# The voltage question...



## christos_s (Jan 16, 2008)

Switch-Variable Power Supply 

If you are slot-experienced and a little electro-mechanically literate please reply.
Why do people consider only expensive, over $100 variable power supplies?

There are plenty of switch regulated (ok not continuous-variable) power supplies around 24-40Watts in the 25-40 euros range = $37-60, lately especially with the proliferation of gadgets, laptops etc. and the need of flexiblility.

After all the original AuroraMM was only 24W (= 1.2amps at 20V) and the new BSRT also 24W (= 2amps at 12V)

The pictured power supply below is 36W (= 1.8amps at 20V or 3amps at 12V) and the switch allows operation in steps between 5-24V at 26.50euros=$39.25


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## AfxToo (Aug 29, 2003)

Linear, lab-grade variable output DC power supplies that can put out 3-10+ amps constant current at voltages above the standard car battery voltage (12 VDC nominal) tend to be more expensive. A power supply like the popular MG PS-10 sold by Alan Galinko can source 20VDC and 10A, or 200 watts. Most racers prefer power supplies with a higher current capacity than what stock wall warts produce. 

When you look at ratings on power supplies you need to look at the VA rating. That tells you exactly how much power they can source. You can't typically multiply the max voltage by the max current because they can never put out more than what the VA rating dictates. The Tomy wall wart has a VA rating of 7 VA, or 7 watts. At 22V you're not going to get more than about 300 mA. Take a look at the VA rating on your old MM transformer. It's probably much less than 24 VA.

As far as pricing goes, we are often at the mercy of whatever the primary market and use of a product, material, or component that we also happen to use for slot cars. There are few, if any, major power supply vendors who build power supplies exclusively for slot car racers. Power supplies are built for some other reason and we happen to use them. When the other reason is a high volume market, say 12VDC battery replacers, battery chargers, or power supplies for a high volume commercial product we benefit and see lower prices. Economy of scale. When the situation is the other way around we usually have to pay more. The tradition of running HO slot cars at 18V-20V costs us more when we use something other than the power supplies that come with the race sets.


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## christos_s (Jan 16, 2008)

No the old 1970 Aurora-MM power supply, the grey metal one, clearly lists 24VA.

Yes I know how to devide VA or Watts by voltage to get the current Amperes. My question was a practical one.

Are 36 VA or Watts enough you think? for a 2-lane running stock cars?


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## AfxToo (Aug 29, 2003)

1.8 amps at 20V is plenty enough for running two lanes. This will work well for anything up to and including superstock magnet cars. 

Those old MM transformers must have pre-dated the toy safety regulations that have kept the power ratings of set power supplies so low. Unfortunately, some of those old units were only half wave rectified.

The output current from unregulated power supplies, like many wall warts, is not necessarily the VA rating divided by the rated voltage. Unregulated supplies often suffer from a significant sag in output voltage when current demand gets close to the maximum rated current.


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## vaBcHRog (Feb 19, 2003)

christos_s said:


> No the old 1970 Aurora-MM power supply, the grey metal one, clearly lists 24VA.
> 
> Yes I know how to devide VA or Watts by voltage to get the current Amperes. My question was a practical one.
> 
> Are 36 VA or Watts enough you think? for a 2-lane running stock cars?



Should work just fine. 

Roger Corrie


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## Grandcheapskate (Jan 5, 2006)

christos_s said:


> No the old 1970 Aurora-MM power supply, the grey metal one, clearly lists 24VA.


 All my old Aurora MM power packs, of different colors, show only 12va.

Joe


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