[ARC5] Army/Navy transmitters from a NON COLLECTOR viewpoint, the ham version...

[Brian Clarke]

Hello Kenneth,

 

Not just transformer-less power supplies. Most Voltage doublers are
effectively half wave; some Cockcroft-Walton designs are full-wave - but
they are quite complex. With consumer-level Voltage doublers, DC does get
back through the mains transformer because with half-wave rectification,
only one half of the mains cycles are loaded. So, the transformer core is
compromised, too.

 

The mains distribution board fuse or circuit breaker is unlikely to trip.
Its main purpose is to prevent fire in the wiring hidden inside the walls
that services the power outlets.  You probably have 60 A fuses / circuit
breakers for each mains circuit with NEMA-code wiring, and a 200 A fuse on
the phase wire incoming from your local pole pig. 

 

However, equipment fuses are there for a different purpose - to protect the
equipment - and hence, should be selected for their time vs current
performance for each piece of equipment, with perhaps a 20% continuous
overload. As you have bumped your smoothing filter capacitor from 150 to 640
uF, you have more than quadrupled the start-up surge load. And if you have
used modern, low-ESR capacitors, all the rage these days, the start-up surge
may be even higher. One hidden effect of such surge loading is that the
turns inside the transformer starts to jump about and may hit the
laminations, especially if an accountant has designed the transformer,
giving intermittent shorted turns.

 

Actually, the average load does increase with Voltage doubling. The
rectification efficiency is lower than with full-wave rectification.

 

When I have made changes to increase the filtering, rather than rely on the
resistance of the transformer primary, I use a very simple soft-start
circuit; I use a mains Voltage rated SPST relay, its solenoid across the
transformer primary, with a resistor (select on test - usually about 10 Ohm)
across the NO contacts in series with the incoming mains. If you don't have
mains rated relays, use a lower Voltage one across one of the lower Voltage
secondaries, eg, a heater winding.

 

Cheers es 73 de Brian, VK2GCE

 

On Tuesday, 11 February 2020 4:15 PM, Kenneth said:

 

On 11 Feb 2020 at 15:35, Brian Clarke wrote:

 

<snip>

 

> Voltage doublers have several problems:

 

...and advantages...

 

> 1. if half-wave, DC is fed back into

> the mains; yes, I know it's done in computer SMPSUs, but sending DC into
the

> mains is against the law in Australia and New Zealand, and if enough

> hornswogglers do it, the distribution transformer (pole pig in USA) gets

> unhappy 

 

I suspect you are talking about transformer-less voltage-doubler supplies,
Brian. I don't know too many technically competent hams who would dare use
such a supply. DC can't get back to the mains through a transformer.

 

> 2. at start-up, increasing the capacitance of the filter increases the
load on

> the diodes, the power transformer and the mains fuse. 

 

The peak load, yes. I don't think the average load is changed much. At least
Duncan Amps Power Supply Designer tells me so. But in the case of my SB-200s
and other rigs I have used this technique with, the diodes I use are
over-rated for this service and the fuses/circuit breakers have never been
changed for higher-rated ones. IMHO, that would be kinda dumb.

 

> 3. increasing the size of the mains fuse increases the risk of taking out
the

> power transformer, and no longer protects the power supply from excess
load. 

 

Yes, but I most certainly have never done that, and won't. As I said, all my
fuses and circuit breakers are exactly as they were originally. Furthermore,
I have never had any problem with any of them.

 

Properly designed and implemented voltage-doubler supplies are quite useful
and work well. They also have several advantages, amongst others, the HV
transformer doesn't have to be rated for the full DC output voltage.

 

Also, FYI, according to Rich Measures (SK) the power transformers in the
Heathkit SB-200 and SB-220 (and possibly others) are specifically designed
to ramp up their output voltage "slowly" so that a "soft-start" isn't needed
with them. I have found this to be true. 

 

Even with my much larger filter caps, the lights don't dim, even
momentarily, when I turn the SB-200 amps on, and watching the HV (2250VDC)
come up on the meter it rises fairly slowly. It doesn't "jump up" to the
full voltage.

 

I have never tripped a breaker or blown a fuse on start-up with any of the
voltage-doubler supplies I use.

 

Ken W7EKB

 

-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://mailman.qth.net/pipermail/arc5/attachments/20200211/a1de0395/attachment.html>