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Floating voltages
Floating voltages
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Old 23rd January 2004, 06:05 PM   #1
eeka chu is offline eeka chu  United Kingdom
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Join Date: Nov 2003
Location: England
Unhappy Floating voltages

I'm having a very bad time trying to think of a power supply that will do what I want it to and not be -unnecessarily- complicated or expensive.

I'll number my questions in case it makes it easier to see where I'm having trouble.

There is one possible method that involves using a lot of seperate windings and, in turn, seperate filter / regulation circuits. But I don't want to go that way unless I really have to, especially if there's something simpler.

I know a lot of you are incredibly good with semiconductors, I am most definitly not! So I expect most of these questions will be real easy for you to answer.

First of all. I read a post on this site a few days about floating voltages and that you theoretically won't get a shock if you touch the output of a transformer that is not referenced to ground.

I have also read a number of pages, one by Textron, that outlines a serious risk involved in measuring floating voltages, suggesting that the theoretical idea is not necessarily what happens in practice.

1) With this in mind, would I be right in saying that it could create a potential shock hazard if things such as LED and fans, that the user could touch, where floated over their normal operating voltages?

2) If I want to float a voltage in a rectifier, filter, regulator supply, does it make any difference whether I connect my imaginary centre tap at the transformer outputs or all the way down the other end at the output of the regulator?

3) Following on from that, if I use a regulator like the LM317, when I connect the floating tap at it's output, will the voltage before the regulator be able to 'see' the ground tap reference at the output?

4) Finally.... if the reference will be visible thru the LM317, is there anyway I can isolate floating voltages from each other so that I can use a multiple of floats from one supply?

I'm really sorry I've asked them in such an impersonal manner, I thought it might make it simpler for you though. Rather than mixing it all into one post.

I've looked for answers to these questions and am having trouble finding any, which is why I've come to ask for your help.

You can probably already work out from my questions what the problem is. I need to reference my low voltage supply to at least a few difference voltages. The first way of doing this, that I thought about, was to use seperate windings and supplies. But that's going to start costing a lot of money.

It would ideal if I could tap my low voltage supply off into seperate channels and then float each individually. I would like to float the voltages for things like the bias and heaters, along with keep one referenced to ground for all the auxiliary bits and pieces.

I have looked at 'isolating floating voltages' on google, but I'm getting pages about measuring them in industrial enviroments where the word isolating is being used in place of locating. I also found a lot of pages on using isolating transformers. I suppose it is essential that I say it just in case, the voltages are DC and need to be stable.

Please help me out on this guys, I'm starting to think about it in bed!!!!

I just want to get all this annoying stuff sorted and move forward.

My best wishes!
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Old 26th January 2004, 09:41 PM   #2
djmiddelkoop is offline djmiddelkoop  Netherlands
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Location: Noord-Holland

I will try to answer your questions, altough they are a bit fuzzy.

1) no, floating means no reference to anything, so as long as you touch only one point, nothing happens.
2) it depends a bit on the circuit used, but mostly it's the end.
3) yes.
4) only if you use seperate transformer windings for each floating voltage.

Oh, and don't use floating voltages for heaters. They must be referenced to the cathode, i.e. there is a maximum cathode-heater voltage.
When using a floating bias voltage, it must be connected , read referenced , to the main voltage powering the tube.

Hope this helps,
“Information is not knowledge Knowledge is not wisdom
Wisdom is not truth Truth is not beauty Beauty is not love Love is not music Music is THE BEST”
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Old 27th January 2004, 09:10 AM   #3
eeka chu is offline eeka chu  United Kingdom
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Join Date: Nov 2003
Location: England
Thanks Dick,

It definitly helped! I should have really said "raised heater voltage" instead of floated.

Time to get my coloured pens and start working it all out!

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