Rheostat ?

I have been trying to find Rheostats that can handle 70 watts (ac side of transformer is 17.5v and 4a) but small enough in physical size to put on control
panel. Want to put about 7 of them on control panel.
Went to Radio Shack and the only thing they had were rated at 25ohm and 3 watt. I did a google search and the only thing that I could find were your normal house type dimmers, to big for control panel.
If that is all there is I was thinking of building a separate panel, but really do not want to do that.
Thanks Chris
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ChrisGW spake thus:

Rheostats at *Radio Shack*? Excuse me while I pee my pants from laughing too hard.
You're not going to get jack shit at Radio Shack these, days, that's thing #1.
Big honking rheostats like the ones you seek will probably only be found at surplus-type, old-line electronics stores. There's one close to where I live, in Berkeley, but maybe not close to where you live.
I did find one at Allied Electronics. It's not exactly what you were looking for (this one is 25 ohms, 25 watts), but just to give you an idea of the size and cost of these things:
http://www.alliedelec.com/Search/ProductDetail.asp?SKU )6-4225&SEARCH=&MPN=RHS25R&DESC=RHS25R&R)6%2D4225&sidE1F050028A3E17F
By the way, what are you planning to use those for? Multiple speed controls? Just curious.
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David Nebenzahl wrote:

http://www.alliedelec.com/Search/ProductDetail.asp?SKU )6-4225&SEARCH=&MPN=RHS25R&DESC=RHS25R&R)6%2D4225&sidE1F050028A3E17F

I am using them for Building lights, street lamps, etc...
As of right now unless I come up with a better idea, I will extend my control panel to fit regular dimmers.
Chris
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As of right now unless I come up with a better idea, I will extend my control panel to fit regular dimmers.<
Why don't you use an auto transformer on the front of the transformer? or If you want to control different sets of lights differently you don't need the rating of the transformer for the pot you only need the wattage of the string you are controlling.
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ChrisGW spake thus:

So, I take it you're controlling, what--maybe one or a few lights with each control? If so, you certainly don't need such big (so far as watts go) rheostats, only enough to control the few lights.
There's another way to do this, which is a little more complicated, but would be cheaper and take up less space. Instead of rheostats, you could get a bunch of multi-position rotary switches (which are easier to get and cheaper than rheostats), and a bunch of small power resistors for each switch. What you'd end up would be a stepped dimmer control; if the switches have 6 positions, for example, you'd have 6 positions between full on and off. Would this work for you? I can tell you how to wire it up (not rocket science by any means) if you're interested.
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David Nebenzahl wrote:

I like that idea. If you would please post or email the wiring instructions. That will work.
Thanks Chris
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ChrisGW spake thus:

OK, here it is: http://www.geocities.com/bonezphoto/misc/RotaryDimmer.pdf
Now, this is missing some information: namely, how big should those resistors be? I haven't worked this out yet, but the short answer is, it depends--on the load you'll be driving with this. Maybe someone else here can tackle this; basically, you want a pretty full range of dimming, from totally on to totally off. The resistors around each switch are all the same value.
So far as resistor size (power capacity in watts) goes, I'm guessing you're going to need something in the 1-5 watt range. Resistors like these can easily be gotten from places like Digi-Key (http://digikey.com).
Once you figure out the resitor values, just solder it up as shown and run it.
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David Nebenzahl wrote:

Thank you that looks good.
I am proud of myself. I was drawing it out last night and my drawing just about matched yours. I did not realize that you have to have a resistor between each lead. I will sit down tonight and try to figure out the the right resistors.
Thanks again
Chris
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.

Chris,
You can also control lights electronically - a small potentiometer on the control panel can adjust the voltage on a remote circuit (an adjustable voltage regulator or throttle.)
An example can be seen at: http://home.cogeco.ca/~rpaisley4/TTRThrottle.html
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kt0t spake thus:

So, "kt0t", I'll bet you can eliminate the following components for the purpose the OP has in mind (operating lights):
- the big (3500 uf) cap, since pulsating DC is no problem; - D3 & D4, which I assume are there to provide a small voltage drop; - the capacitor on the output.
And I assume you can run a bunch of these in parallel from a single power supply (xfmr & bridge rectifier), correct?
By the way, you don't even really need D1 & D2, do you?
This would end up being cheaper and simpler than even my rotary-switched resistor string suggestion. Kewl!
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I suspect you are right David. I'm not a power supply engineer but have played with some regulators with good and bad experiences...

