[ARC5] Cool New DC-DC Converter

[Bob kb8tq]

Hi

Often what we see advertised on eBay as “wide range output supply” was actually 
designed for a very specific output. It may adjust from 30 to 300V when you crank 
the pot. The parts on it may have a 25W spec sheet rating. The guys who actually 
built it knew that it was a 183V, 112 ma part. They designed it to meet that spec. 
They *may* have used a 250V cap here or there to meet that spec …..

Is this pure conjecture? Not at all. I have bought and used a number of little DC/DC
converters. After the first few failures, I learned to start checking the coil and capacitor
values on the board. With a lot of designs, you are supposed to change both depending
on the range you are targeting. This is not unique to the high voltage boards. It applies
equally well to the low voltage stuff. 

In some cases, indeed a 250V cap is quite happy for a very long time at 300V. It’s not
all “great balls of fire” sort of stuff ….

Bob

> On Mar 18, 2018, at 12:34 AM, Brian Clarke <brianclarke01 at optusnet.com.au> wrote:
> 
> Totally irrelevant, Dennis. And spot-on, Peter.
> In multi-output mass-produced switch mode power supplies, eg, for PCs, you can bet that the accountant designers have ensured that only just enough power is available at each output, independent of the input transformer design. So the fantasy that outputs can do a dosy-do while the loads on each change, is just that - pure fantasy.
> 73 de Brian, VK2GCE.
> 
> 
> On Fri, 16 Mar 2018 12:39:39 -0400, Peter said:
> 
> It’s a bit different with switchers, a lot depends on the design.  The input section has a power limitation and each output section has limitations. In a very simple design the output section limitations may be the components, such as diodes, acting like fuses. A lack of load on one section could well allow a lot more output on another section before the input section reaches its limit. However, in this case, filaments may take long enough when cold to cause a diode failure. You may need to replace the low voltage section diode with a beefier version to handle the warm-up overload. 
> 
> A more sophisticated power supply would have overload protection on each output and may limit current or even shut down the supply trying to light filaments. 
> 
> 
> Peter
> 
> On Mar 16, 2018, at 12:19 PM, Dennis Monticelli <dennis.monticelli at gmail.com <mailto:dennis.monticelli at gmail.com>> wrote:
> 
> Brian,
> 
> I think there is a misunderstanding.  I think Scott was only trying to point out that the primary winding and mag core are shared by all secondary loads.  You can only get so many VA out of a transformer before hitting the limitations of primary winding resistance and/or core saturation.
> 
> Dennis AE6C
> 
> 
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