GM70 PP

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izzy wizzy
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Re: GM70 PP

Post by izzy wizzy » Wed Nov 13, 2019 8:11 am

RhythMick wrote:
Wed Nov 13, 2019 7:25 am
Harshest conditions would be at startup, maybe consider a slow start circuit? I've honestly never worried about the choke. Even if it briefly saturated at startup it just makes it a heavy resistor but it will quickly return to normal? Am I missing something?
I think you might be talking about a swinging choke. Current isn't so much the issue here but the AC voltage the choke can support before saturating. It's is a combination of both current and voltage but for the lump of iron I have I suspect it's more the voltage across it that may be issue.

I'm going to have to have a dig to educate myself more on this. How to test etc. Hoping the right piece of cardboard will gap it to support the higher voltage. Expecting to lose some inductance but be left with enough.

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Nick
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Re: GM70 PP

Post by Nick » Wed Nov 13, 2019 9:03 am

Am I missing something
You may be missing what choke input means, its not just a power supply with a choke in it.
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Re: GM70 PP

Post by RhythMick » Wed Nov 13, 2019 9:47 am

Nick wrote:
Wed Nov 13, 2019 9:03 am
Am I missing something
You may be missing what choke input means, its not just a power supply with a choke in it.
Always possible, I'll admit.

I think (correct me if I'm wrong) that choke input power supplies have a choke immediately following the rectifier rather than a cap. Chokes resist change in current. The voltage available from a choke input supply will roughly be 90% of AC RMS transformer value, less rectifier losses, as opposed to cap input which will tend to rise to 1.41 x AC RMS. Choke input supply requires a "critical current", below which the output voltage will rise towards the cap filter levels. For this reason I always use a bleed resistor which draws at least the critical current, thus avoiding voltage rise if a valve fails. Once I know the total current draw (valve + bleed) I choose the highest inductance choke which is rated for the current draw. The DCR value of the chokes in the supply obviously has to be accounted for to ensure the transformer-rectifier-LCLC supply can meet the voltage and current requirements of the valve.

I find this page on JacMusic useful and find Jac to very helpful. https://jacmusic.com/lundahl/html/power-chokes.htm

I have used the LL2743-90 or 70 mA chokes as input choke with good results, as well as the LL1673-20H which is rated for 100mA.

There will be many nuances which bypass me because I'm only a few years into learning this stuff, but have I missed anything important, or misunderstood anything ?

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Re: GM70 PP

Post by izzy wizzy » Wed Nov 13, 2019 10:11 am

RhythMick wrote:
Wed Nov 13, 2019 9:47 am

I find this page on JacMusic useful and find Jac to very helpful. https://jacmusic.com/lundahl/html/power-chokes.htm

I have used the LL2743-90 or 70 mA chokes as input choke with good results, as well as the LL1673-20H which is rated for 100mA.

There will be many nuances which bypass me because I'm only a few years into learning this stuff, but have I missed anything important, or misunderstood anything ?
You seem to have it right from what I can see and that JAC music page is a good one if you use Lundahl. I wish others had the same breadth of info.

The key column in that is Max RMS @ 100Hz for choke input duty coz for that application, there's a lot of AC across that choke. In my case 130V RMS AC. I just have to figure a way of knowing/measuring or summat to see if the chokes I have will do the job. It's a custom would thing so no info. Guess I could just bung it in and see what happens; if the voltages and ripple I get look about right.

Cheers,
Stephen

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Re: GM70 PP

Post by Nick » Wed Nov 13, 2019 10:30 am

Once I know the total current draw (valve + bleed)
That's the bit you may be missing, that's not the total current through the choke. The cap that follows the choke is a short circuit to AC, so you have to add the AC current that will flow through the choke from the rectifier through the cap to ground. So simplify it, 300v, 10H, treat the rectified sine as a 100Hz sine wave you will get a extra 50ma or so of current. 300 / 2 * pi * 200 * 10.
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Re: GM70 PP

Post by izzy wizzy » Wed Nov 13, 2019 11:03 am

Nick wrote:
Wed Nov 13, 2019 10:30 am
Once I know the total current draw (valve + bleed)
That's the bit you may be missing, that's not the total current through the choke. The cap that follows the choke is a short circuit to AC, so you have to add the AC current that will flow through the choke from the rectifier through the cap to ground. So simplify it, 300v, 10H, treat the rectified sine as a 100Hz sine wave you will get a extra 50ma or so of current. 300 / 2 * pi * 200 * 10.
This bit I'm not getting. Sure at start up but not continuously as the cap has a DC charge and the AC content across the cap is ripple so the current draw by the cap is the AC ripple V/cap Z or summat like that.

driverPSUsim.jpg

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Re: GM70 PP

Post by RhythMick » Wed Nov 13, 2019 11:59 am

izzy wizzy wrote:
Wed Nov 13, 2019 11:03 am

This bit I'm not getting. Sure at start up but not continuously as the cap has a DC charge and the AC content across the cap is ripple so the current draw by the cap is the AC ripple V/cap Z or summat like that.