The heavy ripple may give the regulator problems as it is designed for DC. I don't know that 3300 uf is needed; All Electronics has a 2700 uf unit at http://www.allelectronics.com/cgi-bin/category/140300/Snap-in.html for $1.40. That's cheap for the comfort factor. I noticed that Rob specified a 2200 uf for a 1.5 amp circuit.

Yes - the regulator IC only goes down to 1.25 VDC; the added drop of D3 and D4 would take the minimum output to almost 0.

Probably not needed.

I think you are correct with two caveats: 1) When the regulator is at any distance from the transformer/rectifier/filter cap the application note recommends a 0.1 uf capacitor at the input to the regulator IC. 2) As the current draw increases, the filter cap. may need to be larger. Someone reading can probably recommend a size.
By the way, Rob points out that a lower current regulator (the LM 317) is less expensive and still supplies 1.5 amps. ALL has them for $0.50 each. The LM350 3 amp regulator is $3.50.

Probably not.

Pretty easy to wire too; I use 317s several places on my layout for light control.
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I think the rule of thumb was 1000uf for a 1 amp draw gives 1Volt of ripple. Draw more current or want less ripple and you have to increas the size of the capacitor.

Only necessary if you want the output to go to zero volts. For lighting you can probably get away without them.

Yes needed. Helps to stabliize the regulator.

And filter capacitor.

See above about 1 and 1 and 1. As your current draw goes up you increase your filter cap accordingly. Double the current draw, double the size of the cap. And if you can't find a big enough cap, you can wire multiple caps in parallal.
Also keep an eye on the rating of the rectifyer.

Better put a good heat sink on that LM317 My rule of the thumb was always 0.5 amp without a heat sink.

THey protect the regulator from reverse voltage. These regulators tend to fail if the output voltage gets higher than the input. In the event that the output voltage gets higer than the input (because of wireing errors or stored voltage sources [capacitors]) D1 will conduct and route the reverse current around the LM317.

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<SNIP>
Thanks Gordon! Really appreciate your help.
Bob
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I assume from the building lights you mean on the layout, not the scenery lights. If so, any ordinary power pack will provide the necessary amount of power and control of the lights. There are a number of schematic diagrams on the web of throttles that would work. If you're talking about the scenery lighting then I'd just use regular dimmers than a big ol' honking reostat - you generate a lot less heat and the lights can be turned all the way down in a fairly linear intensity with the dimmers. There are also (at a higher cost) remote controlled dimmers if you don't want to have house current at your control panel.
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wrote:

Mouser still carries reostats.... Up to 300 watts at least.
http://www.mouser.com/search/Refine.aspx?Ne%4016+1447464&N%4359%201323038&Ns=P_SField&RefType=Home
Jim
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ChrisGW wrote:

Hi Chris; Something that hasn't been touched with the answers so far --- Are you trying to control the 110Volt side of a transformer?
If that's your aim DO NOT GO THE 'Lamp Dimmer' route. Bad things WILL happen!!!!!
Chuck Davis
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Charles Davis skriver:

Why ?
It's just a matter of getting the right dimmer....
Klaus
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Klaus D. Mikkelsen wrote:

Right!!! The cheap dimmers don't handle inductive loads worth a darn. The dimmers that DO handle inductive loads (florescent lamps & associated 'ballast' components) are on the expensive side. Sometimes double the cost or more compared to the 'cheap' dimmers.
Chuck Davis.
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Charles Davis spake thus:

Wrong, actually; I've got a garden-variety hardward-store/home-improvement-store-type dimmer running a transformer being used for resistance soldering, and it works just fine, thank you. No sparks, no magic smoke escaping, just smooth voltage control.
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David Nebenzahl wrote:

Laid a hand on the transformer recently? The problem is that the power out of the 'dimmer' is 'chopped' AC, not a smooth sine wave (like transformers are designed to operate with.) This 'ragged wave form' quasi AC, creates a LOT of waste heat as it tries to drive the magnetic field of the transformer back and forth (the 'back & forth' is what makes the 'transformer' do it's thing.)
If your happy with your usage, fine, more power to you. But it's a situation just crying for a smoking transformer & possibly fire problems.
Chuck Davis
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