5, 6, etc. volts. It's rated at 1.2A max. I expect that one of the settings should give me what I'm looking for.


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## Bubba 123 (Sep 10, 2010)

Fozzie1952 said:


> Thanks everyone for your responses. I just ordered an inexpensive variable power pack that delivers 12, 9, 7.5, 6, etc. volts. It's rated at 1.2A max. I expect that one of the settings should give me what I'm looking for.


Ok,

"Richard" & "Al', are verifiable in "Rocket-Science" of Electronics/Slot Cars...
That said (IE: They Actually "KNOW" What They are Talk'n about's-is :wink2

That; " 1.2 Amp Maximum Out-Put"..
Has "Me" Grit'n Me Teeth.. 
"2+ Amps (per lane) Rating.. Would be the "Holy-Grail", to most.. :thumbsup:

However, the cars being; "Tyco/Mattel", (Which were designed to Run on a
DC. Battery-Box ( Several (4??) "C" or "D" size Canister types..)

"Should" (The "Electric-Transformer" You bought..) 
do What You are looking for.. "Slower-Cars".. 

(On 22 Volts, 2-ish over-all Amps DC. You can Barely even "SEE" the cars.. @ Literally-Ludacris Mach-Scale Speeds !!) :freak:

Keep Us Posted on 'Yer Results :thumbsup: 

Bubba (The Senile-1) 123 :willy_nilly:


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## Rich Dumas (Sep 3, 2008)

My club seldom races cars that run at light speed, I guess that we are getting old and both our reflexes and eyesight are not what they were. I have measured how much power the cars use on the track, so I know that at least. If there is less power available than that the cars will not run at their full potential, however that might not be important if you do not hold formal races on your track. If the voltage drops when you punch your controller that actually acts like a sort of traction control and will make the cars easier to drive.


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## Fozzie1952 (Jan 1, 2019)

A fast as Amps go, I've been using AFX Tri-Power packs for several months. They are rated at 1 Amp. I'm happy with them at the Beginner setting (8VDC under load), but some Tyco 440X2 and Lifelike T chassis cars still go too fast to stay on the track at full throttle (the only way my 3 and 5 year old grandkids drive LOL).


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## Fozzie1952 (Jan 1, 2019)

Another possible reason that 1 amp is sufficient for me is that the tracks I build are fairly small. I make them portable, to transport to my grandkids and great nephews. I've designed and built several that fit on a 30"x72" folding table. Other track boards are 60"X30", 57"x30", and 48"X39". It's fun and challenging to build small tracks that are still interesting.


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## Rich Dumas (Sep 3, 2008)

A long track does not need more power than a short track, it is the cars that use power. As I mentioned earlier you voltage will drop a lot if the power supply is not regulated or is small. Since a car uses the most power when it starts up or is still accelerating that is when the voltage will be lower. In that case it will be easier to drive the car. If the power supply had a constant output the cars would have a little more snap and would be more difficult to drive.


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## Fozzie1952 (Jan 1, 2019)

I've used my new inexpensive multi-setting power pack, and it seems to do what I need. My faster cars (Lifelike T chassis) are more manageable at the 7.5VDC setting, and other cars (Tyco 440X2) can be run at the 9VDC setting, with the cars staying on the track for the most part. Since I'm only using one power pack for both lanes, we need to run two cars of similar type. Buy this power pack has solved my problem, it seems. When my grandkids master controlling the speed of the cars, we'll race at higher voltages. Thanks for all your responses and information.


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