Rheobus questions

[DaveSauce]

OK, so I think that I have almost all the info that I need to do this. However, a few questions remain.

Does anyone know if 75 ohm rheostats exist? I have a 100 ohm for testing purposes, and it just seems like way too much. It's a 10 turn, but all current is shut off at half way through...or actually, from my calculations, 2/3 through (resistance required to shut the fans off is 66.67 ohm). Thus, It is quite annoying that they are off halfway through turning the thing. I've looked, and I can't seem to find any 75 ohm resistors. Ive tried Allied Electronics and another online site that starts with a J or something, and they dont seem to have them. I'm beginning to wonder if they even exist.....im certain they dont, but any ideas?

And any more good places to buy?

 

[stool]

http://www.surplussales.com/Potentiometers/PotsRheostList.html

Loads of rheostats, $10.00 minimum order.

 

[DaveSauce]

oh wow...i dont think its possible to spend less than 10 there....heh. those are pretty expensive, considering I need about 6-8 of them. Thanks for the link, though.

 

[stool]

One thing you have to realize, all rheostats aren't as cheap as the Radio Shack jobs and the ones Hoot was selling. If you do a search, you'll find some that are really up there.

 

[DaveSauce]

yea I know....i noticed that the higher wattage they can handle, the more expensive they get. I suppose if I needed a 25 watt tolerance, I would get them....but all I need is about 5 watts max....if even...

 

[gahdzila]

Yeah, I feel your pain, dude. I'm tossing around some ideas for a rheobus myself. It seems that the only reasonably cheap, reasonably easy-to-get-yer-hands-on rheostats out there are the 25 ohm, 3 watt rheo's from radio shack, or the 100 ohm rheo's from jameco (and others) that you mentioned.

Then, I saw a few articles on somewhat more complex designs...I'm thinking of building a rheobus based on one of these designs. Here's a few of the articles I've gone through so far...enjoy!!!

switched diode design

pulse width modulator---here and here

voltage regulator circuit---here and here and here and here

And here's an article comparing different methods, with some links to other articles.

Th voltage regulator circuit is probably what I'll use. PWM just looks TOO complicated...the diode idea would probably work well if you want to implement the LED's (I don't) and you have a very steady hand to solder those diodes to the switch (I don't )

good luck!!!!!

 

[Crazy Jayhawk]

I think if you connect a resistor in parallel with the rheostat you can turn it into a 75-ohm unit.

Rt = total resistance.
Rr = rheostat resistance.
Rp = parallel resistor.

1/Rr + 1/Rp = 1/Rt

Solved for Rp, this is (Rr - Rt)/(Rr x Rt)

I might be wrong though.

 

[DaveSauce]

Hm, I see the advantages to using a diobus. Perhaps not as smooth, but it might have an advantage. Only difficulty would be making room for all the LEDs....my problem is that I want to put a bunch on, with limited room. But with that diobus, i will have a tolerance for more fans per switch.....ideally I would have a dial for each fan, but even a rheobus would be pushing the limit on my drivebay....because I plan on putting a little $10 temperature sensor on the bay.....takes up about 1.5-2 inches...so I guess i'll have to cut a few to make space. But oh well, if I use the diobus, I wont have to worry about overloading my circut, heh.

 

[gahdzila]

I think if you connect a resistor in parallel with the rheostat you can turn it into a 75-ohm unit.

but that would give your circuit the resistance of the resistor all of the time...you wouldn't be able to run the fan at full speed.

right???

 

[Overload]

but that would give your circuit the resistance of the resistor all of the time...you wouldn't be able to run the fan at full speed.

right???

actually thats not true as when you parallel resistors you always end up with a smaller value that either resistor you started with. if your reo goes to zero ohm then you short the other and the load get full power.

 

[DaveSauce]

So, if I follow what your saying, putting a 75 ohm resistor in parallel would turn it into a 75 ohm unit? Once the rheo reaches 75 ohms or greater, it would just take the path of the fixed resistor? Or is there some mathematical portion that I am missing?

Either way, the Diobus is beginning to look more attractive....I kinda like the idea of a fixed number of speeds, accompanied by lights for each speed.

 

[gahdzila]

actually thats not true as when you parallel resistors you always end up with a smaller value that either resistor you started with. if your reo goes to zero ohm then you short the other and the load get full power.

ok...now I see. thanks for the clarification.

having a 75 ohm resistor parallel with a 100 ohm rheostat would give you a MAXIMUM resistance of 75 ohms...as electricity would follow the path of least resistance. This solution still would be less than ideal, however, as the actual dial-in control would still be as coarse as a 100 ohm rheo alone...a lower rated rheostat alone would give you finer control of the fan speed.

The thread starter's original complaint

It is quite annoying that they are off halfway through turning the thing.

would not be resolved by wiring a 75 ohm resistor parallel to the 100 ohm rheo.

I'm certainly no electrician, I'm quite new to this stuff...feel free to correct me if I'm wrong.

 

[DaveSauce]

The thread starter's original complaint would not be resolved by wiring a 75 ohm resistor parallel to the 100 ohm rheo.

Yea, your right about that....heh, I forgot all about that, lol.

 

[Veland]

Actually you would need something like 300ohm resistor to total 75 ohm.. And the resistor must handle the same amount of wattage as the reo or it'll burn. As for least resistance, that's correct, but remeber the electricity has two ways of flowing when something is in parallel. Let's go back to school using the tried and tested water example:

Water flows at 10l per second
The reo can regulate from 1l/s -> 10l/s
The resistor has a 3l/s throughput

Someone commented that the resistor sets maximum resistance due to least resistance, but the reo still let's some power, water, pass. So in the example above:

No resistance - 10l/s
Reo at max resistance - 1l/s
As above with resistor - 4l/s

Ok ok it got more messy and a lot less clear then I itended it to be, but I'll let it stand there. Do read some of the "quick-n-dirty" guides to basic electricity and this should be simple..

 

[DaveSauce]

veland, your avatar rocks.

So long, and thanks for all the fish.

 

[Overload]

I did some calculations so if you want to know, to turn a 100ohm reo into a 75ohm max you will need to put a 300ohm resistor in parallel with it. and assuming this is going to be used with a fan having a resistance of around 30ohm a 1/4 watt resistor should work fine. while this is not the best way to do it i see nothing wrong with using it in a pinch or for testing purposes. Hope this helps