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Old 15th May 2011, 12:04 AM   #1
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Default Lundahl and rectifier mismatch?

I have a Lundahl 1683 mains transformer, which is usually supposed to put out 250 volts. It also has a 48 volt tap and I'm trying to get 300 (or 298 volts) out of it. The data sheet is: http://lundahl.se/pdfs/datash/1683.pdf
and I'm using Lundahl Transformers - Hybrid power supply having connected the 250, 48, and 250 windings in series. The rectifier went supernova for five seconds and then died quietly. So it seems my math is wrong and did I just put 548V on the tube? How do I get 300V from this thing, and what happened to the good old centre tap??
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Old 15th May 2011, 01:30 AM   #2
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Phase up the pair of 250 VAC windings. Connect those windings in parallel and bridge rectify the 250 VAC with 4X 600 PIV Schottky diodes. Use a CLC filter. Start with 1 μF. in the 1st position. With the supply loaded down, add small amounts of capacitance in the 1st position, until the rail voltage is where you want it to be. A 10 H. choke and a large 2nd filter cap. are in order. Slow B+ rise down a tad by installing a negative temperature coefficient (NTC) inrush current limiting thermistor between the bridge rectifier and the PSU filter.

Either tie the 48 VAC winding off or bridge rectify it for a bias supply.
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Old 15th May 2011, 01:45 AM   #3
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OKay, thanks, Eli, I'll run with that. I decided to try connecting it as recommended in the datasheet (2 turns of 250) using the centre tap as ground and killed another rectifier. Frustratingly, I get music for about a minute until the rectifier arcs.
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Old 15th May 2011, 02:39 PM   #4
DF96 is offline DF96  England
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The first time you tried you put 548V into your rectifier. The second time I'm not sure what you did, and maybe you are not sure either. I am concerned by the question you raised in post #1, as it suggests you are out of your depth here. Main HT PSUs are not a good safe place to find yourself out of your depth.

Are you after 300V AC, or 300C DC? They are two quite different things. Eli Duttman has described how to get 300V DC. If this is what you want then follow his advice, and build a PSU which is partway between capacitor input and choke input. If you really want 300V AC then you need to buy a different transformer.
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Old 15th May 2011, 03:15 PM   #5
M Gregg is offline M Gregg  United Kingdom
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Just a thought,

The AC from the transformer is in RMS value. your DC will rise to the peak value of the AC. So you don't want 300V AC to get 300V DC.

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Old 15th May 2011, 03:20 PM   #6
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Thanks for the reply DF96. My aim is to get 350VDC. I breadboarded this with 300-0-300 Parmeko iron (which has a centre tap) and everything went swimmingly, and I got about 330 after the rectifier, cap, choke, and cap, which is close enough to the 350 VDC I want to get. I'm baffled by the various windings of the Lundahl, though. I've connected the end of one 250VAC winding to the opposite phase of the other 250VAC winding (as recommended in the datasheet) and I am thinking this will serve as a centre tap -- but I don't know why I'm still getting arcing (perhaps I blew the first cap the first time?).

Another way of getting 300V might be to series a 250 winding and the 48 winding then do the hybrid rectification to get a ground rail? I'll wait to get good advice before trying the valve rectifier again!
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Old 15th May 2011, 03:27 PM   #7
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Thanks M. Gregg. Yes, am trying to get around 350VDC, even up to 400 B+ after the CLC.
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Old 15th May 2011, 03:28 PM   #8
M Gregg is offline M Gregg  United Kingdom
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Looks like,


15 connected to 24

22 connected to 17

fullwave bridge off 24 and 17.

or

22 connected to 24 gives center tap power off 15 and 17

for tube with correct value of first cap.

I would forget the 48V to start with and get it working with the 250V windings.
Then see what rectified and smoothed voltage you get.

It seems if you use CT then you cannot use the 48V winding because you will off set the supply on one side this will stuff the rectifier!

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Last edited by M Gregg; 15th May 2011 at 03:33 PM.
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Old 15th May 2011, 03:35 PM   #9
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Thanks! It's a 47uF.
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Old 15th May 2011, 03:41 PM   #10
M Gregg is offline M Gregg  United Kingdom
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If you rectify with SS you will get 300+

Just for fun here is a link for the RMS Vs DC

AC, DC and Electrical Signals

Your meter will read RMS AC this is not true peak value.

So if you rectify your 250V you will charge to peak. This also depends on current drawn from the circuit. Be carefull do not power your capacitors without circuit connected you may exceed the working voltage due to no current drawn by the circuit!

You will drop more voltage across tube rectifier so B+ will be lower than SS.

Sorry if already know this info.

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