Forgot the link. Look under potentiometers rheostats I have bought from this company.
http://www.surplussales.com
Rich
If you ever fall over in public, pick yourself up and say “sorry it’s been a while since I inhabited a body.” And just walk away.
misterconsisterThanks for the detailed answers everyone. I'm going to have to digest some of this a bit before I can formulate a reasonable response as some of it is a tad over my head. On the road right now but will think about it in the interim. I guess its fair to say I shouldn't buy the potentiometer at Digikey. Back soon. Eric
Thanks for the detailed answers everyone. I'm going to have to digest some of this a bit before I can formulate a reasonable response as some of it is a tad over my head.
On the road right now but will think about it in the interim. I guess its fair to say I shouldn't buy the potentiometer at Digikey.
Back soon.
Eric
One solution I use a lot in a situation like this is a Googel search for 10 ohm rheostat.
One result was for Surplus sales. $9.95. It is a 15 ohm 12 watt rheostat.
(RWA) RP061UB150KK Rheostat, 15 ohm, 12.5 watt, 0.91 amp, 10%. Panel mount. 0.879" diameter body, 0.71" behind. 1/8"D slotted shaft with locking nut. Made by Memcor. NSN: 5905-00-880-7186.
I'm kinda likin this stuff
misterconsisterI have a 1.5V, regulated, 1500mA PS from Miniatronics and I want to use 20-30 1.5V 30mA incandescent lamps on it and regulate the resistance with a dimmer switch. There is a linear scale rotary potentiometer at Digikey that I like but it comes in quite a number of "base"(?) resistance values. If any one cares enough, it's Digikey part# P16NP-100-ND. I'm thinking any base resistance at all will decrease the maximum voltage to the lamps so that they'll never even get to 1.5V. Is this so? Should I go with the smallest base resistance one at 100 ohms? Thanks, Eric
I have a 1.5V, regulated, 1500mA PS from Miniatronics and I want to use 20-30 1.5V 30mA incandescent lamps on it and regulate the resistance with a dimmer switch.
There is a linear scale rotary potentiometer at Digikey that I like but it comes in quite a number of "base"(?) resistance values. If any one cares enough, it's Digikey part# P16NP-100-ND.
I'm thinking any base resistance at all will decrease the maximum voltage to the lamps so that they'll never even get to 1.5V. Is this so? Should I go with the smallest base resistance one at 100 ohms?
Thanks, Eric
I was wrong in my calculations.
I made a 1.5 vdc regulator and connected the equivalent load for about twenty two 1.5 volt, 30ma lamps that amounted to about 0.7 amps at 1.5 volts.
I put a 10 ohm surplus Ohmite rheostat in series with the load to lower the voltage across the load to 1.0 volts. The resistance was 6 ohms. The current was .51 amps through a 6 ohm resistor. That would dim the lights some. The power calculates to about 1.8 watts.
richg1998 Hamltnblue He would actually need a rheostat to handle it directly. The 25 ohm radio shack one is a rheostat. A regular household lighting dimmer might do the trick. A rheostat is a low resistance pot you might say. It is 25 ohm, It is only 3 watts. If 20 lamps using 30 ma, the current is 0.6 amps. With a 25 ohm rheostat, the wattage will be 9 watts if I calculated correctly. More current/wattage if using 30 lamps like he mentioned. Rich
Hamltnblue He would actually need a rheostat to handle it directly. The 25 ohm radio shack one is a rheostat. A regular household lighting dimmer might do the trick.
He would actually need a rheostat to handle it directly. The 25 ohm radio shack one is a rheostat. A regular household lighting dimmer might do the trick.
A rheostat is a low resistance pot you might say. It is 25 ohm, It is only 3 watts.
If 20 lamps using 30 ma, the current is 0.6 amps. With a 25 ohm rheostat, the wattage will be 9 watts if I calculated correctly.
More current/wattage if using 30 lamps like he mentioned.
