[Q] dimmer,rheostat,potentiater

[plumber]

Can you use a dimmer switch to increase or decrease resistance on a dc system? in other words,manually regulating current (mA) ? Are there dc dimmer switches? I want to regulate the dc out put from a wall adapter transformer, lowering the mA as it climbs due to a progressive drop in resistance.

Thanks, plumber

[UncleAl]

Dimmer switches operate by phase regulation. The power circuit is switched on with a variable time delay following zero-crossing of AC sine wave input. At the end of a half cycle it repeats. Variable level power is thus sent to the load, decreasing with increasing delay. Dimmer switches use solid-state devices such as triacs, silicon-controlled rectifiers, or field-effect transistors for varying the phase angle of an applied AC sinusoidal voltage.

 

Regulate AC power then rectify and smooth. Cutting the sine wave input launches high harmonics downstream. Your rectifier may not like that, or be capable of smoothing out the high frequencies. or it may act as a bandpass filter and not forward all the input power.

 

--

Uncle Al

UNDER SATAN'S LEFT FOOT

Vote a 10 for doing the experiments!

[lawcat]

Yes, there are. But first, I don't understand what you mean by progressive drop in resistance? Generally, resistance only increases, or in other words voltage drops. To keep the voltage at the necessary operational levels, i.e. 22-24 VDC, you must increase the wire size to reduce resistance. But once you select the proper wire size, considering that voltage output is constant 24VDC, your current will not vary--it will be constant.

 

Now, you want to control the current output by adding resistance at the source--the transformer; you want a dimmer. Do not manufacture the dimmer yourself. There are several problems with dimming DC as Al pointed out above--one being harmonics. But more importantly, a manufacturer of your final product, whether it is a speaker, a ceiling light, or a motorized curtain, will have a dimmer and transformer to sell you. Those dimmers are tested to work in conjuction with the lights. You can only have so many lights on the system, you have to keep the wire length below 20' for example; in other words dimmers have certain operational ratings. It's not worth it to mess with this yourself. Contact the manufacturer.

[plumber]

Thank you Lawcat, your verbiage is a little easier to understand. I'm sure I must have things jumbled, but: if you have a positive and negative electrode inside of pure distilled water you have a very very slow start at ion transfer because distilled water is a lousy conductor. Isn't that resistance? please bear with me. However, as the water becomes infused with metallic ions and metallic particles, the water begins to conduct better resulting in a progressively faster process. Along with this an in- line amp meter begins to climb, producing more milliamps...isn't that because of "less resistance"? I probably have a few things wrong, but for sure...the amps increase as the process continues. Now. What I need to do is keep the mA's below a set number. To start I would like to do this manually, and I can sacrifice voltage if that is what it takes. Will a dimmer do this at all? When I am all growed up, I'll buy a regulated power supply that can keep the current constant regardless of factors. One of those test bench AC/VDC power supply's with regulated variable out puts. That's what I really need...but until then? For instance, mechanical stirring produces a profound drop in milliamps as the suspended metal particles are dispersed away from the electrodes while the process is happening. That is one way of "regulating" the amps. So...when I see two electrodes using a stream of metallic particles traveling a short distance through water to complete the circuit I see something very similar to a simple incandescent circuit...for which a dimmer does work. I would really appreciate any and all correction and input...I am less than a novice, but very determined. Mechanical stirring works, but I need theory under my belt for sure. So, how to regulate amps while mantaining a constant dc voltage with out going broke or crazy.

Thanks again for your last reply, Hope you reply again.

Plumber

[lawcat]

EDITED:

 

I gotta run. But long story short: You want to regulate current through your metallc solution. Metallic solution is the resistance, and in your case it appears that it varies due to stirring and saturation. As the resistance decreases due to higher metal content, the current increases.

You do not like this. You want to keep the current at some limit. You can (1) regulate voltage manually by monitoring the current, or (2) you can limit the current. Check the Constant Current Source, as I think it will limit your current nicely: Current source - Wikipedia, the free encyclopedia You should be able to limit your DC current from a breadboard.

[DFINITLYDISTRUBD]

How much voltage are you dealing with?

 

There are many cheapy ways to handle power regulation @ 12volts or lower.

If power is in the mili scale a plain ol potentiometer will work fine (though as other variables change you might have to tweak it periodicly)

 

Above mili there are slot car speed controls (typicly very cheap)

model train transformers (range from really cheap to ungodly expensive, or of course with a basic electronics book (or you could look up the schematic on the web) you could build one on the fairly cheap side of things.... cheaper if you've got lots of unused or dead electronics lying around (Ie. old radios, a dead TV, old toys, an outdated computer etc.) but an A/C dimmer is not likely to work to well if at all in a D/C circuit.

[plumber]

Ok. When you get back...what is a "breadboard"

[plumber]

Dear distrubed, didnt catch your reply before asking lawcat about a bread box.

Alrighty then! you be speaking my language. Thank you. It is 30 vdc and I want to limit the current to

6 mA max. Or set the current at a constant 4 mA. Preferably with out lowering voltage.

Thanks for the reply. Plumber

[lawcat]

A breadboard is this: Breadboard - Wikipedia, the free encyclopedia

and this

YouTube - Intro to Breadboard Electronics http://www.youtube.com/watch?v=HteDBfSJ9zo.

 

Anytime you are constructing a circuit, you need some tools. Go to any hobbyist store, maybe Radio Shack, a college campus bookstore, or online, and purchase a student's electronics kit that includes resistors, transistors, and diodes, and leads, and the breadboard. Electronic Components and Parts at ApogeeKits

 

Then, you can try building this circuit http://www.physics.unlv.edu/~bill/PHYS483/current_lim.pdf

 

Vs is your voltage supply. R1 nad R2 are resistors. You need two transistors 2N3904.

R1 comes out to be a 175 ohms resistor, and R2 is 48 kiloOhm resistor.

 

You will likely have to readjust the resistor sizes until the circuit functions the way you want it: 6 miliAmps through the load.

 

Your problem is specific, and I am not aware of any premade devices that will accomplish exacly what you need: 30VDC with constant current of 6mA accross the varying load. You have to build it using the breadboard.

 

OR, better yet, go to local university and offer a senior electrical engineering student a few bucks to build it for you. You need a consultant on this one.

 

EDIT: But honestly I don't know why you need limited current. Buy a potentiometer, or variable resistor, wire it in series with the lead into the anode, and control it manually.