[Amps] MFJ products

Jim Garland 4cx250b at miamioh.edu
Tue Dec 5 10:23:32 EST 2017

Jim makes very good points. In my station, I have several linear and 
switching power supplies. Over the years I've scavenged some "crowbar" 
overvoltage modules from old Lambda linear supplies and use them 
routinely with all my large 12V supplies. These are small black plastic 
modules with two leads that attach to the power supply output terminals. 
An internal SCR in the device fires instantly when the voltage exceeds 
the module's threshold setting (I set it at about 15V) and shorts the 
power supply output, blowing a fuse and protecting the connected radios. 
The devices reset automatically.

I generally prefer linear DC power supplies, for a variety of reasons, 
even though they are heavier than switching supplies. Aside from their 
increased weight, their only disadvantage is that a failure of the 
series regulator transistors can dump the high unregulated voltage on 
connected equipment, which is the problem the crowbar modules fix. 
Switching supplies tend to be more complicated than their linear 
counterparts, and significantly less reliable in my experience Many are 
very inexpensive but also cheaply made, which is why desktop computer 
power supplies fail so frequently.

I try to buy lab-grade linear power supplies for my station. Consumer 
grade power supplies designed for the ham market are often very poorly 
made. (Truthfully, I can hardly stand to look at the poor workmanship 
inside the larger Astron linear power supplies.)  By contrast, used 
inexpensive Lambda and Kepco supplies are always available on auction 
sites and seemingly last forever. These companies also make high quality 
switching supplies, so if you really need the small size and weight, 
these may be the way to go. I have two Lambda switching supplies (12V 
and 24V) in my computer wiring closet (where I run a microwave access 
point for our local wireless internet provider) that I bought used for 
$20 each, and they've been running 24/7 for almost eleven years now.


Jim W8ZR

On 12/5/2017 7:44 AM, Jim Thomson wrote:
> ##  You forgot to tell us that it was a 40 A   13.8 vdc, switching type power supply !
> Found it on U tube, where the vdc pegged past 16 vdc.  Another fellow had his
> way over 45 vdc...and fried his yaesu.    The u tube fellow says his  did not have
> any over voltage protection built into it !   Since his was under warranty, MFJ will
> repair it..... but not the very expensive yaesu xcvr.
> ##  Fuses on radios  will only protect against excess current, not excess voltage.   Your
> surge protector on the input of the 120 vac.. feeding the MFJ switching supply, will  not help at all.
> The issue is the lack of over voltage protection in the mfj switching supply, and what ever caused
> it to crank out excess  vdc in the 1st place... frying everything connected to it.
> ##  Either   replace with a better quality switcher from a different manufacturer, or  use
> an analog supply, like a  50 A  CCS astron.
> ##  way too many issues with the MFJ-4245  switcher imo.  if the supply bows up, who cares... but when the
> same supply  blows up  a bunch of expensive gear connected to it, then its a serious issue.
> Jim   VE7RF

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