Power transformer Mil-spec Ability?

[KI4YAN]

I have an obviously mil-spec, NOS, never used power transformer that is well marked as follows:

TF1RX03MA

115V/230V 50 to 400 CPS

980VRMS CT CHOKE INPUT FILTER
0.250 AMP DC

5VRMS CT
2 AMP

6.3VRMS
5 AMP

PWV=1070
PWV BETWEEN TERMS
5-6-7 (HV winding) & 8-9-10 (5v winding) = 1400V
MAX ALT 10000 FT

CHICAGO STANDARD TRANSFORMER CORP

Now, I want to know what that "choke input filter" comment means, I want to run this transformer into a full-wave bridge for 1300V B+ for a pair of plate-modulated 812A's, drawing about 220mA. Obviously, this transformer will do so, but I am worried as I had planned on a CRC filter, rather than a choke input filter. I'm a little short on chokes of the required insulation values, and all the ones I have are 5H at 200mA.

[ab3al]

errrrrrrrrr little rusty but the choke rating i think was the supply current it could supply after taking into account for the draw of the choke. shouldnt be a problem to use crc

[WA1GFZ]

You will have higher peak current but it should not be a problem at room temp. Most mil transformers are designed to operate up to a 40 to 45 degree C temp rise. At ham duty cycly this should be ok but check it to make sure it doesn't get too hot.

[KI4YAN]

Well, ok. Does this mean that I should really look into getting a choke for the supply, or will the difference be unnoticable in my application?

[N0WEK]

Well, ok. Does this mean that I should really look into getting a choke for the supply, or will the difference be unnoticable in my application?

If you go to a choke input filter you'll only have about 1000 vdc out of the supply.

With a cap input filter you'll bet closer to the peak voltage out, about 1300 vdc. I'd use some sort of soft start for the supply if you go with the cap input filter since the starting current surge is pretty high into the cap.

[Opcom]

The real answer to that depends on your DC load resistance and the impedance of the transformer and the filter cap size. Can you measure the DCR of the secondary and of the primary?

Just guessing, impedance of a transformer supplying 980V at 250mA (after dropping 10% (108V) from a presumed no-load voltage of  1088V), looks like 432 ohms. Could be way off, but 10% drop is the usual suggestion.

At issue is the amount of time the current is drawn from the transformer for. With a choke input, it is drawn for a much greater percentage of the half-cycle than with a capacitor input filter. Since in a given output current from the supply, the same power must be drawn during these rectifier conduction times, the capacitor input filter draws much higher currents but for shorter times, nonetheless it can heat up the transformer windings.

This is dramatically shown in the two images. Note how all the power is taken from the transformer during the time of rectifier conduction. The IT1 waveform shows the angle of conduction of current from the transformer. With the choke input, the current is taken over the entire half-cycle and is a gentle 0.2 to 0.3 amps. With the C filter, current is taken for only about 1/2 of the half cycle, and the peak current is 0.7A

Also notice secondary PWV is 1070V to GND. This could make a problem if you use a bridge rectifier. People do it, and it will probably not cause problems but it has been known to cause carnal damage.

[N2DTS]

Yes, I always thought to use a trans in the bridge mode, the maximum working voltage needed to be higher than the output voltage of the transformer in bridge mode.
Some are, they were designed to be able to be used both ways, but most were not and would zorch in the bridge mode....

Brett
N2DTS

[Opcom]

I should have mentioned one more nutty way to get the bridge-type DC voltage from it without violating the center tap gound rule.
You could use a half wave voltage doubler on each leg. The regulation won't be too good unless you have pretty large capacitors but it will work. My little program does not simulate this lash-up. I have used it where the load is pretty constant in a 100W AM linear with two 6146's and a 50W AB1 audio amp.

C3 and C4 can be combined. All can be the same size, but C1 and C2 are subject to more current and heating, so making them oversize 2-3x to lower their impedance is a good idea.

It takes two cycles to make a complete charge.

half cycle 1:
top of T1 negative, bottom positive.
C1 charges through D1
C4 charges through D4 and C2 (C2 momentarily reversed)

half cycle 2:
bottom of T1 negative, top positive.
C3 charges through C1 and D2 (C1's voltage is added to that of T1)
C2 charges through D3

half cycle 3:
top of T1 negative, bottom positive.
C1 recharges through D1
C4 charges through D4 and C2 (C2's voltage is added to that of T1)

half cycle 4:
bottom of T1 negative, top positive.
C3 charges through C1 and D2 (C1's voltage is added to that of T1)
C2 recharges through D3

maybe it is silly but it does work.

And it should also be said that the ratings of the transformer as stated indicate that it was to be used as a FWCT circuit where only half the secondary was operating at a time with the 250mA, so the total VA rating is not for 980V*0.25A=245 "DC watts", but for 490V*0.25A=123 "DC watts". Or something along that line. It will be the primary that will suffer the most. But being mil spec you can probably get away with it.

[KI4YAN]

Being marked .250A DC, i would assume i can draw a full quarter amp from it. the primary is a 115/230V dual winding setup, and it is appropriately sized for 300VA. I'll run a load test on it tonight and find out.

[WA1GFZ]

a voltage multiplier will not increase the VA rating. You will have high peak currents charging the boost caps.

[Opcom]

Please post the results, they'll be interesting as a base line or reference!