LD1084 vs LM317 @ 24v

Hello,


I am building the Dual/Biploar LV-Reg power supply from glassware/Tubecad and have two questions. I did email [email protected] a few days ago but I understand of course he might be really busy and hasn't had a chance to reply...I am itching to continue my build ;)


I am confused which regulator to use. The bag the LM317 comes in says use if Raw V is >30. As I am building this supply for 24Vdc, raw input is about 22Vdc. In several places online and in the instructions, he states "If the output voltage is greater than 20Vdc, then the LM317-HV will have to be used as the LDO three-pin regulators present a 30V maximum input voltage". Since this kit is rated for 24V maximum output, input voltage will always be lower than 30V....so why use the LM317? I looked at the spec sheet for the LD1084 and it seems capable of 24Vdc output...the input voltage and output differential are within spec.



I would just use the LM317 and keep building, but he states the LD1084 is a great positive regulator and I am curious why it cannot be used in this application.



I am running the FirstWatt BA-3 as a preamp and I don't know what load it presents, but the article talks only about mA's....


Second question, which you may not be able to answer without seeing the schematic, but R3 is part of a CRC filter but also reveals the current flowing into the load via measuring voltage drop. R3 can be .33, 1, 3.3 or 10 via the provided parts. The instructions state I could use 10ohms if the load is light. What is 'light' exactly? How do I pick which resister to use?
 
LM317 needs about 3V of room to properly regulate, while the 1084 about a volt. But it also depends on ripple voltages at the input of the reg. Any reg. can't regulate the output if the ripple minimum is below the output plus voltage drop. In any case, you will need enough room for line drops/sags, which at 24V, a good compromise is about 4 or 5VDC.
 
the higher R the more voltage drop and the greater ripple repression. This depends on current. For instance, if draw is 0.1 A you drop 1 V across r=10. You also have to factor dissipation of that resistor. For the BA3 pre I don’t think you should go above 3.3, but again it depends also on the transformer used as you need to ensure 3-4 V headroom for the regs.
 
My transformer is a dual 20v secondary rated at 50VA....because that is what 6L6 used when he built his. Another member told me it will be fine because the load is so light I will pick up a few volts and should have the headroom needed. The BA-3 works with voltages between 20-32v anyway.



So definitely use LM317 because it will more reliably work at 24Vdc. I did see the LM317 data sheet said it was good up to 57Vdc, but I didn't see anything like that on LD1084's sheet.



And R3 should be 3.3ohms as a good compromised between voltage drop and load rating. 1 ohm would work cause it would allow plenty of headroom, but 3.3 would also help in the filter part cut more high freq ripple than 1 or .33 would?
 
Transformer is OK, will give you 28 V after rectification, so thats fine to get 24V from LM317.



Ripple is the frequency of your mains AC, so not high frequency. As mentioned, the larger the R the more effective the dampening of the ripple. I don't remember the draw of the BA3 off the top of my head, but stock i think it is around 100 mA per channel, so i think 3.3 should be fine (10R would probably also be OK for that matter, just use 3-5W or rated resistors).
 

Vovk Z

Member
2011-10-30 10:32 pm
Kyiv
If transformer secondary is 20V (I think it is on the edge for 317), then exact decision can depend on real DC voltage after rectifier under load, because of possible small voltage drop. If voltage drop is 3 V or less - LM317 became a bad regulator, then low-drop LD1084 will be great again.
For such situations I have both regulators and use 317 first (preferably), if not - I use low-drop. Or I use Shottky diodes in the rectifier, it can give additional 0.5-1.0 VDC for 317.
 
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dotneck335

Member
2012-12-11 10:10 pm
The LM317 is rated for a maximum of 40 volts of input-output differential---ONLY. There is no 30 volt maximum input as has been suggested. An allowance for a 3 volt dropout is indeed desirable. The LD1084, however, DOES have a 30 volt max input, but offers a 1.3 volt dropout----which could be a critical requirement in some applications.
 
Hello,
I am building the Dual/Biploar LV-Reg power supply from glassware/Tubecad and have two questions. I did email [email protected] a few days ago but I understand of course he might be really busy and hasn't had a chance to reply...I am itching to continue my build ;)

I am confused which regulator to use. The bag the LM317 comes in says use if Raw V is >30. As I am building this supply for 24Vdc, raw input is about 22Vdc. In several places online and in the instructions, he states "If the output voltage is greater than 20Vdc, then the LM317-HV will have to be used as the LDO three-pin regulators present a 30V maximum input voltage". Since this kit is rated for 24V maximum output, input voltage will always be lower than 30V....so why use the LM317? I looked at the spec sheet for the LD1084 and it seems capable of 24Vdc output...the input voltage and output differential are within spec.

I would just use the LM317 and keep building, but he states the LD1084 is a great positive regulator and I am curious why it cannot be used in this application.

I am running the FirstWatt BA-3 as a preamp and I don't know what load it presents, but the article talks only about mA's....

Second question, which you may not be able to answer without seeing the schematic, but R3 is part of a CRC filter but also reveals the current flowing into the load via measuring voltage drop. R3 can be .33, 1, 3.3 or 10 via the provided parts. The instructions state I could use 10ohms if the load is light. What is 'light' exactly? How do I pick which resister to use?
Check out the The LT®1083 series from Analogue Device. 30 volts rating up to 7.5 amperes.
https://www.analog.com/media/en/technical-documentation/data-sheets/108345fh.pdf
 
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