# Nice little power supply on sale



## Pomfish (Oct 25, 2003)

Ran across this today;

http://www.omnitronelectronics.net/...Supply-Variable-0-15-Volts-at-2-Amp-1095.html

Nice for a small track with kids or a nice break in supply.
Thanks,
Keith


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## wheelszk (Jul 8, 2006)

That's a good price.


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## ParkRNDL (Mar 20, 2002)

ok, you got me looking at the other stuff on that site and thinking...

how much amperage does a Tjet draw? or a stock magnet car? many of the supplies there are rated at 2, 3, or 5 amps. for a 4-lane track that will never see anything hotter than a stock 440, LL, or G-Plus, would a 3-amp be enough? or would I need a 5-amp to cover myself? 

i remember reading that magnet cars draw the most on startup, and i THOUGHT i remember they can draw up to an amp at that point, but I don't remember if that was a stock magnet car or like a Patriot or Modified...

--rick


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## fastlap (Nov 11, 2007)

*for me?*

I run mostly inline magnet cars up to super stock range. I'm thinking these would be more than enough for my 117' Bowman.

http://www.omnitronelectronics.net/...er-Supply-Digital-0-to-30VDC0-20AMP-1367.html

or;

http://www.omnitronelectronics.net/...er-Supply-Digital-0-to-30VDC0-20AMP-1367.html


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## martybauer31 (Jan 27, 2004)

ParkRNDL said:


> ok, you got me looking at the other stuff on that site and thinking...
> 
> how much amperage does a Tjet draw? or a stock magnet car? many of the supplies there are rated at 2, 3, or 5 amps. for a 4-lane track that will never see anything hotter than a stock 440, LL, or G-Plus, would a 3-amp be enough? or would I need a 5-amp to cover myself?
> 
> ...


You definitely want to get as much as you can afford to... I have a 0-20V 5 Amp PS and it can't handle 4 G3 Super Stock starting at the same time. Once they are all running it's great and probably sits at about 3 Amps. I'm looking at finding myself something with 10 Amps to cover what I would use on my track.

If you aren't doing anything hotter than LL or G+ it will be close at 5 Amps, but you might be ok.


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## Dragula (Jun 27, 2003)

Just throwing this out there..if anyone is looking to race outlaw drag t-jets,you should get a supply for each lane,these cars are amp suckers. Outlaw t-jets= 0.2 to 0.09 ohm arm.M2C
Christian


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## demether (Aug 26, 2008)

I don't use anything fancy as modified slot cars, but I saw that my tjets (JL/AW) were more comfortable with my new powersupply (14volt, 8amp for 4lanes) than with my old tyco wall packs.

I don't know if it's real or not, but since I've got more amps now, the cars seems to be more reactive and the handling seems diferent.


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## AfxToo (Aug 29, 2003)

> i remember reading that magnet cars draw the most on startup, and i THOUGHT i remember they can draw up to an amp at that point, but I don't remember if that was a stock magnet car or like a Patriot or Modified...


At startup, the maximum draw for any car is the voltage divided by the total resistance in the circuit. The total resistance in the circuit includes the static resistance of the armature, brush resistance, all contact resistances (shoes, hangers, brush tubes, etc.), resistance of the wiring to the car, resistance of the rails from the tap points to where the car is on the track, and of course any residual resistance in the controller, including wiper and full throttle contact resistance. If you ignore everything except the static resistance of the armature windings it would be voltage divided by armature resistance. For a typical 6 ohm armature with an 18V power supply, the instantaneous current draw while the motor is not turning would be:

Current = 18/6 = 3 amps

With 4 lanes in perfect synchronization due to a power relay starting the race with everyone at max throttle the instantaneous current demand would be around 12 amps. This peak demand would last a few milliseconds and once the motors are turning the demand drops significantly. If the power supply cannot produce the peak demand, in this case 12 amps, it produces what it can and the cars get less current than they demand, with the amount of current each car gets being inversely proportional to its resistance. Lower resistance arms will get more of the available current.

