Re: Remember Mr. Bill's servo lite?

There are several potentially sertious problems with using a "non isolated" variac. Even if you are careful to ensure that the neutral is made the common line through the variac. The neutral will be at or near ground level, so there is a measure of safety in that but there is never any gaurantee that the neutral will remain at near groung level, it is not the same as a true ground. Another big problem with a "non' isoltated variac is that if you use a bridge rectifier to derive a DC source for motor and circuit logic you cannot ground either the plus or the negative side of the supply, you will short the secondary winding with spectacular smelly result!. Thirdly because the neutral is not true ground and you cannot ground either side of a DC supply powered by the variac out put you will end up with no control over any electrical "noise" that hydro is cursed with. Simply put your logic will have an unpredictable behavior. Lastly but by no means lest of all any lightening that hits the hydro will probly take out your electronics.

If you want adjustable voltage attach the variac to the output of an isolation transformer, then you can arounge your grounding with less concern for ground loops.

Mike

--- In CAD_CAM_EDM_DRO@yahoogroups.com, "Tom" <thom@...> wrote:
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> --- In CAD_CAM_EDM_DRO@yahoogroups.com, Andy Wander <awander@> wrote:
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> > Again, you are only saying the "conventional" stuff, Jeff. I don't mean to pick on you, but can you tell me why a direct path from mains to earth is more dangerous than a magnetically coupled path from mains to earth?
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> > I can't see any reason why 100V, say, should be more dangerous whether it is through a transformer or directly coupled to the mains.
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> > Andy Wander
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> Andy you are right. 100 volts is deadly no matter which way you receive it. As long as ALL of the connections are correct and the saftey grounds are valid and the primary side does not end up with the Hot tied to the neutral then it is fine. However sometimes builders can miss things and end up with a hot chassis. With a true transformer you can redefine the circuit common. You are dealing with line neutral (0 volts if the transformer on the pole has a good solid ground) and Safety ground that may not be a real ground at all. If there is no ground rod at the main breaker and the Neutral and safety bonded as they should be you can have problems.
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> I think the need for a transformer for isolation diminishes as the voltages increase. What does need to be done is to make sure there is NO WAY for the user to get across a HOT and a building ground on the load. A hand drill is an example. Double insulated.
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> All it takes to kill is a few Mils of current across your heart. It does not matter where it comes from. depending on skin resistance voltages above 48V can start to be dangerous. Dry skin can take 60 to 75.
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> If you are building a power supply with high voltages and no isolation then make sure the AC side is connected properly, that neutral on the AC side is REALLY zero volts in relation to earth ground and that you protect any person from touching ANY wire
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> In a servo circuit it is not required that anything is tied to a common chassis and in fact may work better if it is not. All that PWM noise does not get pumped back across a common connection to the logic circuits at the PC. The servo motor is a coil and just needs voltage across it to spin. One side does not need to be common to the chassis or the AC ground.
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> We find in industrial settings that building wiring is not always pristine. We also find that noise is rampant and when plasma cutting (high voltage and high current) are involved grounds connected locally may become major noise conduits.
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