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Old 12th Apr 2018, 11:14 pm   #1
Thatvalveguy
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Default DC heater supply valve tester.

I decided against posting this in the Sussex topic, to keep that topic uncluttered.

Anyway, so here it goes, i was thinking about designing a DC supply for the Sussex homebrew valve testers as transformers tend to have relatively poor regulation. DC can easily be stabilised with one of the abundant three terminal regulators. These however tend to be quite inefficient to say the least. A friend of mine told me you could build a efficient switching pre-regulator for linear regulators see the attached schematic. I ended up using a BC558 and a 1n4148 instead of the P channel Fet.

This ended up keeping the voltage over the linear regulator at about 1.1V, the LT1084

Today i tested a LT1084 based regulator, I had a board with LT1076 laying around, this isn't the most efficient device, but for testing functionality it should suffice. Surprisingly enough it works quite well, noise is quite high, partly because its a switching regulator and partly because i didn't have the correct inductor for the LT1076

Attached you will find a scope plot of the supply starting up into the cold heater of a 6P3S-E, a 6,3V 880mA tube.

Calculated efficiency is about 68% at 20V in 6.3V out. With a better device than the LT1076 efficiency would be over 80%
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Last edited by Thatvalveguy; 12th Apr 2018 at 11:44 pm.
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Old 13th Apr 2018, 6:16 pm   #2
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Default Re: DC heater supply valve tester.

An interesting approach. With mine, I use an ex computer high current switch mode external power unit (I think 13A, if I remember correctly). This feeds a standard linear regulator configured for slow start. I have a number of switched ranges which suffices for everything I have got. My high voltage supplies are via inverters and backward run transformers. Probably not very efficient but it does work well.
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Old 13th Apr 2018, 8:15 pm   #3
Thatvalveguy
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Default Re: DC heater supply valve tester.

I have a couple 300V 100mA supplies, voltage and current limited for playing around with tube circuits. And a triple output bench supply 0-20V 0-2.5A 0-10V 5A.

eventually i will build a 1.25V-20V 5A unit, for my tester project. this was just a little prototype that i put together out of things i had laying around. The aim of which was to design it so it can be used in new designs.

I just finished a Layout for a prototype, single sided, so the electrolytics will all be mounted on the backside
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Old 14th Apr 2018, 6:39 am   #4
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Default Re: DC heater supply valve tester.

Tend not to think about efficiency when building linear PSU's, it's a good idea to though. So my heater supply on my valve tester used a LM317 and two TIP3055's to get over 3A. I made mine to be adjustable with fixed 5,6.3,12.6 and 0 - 30v variable. I only use the variable bit now as the voltage varies with different valves.

A.
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Old 14th Apr 2018, 8:23 am   #5
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Default Re: DC heater supply valve tester.

I have concerns about using a variable DC supply since if the pass transistor goes SC or a fault occurs elsewhere you could end up with the full raw DC on your filament. You could argue that you could incorrectly set the fil volts on any tester but still.
Accidentally connecting the HT to the filament supply I would want to make sure the regulator was bullet proof against such accidental connection.

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Old 14th Apr 2018, 12:39 pm   #6
Thatvalveguy
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Default Re: DC heater supply valve tester.

Pass transistors shorting out shouldn't happen in a well designed supply, this supply uses the LT1084 5A regulator, the LT1076 internally limits the switch current which gives the ramp voltage on the output.

A diode anti parallel over both devices mean that if the HT would be inadvertently connected to the positive terminal of the regulator the current would be eaten up by the relatively low impedance of the supply.

The Sussex limits the output current, and if some load is connected to the supply, nothing should break.

The switcher keeps the voltage over the 1084 at about 1.2V, by taking a zener diode as opposed to the 1n4148 one can choose higher differential voltage over the regulator chip.

dissipation in the 1084 device is about 1.2 watts per amp of output current, overall efficiency depends mostly on the switcher used, if a newer device is used that uses Fets instead of bipolars an overal efficiency in excess of 85% should be possible.

I etched the prototype PCB, the circuit works much better without the long leads present, the main switching frequency is propagating into the output somewhat, however the optional output filter of 10uH 220uF seems to reject all the choppy harmonics.

All in all, it works relatively well, the LT1076 sometimes runs into switch current limiting starting into a cold heater, which is to be expected because the inductance 47uH instead of 100uH.
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Old 18th Apr 2018, 12:22 am   #7
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Default Re: DC heater supply valve tester.

Quote:
Pass transistors shorting out shouldn't happen in a well designed supply, this supply uses the LT1084 5A regulator, the LT1076 internally limits the switch current which gives the ramp voltage on the output.