For TJets with 15 ohm arms, the maximum instantaneous current under the same conditions would be 1.2 amps per car, or 4.8 amps total. 

So that's the maximum possible demand and it only occurs for a few milliseconds. Once the motor starts turning, its current demand goes way down as the motor starts generating a voltage that opposes the power supply voltage. How far down does it go?

Ideally, the current draw for an operating motor would be zero. But since we all know that a motor's generated torque is directly proportional to motor current, it would generate zero torque. It would be highly efficient and totally useless. So this tells us that any current draw after the motor is turning is what is needed to produce torque. 

How much current? Easy answer, how much torque do you need? The amount of torque the motor needs to produce depends upon the work you are trying to do with that torque. If you want to move a slot car that has big 'ol traction magnets trying to keep it from moving, you'll need more torque. If you have sloppy gear mesh and friction on your drive train bearings, you'll need more torque. If your motor is wobbly, you'll need more torque. If your tires are rubbing the body or chassis, you'll need more torque. Brushes too tight, need more torque. Anything that places a load on your motor increases the amount of torque you'll need to move that load. More torque equals more current. Plain and simple.

Based on empirical data, most box stock magnet cars only demand around 0.3 amps to 0.5 amps while running. TJets should demand even less, but until you get all of the friction out of them, they can demand more current than you'd think. If your TJet is pulling more than a half amp you probably have a friction problem.

Drag cars with 0.09 ohm arms and races lasting a few hundred milliseconds? Nothing short of a battery, supercap, or power supply with a robust stiffening capacitor is going to provide a power source that does not detract from the raw potential of the car.


- That is a nice little power supply for bench top tuning and break-in. The one thing it's missing is a built-in ammeter, but you can use your multimeter. The ideal break-in and tuning supply needs both a voltage and current readout. But for $30 I can compromise.


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## ParkRNDL (Mar 20, 2002)

AfxToo said:


> (snip) If you ignore everything except the static resistance of the armature windings it would be voltage divided by armature resistance. For a typical 6 ohm armature with an 18V power supply, the instantaneous current draw while the motor is not turning would be:
> 
> Current = 18/6 = 3 amps
> 
> ...


This is good. It's simple, and I can understand it. It also says a 5 amp supply would be adequate for stock Tjets or JL/AW cars. One question, because I don't happen to have any 440 armatures out of the car: Did you pick the 6 ohm number first because that's about what stock magnet car armatures ohm out at? If not, what is the average resistance in ohms of a 440 or LL or G+ armature? 

Just asking because I have a whole load of stock yard-sale 440s and LLs, and the kids like to run them even if I don't... so sometimes we do have 3 or 4 running at a time, but I don't exactly have the scratch for a 12-amp supply. for now we have a 20-volt Aurora DC-2 powering each lane independently.

thanks for the explanation broken down into layman's terms...

--rick


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## AfxToo (Aug 29, 2003)

Yes, stock magnet car motors are usually in the 6 ohm range. The Life-Like T car motors are the exception, they are in the 4.5 ohm range. Aurora A/FX and TuffOnes motors can also be in the 6 ohm range and the much coveted quadralams are in the 4 ohm range. 

Keep in mind that it doesn't matter how many amps the cars want, the power supply can never put out more than it is capable of putting out. As with many things in life, you can't always get what you want. And with all due fairness to the 'Stones, you don't always get what you need either. You only get what there is to get, and you deal with it. This is the case when running two 6 ohm cars with a stock wall wart that's capable of putting out 0.3 amps. You deal with it. Spongy starts and car-off surges are part of the "dealing with it."


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## Dragula (Jun 27, 2003)

I recently bought a flux capacitor for my dragstrip power supply,big improvement.Find them on ebay under user i.d. hellomcfly


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## slotnewbie69 (Dec 3, 2008)

cheaper solution.we use two variable voltage laptop transformers that supply one amp per lane.enough for out of the box i/32 and ho with slideguides.the only drawback is we can't run the higher amperage parma motors.(4 amp deathstar )


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