floating power supply

[labview1958]

Is it true that some circuits can only work on a battery and not on a ordinary lab power supply connected to the mains. If so, why the difference?

[Externet]

Hi.
If it is 'ordinary' , may have its negative terminal grounded and can give noise or problem to certain balanced or isolated circuits.

A truly floating power supply with no ground connection nor capacitive bypassing should be able to work just as a battery.

Telephony circuits get specially angry when fed by wrong supplies.

Miguel

[MrAl]

Hi there,

You can sometimes use a wall wart and regulator to act as a floating
supply. I do this with one of my voltmeters that normally takes a
9v battery. I have it set up to feed the voltmeter with a small
cap to filter ripple a bit better than the wall wart already does.
Works great, and i can measure anything...even circuits connected
to the mains because the wall wart output is isolated from the mains.

BTW Externet your icon is very annoying...cute though, but it makes
it harder to read your posts. Most people dont like that.
Sorry.

[Externet]

Thanks, Al. Did not think about that.
Will replace it soon
Miguel

[Robert Reed]

OOPS! double post - deleted.

[Robert Reed]

Any quality lab supply is floating and will have a third terminal for grounding.This gives you the option to ground either the positive or negative terminal if desired. The supply should look like a battery to the load. One situation comes to mind that may have instigated your post is digital panel meters ,which in some cases cannot share a common ground with their input terminals. In this case the DPM power must be "floated " separately from all else.
BTW I agree with Mr Al on Exter's icon. Had mentioned this before.

[labview1958]

The lab power supply in my lab has a positive, negative and a ground. A battery has a positive and negative terminal. If I connect my circuit to the positive and negative supply of my lab power supply, then it acts as a battery. Is that right? What use is the ground then? The battery does not hve ground, yet it works. I am a bit confused.

[Chris Smith]

No, it becomes a floating power supply, independent of EARTH or ground.

[Robert Reed]

Labview
You do not need a "ground" to complete a circuit. You only need a positive terminal and a negative terminal and a load to complete a circuit. Frequently "ground "is referred to as a reference point to which all other voltages are measured . These voltages can be either positive or negative or both in relation to the "ground". The ground terminal on your lab supply refers to earth ground which is common to power line AC power all the way back to the power plant. This not only is for safety's sake in these applications but also establishes a common reference point between various pieces of equipment. That is one use for the term-ground. Each individual peiece of equipment would function without the ground just fine as it still has a complete internal circuit. A battery circuit is just that--a complete circuit with no "ground" as in earth ground. However, an ungrounded (earth) peice of equipment can still have its own ground by virtue of a common level which is usually zero volts in relation to all others. For instance. in an auotmotive environment. we usually refer to the chassis as "ground" and the 12V battery term as "hot", even though there is no actual earth ground attached. We could just as easily tied the pos batt. terminal to ground and made the neg. term "hot" if the load were designed to accept this. When we say "ground", its all a matter of perspective.,

[dyarker]

Well said.