Bergeon Demagnetiser Switch Replacement

[JBerry]

Hey folks,

I recently picked up this 2321 Bergeon Demagnetizer, the plastic ring that secures the momentary switch to the top of the wooden block had perished and snapped off, and no longer retains the button.

I'm fairly clueless when it comes to electronics. I picked up a new switch of the correct diameter, problem is I'm not sure if the switch is of the correct rating.

The switch is rated 3A 250V/AC, but I'm not sure about the amp rating of the demagnetizer. Anyone have any ideas?

[nickelsilver]

If you have a multimeter you can find the resistance of the coil, then divide the wall voltage by that to get your amp draw.

Amps = volt/ohms

If they had listed the wattage, you could use amps= watts/volts. By the same formula if we imagine it draws 2 amps, you'd be at almost 500 watts which I'm pretty sure it doesn't use. I think you are safe with your 3 amp switch.

[JBerry]

Thanks very much! I see a similar model listed at 1200 ohms, so I think we're good by your calculation

[AndyHull]

The only similar demagnetiser I could find on line states it is 80 Watts. That would be approximately 1/3 A at 240V or about 700 mA at 110V. A 2A or 3A switch should be fine.

[JBerry]

5 hours ago, AndyHull said:

The only similar demagnetiser I could find on line states it is 80 Watts. That would be approximately 1/3 A at 240V or about 700 mA at 110V. A 2A or 3A switch should be fine.

Thanks for the advice Andy. I fit the switch and it's working well!

[measuretwice]

19 hours ago, nickelsilver said:

If you have a multimeter you can find the resistance of the coil, then divide the wall voltage by that to get your amp draw.

hmmm careful, multimeters measure resistance and you can't figure current knowing resistance and volts if its an AC signal with a coil .  I agree, if the power is given, then yes Ohm's law works, but we're dealing with with AC so the ohms has to be impedance not resistance....and you can't measure impedance with a multimeter (they measure resistance only, whereas impedance is resistance, inductance and capacitance, you need a fancy LCR).  

Look at it this way, if you connected a wire between the legs of AC mains, bang you blow the breaker.  But if you coil that same wire, it now has inductance.  That's why say a transformer (which is a wire connecting the mains but in a coil) doesn't blow the breaker.

I think the right answer was given on the switch so all is good, just saying with coils and AC keep in mind inductance has to part of applying ohms law to get power

 

Good job on the repair, it looks very professional

 

Edited March 13, 2019 by measuretwice

[AndyHull]

3 hours ago, measuretwice said:

hmmm careful, multimeters measure resistance and you can't figure current knowing resistance and volts if its an AC signal with a coil .  I agree, if the power is given, then yes Ohm's law works, but we're dealing with with AC so the ohms has to be impedance not resistance....and you can't measure impedance with a multimeter (they measure resistance only, whereas impedance is resistance, inductance and capacitance, you need a fancy LCR).  

Look at it this way, if you connected a wire between the legs of AC mains, bang you blow the breaker.  But if you coil that same wire, it now has inductance.  That's why say a transformer (which is a wire connecting the mains but in a coil) doesn't blow the breaker.

I think the right answer was given on the switch so all is good, just saying with coils and AC keep in mind inductance has to part of applying ohms law to get power

 

Good job on the repair, it looks very professional

 

A very valid point. Also in some cases the power goes through other components before it reaches the coil. There may be some form of inrush current limiting device , an NTC or something similar,  that will also mess up your calculations if you simply try to gauge the current by measuring coil DC resistance, especially if you simply measure across the mains plug.

You can use a clamp meter to measure current, but you then need to clamp it round only one of either the live or the neutral. Simply clamping it round the whole mains cable wont give you a valid answer either.... and then we come to what do you mean by AC power, are you talking about average AC voltage or true RMS...etc...  but that is a whole different story.

In some simple cases, you can get a rough estimate by measuring the coil resistance, but it will only give you a possible ballpark figure.

In this particular case, it is probably near enough for our purposes. Anything more complicated (motors, active electronics or whatever), and you could be miles out.

Edited March 13, 2019 by AndyHull

[praezis]

All correct, and I admire your knowledge, but why make the matter over complicated?

Here we have a coil without iron core and a low frequency of 50 or 60 Hz. You will find no relevant difference between AC or DC resistance...

Frank 

[AndyHull]

38 minutes ago, praezis said:

All correct, and I admire your knowledge, but why make the matter over complicated?

Here we have a coil without iron core and a low frequency of 50 or 60 Hz. You will find no relevant difference between AC or DC resistance...

Frank 

It may seem that we are being pedantic, but it is more a case of highlighting and understanding the limitations of this technique, after all, for small values of flat, the earth is flat.

[measuretwice]

11 hours ago, praezis said:

All correct, and I admire your knowledge, but why make the matter over complicated?

Here we have a coil without iron core and a low frequency of 50 or 60 Hz. You will find no relevant difference between AC or DC resistance...

 

good point, without the iron core they will be very close.   No interest in over complicating things, I overlooked lack of the iron core.  Point about caution on doing AC/coil/current calcs based on resistance stands as so often inductance is most of impedance

Edited March 14, 2019 by measuretwice