Isolating an SMPS from earth - Page 1
Forum section: electronics / projects

author: BiOzZ
author_rank: Regular Contributor
timestamp: April 14, 2013, 09:14:29 am
content: I need a simple high current SMPS that's ground isolated ... i have none of theseim going to do the age old conversion of a desktop power supply (not to concerned about ripple) and my plan is to simply cut the tabs that connect to the LV ground side of the PCB and insert plastic standoffs and hot glue than use a HV isolated power switch and mount it on the casei have all these parts sitting about but im afread im missing some sort of RF ground *does magical movments with hands* voodoo that might make this not work so if i can get some conformation that would be great!i could just unhook and snip the ground lead but i want atleast some protection XP ... i dont trust these no name PSUs as far as i can throw them even tho i checked this one out and it looks legit

author: eevblogfan
author_rank: Frequent Contributor
timestamp: April 14, 2013, 09:27:32 am
content: heyI did disconnected my earth ground lead and the SMPS works , but gave me nasty shocks when touched the 12V line . please be careful and use Y caps from the - + to the ground , and open your SMPS and check for any cap between the 12V side and mains side , if you didn't found , you should be concerned since they sometimes apply some winding and earthing it and using the capacitance between the windings as capacitor , so you should un-wind that single winding ot tracing it down and de solder that tap ,good luck and hope to see updating on that because me and other can help you ,P.s what's the project ? shear it with usCheers !

author: SeanB
author_rank: Super Contributor
timestamp: April 14, 2013, 09:38:44 am
content: You can do that, but what is the voltage you intend to float it to? If it is over 300V then you have a problem, as the insulation on the transformer is really only rated for that. As well remember the fan will have a limited standoff voltage ( which may not be specified anywhere) if there is one used for cooling. It too will require insulating as well, as often the plastic used in the housing may be conductive somewhat.

author: BiOzZ
author_rank: Regular Contributor
timestamp: April 14, 2013, 09:56:22 am
content: Quote from: SeanB on April 14, 2013, 09:38:44 amnothing over 48 volts will go threw this so i think im fineQuote from: eevblogfan on April 14, 2013, 09:27:32 amthats why i want the case still grounded XPand im working on a high voltage CCPS powersupply but it takes -12v and 12v at like 8 amps to run in my current design and i think this is the easiest way of going about this

author: Alana
author_rank: Frequent Contributor
timestamp: April 15, 2013, 12:16:05 pm
content: In case of AT/ATX computer PSUs typically there are 2 earth connections by means of mounting screws.This connection on the HV side is needed for EMI - it grounds input mains filter. But i think you can insulate the other connection - this one on LV side. This leaves case grounded and gives you secondary side insulated from mains earth. On PCB there is no other ground connection between HV and LV sides - just contacts near mounting screws that touch PSU metal case.If you are talking about some other equipment try looking for something similar. I have a hunch that there will be something very much alike.

author: kt315
author_rank: Regular Contributor
timestamp: April 17, 2013, 12:24:30 am
content: I might be wrong, but I do not believe -12V on comp PSU is typically rated for 8 amp ...

author: Fsck
author_rank: Super Contributor
timestamp: April 17, 2013, 12:27:43 am
content: Quote from: kt315 on April 17, 2013, 12:24:30 amI've only ever seen 0.5-1.0A these days.

author: Alana
author_rank: Frequent Contributor
timestamp: April 17, 2013, 01:02:02 am
content: Yes but if you have 2 ATX PSUs with both grounds LV insulated from mains then you can conect +12V outputs in series and get +/-12V 8A.And one more thing - if you use older models, those with 5V rail rated for 30+ amps its best to re-wire feedback circutry from controling 5V rail to 12V rail. Alternative is loading 5V rail with couple of watts.

author: peter.mitchell
author_rank: Super Contributor
timestamp: April 17, 2013, 10:07:55 am
content: If you need +/- 12v or 24v, it's best to only isolate/float one for safetys sake, and then when you connect them in series, neither will be "floating", one will be below and one above mains earth.