A diode anti parallel over both devices mean that if the HT would be inadvertently connected to the positive terminal of the regulator the current would be eaten up by the relatively low impedance of the supply.
In power supplies in some fixed installation in equipment, there is something that practically never happens, that is the output of the supply forced higher than its running voltage. So precautions for this event are mostly never taken, usually only overloaded down or short circuit protection is incorporated.

However, if there is a higher voltage low Z supply, elsewhere in the apparatus or in some external device connected to it (or even just a moderate size capacitor charged to a higher voltage) and it gets inadvertently connected to the regulator output, it is usually very destructive to the pass transistor, especially if an emitter follower configuration as it heavily zeners the E-B junction.

The best way to avoid this is a series (rather than a shunt diode) on the supply output and that can be included in the feedback loop so the diode's temperature coefficient and voltage drop can be compensated out.
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Old 19th Apr 2018, 7:47 pm   #8
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Default Re: DC heater supply valve tester.

Quote:
However, if there is a higher voltage low Z supply, elsewhere in the apparatus or in some external device connected to it (or even just a moderate size capacitor charged to a higher voltage) and it gets inadvertently connected to the regulator output, it is usually very destructive to the pass transistor, ...

The best way to avoid this is a series (rather than a shunt diode) on the supply output and that can be included in the feedback loop so the diode's temperature coefficient and voltage drop can be compensated out.
Neat solution. For a few pennies, the whole shebang can be saved from this failure mode. The external cap charged to a higher voltage is an easily imagined scenario.
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Old 19th Apr 2018, 10:56 pm   #9
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Default Re: DC heater supply valve tester.

Quote:
For a few pennies, the whole shebang can be saved from this failure mode. The external cap charged to a higher voltage is an easily imagined scenario.
Al,

Scroll to page 29, the third to last page of this article:

http://worldphaco.com/uploads/THE_GREBE_MU-1.pdf

You will see the diodes in series with the emitters on the schematic. What happens here if say you have the radio's HT plugged on to the 135V output (giving the filter caps in the radio that potential), and while its running, then say pull out the plug and plug it on to the 90V connection, the energy stored in the radio's capacitors destroys the pass transistor. So you can see the diodes there to save them.

Hugo.
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Old 20th Apr 2018, 4:26 pm   #10
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Default Re: DC heater supply valve tester.

Quote:
You will see the diodes in series with the emitters on the schematic. What happens here if say you have the radio's HT plugged on to the 135V output (giving the filter caps in the radio that potential), and while its running, then say pull out the plug and plug it on to the 90V connection, the energy stored in the radio's capacitors destroys the pass transistor.
Nice one, Hugo. Neat idea. I also like other aspects of your battery eliminator - have you put it elsewhere on the forum?
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Old 20th Apr 2018, 8:59 pm   #11
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Default Re: DC heater supply valve tester.

Quote:
I also like other aspects of your battery eliminator - have you put it elsewhere on the forum?
Al,

Only as part of other articles. I have only made two battery eliminators, the other one is in this article on page 23, again three pages from the article end:

http://worldphaco.com/uploads/UX-171...amplifier..pdf

It has a -44 and -13.5V bias supply and again it required the same protection, to stop the pass transistor failing if say the -44V got accidentally shorted to the -13.5.

Another important thing for battery eliminators I think, there should ideally be an over-voltage protection system on any electronically controlled regulators to run valve filaments/heaters. Battery valves often have pretty fragile filaments. Shunt zeners/TVS and fuses are ok. It is a real shame to take out vintage valve heaters if an electronic regulator or sub circuit fails.

Of course in valve testers, there is always the opportunity for the operator to make an error and select the wrong tube/voltage.
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Old 21st Apr 2018, 10:06 pm   #12
Thatvalveguy
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Default Re: DC heater supply valve tester.

Putting a diode in the feedback loop of the linear regulator starts to defeat the purpose of this experiment (High efficiency)

If two parallel , relatively fast diodes are used to protect both the switcher and linear regulator from reverse polarity.
The Sussex tester uses current limiting, as do all my homebrew testers, i wouldn't design without it to be honest. if heaters are lit up, the heaters will eat up the extra current.

Internal shorting is rare in my experience, a tube should be rejected for shorts before HT is applied.
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Old 22nd Apr 2018, 3:39 pm   #13
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Default Re: DC heater supply valve tester.

Quote:
Internal shorting is rare in my experience, a tube should be rejected for shorts before HT is applied.
I would agree, in practice, but in principle a complete tester should test for all problems, not require pre-testing before use.
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Old 22nd Apr 2018, 7:31 pm   #14
Thatvalveguy
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Default Re: DC heater supply valve tester.

This problem occurs primarily with directly heated tubes, indirectly heated ones are more inclined to short G2 to G1 or G1 to cathode.

At any rate, this is a DC supply, not really suitable for testing directly heated tubes anyway. This is due to the fact that one part of the heater is going to be more positive with respects to the reference.
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