Agilent/HP E3612A "Floating Ground" Question

[sonicj]

hi! i recently scooped up a HP E3612A from the local flea market, and unfortunately, the user manual assumes the operator knows what he/she is doing and fails to give specific examples of the product in use. i comprehend the basics, but what im having trouble figuring out is the "floating ground."

from the user manual:

The output of the supply is isolated from earth ground. Either
output terminal may be grounded or the output can be floated
up to 240 volts off ground.

does this mean i can get 240v dc from the PS? if so, how? if not, can it be done with external circuitry?

in case it matters, im trying to light up some nixie tubes my dad gave to me. any insight/guidance is greatly appreciated! thanx!
-sj

HP E3612A Product Page

Data Sheet <==69 KB

E361xA User Manual <==12.1 MB

[DJPhil]

The short version is that it means you could use another supply of the same type to hit 240V.
Imagine two 9V batteries (floating voltage sources) connected in series. You could get 18V from the outside terminals, or +9V/-9V with the shared terminal acting as ground. Now if the same two batteries had their negative terminals referenced to earth ground (think connected to a chassis, for example) then you're stuck with a 9V supply, albeit at twice the current.
Floating in this sense means that these supplies act like batteries. If you took two of these supplies (or two sections of a dual supply) and wired them in series you could get double the voltage safely. The protection circuitry within is likely not designed for more than 240VDC, so if you wire a higher voltage supply than that you must ensure that it's not ground referenced, i.e. that the whole circuit is never referenced to ground. I'd avoid mucking with voltages that high if you can help it.
In the end, ground is where you say it is. Earth ground is, for the most part (wiring losses, lightning strikes, and dry soil aside) an absolute reference on the scale of a building, so it's handy to reference to.

As a side note, most nixie tubes won't require more than about 80V to trigger. See here or here for details. Be gentle with nixie tubes, they have a finite lifetime and are no longer being manufactured!

Hope that helps.

[sonicj]

hey thanx DJPhil! the 9v battery analogy made a lot of sense!

so basically, i need another e3612a or a dual output model to be able to series the outputs for double the voltage. sound correct?

is there a external circuit i could build to boost the 120v to the minimum striking voltage? (160-170VDC) i found what i think is the data sheet for those nixie tubes... for longest life, they recommend 300v w/a 82k resistor.
-sj

[DJPhil]

hey thanx DJPhil! the 9v battery analogy made a lot of sense!

Excellent.

so basically, i need another e3612a or a dual output model to be able to series the outputs for double the voltage. sound correct?

Sounds right to me. However, you probably won't need more total power than one supply can provide, so there are other options.

is there a external circuit i could build to boost the 120v to the minimum striking voltage? (160-170VDC) i found what i think is the data sheet for those nixie tubes... for longest life, they recommend 300v w/a 82k resistor.
-sj

You have a datasheet, excellent! I found the full document from that sheet, very useful! Here's the link just to be sure in case you don't have the whole thing. My memory isn't the greatest, as you can see.

This is a perfect job for a switchmode power supply. You only need a few mA per digit, so the total power is very low, about 1W. To put this in perspective you could drive a tube for about four hours with a 9V battery and 80% efficiency! A switcher would allow you to use a standard 5V or 12V supply for the rest of your circuitry. The tubes themselves won't care very much about power supply noise. You could even make a simple switcher out of a 555 like this. It's a good project to start with, just be safe around those voltages.

Hope that helps. I'd have gone on a bit but I'm late for somewhere.