Screen grids zener shunt regulator project

[catalin gramada]

Hey. I started this build as an alternative at a power voltage divider able to offer reasonable voltage to supply the screen grids into a 6x6550 project. The available input voltage is 480-490V around and my intention was to reduce it to 360V somehow, keepin' the screen dissipation within the limit. At this voltage the screens draw at max power of 160W a total curent of 75 mA so the regulator should be able to support this current and bit more. ( roughly 30W).
I started inspired by this tubecad article :

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Using 1K series resistor and 12 pcs of 30V / 5W zeners I was able to make it stable for 100 mA draw within 15V regulation, meant less than 5 percent, which I consider pretty acceptable for application.
Assuring thermal conditions was a challenge. I ended with an unusual build, who helped by fan cooler ventilation keep the diodes at almost environmental temp. Measuring 30*C around on the diodes leads in a pretty proximity of diode case. It works very well for 2 days powered on my bench but I.m not sure I use it. It dissipate 40w. The unit is bulky and need to be fully ventilated, supposed will be mounted outside on the back of chassis. It have 70x70x210 mm.





A compulsory safety cage was considered...kind of...

[Chuck H]

Overkill for sure but it looks great Those copper fins with the big solder would probably be suficcient on their own without the fans and are inspiring me for design matters on future builds for components in heat sensitive circuits.

[catalin gramada]

Overkill for sure but it looks great Those copper fins with the big solder would probably be suficcient on their own without the fans and are inspiring me for design matters on future builds for components in heat sensitive circuits.

Thanks without fans the zeners get 50*C around for 80mA draw. Carefully measure should be taken when soldering silicons on heavy hardware. I used a hefty 60w iron , a surgical tweezer as thermal shunt and a wet cloth thimble on finger ,like a wet sponge, over diodes case.

[Chuck H]

Thanks without fans the zeners get 50*C around for 80mA draw

Well I have it right then. The 30V they're dropping at that current would amount to about 2.4 watts. So at 5 watts you still have a reasonable over spec and with thse fins there's very little chance ove them desoldering themselves. Love the design.

[catalin gramada]

Thanks!

[g1]

I don't see any insulators, what keeps all those copper fins from being shorted to each other?

[Chuck H]

I don't see any insulators, what keeps all those copper fins from being shorted to each other?

It looks to me that he has the fins mounted on an insulative substrate.?.

EDIT: I'll grant that the aluminum housed resistor is much better mounted to aditional heat sinking (metal chassis). But the copper fins on an insulator for mounting the Zeners is clever.

[catalin gramada]

There are individuals fins mounted on FR4 board. Those big Arcol 1K is 14w rated without sink. I get 100*C on it for 8W dissipation. The temp of those ceramic 10w mounted in parallel is 60*C without fans. And yes, mounted on 100 square cm brass plate is much better- only 50C but I want to keep it as compact as possible

[catalin gramada]

The fins are bolted with screws on fr4 board so any quick replacement of any diode on board is fast possible...
Back side mounting details.

[catalin gramada]

There is a comparison by size of the unit with a light mesh safety cage without fans ( not necessary required). It is rigid enough and to avoid any accident will be hard wired directly to earthing nut. Posing on the back of chassis

[Chuck H]

It is rigid enough and to avoid any accident will be hard wired directly to earthing nut. Posing on the back of chassis

Yes, that's an important safety measure. But you sure can't put it into a chassis conveniently. I guess if someone really wanted a shock they could poke something conductive in there. But then they're just asking for it. Besides, chances are any probe poked in there would ground against the cage before causing any human damage.

I like it and approve.