[GreenKeys] energizing relays and questions on voltage and current

Thu, 10 Apr 2003 16:56:00 -0400

Hi there. I have a question that is kinda more electricity oriented than greenkeys per se, but it involves a stock ticker (new york quotation) that I recently acquired and afterall, these are the predecessors to our beloved greenkeys (so I hope this message gets through--besides, this is the best list I belong to, no other list has as much quality give and take)...here goes:

The ticker has an electromagnet (it actually has three, but my question is general).  Last night I wanted to see if the darn thing had any life left in it, so I measured the resistance of one of these electromagnets and came up with about 10 ohms.  The point was to attempt to re-energize these electromagnets and see what happens!.  I have no technical information on these and was afraid that I might burn out the coil.

Now as I understand it, an electromagnet only cares about the current through it, not the voltage applied to it, of course I know I can control the current through it by dividing the applied voltage by the resistance of the coil (I think 10 ohms).  So not knowing what I was doing, I found a 7.2V camcorder battery and tried to touch it to the ticker contacts just momentarily to see what would happen (figuring I was generating less than 1A).  Sure enough! The electromagnet energized and it "ticked!" (actually it just blotted the type wheel on the paper and advanced, which is awesome!).

Here's getting to my question...I want to be careful with energizing this, so I figured the best approach would be to start off with the lowest amount of current and slowly, increase the current until the coils energize and not use much more than that.

Is this best done with just a variable voltage supply? I started looking for power supplies on ebay and saw some that had a CC option (which I understood to mean constant current).  Is this similar to a current look of a tty?

Should I be testing this by using some kind of current source like one of these bench supplies, or should I forget all about that and just use a variable voltage supply? Would there be any need to place a resistor in series with the ticker so as to limit something!? I just don;t know if its safe to simply hook it directly to a voltage source..should I protect it somehow with some kind of current limiter? Or maybe a voltage limiter?

What do you think here? 
Thanks in advance