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In Reply to: RE: I'll be surprised... posted by Triode_Kingdom on June 28, 2015 at 15:18:33
You might be right but I hope that working with a regulator circuit will help me understand how to use it correctly. Then I can more comfortably implement it in an amplifier where its use is more beneficial. If you have a circuit with the LR8 please give me the site. tyvm, dak
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The information needed to design the circuit is in the datasheet that Eli linked. You would use the "Typical Application Circuit" shown in figure 3-1.1. Input voltage must be at least 12V greater than output voltage.
2. Output current cannot exceed 20mA and must be at least 500uA.
3. The formula for R1 & R2 is shown in figure 3-2, but there's a typo. Where the formula refers to the constant "+ Iadj", it should in fact read "+ (Iadj X R2)". In case you're not familiar with three-terminal regulators like these, adjust current (Iadj) is a constant. In the case of the LR8, it's 10uA.
4. The two resistors are effectively connected across the output of the regulator. Therefore, they must be selected with due regard for the dissipation of both the resistors and the regulator itself.
5. The TO-92 version of the LR8 can dissipate 740mW in free air. Note - this is another typo in the datasheet. I'm sure the rating for dissipation is at 25C ambient, but the sheet reads "2.5C."
6. I use a spreadsheet for calculations like these. That lets me enter values until I reach the results I want. Here's what I get for a pair of EL84s with screens at ~250V and drawing a total of 11mA :
R1: 1.5K
R2: 300K
Exact Output Voltage: +244.3
Current through R1/R2: 0.81mA
Pdiss R1: 0.98mW
Pdiss R2: 197mW
Pdiss Regulator: 660mWThe above regulator dissipation assumes an input of +300V. Under these conditions, either a small clip-on heatsink is needed, or two regulators should be used, one to each screen.
Hope this is helpful.
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Buy Chinese. Bury freedom.
Edits: 06/28/15
Thanks for doing the math, TK.
FWIW, my seat of the pants suggestion was 1 heatsinked LR8 per channel. "Sand" is not at all tolerant of published limits being exceeded and I (no surprise) like reliable circuitry.
IMO, the TO92 case LR8 is a gem, which can be of great utility in tubed circuitry.
Eli D.
I hadn't seen this part before. It's basically a HV version of the LM317. I can think of a lot of uses for this!
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Buy Chinese. Bury freedom.
Thanx TK, I will be ordering some regulators from Mousser and will give you an update when I get them situated. cheers, Dak
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