been looking for an answer for 40 days.

Thread Starter

fisal

Joined Jul 3, 2010
10
First hello ,this is my very first posting in any web site ever since I start using the internet 14 years ago ,i have been looking for an answer for 40 days and still counting.
i am using a signal generator to drive a coil back and forth on a pole between 2 magnets ,but the current i am getting is too small 4mA with square wave voltage +- 20v.
is there any way i can increase the current say to .5A or maybe 1A, but still using the signal generator, or if anyone know a way using switches to reveres polarity so I can use an external current source.
plz check this link to say what I am trying to replicate but i dont wanna use any more power than i should .http://www.youtube.com/watch?v=Famr4YWBE7g&playnext_from=TL&videos=YBHThnM-Fzg
thank you in advance.
 

bertus

Joined Apr 5, 2008
22,266
Hello,

You could use a power opamp to amplify the current.
A LM1875 will probably do the trick.
See the datasheet for more info.

Bertus
 

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someonesdad

Joined Jul 7, 2009
1,583
With a 20 V square wave and a 4 mA current, the load resistance is 20/0.004 = 20000/4 = 5 kΩ. To get 0.5 A, you'll need to raise the voltage to 0.5(5000) or 2500 V. Not a likely scenario...
 

Thread Starter

fisal

Joined Jul 3, 2010
10
I think u r right ,i should make a better coil with much lower resistance ,thank but I first have to try an opamp idea cause it is much easier
 

Norfindel

Joined Mar 6, 2008
326
You could use a darlington emitter follower or a MOSFET to boost the current to the required level. The signal generator is unlikely to be able to supply so much current. You also need a diode to protect the transistor from the coil's back-emf.

If you need to reverse the polarity on the coil, you will need an H-bridge circuit, like the ones used to drive motors. There are several of them here, some easier than others: http://www.epanorama.net/links/motorcontrol.html
 

sceadwian

Joined Jun 1, 2009
499
someonesdad. I'm going to take a shot that the output of the signal generator is 20V peak to peak unloaded only, the voltage probably drops dramatically when connected to the coil. You would have to be using something like 40 gauge wire and a LOT of it to make a 5kohm coil.

It really depends on the actual DC resistance of the coil and the reasons for you doing this. I don't know how big this experiment is but if it's small scale you can use the voice coil from a small speaker as the coil, which can be driven directly from an audio power amplifier.
 

Norfindel

Joined Mar 6, 2008
326
Guys, it's a signal generator. He needs something that is capable of delivering current. The power opamp idea would be easy to implement, and is probably going to suceed.
The frequency should be pretty low, unless he tries to make the coil move up and down 1 million times per second :p
 

sceadwian

Joined Jun 1, 2009
499
A power opamp requires you to make a circuit, using an already existing power audio amp is problem solved without any messy design issues. There is however NO solution that can can make any sense till the TRUE DC resistance of his coil is KNOWN, the only statement of resistance so far was an estimate from possible incomplete information NOT directly measured.
 

sceadwian

Joined Jun 1, 2009
499
With that much fine wire impedance may kick in far before resistance, especially at any appreciable frequency. I think the entire coil has to be rethought, then again we know so little about it. I'm giving this thread 5 days of silence if the original poster doesn't step back in with some real details to work with.
 
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