Using the proper definitions, the difference between a potentiometer and a rheostat is that a potentiometer has 3 termianls and a rheostat has 2. A rheostat is a true variable resistor, whereas a poteniotemer is two variable resistors in series. Not a potentiometer can be used a rheostat but a rheostat cannot be a potentiometer. I've also seen folks call any variable resistance device that is wire wound a rheostat, regardless of the terminals. It is not technically correct but not uncommon. Radio Shack did this very thing with their 25 ohm device.
Engineer Jeff NS Nut Visit my layout at: http://www.thebinks.com/trains/
The series regulator I mentioned might drop the voltage too low as I believe there is a minimum 0.7 volt drop at a diode junction which might be too dim. That would alllow 0.8 volts to the lamps. No more.
If you were using 12 volts lamps, this would not be an issue. With 1.5 volt lamps, not much to play with.
I checked with my best friend, Google and found the below links for rheostats. Have fun.
http://tinyurl.com/27jehu9
Below is a online claculator.
http://www.opamplabs.com/eirp.htm
Let us know ahat you come up with. Ohters may be interested. I have a link to a circuit for a 1.5 volt regulator that can be fully adjustable. Zero volts up to 1.5 volts using low wattage pot.
Here is what I think will work. I have never tried this as I just bust right into the circuit. Put the potentiometer in where the zener diode is. Adding more lights will lower the brightness of all. The pot will become the “zener diode” you might say. If you add more lights, re-adjust the pot until you have the brightness you need.
I am on the road so not much time to give this much thought. Radio Shack should still have TO-220 case NPN power transistors.
http://www.satcure-focus.com/tutor/page5.htm
Springfield PA
You may have trouble finding a potentiometer to handle the current. Have you calculated the wattage necessary to handle the current?
You can use a power transistor as a series current handling device and use a common two watt potentiometer to control the base current of the power transistor. Get a transistor in the TO-220 case which is easy to bolt to a piece of aluminum if it needs to be heat sinked. Radio Shack has them.
0-100 They have them down to 25 ohm.
Oh and I'm confused by how the potentiometer works too. For a 100 ohm potentiometer, does it provide a resistance of 100 - 0 ohms or 100 - infinity?
Thanks for your responses.
I got the 1.5V PS so that I could run all the 1.5V lamps without using resistors at each one to drop the voltage. Since the voltage is the same, seems there's no need to drop the voltage for the lamps, so no need for resistors. Yes?
But I want to be able to dim that circuit too. The lowest ohm rating for the rotary potentiometers I was looking at was 100 ohms. I think that means the switch has 100 "base" ohms of resistance for the cuircuit through it (but I could be wrong). Since this will drop the 1.5V further (no?), will that have a (too) large dimming affect on the 1.5V bulbs?
I'm not planning on opening up the PS, that's way beyond my skills at this point.
It didn't sound like he opened up the power supply, but if he did - assuming they used an adjustable voltage regulator like the LM317T, the MINIMUM voltage they work at is 1.2, so that's not a whole heck of a lot of dimming.
I'd try a single diode - that will put .8 to .9 volts to the bulb - might be too dim, but worth a try. If it works, a single 3 amp diode will dim all the lights that power supply could handle. And not generate the heat a resistor will.
--Randy
Modeling the Reading Railroad in the 1950's
Visit my web site at www.readingeastpenn.com for construction updates, DCC Info, and more.
It sounds like you are digging into the circuit. There is one resistor that determines the voltage of a regulator but not knowing the circuit, this is only a guess. I do this with the LM317 voltage regulator. With this resistor, the wattage is not an issue. Have your checked the regulator circuit and drawn it out?
Radio shack has several available and they're cheap. I would go there.
Eric:
The only way to tell what resistance in ohms you are going to need to get the brightness you want is by some experimentation. If it is something less than 100 ohms then a 100 ohm pot will work IF the wattage is enough to handle the load. I would suggest that you get some fixed resisters in various values and place them in your circuit to bracket the desired brightness. Then you can buy an appropriate pot or you may find that a fixed resister combo fits the bill for you. Remember that, in any case, to get resisters or a pot with the appropriate wattage rating and that they will get warm to hot.
Joe