[Lowfer] A power supply head scratcher

[Clint Turner]

Putting SMPS's in series is rather tricky and isn't usually recommended 
as you can't be guaranteed that either one will start properly or remain 
stable under a variety of load conditions - especially if they aren't 
identical to each other.

Your AC voltage variations may be causing one of the SMPS's to drop out 
of regulation or briefly stall - but since they can have rather small 
output caps, it may up being "reversed" by the one that is still 
operating and can't get re-started.

You didn't mention if you have to unplug the SMPS's or simply remove the 
load to "reset" them - or a combination of the two.  It is not uncommon 
to see an SMPS that will not start under the rated load if there is ALSO 
a large capacitor on the input:  The "dead short" of the cap - plus the 
load (particularly if it's feeding another device with an SMPS) can 
cause the P/S to shut down due to current overload. and never really 
start up.

Two possibilities to remedy this:

- Put a reverse-biased diode across each SMPS.  This will prevent any 
reversal and *may* solve the problem and is recommended ANYTIME one puts 
two supplies in series.  If this doesn't work, then you'll need to do a...
- Delayed start.  If voltage drops below nominal, have a relay that 
dumps the load entirely for a second or so before re-closing.  This may 
allow the SMPS to recover - unless they are of the "need to unplug them" 
variety.  Then again, if connecting the load causes one to shut down, 
you'll have to be more-creative - like implementing a "soft start" - 
which can have its own problems when using an SMPS unless you jump 
through a few more hoops...

Best of luck,

Clint
KA7OEI


Andy - KU4XR wrote:
> First, Thanks for all the responses to the antenna insulator question.
> J.B. had the one i'd never of thought up, The broken pole vault pole.
>
> I do have a head scratcher here. To get around 35 volts DC, I have
> connected 2 wallwarts in series. One is a 10 volt, and the other,
> a 24 volt. Connected together I have 37 volts DC. *** This is the first
> time I have ever tried this method before, so I have no previous
> experience to go by. I simply connected them 10 volt positive open, and
> the negative connected to the 24 volt positive, with the negative open.
> The problem I'm having is that everytime something in the house turns
> OFF, it kicks off the beacon....hmmmm. AC goes off, and so does the
> beacon, fan in the room goes off, beacon does too. connect the HF ant.
> to the radio and it kicks the beacon off. I have filtered the hound out
> of it. But can't stop this problem. It seems to be coming in on the
> ground line. These 2 wallwarts were 2 pronged plugs, and would not work
> the circuit until I connected a 3 prong plug with the ground wire going
> to the DC negative wire. The warts are SMPS. Even though I have the
> voltage I want, something is amiss. It is getting spiked somehow.
> Has anyone tried this before, and had a similar experience ?? If so,
> were you able to fix it, and how did you do it ?? I may have something
> wired the wrong way, or maybe missing a capacitor where one should be.
> I don't have a schematic for a circuit like this, and can't find one
> on the net. I'm wondering if I have a floating ground issue between the
> 2 wallwarts.
> Any input will be appreciated, and thanks for your time.
>
> 73 everyone :::
>
>
> Andy - KU4XR - EM75xr - Friendsville, TN. USA
> LOWfer Beacon " XR " @ 185.29875 KHz ( QRSS-60 )
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