Need voltage regulator 20v at 3A - ideas?

I have an application (tube DC filament supply for 10Y DHT in filament bias) requiring 20 volts adjustable +/- 2v at 1.25A steady current. I'm planning on at least 3 amps turn on surge. Currently using a LM1084 which though rated 5 amps is only rated 15v, and it cuts out on turn on.

So what's a simple circuit for achieving a comfortable 20v at 3A with plenty of margin? Supply voltage is choke input and can be anything from 24v to 29v depending if I put a small cap in front of the choke (280mH).

Need to get this built, so all ideas, links and schematics very welcome!

andy
 
Many thanks for your contributions! First the LM108x series - the LT1084 may work better than the LM version - possibly it's better built to a higher spec, though generically it should be pretty similar. The LT1083 does look as if it would do the job - but ouch! Expensive at over £12 from Farnell. There is a LM1083 version if I can find a supplier in the UK.

I don't want to eliminate the choke - with filament bias the signal goes through the filament supply so the quality of the filament supply is absolutely critical - it doesn't just have to do the job it has to be sonically as perfect as possible!

I'll look at the zener option - that might help. I was discussing this with the London Audiocircle in the pub last night and suggestions were:
- a Darlington pair instead of a voltage reg
- a thermister input so the voltage would come up more slowly and maybe not trigger the cutout on switch on, which is the current problem.
- a 1 ohm resistor in series with the positive leg before the reg, again to try and avoid triggering the cutout.
- using the LM1084 as a CCS

More suggestions?

andy
 
Hi,
if the circuit needs 1.25A then I cannot see any reason for supplying a constant (adjustable) voltage.

I would use a Constant Current Source (CCS) set to 1.25A to feed the circuit.

A 317 can meet this current requirement but if the voltage drop across the 317 increases then the total dissipation of the 317 also increases.
Might be better to use a pair of 317 in parallel each set to ~630mA

The big advantage of the CCS feed is that current surge into the cold filament does not exist.
 
I recently built one of these to power a laptop computer(3A). With the 20 ohm pot it will give you 17.9 to 21.7V. You can add an output cap if you want to improve HF rejection but this will give you about 68db power supply rejection at 120hz without any capacitors.

M2 needs a good heatsink.

Needs about 3.5V Vin to Vout.
 

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Can't edit any more. You can use just about any N-channel MOS fet 50W or more
I just used that model because i've got a bag of IXTH11N100's and it seemed close. I added a couple of thousand of pf's to the g-d, g-s and it didn't seem to affect the simulation. Anyway it works.
 
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You can use 15 v regulator with 5v zener attached to its ground pin making "ground pin" of 15v regulator 5 v higher from common ground. now you have 15 + 5 = 20v regulator.>>

A friend recommended using the LM329DZ as being very low noise. That's 6.9v so I would have to revise something. I certainly need some solution - will try out a CCS next. I haven't got round to a Darlington pair yet - that was another suggestion

andy
 

juma

Member
2005-11-08 11:27 am
Zemun
...I would use a Constant Current Source (CCS) set to 1.25A to feed the circuit. ...
Yes, that's a good solution for tube filaments - fix the current and voltage will respond to Ohms' law.

... A 317 can meet this current requirement but if the voltage drop across the 317 increases then the total dissipation of the 317 also increases.
Might be better to use a pair of 317 in parallel each set to ~630mA
This is true only if LM317 is used as a series voltage regulator. If you use it as a CCS it becomes very unreliable for currents greater than couple of hundreds of mA (Rset becomes too small).
 
The LM1084 makes a perfectly good CCS - for the 10y tube you need 1.25A which gives you a 1 ohm resistor at 3.2W so you need a 12 watter or more.

I got frustrated with not finding a solution to my shutting-down LM1084 in voltage reg mode, so I went back to a LCLC supply ending up in a common mode choke. Listening now - it's probably the most detailed version I've used but may be a trifle hard sounding compared to the solid state stuff. The supply is really much too big, so I'd still like to only use one choke.

The hunt for the perfect solid state solution goes on. Haven't tried the LM1084 as CCS yet - just wanted some sound!!!

andy
 
I'm sceptical of anything less than 5 amps (LM1084) and the LM350 is rated 3 amps. On the contrary it may be necessary to go up to a LT1083 at 7.5A. Why I don't know, but the damn LM1084 just shuts down. The steady current is only 1.25A but maybe it's the initial voltage that triggers the shut down - the supply is 33v off load - or the current surge on switch on or something. It's frustrating.

andy