Elecraft

Fw: Power supply?

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Richard Fjeld

Fw: Power supply?

Just a comment and a question.
I take care of our club's repeaters. One of the power supplies failed this
weekend. I watched as smoke came out of it. It is 20+ years old. Here is
the comment; make sure the fuse on the primary is properly matched, even if
you choose to go lighter. (It's not good to work the PS to the max anyway.)
It may not be possible to fuse a power supply to protect everything in it,
but at least you can protect the transformer. (I realize this thread is
talking small for the KX3, but the subject is power supplies.)

Question; Elecraft radios are protected against low voltage, but what about
high voltage? I'm wondering if a FUSED Crowbar circuit would be practical.
The fuse (if any) on the PS 12V leads is behind the crowbar. It seems to
me that a versatle fused crowbar circuit would be good.
Has anybody been there and done that? Maybe someone sells one. I'm past the
age when I quit keeping up on components and their specs, so I appreciate
comments aye or nay.

Rich, n0ce

----- Original Message -----
From: "William Levy" <[hidden email]>
To: <[hidden email]>
Sent: Tuesday, September 18, 2012 12:02 PM
Subject: [Elecraft] Power supply?

> Don't think we need Elecraft to build us a power supply. It's a pretty low
> tech endeavor and the supply market is infinite. It makes the same sense
> as me going into the car biz!
>
> Bill N2WL
>
>
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Phil Salas

Re: Fw: Power supply?

“...It seems to me that a versatile fused crowbar circuit would be good.Has
anybody been there and done that? Maybe someone sells one....”

See the Voltage Protector article in the “Articles” section of my website at
www.ad5x.com.

Phil – AD5X

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Help: http://mailman.qth.net/mmfaq.htm
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David Cutter

Re: Fw: Power supply?

In reply to this post by Richard Fjeld

Power supply fusing can be tricky. It has been my practice to use ptc
devices embedded into a transformer primary as the "fuse," they're sometimes
called everlasting fuses. They detect temperature in their immediate
environment (eg transformer winding) and due to self-heating. They trip at
a particular temperature and suddenly go high resistance, reducing current
flow to very low (safe) level. They can only be reset by removing the power
completely for a short time, if the environment has cooled down enough.
They enable very stringent Eu safety regs to met with relative ease. Mass
produced transformer bobbins are often made with a little pocket in the
moulding to hold the device.

Ordinary thermal fuses are often mis-used. Some common fuses are rated to
blow in 30s with 2.1 times their current rating. Many circuits cannot
generate sufficient current so the fuse will never blow except in times of a
dead short. In a supply primary, thermal lag fuses are fitted to prevent
them blowing at start-up which exacerbates the problem. Answer: use a ptc
device, it will protect that expensive transformer from cooking itself to
death.

David
G3UNA

> Just a comment and a question.
> I take care of our club's repeaters. One of the power supplies failed this
> weekend. I watched as smoke came out of it. It is 20+ years old. Here is
> the comment; make sure the fuse on the primary is properly matched, even
> if
> you choose to go lighter. (It's not good to work the PS to the max
> anyway.)
> It may not be possible to fuse a power supply to protect everything in it,
> but at least you can protect the transformer. (I realize this thread is
> talking small for the KX3, but the subject is power supplies.)
>
> Question; Elecraft radios are protected against low voltage, but what
> about
> high voltage? I'm wondering if a FUSED Crowbar circuit would be practical.
> The fuse (if any) on the PS 12V leads is behind the crowbar. It seems to
> me that a versatle fused crowbar circuit would be good.
> Has anybody been there and done that? Maybe someone sells one. I'm past
> the
> age when I quit keeping up on components and their specs, so I appreciate
> comments aye or nay.
>
> Rich, n0ce
>