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Old 14th January 2011, 08:10 AM   #1
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Default use more then 'maximum capacitance' after tube rectifier

I wanted to use more than the maximum allowed capacitance of 47uF after an EZ80 rectifier. The maximum capacitance is limited by the amount of inrush current the EZ80 can take upon startup. The maximum current of an EZ80 is about 100mA.

So I've used a simple current limiting circuit to limit the current to 100mA (see attached schematic).

With luck, you can find all required components in an old PC power supply.

The circuit is positioned directly between the tube rectifier and the capacitance.
I've used 340 uF without any problem.
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File Type: png curr_limit.png (84.5 KB, 245 views)
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Old 14th January 2011, 10:07 AM   #2
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I guess one issue is that the maximum power throughput into the caps is limited by the duty cycle of diode conduction (during which only 100mA can conduct) - so the continuous power rating of the DC supply before droop is likely to about 30-40% of 100mA.

Another way to alleviate turn-on peak is to do a series R with a relay coil across the HT DC that shorts the series R. Another way guitar amps do it is to use a bleed resistor across the standby switch.

Ciao, Tim
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Old 14th January 2011, 10:34 AM   #3
DF96 is offline DF96  England
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You may be confusing average current and peak current. The maximum DC current from an EZ80 supply is 90mA. The maximum peak current will be significantly greater than this. The effect of this current will depend on the shape of the pulse, the duration, repeat rate etc. The data sheet gives values for max cap, min series resistance etc. which if adhered to should lead to a reasonably long happy life for the rectifier. Exceeding them will shorten its life. Demanding less will lengthen life.

A big cap means a short (in time) but large (in amplitude) charging pulse, unless you use a big series resistance. Your current limiter is even better than a resistor, but you will end up with a supply with quite a high output impedance i.e. bad droop on load. You could use a higher current limit - 200mA?

On the other hand, why on earth use a vacuum rectifier then pad it out with lots of SS? You don't have to use big caps as reservoirs. You might end up with a better supply using a smaller cap and just a series resistor. Fewer bragging rights, though!
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Old 14th January 2011, 12:41 PM   #4
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Imho, I think Michaelis is trying to electronically remove the stress of power-up and also 'smooth' the rectified current through the valve diode - which are credible design aims.

One design issue is that it is difficult to achieve good power conversion when you have AC on one side, and you want DC on the other. The ss purist ends up with a unity power factor correction stage, but you still need to add isolation, so end up with another ss switchmode stage afterwards.

If you want to design something in between then you end up in a design whirlpool, and still have the question hanging: what are you trying to achieve. My suggestion is that if you want to use valve diodes, then enjoy implementing them in the time-proven manner. If you can tweak around the edges with modern tech, that's good. I think a circuit that alleviates turn-on stress but otherwise had no effect is a good tweak. However the proposed circuit may well dramatically change the rectified current waveshape during normal operation.

Tim
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Old 14th January 2011, 02:04 PM   #5
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hmm interesting views.

The main reason for using a valve rectifier is slow turn on upon startup, and because I had a transformer that in conjunction with a valve rectifier just gave me the right voltage, without having to dissipate any to match the circuit.

One downside, however, is that because of the current limitation, the ripple of the power supply increases, for which I'll have to work harder to remove that...

In normal operation, the circuit only uses about 60mA, so the current isn't limited. Since DF96 suggested I can increase the current limit, I'm wondering up to how much I could increase the current.

Since the datasheet advises a limiting resistor of 300R for 325-0-325 into 47uF, rendering a peak current of 1.5A, that if I can increase the current limit a great deal more.
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Old 14th January 2011, 02:09 PM   #6
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Ah I did find the peak current. It's 270 mA per Anode. I guess I could then still increase the current a bit.

That would render extra capacitance more useful, since a current limiter reduces the benefit of adding more capacitance..
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Old 14th January 2011, 02:24 PM   #7
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CLC, the first C as rated for the tube, then with a large cap after the L, I find this provides extremely good hum free power with the added advantage of the L stabilising the power. Simpler, less likelihood of failure and gives a very good sound.
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Old 14th January 2011, 03:01 PM   #8
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Thank you quickie, I might try that.

I was also thinking about using a voltage stabilizer. (attachment)
That would also give me some more slow-start when turning on the amp.

cheers, Michaelis
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Old 14th January 2011, 03:28 PM   #9
DF96 is offline DF96  England
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270mA peak probably assumes that it won't be at 270 for long. Your limiter gives a flat-top pulse, so I would not go above 200mA. Better to do as quikie22 suggests - small C reservoir, then LC or RC smoothing with as big a C as you like.
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Old 14th January 2011, 05:17 PM   #10
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I've tried limiter - 47uf - choke - 340uF.. But the choke I have isn't very big, so that didn't help much except make the hum 'nicer to listen to', if that makes sense.

I've also tried doubling the limiting current to 200mA (soldering an extra 10R resistor on one leg and then pushing it on with an insulated screwdriver), and that didn't give any audible difference.

I'd rather use a voltage regulator, because I've got some PC PSU components and I don't have to use a bulky and relatively expensive choke.
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