[ARC5] (no subject)

[Scott Robinson]

Hi Mark,

First off, if you use a capacitor input filter on the DC supply, you'll 
get about 40V output (29 x 1.414) at no load, nd s bit less as teh load 
increases. I f you have a great big choke, you  can get about 25V from a 
choke input filter. The choke will need to have an inductance of at 
least 1.1 mH per ohm of load resistance. Fir instance, for a 14 ampere 
drain, you'd need 2.2 mH.

If no choke, you'll have to reduce the voltage some other way, and a 
*big* regulator is one way to do that...or turn the Variac down. Do be 
aware that the current rating of a Variac is for *RMS* current, and that 
capacitor input filters generate an RMS input current between 2 and 
three times the DC value. For a 14A 28V load, the secondary RMS load 
will be between 28 and 42A, and the primary current between 6.75 and 
10A. The 5V variac you may have will not like this...but you can cheat 
with a fan.  The thermal weak spot in a Variac is the interface between 
the brush and the winding, so aim a small fan at that area and it will 
make a big difference.

How did I learn this? I toasted several 3A Variacs in a car battery 
charger over a span of time; the 5A one has lasted nicely.

Other possibility: find a 120V to 24V transformer of a suitable current 
rating and wire it as a bucking auto-transformer. Th will give you a no 
load voltage of about 32V.

Fo course, if your dynamotor drain is other than 14A, scale appropriately.

For filter caps, you'll need a ripple current rating to match the RMS 
secondary currents given  above. The big old ones with screw terminals 
that everyone seems to have lying around are a good bet here. This 
rating is thermal, so for intermittent use--you probably dont transmit 
for more than a minute of three at a time) you can push this.

Also, use  *really* big diodes, certainly 70A or better rating. Doesn't 
cost much, and they won't break.

Enjoy!

/scott robinson

On 7/28/17 7:08 AM, Mark K3MSB wrote:
> Good morning gang...
> 
> My 28V power supply for my SCR-274N system isn’t quite stout enough so 
> I’ve been using a standalone HV supply in place of the BC-456 
> dynamotor.   I’m planning on getting the 274 on AM and would like to use 
> the dynamotor (which I’ve rebuilt and runs fine).
> 
>   I have a 29-0-29 50A transformer in the pile that I’m planning on 
> using for a new supply (variac controlled) to power the dynamotor.
> 
>   Does a dynamotor need a regulated input?   I recall reading that if 
> the supply for a dynamotor is stout enough a regulated input isn’t 
> needed,  but I can no longer find the reference.
> 
> Thanks!
> 
> 73 Mark K3MSB
> 
> 
> 
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