Floating power supply - Page 1
Forum section: electronics / beginners

author: chimera_786
author_rank: Regular Contributor
timestamp: April 11, 2012, 06:24:10 pm
content: Hello!I have a floating power supply; this means that it has two output lugs (positive and common). It does not have negative voltage output becaz I tend not to work with negative voltages as much.Now, when I look into the circuitry, there is a EARTH wire however its connected to the casing of the power supply. That's understandable becaz you dont a shock hazard present. However, what if I connect the COMMON of my power supply to the EARTH connection?That would give me true 0V reference. Is that suggested or am I creating a problem for my self?

author: sacherjj
author_rank: Frequent Contributor
timestamp: April 11, 2012, 06:45:41 pm
content: That is generally fine.Your idea of a "true 0V reference" isn't really possible. Voltage is simply a measure of different potentials between two points. Ground or 0V is whatever you define it as. Earth ground doesn't have to be though of as 0V.If you define your positive output as 0V, then you indeed have a negative output power supply. If the power supply is properly floating, you could tie this output to ground and make the negative be a voltage below ground.All of it is based on what you set as the reference point.

author: chimera_786
author_rank: Regular Contributor
timestamp: April 11, 2012, 07:45:42 pm
content: Please see the attached schematic. While viewing it, you can see what I mean when I say connecting COMMON to GROUND. Like I mentioned earlier, the EARTH connection is connected to the housing.If I am to proceed with it, I would simply solder a wire from COMMON directly to the point where the EARTH connection is being connected to the housing. Does that seem to be correct/safe? Would doing so help the accuracy of the power supply?

author: amspire
author_rank: Super Contributor
timestamp: April 12, 2012, 03:11:56 am
content: You can connect the common to mains earth, but do you actually want to? We don't know what you are going to use the supply for, so it is hard to give advice.There is a good reason why power supplies usually have a floating supply and a separate earth terminal that you can connect if you choose.Not earthing the supply may be important if you need to connect it to a circuit that cannot have the 0V earth. For example, if you need to use it as a negative supply with the positive rail earthed, or if you want to put it in series with another supply to increase the voltage. If you want to keep the option open, leave the mains earth as a separate terminal.A second reason is earth loops can cause measurement problems, so there are times when you need to be able to control the earth on the circuit at the load rather then having multiple earth connections through the power supply, oscilloscope, etc.A third reason is at times, you might need to connect an oscilloscope between two points of the load that are not connected to the negative rail. You can do this with an isolated supply, but you cannot with an earthed supply.If none of the above matter, then there is probably no reason not to connect the negative supply rail to mains earth.Richard.

author: ejeffrey
author_rank: Super Contributor
timestamp: April 12, 2012, 07:45:10 am
content: Quote from: chimera_786 on April 11, 2012, 06:24:10 pmActually it doesn't. Floating power supplies have two terminals: + and -. Either one (or neither) can be designated as zero volts DC in your circuit. Likewise, you can connect the ground terminal to either + or -, to neither, or even to a different node in the circuit. If your circuit is isolated, it doesn't matter at all, the ground connection has no effect.The ground terminal only matters when you connect cables between your circuit and some other device with its own power supply.

author: T4P
author_rank: Super Contributor
timestamp: April 12, 2012, 09:21:07 am
content: What's the exact way of paralleling floating supplies ? And in series .Did a google search and no relevant images came up .

author: caroper
author_rank: Regular Contributor
timestamp: April 12, 2012, 09:47:52 am
content: Try this DC POWER SUPPLY HANDBOOK from Agilent for a very detailed look at power supply's:http://www.google.co.za/url?sa=t&rct=j&q=&esrc=s&source=web&cd=1&ved=0CCYQFjAA&url=http%3A%2F%2Fwww.wfp76364.w1.com%2FHP-AN90B.pdf&ei=maOGT-f-CJOIhQf5ke3fCA&usg=AFQjCNEDaz8H4e2h3OkPP-3LWqY9MdBETQCheersChris

author: Rerouter
author_rank: Super Contributor
timestamp: April 12, 2012, 09:54:38 am
content: to connect them in series is trivial, think of both like batteries, positive of one to negative of the other, and you get double the voltage, or +- that voltage depending on where you tap off it,as for in parrellel, that gets a tad complex, as you need to both balance the voltage and current, which for low loads can be done with a resistance value, e.g. 1 ohm, but for larger loads, needs an active current muxxing system, from memory there are a few online from people parrelleling ATX power supply rails, which covers the worst case stuff, as atx supplies are both noisy and 2 different models can have radically different loading profiles,

author: chimera_786
author_rank: Regular Contributor
timestamp: April 12, 2012, 03:05:35 pm
content: Quote from: amspire on April 12, 2012, 03:11:56 amThanks for breaking it down Richard. None of the above apply to me as yet. I do have a EARTH connection in my power supply which is connected to the housing of my supply. I'll leave it alone then till the time I have some definite reason to attach it to the common return.Quote from: ejeffrey on April 12, 2012, 07:45:10 amFinally! thank you for explaining it over in some relevant detail! Yes, my power supply has a step down transformer; so it is isolated. I do not plan on connecting multiple power supplies in series/parallel and nor do I plan on connecting an external circuit with its own power supply.SO, therefore verdict: I will be modify my current power supply by adding a EARTH (ground) lug in the front where the other two lugs (positive and return) are present. In this way, I will eventually have an option available to me, should a need ever arise.Thanks for all the help every one!