negative voltage supply

[Afifa]

Hi all,

Can anyone clarify why a negative voltage supply is used in B-N transducers?

Is the use of NPN transistors has made this the industry standard, or is there any other reason related to the eddy currents phenomenon for using the negative supply?

Kindly help me in understanding this point, as googgling has not satisfied me uptil now!

regards,
Afifa

[constovich]

Hello Afifa,

If a transducer designer could answer that would be great, but this is my interpretation:

We as humans like seeing larger numbers when the process get larger (in this case distance). With the current "-24 VDC" input that most people use, the signal when referenced to return would become more negative as distance increases - obviously, ignoring the polarity as most people do.

If they were to "reverse" polarity and use a 24 VDC power supply (which you can do anyway with the leads reversed), when the signal is referenced to +24VDC Return, the signal's amplitude would get smaller respect to the +24VDC Return.

-24 VDC ===== 24VDC Return
Signal ===== Signal
-24VDC Return ===== 24VDC

Amplitude INCREASES with distance between Signal and -24VDC Return and DECREASES with distance between Signal and +24VDC Return.

[ronbow]

That sounds great...but the real reason is because Don Bently wanted it that way... Really!!!

[constovich]

Hey ronbow,

I guess it wouldn't be the first time that some arbitrary value is used for something with no basis. Another example would that the width of the Saturn V rocket is proportional to the width of a...erhm...horse's rear. It is because the rocket was ferried on train tracks and the width of standard gage train tracks is the result of making the rails far enough apart so horses wouldn't trip when pulling carts on the rails (before the days of steam trains of course).

So was Bently's original choice -24 VDC or -18 VDC? I would still assume that his decision was based on the availability of parts / logic families available when he built his first drivers.

[ronbow]

True...true...well at least the saturn rocket story has somewhat of a reason...

[ronbow]

it was orginally -18 and was based on nothing...

[Steven Sturm]

There actually is a reason for the negative voltage. Don Bently designed the first proximitors back in the early days of transistors. Most electronics at the time were still vacuum tube based, but the proximitor had to be small and relatively low power for the application so a transistor was used in the oscillator circuit. Transistors were quite expensive at the time and PNP transistors were easier to manufacture than NPN and therfore cost less. The nature of a PNP dictated that the supply be negative and therefore the first proximitors were negative voltage supply. No electronics or eddy current related reason for the negative supply to still be used today, but once the standard was set it would have caused too much trouble to change with the large installed base of sensors and monitors.

[Afifa]

thanx for the comments everyone !