That's how I see it, hence my original comment about perhaps a slow start circuit (thermistor in the mains primary, bypassed by relay after a set time period). After start-up the AC across the 1st cap should be 40% ish of the rectifier output, right ?

Nick - thanks for that, yes you're quite right I hadn't considered that. That's why I love this forum. I am currently rebuilding my PSU and as it happens I am including just such a slow-start circuit based on this: https://sound-au.com/project39.htm. That should reduce the inrush, but won't eliminate it.

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Re: GM70 PP

Post by Nick » Wed Nov 13, 2019 1:16 pm

So in the numbers there I(I1) is 60ma, and I(C1) is 13ma, I know you are adding complex currents, but I would simply assume I(L1) of 73ma, not the 61ma that the sim shows. I don't have my copy of MJ to hand, but its in there. It may not be as bad as I remember.
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Re: GM70 PP

Post by izzy wizzy » Wed Nov 13, 2019 1:37 pm

Nick wrote:
Wed Nov 13, 2019 1:16 pm
So in the numbers there I(I1) is 60ma, and I(C1) is 13ma, I know you are adding complex currents, but I would simply assume I(L1) of 73ma, not the 61ma that the sim shows. I don't have my copy of MJ to hand, but its in there. It may not be as bad as I remember.
I think you're right in terms of peak current but it goes the other way as well so RMS values means little change. And as the choke specs are for RMS voltages and current, we can ignore the peaks for specifying but worth keeping in mind if going for little headroom.

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Re: GM70 PP

Post by izzy wizzy » Wed Nov 13, 2019 7:03 pm

Nick wrote:
Wed Nov 13, 2019 1:16 pm
So in the numbers there I(I1) is 60ma, and I(C1) is 13ma, I know you are adding complex currents, but I would simply assume I(L1) of 73ma, not the 61ma that the sim shows. I don't have my copy of MJ to hand, but its in there. It may not be as bad as I remember.
So dragged MJ of the shelf and he is talking about peak currents so your example Nick would seem to follow what he says and the PSUD sim is useful in that regard. For the current draw of 60mA and that choke, it should be rated higher than 78mA.

Cheers,
Stephen

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Re: GM70 PP

Post by izzy wizzy » Thu Nov 14, 2019 8:34 am

I find these discussions fascinating as they challenge how you get to where you are for one. How did I use choke input filters for this long and not know how to fully spec the input choke current? Had a sleep on it and realised, in the 17 odd years I've used them, never had to select one off the shelf. All I have are custom wound from around 2002/2003 by Majestic and designed for that purpose. Good stuff their iron is too.

The intricacies of having to figure it all out was removed from me by them and now going though this made me realise there was a big gap in my knowhow. Every day's a school day :)

Cheers,
Stephen

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Re: GM70 PP

Post by Nick » Thu Nov 14, 2019 9:30 am

and he is talking about peak current
Also, don't conflate "peak" with "startup", the equations he gives are for steady state operation.
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Re: GM70 PP

Post by izzy wizzy » Thu Nov 14, 2019 9:49 am

Nick wrote:
Thu Nov 14, 2019 9:30 am
and he is talking about peak current
Also, don't conflate "peak" with "startup", the equations he gives are for steady state operation.
Indeed they are. Peak current at startup can look very scary on PSUD. Not sure about the inductor here but the rectifiers come under a huge strain. For the supply I'm talking about, 630mA and the first cap will see 300V for a bit before settling to 240V.

So I'm thinking rather than butcher a good choke, just rang Majestic for a quote on a Robinson01 to go with the one I have in the shed which was my first 10H 100mH choke; they still have on record from 2003 and for £36, reckon that's the way forward if my trials with seperate driver PSU work out.

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Re: GM70 PP

Post by RhythMick » Thu Nov 14, 2019 9:54 am

Why not think about a slow start option?

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Re: GM70 PP

Post by pre65 » Thu Nov 14, 2019 10:04 am

RhythMick wrote:
Thu Nov 14, 2019 9:54 am
Why not think about a slow start option?
One reason why I like 6D22s damper diodes. :)
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