Re: [neonixie-l] Re: High current 1.5A x150V power supply design for 100 IN9s

Wed, 10 Feb 2016 18:18:06 -0800

Yes!!! And that supply, like most other HP gear, will probably out last the both of us. Ira. 


On 2/10/2016 5:53 PM, chuck richards wrote:

Several years ago I needed about 1.5 amps at 175 volts to
power up my array of (16) B-7971 tubes.

Found on Ebay a very nice old Hewlett Packard model 395A adjustable
power supply.  It puts out 0 to 1.5 amps over a range of 0 to 320
volts DC.

It is a 19 inch rack-mountable unit.

It was in service at Los Alamos, NM at one time then sold as surplus.

After receiving it, I opened it up and vacuumed out a layer of
western desert fine red dust, cleaned it all up and put it in service
here to light up my scrolling B-7971 array.

Even contacted Agilent Technologies and found a guy there who
copied off the complete manual for it and sent it to me.

That is what I did when I needed a real power supply!  :)

Chuck Richards








---- Original Message ----
From: [email protected]
To: [email protected]
Subject: RE: [neonixie-l] Re: High current 1.5A x150V power supply
design for 100 IN9s
Date: Wed, 10 Feb 2016 17:37:02 -0800 (PST)


All excellent points.
Having been on the fixing end of this stuff but never design I never

really

thought about the how's and why's of certain layouts.
They just worked or they didn't.
Electronic design engineer I am not.

Though I am very well aware of just how dangerous electrolytics are

...

having been responsible for letting the smoke out of them on various
occasions.
Choosing cap's seems to be a black art.

I think I need a pro. The link Jonathan F pointed too has opened my

eyes.

Flyback is needed here I think.

On Thursday, February 11, 2016 at 1:46:38 AM UTC+8, gregebert wrote:

There's nothing inherently unsafe about non-isolated supplies;

like any

high-voltage project, you have to be careful about keeping

everything

properly insulated and follow minimum-spacing rules carefully.

The most-dangerous item in a high-current supply is the

electrolytic cap.

You *must* handle all of these concerns

    - Extra margin for voltage-rating. I use 450V (500V when

possible)

    caps for ~200V supplies. Higher voltage ratings also reduce

leakage

    current. Caps designed for solar-energy inverters are perfect

because they

    come in high-voltage + high temp + high ripple-current.
    - Be very conservative with rms ripple-current, because it

directly

    affects self-heating. Obviously you dont want any warm

components near your

    capacitors. Remember: Lifetime is dramatically impacted by

temperature.

    - You must have a discharge resistor; I even put a flashing

neon bulb

    across my large caps to indicate they have dangerous voltage.

Larger

    resistance values take longer to discharge, but they reduce

wasted energy

    (heat)
    - Series fusing. In case the cap fails, you want to blow a

fuse, not

    the cap. The fuse must be a small as possible, and dont forget

the RMS

    charging current is not sinusiodal. BTW, this fuse is for the

cap; it's

    in-addition to the fuse at the AC input.
    - Charging-current needs to be limited during power-up. For the

1.5

    amp supply, a 1500uF cap will have 10 volts of ripple at 50Hz

when using a

    full-wave rectifier. I'd suggest a series charging resistor of

about 200

    ohms to charge at power-on, then 'shorting' the resistor with a

relay after

    charging is done.
    - Surge protection at the AC input. I always have a fuse on

both AC

    lines, then a varistor and 0.01uF capacitor (for filtering hash

noise if

    it's present). If your current is 'low' (which isn't the case

in this

    design), adding series resistance to create an RC filter is a

big help, and

    furthermore the resistors will act as secondary fuses if you

pick

    low-enough wattage.
    - Reverse-polarity protection diode. If you dont use a bridge
    rectifier, be sure to put a protection diode across the cap.

BTW, a

    bridge-rectifier gives you reverse-polarity protection down to

1.4V,

    whereas a single diode is 0.7V. Polarized electrolytics caps

can be damaged

    with as little as 1 volt of reverse voltage, so you may want to

add the

    diode even if a bridge rectifier is used.



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