seeking profesional help!

Kenny · Post by **Kenny** » Wed Jun 28, 2006 7:46 pm

I have two electronic problems, one is how to calculate the resistor required to use a 3V motor off a 9v battery, while in parallel to many LEDS (quite comfortable with Ohms law here)

and second how to supply power to a revolving section. I have thought this:

http://community.webshots.com/myphotos? ... ity=PhRwIZ

red and blue being wires, black being the shaft (insulation not drawn.)

but I don't think it will be too robust or reliable.

Umi_Ryuzuki · Post by **Umi_Ryuzuki** » Wed Jun 28, 2006 10:21 pm

Almost how I would do that....

For supplying power, make the red leads into contact rings around the shaft. then the blue leads would be point contacts. The blue leads could be flat brush type contacts or pin contacts that would use a small spring behind them to provide a constant pressure on the contact rings.

Something like this...
http://img523.imageshack.us/img523/8872 ... cts8qn.jpg

jwrjr · Post by **jwrjr** » Wed Jun 28, 2006 10:23 pm

First problem - if you know how much current the motor will draw, then calculating the resistor is easy. However, don't forget to determine what power rating will be needed for the resistor. Just remember that the more current you draw from the battery, the sooner you will have to replace it.
Second problem - Normally you would have copper rings on the rotating part with stationary sliding contacts. That's how they do it for rotating lighted signs.

Kenny · Post by **Kenny** » Thu Jun 29, 2006 1:02 am

jwrjr wrote:First problem - if you know how much current the motor will draw, then calculating the resistor is easy. However, don't forget to determine what power rating will be needed for the resistor. Just remember that the more current you draw from the battery, the sooner you will have to replace it.

ok,

Current (no load): 0.25A
Current (max eff): 0.81A

so drawing 0.81a and voltage required at resistor of 6V
gives 7.4Ohm.

and power rating of 4.86W

How does that sound?

Ps thanks for help

Sparky · Post by **Sparky** » Thu Jun 29, 2006 2:17 am

Prehaps a stero connector

http://www.kc6sye.com/techmages_12_28_03_b.html

http://www.kc6sye.com/techmages_1_3_04.html

If you're moving power through it (lika a wind generator) then you need to hang on the otherpower board, that's where I confirmed that the stero jack will work as long as you don't over current it.

http://www.fieldlines.com/story/2006/5/6/102016/2184

They had a good starter article on 'slip ring' construction, I have it saved somwhere. . .

For the motor I suggest a voltage regulator. . .fixed should be simple hook up (in votlage, ground and out voltage) with maybe a filter/in-rush current capicator. Get one that can supply the peak current (startup current). You may need a diode across the motor to protect agianst back emf (when power is turned off and the motor is spining down).

We used a 6 volt regulator to drive the 12 volt motor, it geared dwon to turn very slowly so there wasn't much issue with back emf, just current draw.

Kenny · Post by **Kenny** » Thu Jun 29, 2006 6:40 pm

Sparky wrote:Prehaps a stero connector

http://www.kc6sye.com/techmages_12_28_03_b.html

http://www.kc6sye.com/techmages_1_3_04.html

That sounds good, maybe I can use a jack at one end instead of a bearing.

jwrjr · Post by **jwrjr** » Thu Jun 29, 2006 10:27 pm

Kenny - I agree with your calculations. I DEFINITELY wouldn't use a 9 volt battery for this, as the average 9 volt 'transistor radio' battery will last less than 1/2 hour at max load (and less than 1 hour at no load). If you can find a power supply with outputs at 12 volts (for the leds) and 3.3 volts (for the motor), that would be good. Actually, the 12 volt output is not fixed in stone. Aa 5 volt output would also do well. However, powering white or blue leds from a 3.3 volt output would work marginally at best.

Kenny · Post by **Kenny** » Fri Jun 30, 2006 2:11 am

Thanks jwrjr, I've always used 9v for leds as they can be easily fited in to the base, so that sucks to learn it won't last long.

And Sparky that Idea was just what I was looking for. Got a good mono-stereo plug today for a couple of dollars.

Now just the power source and building (the good part) to go

Sparky · Post by **Sparky** » Fri Jun 30, 2006 9:19 am

Hehe we used the sterol because I passed +12 volts, Gnd, and Audio into the station. (There was a speaker in the saucer playing the display screen sound effects from TOS Star Trek, mixed into that at various intervals was the navigation and bridge sound effects). The TOS sound track was played out the other channel on a speaker at the other end of the display (hidden in the motor housing lump).

Took some editing of the sound Fx but that was fun too.

Kenny · Post by **Kenny** » Sun Jul 16, 2006 4:00 pm

Ok, all 30 ish LEDs are wired up, and the headphone jack is supplying power to the rotating section, BUT... my 10ohm 6W resistor is overheating and causing the battery to overheat, and if memory serves increase the internal resistance and cause the section to stop.

Anyone have any isead to help?

jwrjr · Post by **jwrjr** » Sun Jul 16, 2006 10:32 pm

There is the obvious solution - use a resistors for each led. That way each resistor dissipates a little power while avoiding 1 resistor dissipating a lot of power.
I wouldn't use anything smaller than a motorcycle battery to power 30 leds at once. 30 leds can draw as much as 3/4 amp. most batteries can't supply that for long.

Kenny · Post by **Kenny** » Mon Jul 17, 2006 4:54 pm

Sorry, the led's and resistors are working just fine, and due to size I am not worries about the battery life (may build in a mains powered supply later). The resistor attached to the motor is overheating even thought the power rating is greater then the calc

Current (no load): 0.25A
Current (max eff): 0.81A

so drawing 0.81a and voltage required at resistor of 6V
gives 7.4Ohm.

and power rating of 4.86W

I have tried putting two in parallel to reduce the current going through each branch but still get very hot resistors

Sparky · Post by **Sparky** » Mon Jul 17, 2006 5:02 pm

Can you get 10 watt resistors? Also if you hook resistors in parallel you are changing their effective resistance (for identical resistors its 1/2 the original reistance).

It might be best to try a 6 volt regulator at this point, only 3 connections to make, minus from the battery as ground, plus voltage from battery (Vin) and power to the motor (Vout). Motor minus is connected to the minus of the battery (and the ground of the pin).