LT1085-based regulated 6.3v heater supply - diyAudio
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Old 18th October 2008, 10:47 PM   #1
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Default LT1085-based regulated 6.3v heater supply

Hello,

I have drawn a schematics based on an LT1085 regulator, which is intended to power up the heaters of 2 E88CC/6922 triodes (to be used in a tube line-stage, one tube per channel).
The large capacitors (1000uF) are regular electrolytic capacitors. The smaller ones (10uF/25uF) are tantalums.

Any comments/suggestions?

Thanks.
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Old 19th October 2008, 04:39 AM   #2
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Not familiar with the LT1085............Lets' see... did you do the calculations for:
Ripple voltage
Ripple current (Can the TX handle the peaks)
Regulator dropout Min.
Regulator Absolute Max.
Output current capacity (Well above current draw)
Dropout Min. current draw
..........I see the carefully calculated ADJ. values...
Only thing is the use of Tantulums for ADJ & output legs, there is a profound mistrust of Tantulums in about every application here in the Forum. I'm hearing really bad horror stories about cascading failures attributed directly to Tantulums. Its' an easy fix obviously to use plain electrolytics in this application.
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Old 19th October 2008, 02:31 PM   #3
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Looks good to me...
I built a very similar regulator using the LM1084 to supply 2.8A of heater current for a headphone amplifier. It works very good and reliable and I don't have any noise issues.

I also would avoid the tantalums and use film caps instead (or electrolytics like Richard says...)
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Old 19th October 2008, 04:18 PM   #4
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Tantalum caps are to be avoided unless you like small fires.. My employer has banned them in all new designs because of their proclivity to catch on fire due to small transients, and general aging effects. I have several pieces of test equipment here where the tantalums have failed, and they usually burn at the moment of their end of life. Conventional electrolytics are really quite a bit safer..

Tantalums subjected to transients from the AC line are the most likely to fail catastrophically, (and quickly) although even those protected by good regulators have failed in a similar manner - it usually takes just a little longer.

The LT1085 is a fine choice, just make sure it is adequately heat sinked and for best performance even at 10% low line make sure that you have around 2V across it minimum (even though drop out V is lower, loop gain falls at low differentials across the device and with it goes your ripple rejection.)

You don't need the 10uF at the input unless you are more than a few cm away from the input supply filter caps. Don't use more than 22uF at the reference pin without a reverse biased diode to the output. (Incidentally within reason more capacitance is better here, 100uF not being unreasonable.) I like to use >470uF on the output and also add a reverse biased diode from output to input so cap discharge currents do not flow through the device in the event input power fails or is just shut off. (Particularly important if you use really large caps on the output and the one of the input caps or rectifiers fails - this will limit damage to the just the failed components.) Linear Tech has good app notes on using these, just follow their guidelines.

I assume you are using the TO-220 package, silpad insulator should be fine at the Pd encountered here.
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Old 19th October 2008, 11:56 PM   #5
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Thank you for the advises.
I will perform some modifications, namely include the protection diode and get rid of the filthy Tantalums. I recall my dad saying they are horrible long long time ago when he explain some electronics to me

If and once I get the amp running, probably a few months as delivery of parts/pcbs takes ages to here I will post the results.

Thanks
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Old 29th December 2008, 04:38 PM   #6
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Got the PCB and assembled it - measured 6.05V on the output. My guess is that this is due to tolerance of the LT1085 resistors...

Other than that, you can see I didn't leave enough space for the output 10uF capacitor.. how critical is it?

I think for a first PCB I will call this a success. I still need to measure it with a scope and heater resistor connected.

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Old 29th December 2008, 06:08 PM   #7
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I'd call that a nice PCB...

6.05V is fine unless you want to finetune the resistors some more, like by adding a parallel resistor to R1.

I would try to fit at least an electrolytic cap on the output, the longer leads will allow you to do so.

Good work!
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Old 30th December 2008, 02:40 PM   #8
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It's the nice PCB.Can you sent me this layout ?
Thank you.
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Old 30th December 2008, 03:19 PM   #9
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Quote:
Tantalum caps are to be avoided unless you like small fires.. My employer has banned them in all new designs because of their proclivity to catch on fire due to small transients, and general aging effects.

I avoid them like the plague for that very reason.
I once had a tantalum cap explode,fly across the room,land on a pile of papers on the bench (schematics,datasheets,etc) and nearly burn my house down!
Personally,I will never use tantalum caps in anything I build.Period.
There are still smokey scorch-marks running up the wall.
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Old 30th December 2008, 04:25 PM   #10
kevinkr is offline kevinkr  United States
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Quote:
Originally posted by DigitalJunkie



I avoid them like the plague for that very reason.
I once had a tantalum cap explode,fly across the room,land on a pile of papers on the bench (schematics,datasheets,etc) and nearly burn my house down!
Personally,I will never use tantalum caps in anything I build.Period.
There are still smokey scorch-marks running up the wall.

I've had a few go up in smoke in older test equipment, and unfortunately a lot of my test equipment still has them - just too many to change out. (HP, Fluke, Wavetek, and Amber all used these.) There is a class of newer organic polymer tantalum caps that are quite safe (they will not burn even if internally shorted) and offer similar electrical performance to the old tantalum cap without the risk - they are however rather expensive and must be operated at less than 80% of their rated voltage to be reliable. Kemet is one major vendor that makes these.

I've experienced burning tantalums, exploding axial aluminum electrolytics, bulging radial types so hot they were on the verge of exploding, and shorted HV paper in oil types. AND here is the clincher - none were operated anything other than conservatively in terms of voltage, ripple current or temperature ratings. Keeping a few spares around is a smart thing to do. Nothing I've bought recently has failed, it seems that low voltage electrolytics in particular have gotten more reliable over the past decade or so.
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