Floating chokes?

exeric

Member
2008-03-24 6:23 am
I'm having a problem finding a choke with a small enough current rating for my HV application power supply but which still has a high enough voltage rating. I understand that the choke voltage rating equates to the resistance to arcing of the coil to the frame of the choke.

I thought of putting the choke in the return line but that does not seem to work so well. When I simm it on LTspice it causes a 12 to 16 volt unfiltered voltage on the line connecting the bottom half of the power supply capacitors. Am I correct that the load for that power supply is still connected to regular ground and not the lower end of those capacitors? Sorry if this is a dumb question.

The other alternative that I though about was just floating the frame of these small chokes. As long as there is no danger of fingers or conductive parts in close proximity it seems like I wouldn't need to worry about it. These chokes are close to the range of what the power supply will be, i.e. 300 volt rated choke in a 500 volt circuit. Is this a viable idea? It won't be in a metal chassis, but a wood one.
 

rayma

Member
2011-04-29 8:37 pm
The other alternative that I though about was just floating the frame of these small chokes.
It won't be in a metal chassis, but a wood one.

Can you post the schematic, and is this a choke input supply? How much current do you need?
Don't float the choke frame unless it's sealed in an insulated enclosure, with the mounting bolts
isolated from ground and touch as well.
 

exeric

Member
2008-03-24 6:23 am
Well, in the absence of illumination from someone yet on uploading an image I'll state the obvious. It's a capacitor input circuit. The choke is between the bottom of the two power supple capacitors. Ground is located on the rectifier side of the choke.

I did the simulation and it appears that chokes on the return side just don't work as advertised. The regulation of voltage in simulation changes from about +- 1 mv to about +- 3 volts between ground and HV. Voltage level itself is about right. I trust LTspice on this. The only thing that was changed was the placement of the choke above to below in between the caps.
 

exeric

Member
2008-03-24 6:23 am
Well, all it says is that is that it is rated at 300 volts DC. It is a Hammond choke. Here is the page:Hammond Mfg. - D.C. Filter Chokes - (153 - 159 Series)
You go to the pdf page for the one I'm interested in - the 154ea - and the curious thing is that it there says it's rated at 500 volts. And you go to the introductory remarks and it says each individual choke is hi-pot tested at 1500volts ac. It will live in a 500 volt dc environment.

I'm really feeling that my best bet is to just leave the choke frame ungrounded and that it will probably be fine. It's going to be buried in the amp at a place no one can touch it without disassembling the amp. It will be wood screwed to the wood chassis so at no point will it have contact with something that conducts. I don't own an insulation resistance tester so that won't work for me. Here's to living dangerously.:drink:
 
Chokes in the return side work fine, provided that you get the grounding right. PSU grounds should always be at the output end, not the rectifier end. This is true for any PSU, but the effect of getting it wrong is far worse when the choke is in the negative side.

The safe way to use an underrated choke is to put it in the negative rail.
 

exeric

Member
2008-03-24 6:23 am
DF96, unfortunately I misspoke. I would never place a ground at the rectifier. Unfortunately a picture is worth a thousand words and no one here has yet seen fit to lead me to the forum location that instructs one how to post images. That would have shown better than I could ever say what the problem is.

LTspice, for whatever reason, definitely shows it not to work right having the choke on the return side. It does not seem quite fair to me to argue against a person with one arm (no image posting) tied behind his back.
 

rayma

Member
2011-04-29 8:37 pm
I would never place a ground at the rectifier.

If you think about it, as long as you connect the ground after the choke, the circuit operation is identical.
The rectifier/input capacitor circuit block is in series with the choke. It doesn't matter if the choke is at one end
or the other of the circuit branch, the result is the same. There must be an error in how the circuit model is connected.
 

exeric

Member
2008-03-24 6:23 am
Rayma, thanks for the help. It appears whatever is causing the problem will take some time to figure out.

On the happy side I just figured out what I was doing wrong in LTspice. I was putting ground on the side of the choke closest to the center tap of the transformer. When I placed ground on the other side of the choke everything is resolved and the voltage is well filtered again. Posting images would have resolved all this drama earlier, but at least it's resolved.

To clarify: I'll be putting the choke on the return side and will refrain from taking unnecessary risks.
 

exeric

Member
2008-03-24 6:23 am
Note that as the secondary and the rectifier are in series placing a ground at the secondary is equivalent to grounding the rectifier.

I doubt that many people would agree with this. I certainly don't. All center tapped transformers with chokes on the hv side have ground located at that center tap. Do you consider that grounding at the rectifier, i.e. before the load resistance?
 

exeric

Member
2008-03-24 6:23 am
That's a whole separate issue from what we were talking about. That's about ac noise on the ground line. For example, John Curl's principle of having his products double insulated and not having a ground link at the plug. I think it's better to not conflate ac and dc. For dc you always ground at the center tap except in special cases like what we were talking about.
 
exericc said:
That's about ac noise on the ground line.
No it isn't. Its about charging pulses in the PSU ground.

For dc you always ground at the center tap except in special cases like what we were talking about.
As I said, you directly ground (to chassis or signal ground) the CT if you like buzz. If you prefer quiet then connect the CT to the reservoir cap negative. Then connect this to the smoother negative (in your case using the choke instead of a wire link). Then ground the smoother negative. Most people get this wrong; most people are then amazed how quiet it becomes when they then get it right.