Friday, 31 May 2013

Inrush Revisited

A while back I reported the problems I was having with a SMPSU I planned using to power the heaters and how it would not power up with a full compliment of boards. I suspected this was due to inrush current which occurs because the heater resistance is much lower when cold.

I have now investigated this further. First I purchased an International Power linear 12 volt power supply rated at 5 amps. Another EZTubeMixer builder had reported that even this power supply would not power up with six boards (check out Pierre's blog here):

I suspected this might be due to a fold back current limit circuit in the power supply. The manufacturer does not supply a schematic so the first thing I did was take mine apart to try to trace out the current limit part of the circuit. The main regulator element of this power supply turns out to be the ancient LM723 which was introduced in the 1970s!!! Together with three equally ancient 2N3055 power transistors wired as darlington series  pass transistors this forms a linear regulator capable of at least 5 amps output current. So why would it not power up six boards? Further investigation showed it includes a current limit pot which, when somewhere in the middle of its range, implements a foldback current limit, but if turned fully clockwise should revert to a regular current limit. So in theory, turning this pot clockwise should fix things.

First I tried it on a six board load with the pot untouched. It did power up but very slowly. It was at least 15 seconds before you could see the glow from the 6922 tubes. Of course, once the tubes have been powered up, if you turn off and then turn it on again, it powers up straight away because the heaters are still warm. I found I had to leave it for at least 15 minutes before you could repeat the test. Next I hooked up a DVM, set to its 10 amp range, in series with the supply to the mixer. On powering up, the current started out at about 0.8 amps then slowly rose to about 3 amps when it suddenly peaked at over 6 amps and then settled slowly down to about 2.75 amps as the tubes lit up. I repeated this several times at 15 minute intervals with the same result. This seems to clearly demonstrate the foldback current limit operating but allowing enough current through to to slowly heat the tubes until a point is reached when the foldback limit shuts off and the regular current limit takes over.

The odd thing was that my supply worked on 6 boards but it did not work for Pierre. This could be differences in the tubes and also the tolerances in the power supply. Either way it seemed marginal. So next I tried with the pot fully clockwise and I was very pleased to discover that the foldback current limit was no longer operational. Instead the current rose rapidly to over 6 amps then slowly fell back as the tubes lit up. The time from switch on to seeing the heaters glow was less than 5 seconds. I told Pierre who tried this with the same result. So it looks like rotating the pot clockwise does turn the supply into a regular current limit and allow you to power 6 boards.

The international Power 12V, 5 amp supply is large, heavy and expensive so I wondered if I could come up with something smaller, lighter and cheaper. After looking at a range of regulators I finally realised that the LM317 regulator in its TO220 package, has the same pin out as the TL783 I use in my phantom power supply PCB. Using this chip, changing a couple of resistor values and changing the smoothing capacitor would allow the same PCB to be built as a 12V heater supply capable of supplying one and perhaps 2 amps of current. Quite handy for a couple of tube mic pres or a tube gain make up in a passive summer, but not enough for this mixer. To provide in excess of 3 amps really needs the TO3 version of the LM317 and a bigger heat sink. Perhaps this part with is chunky heat sink could be mounted on the rear of the PCB and wired by flying leads to the PCB? The TO220 heat sink I use on the PCB has two mounting holes. Not surprisingly, these are not the same spacing as a TO3. However, by chance, the heat sink is off centre and if you drill a hole a few mm past the off centre fixing hole you can fit a pair of pillars to the rear of the PCB that mate with a TO3 and it sits almost central to the PCB.

So I built one. I had a spare 12V 5A toroid transformer, not ideal but good enough for an experiment, I built the PCB and mounted the TO3 version of the LM317 regulator  and its heat sink from the rear of the PCB, like this:

Then I connected it up to the mixer via an ammeter as before and switched it on. And it worked! The meter flipped up to just over 6 amps then slowly settled down to just under 3 amps as the tubes lit up. I ran it for half an hour and the heat sink got quite warm but no warmer than the International Power supply did. I will probably use the International Power supply for this mixer because it works and I would need to get a higher voltage toroid to ensure the LM317 has enough operating voltage but at least I know an LM317 can be used.

The PCB will need to be updated though for two reasons. First, the IN5400 series rectifiers it uses are not really up to supplying more than 3 amps continuously so I plan to modify the board to accommodate readily available 4 amp and 6 amp bridge rectifiers. The other problem is the power tracks were never designed for 4 amps; they were designed for 100mA of phantom power current so they need to be replaced by some copper areas. The good thing is the new PCB can still be used for phantom power as well as heater supplies.

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