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Author Topic: Checking output current from unknown power supply  (Read 6325 times)
K3STX
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Posts: 956




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« on: September 24, 2013, 05:11:29 PM »

OK, I'll embarrass myself. I am building a power supply for simple transmitters (6K6, 6AG7 or 6L6) and I don't know how to measure the current output of the supply. The transformer is from an old Delco R-1243 receiver, the schematic says it has a 5V winding, a 6.3 V winding, and secondary winding that delivers about 290 volts to the plate of the output tube of the R-1243. I measured 615V AC (CT) at 115 volts with Variac (650 V at 122 volts to my house mains) to check the primary. But how do I figure out the OUTPUT current for these windings? It is a no-name transformer.

While we are on the topic, would you suggest choke-input filter (choke first) or capacitor input (electrolytic then choke then electrolytic). I read that voltage regulation is better with choke input but B+ out is lower.

paul
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KD0REQ
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« Reply #1 on: September 24, 2013, 05:23:01 PM »

the Q&D way, rather than the R&D way of increasing load on the transformer winding by winding until the voltage drops 10%, is to blow the dust off some of the old Newark or transformer catalogs, find a unit with nearly the same voltages and size, and the current ratings are right there on the same line.
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G3RZP
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« Reply #2 on: September 25, 2013, 02:00:02 AM »

What were the tubes in the Delco? That will tell you the filament current, and adding up the typical plate and screen currents will give you a minimum current rating for the HV after rectification. You can usually figure for a 5 tube radio that 5 volt winding is 2 amps for 80 or a 5Z4 or 5Y3, the 6.3 will be a minimum of 2 amps and the HT capable of 80mA, give or take. More tubes, more current.

Because when transmitting, it's not a 100% duty cycle, you can usually push things another 50% - see how hot the transformer gets. Shouldn't go above about 70 degrees Centigrade.

If using silicon rectifiers, choke input will give about 240 volts: capacitor input will be  450 or a bit more off load, so you will need to watch the capacitor voltage ratings. For choke input, you'll need to figure the choke inductance needed and the bleed resistor. If you use a tube rectifier, the capacitor input volts off load will be a bit lower but not much, while choke input may see you lose 40 volts or so depending on the rectifier tube.
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K8AXW
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« Reply #3 on: September 25, 2013, 08:43:42 AM »

STX:  Once again, Google (Or Bing) is your friend!

http://home.comcast.net/~stphkeri/meader/rewinding_transformers.html

While this site provides a great deal of information.... some of it over most of our heads, it does show a way to estimate the power handling capability of a transformer.

Scroll down until you see the drawing of the transformer core and note how to measure the cross-sectional area of the core.  Then apply your answer to the chart to the right of the drawing.  This will put you in the ball park for estimating how much current you can draw from your power supply.  Ohms law involving voltage/current/power is applied.

Of course the on/off time of the power supply must be considered to understand if you can push these limits.

Al - K8AXW
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WX7G
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« Reply #4 on: September 25, 2013, 08:52:43 AM »

The available HV current is driven by how hot you want to run the transformer. The winding should be fine at 100 degrees C and that can be measured using the change-of-resistance method. At 100 deg C the HV winding resistance will increase over the 20 deg C case by 30%, which is 0.4% per deg C. The test method is:

1. Measure the HV winding DC resistance
2. Load the transformer with a resistive load, apply primary power, and allow the temperature to stabilize. This will take an hour or two.
3. Disconnect the load and power and measure the HV winding DC resistance
4. Adjust the load resistance and repeat until the winding stabilizes at 100 deg.


If you want to rough it in more easily the case temperature method can be used. A case temperature rise of 50 degrees Celcius is conservative. It you don't have a thermometer a finger will do. If you can hold your finger on the metal case for a few seconds before having to remove it the temperature is roughly 70 degrees C and the temperature rise is 50 deg C. The test method is:

1. Load the transformer with a resistive load, apply primary power, and allow the temperature to stabilize. This will take an hour or two.
2. Measure the case temperature.
3. Adjust the load resistance and repeat until the case stabilizes at 100 deg.

You now have the allowable RMS current. For a choke input power supply you can draw DC current equal to the RMS current found by one of the methods. For a capacitor input power supply you can draw a DC current equal to 60% of the RMS current found by one of the methods.

If you are going to use the filament windings they too need to be loaded for either test method.
« Last Edit: September 25, 2013, 08:55:12 AM by WX7G » Logged
K3STX
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« Reply #5 on: September 25, 2013, 09:56:11 AM »

All these answers are WAY over my head! I don't know what a resistive load is, so how can I do this. I'm sure not gonna just hook up a resistor to the HV windings.

I'm just gonna go for it. If I add up ALL values of current for the plate and screen grid of the Delco 6 tube set this transformer came from (it used a 5Y3 with 6SA7, 2X 6SK7s, 6SQ7, and 6K6)  specs from the RCA catalog I get about 76 mA total. A single 6AG7 or 6V6 for my "novice special" only requires about 45 mA; I think I am fine. Not so sure I am fine  if I want to run a 6AG7/6L6 transmitter; those plans call for a 110 mA transformer. I will start on the baby one tube project, and as (if? when?) I learn more, I can always buy a used XFMR whose ratings are known.

"going for it" sounds just about as scientific as the hot finger test to me; if my finger "burns" at 70C how would I know the XFMR gets to 100C?

paul
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KD0REQ
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« Reply #6 on: September 25, 2013, 12:13:58 PM »

while the CW envelope duty cycle while transmitting is 100%, the averaged cycle of current drawn is probably no more than 50 percent (dit spaces and dah spaces between words).  so that transformer will run it fine.  if you can rest a finger on it, it's not too hot.  if it stays hot while receiving, put a cheapo desk fan, one of the $10-15 jobs, near the transformer.  I bet you don't have to sacrifice your fan for transmitter cooling.
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G3RZP
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« Reply #7 on: September 25, 2013, 01:31:40 PM »

Are you going to use a tube rectifier or silicon diodes? If the latter you should be good to 120mA with capacitor input, two 450 volt electrolytics in series each side of the filter choke, each with a 100k 2 watt resistor across them. You'll get over 400 volts off load.  For a lower voltage, just unhook the reservoir capacitors.

If you spit on your finger and dab it on the transformer, if it's at 100C or much over, the spit will sizzle.
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N3DT
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« Reply #8 on: September 25, 2013, 07:08:34 PM »

Measure the diameter of the HV output windings and look in the back of any ARRL handbook, or engineers handbook for the current capacity of the wire sizes in bundles.  That will tell you pretty close what the current capacity of your HV winding is.  No transformer maker is going to use too small a wire for the capacity, or too large either, wastes money.

Now you've got to measure the wires that are actually in the windings, not the wires that tap off the windings, sometimes they're different.
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