Screen trip circuit

[N4LTA]

Anyone ever tried to build an overcurrent trip circuit with one of the low current opto isolators?

I was looking at the 6N139 which will trip at .5uA. It is rated at 2500 volts RMS isolation and has a darlington output. It looks like it can be wired as a latch and would be a good way to shut down the screen supply on overcurrent and also you could use it to sense plate voltage and drop the screen if the plate supply drops out.

I have ordered a few for testing.

[Opcom]

The low current requirement would certainly allow it to be well isolated from whatever it was measuring. That would keep to a minimum the amount of dangerous power that could get to your monitoring board. Beware RF will play the devil's bagpipe with that if not well shielded.

[N4LTA]

I plan to rig up a couple  - one  to detect plate voltage and one across a series resistor to serve as a screen trip. I'll report back when I get it tested.

I am putting together a power supply board for grid and screen supply for a single 4-400A class c amp and the control circuits. The grid supply will be regulated with a zener string. The screen supply will have a small variac and a stiff supply with a large bleeder resistor.

I purchased a large quantity of 240uF 400V snap in caps a while back on the cheap (89 cents each). These will be used as filter caps on the board and a string of 5 will be used for the 1250 volt modulator supply with a 10 Hy 500 MA choke input.

I have been using a string of 8 in a 2200 volts supply and they have worked very well and have held up. Thats  a pretty cheap filter cap - and so far no problems.

[N2DTS]

Why regulate the bias?
If class C, its better to grid leak it, and add protection bias through a diode.

I would never use electrolytics in series or over 400 volts.
Just a bad idea in my book, when a good oil filled cap will last forever if used under its voltage rating.

About 30 years of use on my homebrew stuff and no crapouts!

Brett

[KM1H]

With quality electrolytics lasting 30+ years I dont see any reason to not use them especially when space constrained and doing away with a choke.

I use both versions here and the pair of 12 uF @ 4Kv oils in the 2M amp were NOS and almost free 25 years ago and big electrolytics were damned expensive. OTOH, the PS takes up twice the height as the amp and if I decide to compress all the racks around here into a smaller footprint everything will go electrolytic. There is also a 53.5uF @ 10kV oil that I cant even lift holding down a shelf; its been there 20 years. Maybe someday I'll build the steamer around that tube I showed on here awhile back

IMO there is no reason for one over the other except personal choice.

Carl
KM1H

[N4LTA]

I have heard the negative criticism on the electrolytics but have never had one go bad.

I make my own G10 boards and am conservative in applying them. Most never see more than 60% of rated voltage.

In my modulator, I will have 1200 volts on the 2000 volt bank - so the 400 volt rated caps will have 240 volts on them. I will have a 48uF filter that cost less than $10 and can easily fit under the chassis. In fact the entire power supply for the modulator, the grid bias and the screen is all on one chassis.

I use the BY228 diodes  in the strings. They are cheap (50 cents) and are rated 1500 volts PIV and 3 amps - 50 amp surge. Heavy duty glass encapsulated  and tough.  Get a bit more PIV than the 1N5308 and a little more cost.
I quit using balancing caps a long time ago.

The bias supply is switch selectable from Class C to Class AB as I might as well make it linear capable.

We will see how it all works out.

Pat