Hammond Choke Voltage Rating

[N4LTA]

I have a brand new Hammond 10Hy choke rated at 500MA. I plan to use it in my modulator supply at about 1100 volts. Hammond rates it at 1KV but they are very conservative. I have some 1" HV standoffs that I can mount it on. Would you go to the trouble or just mount it on the chassis?

Pat
N4LTA

[WQ9E]

Pat,

I would mount it as normal and it should be fine since it is new and conservatively rated.  I really do not like having things that are supposed to be grounded floating above ground.  If there ever was a failure then you could easily be fried by touching something you forgot wasn't at ground potential and we all expect choke and transformer cases to be at ground.  Replacing a crapped out choke is easy and also far cheaper than funeral services for a Kentucky Fried operator.

If you do float it above ground, paint it safety orange or something so you or the next owner does not get bitten.

Rodger WQ9E

[KE6DF]

Pat,

I would mount it as normal and it should be fine since it is new and conservatively rated.  I really do not like having things that are supposed to be grounded floating above ground.  If there ever was a failure then you could easily be fried by touching something you forgot wasn't at ground potential and we all expect choke and transformer cases to be at ground.  Replacing a crapped out choke is easy and also far cheaper than funeral services for a Kentucky Fried operator.

If you do float it above ground, paint it safety orange or something so you or the next owner does not get bitten.

Rodger WQ9E

Pat,
I agree with your point about not floating things above ground. I realize all the good electronic reasons for doing so, but I too am worried about how years from now, when I'm an SK and someone sells my rig to a 17 year old kid, and he plugs it in and touches the transformers to see if they are getting warm...

Rodger,
Another option you have on the hammond choke is to put it in the negative lead of the supply.

In the case of a FW CT circuit, put it from the CT to ground.

This keeps the DC voltage on it at zero. There is still an AC voltage across the choke -- particularly in a choke input design -- but this will extend the range of voltages you can use a choke for.

Or course this puts a non zero voltage on the transformer CT,  so you need to be sure that's OK.

[N4LTA]

I'll put it in the negative side. It is a FW bridge, choke input. That keeps the DC off and should help Thanks for the input.

Pat
N4LTA

[k4kyv]

Unless you are running the choke well within its voltage rating, I would put it on insulators and float it above ground.  If you are afraid of "forgetting" and touching it after the insulation has broken down, affix a bright red adhesive tag to the choke to carry a conspicuous warning to remind you that the case or frame may be "hot" with the full high voltage.

Particularly with older components that may have questionable insulation, I always float filter chokes, modulation reactors and modulation transformers. 

I don't see any justification for putting unnecessary stress on the insulation between winding and core, which usually is nothing more than a few layers of paper impregnated with some kind of varnish, and this paper often becomes brittle as it deteriorates with age and may soak up moisture from the air.

In this case, you have a new choke, but you are running it above its maximum rated voltage.  In addition to the +HV, you have to add the a.c. ripple voltage that appears across a filter choke.  Often the peak ripple voltage exceeds the steady DC voltage, so you could have over 2000 volts peak at some point between the winding and the core.

The problem with running the choke in the negative lead is that the full a.c. ripple voltage appears at the mid tap of the plate transformer.  Some HV plate transformers were not insulated well enough to carry any significant voltage between the mid tap and ground; the manufacturer may have assumed this would be directly grounded and the insulation may not be designed to carry more than a few hundred volts.  I have even seen transformers with the midtap grounded to the case internally. 

I always treat a floating transformer or choke the same as if the case were hard wired to the +HV and known to be "hot".

[W3RSW]

Then again, a bridge rectifier circuit with the choke 'isolated' from the windings by two diodes and still in the negative side of DC is nice.
- from a transformer designed for bridge output, of course.

[N4LTA]

Thanks Don.

That was what I thought would be best for the choke.

I have some Storm Copper 1" standoff insulators with 1/4 -20 inserts and that was what I was thinking about using. The plate transformer is a new Hammond 720 rated at 1250 at 300 Ma CCS or 150% more at ICAS. The insulation is OK there as the Mfg recommends either a tapped FW or bridged FW connection.

With a choke input filter, I figure to get 1125 volts DC at 350-400 Ma max.  That would give me close to 400 watts input to the pp 811As and should get me 250 watts out which is the rating of the Modulation transformer.

The modulation transformer is a UTC S22.

Pat
N4LTA

[W7TFO]

Floating a choke or other HV device works well but may be a hazard down the road.  Here's what an old-timer I knew did:  put a NE-2 neon bulb and resistor from the case to ground, mounted on the side in a fuse clip.  If it glows, you know it has HV on it so beware.

[w1vtp]

Floating a choke or other HV device works well but may be a hazard down the road.  Here's what an old-timer I knew did:  put a NE-2 neon bulb and resistor from the case to ground, mounted on the side in a fuse clip.  If it glows, you know it has HV on it so beware.

I LIIIIKE IT!  There are some pretty impressive neon bulbs (besides NE2's) out there that would grab your attention. 

Al

[N4LTA]

We old guys like the old light bulbs don't we?

I was designing a control system for my transmitter today. I am using solid state opto- isolators to detect plate voltage and screen trip current. I just finished the PC boards and was doodling out a transistor circuit to drive jeweled #47 pilot lamps on the front panel.