Electronics Questions

[goobee]

Will a 110AC light dimmer switch (ie: the kind with a knob) work on a 6 volts DC? I'm working on a robotic project and I need to be able to vary the voltage? Thanks.

 

[So]

Depends on the dimmer. Probably yes. But if you don't know what a potentiometer is, and you're working on a robotics project, I strongly suggest you talk to an EE friend.

 

[nakedfrog]

Originally posted by: So
Depends on the dimmer. Probably yes. But if you don't know what a potentiometer is, and you're working on a robotics project, I strongly suggest you talk to an EE friend.

Yes, if for no other reason than safety.

 

[goobee]

No worries, it's a very simple and small project. Thanks.

 

[FoBoT]

the current rating is just as important, make sure they are similar

 

[Demon-Xanth]

No, an AC dimmer switch will NOT work on DC, especially low voltage.

AC dimmer switches use SCRs to narrow the waveform, thus decreasing the time the device is under power. With DC, if it was able to switch on, it would stay on (minus a small voltage drop) until power was removed.

edit:
Diagram of what I'm talking about:
http://home.howstuffworks.com/dimmer-switch2.htm

 

[Demon-Xanth]

The reason why AC dimmers aren't potentiometers is that if they were, the dimmer switch would be burning up as much power as the lights they are dimming at half brightness. I don't know about you, but 100W or so in an unventilated box would be really really hot. They are not variacs as well, as those are too large and too expensive for the normal household applications.

 

[goobee]

Demon, what size potentiometer would I need to pick up from say Radio Shack for 6 to 12 volts?

 

[Demon-Xanth]

What sized load?

You might be better off using an LM317 or other off the shelf voltage regulator with a pot between the output and control pins. It would let you get from about 1.2 to about 11V without having to worry about the load too much.

Basic wiring setup:
pin 1=+12V
pin 2=center of a 10k pot
pin 3=output and one end of the 10k pot
ground=other end of the 10k pot

Granted, it's not an ideal circuit but it's a quick and dirty two part regulator.

 

[Colt45]

won't work..

what sort of load do you have?

 

[goobee]

Not much, prolly around 600 - 900 miliamps.

 

[Demon-Xanth]

If you use an LM317 for that kind of drop and load, make sure you have a good heatsink for it since it'll be dropping about 4W.

 

[Demon-Xanth]

Easy Ohms/Watts law calculator:
http://www.angelfire.com/pa/baconbacon/page2.html

 

[So]

Originally posted by: Demon-Xanth
No, an AC dimmer switch will NOT work on DC, especially low voltage.

AC dimmer switches use SCRs to narrow the waveform, thus decreasing the time the device is under power. With DC, if it was able to switch on, it would stay on (minus a small voltage drop) until power was removed.

edit:
Diagram of what I'm talking about:
http://home.howstuffworks.com/dimmer-switch2.htm

IIRC the old dimmers were just pots, weren't they?

 

[Demon-Xanth]

If by "old" you mean "before Home Depot was founded". I can't say that I've actually seen one in person.

 

[goobee]

I found the parts on Radio Shack's website. Can you clarify what this step means in your instructions?

pin 3=output and one end of the 10k pot
ground=other end of the 10k pot

Both the pot and the regulator appears to have 3 connections.

Thanks.

 

[Demon-Xanth]

btw, you might want to check the manufacturer's datasheet on that part before going with my pin numbers. Often manufacturers use different pinouts (for some reason I have never understood)

An example is National using pin 1 for adjustment, 3 for input, and 2 for output.
http://www3.national.com/mpf/LM/LM317.html

 

[goobee]

Will do. I'll hop down to Radio Shack later this evening and snap them up.

 

[goobee]

Strange, I got it put together but it won't go lower than 5.68 volts according to my multimeter. The specs on the LM317T from Radio Shack indicate that it should be able to go down to 1.2 volts. From an input voltage of 12 volts, I can vary it betweet 5.68 volts to 11.5 volts.

Thoughts? Suggestions?

 

[goobee]

Anybody?

 

[Demon-Xanth]

Did you heat sink it?

It is likely getting too hot and thermally limiting itself.

 

[goobee]

No but I checked it and it was barely warm. There's no load on it right now except the multimeter.