# Trying to Set up LR8 High voltage Regulator

#### Dynaman

Joined Jan 17, 2008
94
Hello Fellow AAC'ers. I am having a heck of a time setting up the LR8 T0-92 regulator IC. It's a lot like an LM317, but I just don't know where to start. Input voltage=350VDC. Output Voltage 250VDC @ 2.5mA. Can't figure out what the variable I-Adj is.

Can't seem to understand what variable I-adj is. Have no clue what values to start off with for R1 and R2. Any assistance?

#### ronsimpson

Joined Oct 7, 2019
2,515
Iadj is from 5uA to 15uA. Typically closer to 10uA.

Example R=6.04k and 1.25meg to get 250V if there was no Iadj problem.
Roughly speaking, you will get a error of 1.25volts for every 1uA of current. Typically 10uA=12.5V of error. That makes your output voltage not very predictable.
If you used 604 and 125k resistors the effect would be 1/10 as much. 10uA will cause 1.25V of error. Now the 125k resistor will burn 500mW and the regulator will see approx., 200mW more heat.
Play with the numbers and see what happens. Maybe R1=1200 ohms then R2=250mW.
Just remember the current could be as low as 5uA or as high as 15uA.

Last edited:

#### AlbertHall

Joined Jun 4, 2014
12,186
If you stick with R1 as 6.04k then for 250V R2 should be about 1.2MΩ.

#### Dynaman

Joined Jan 17, 2008
94
Hmm.. okay gentleman.. Why stick with 6.04K?

#### Dynaman

Joined Jan 17, 2008
94
Well using the equation above for Vout, I calculate 2875V. Where did I go wrong?

#### Dynaman

Joined Jan 17, 2008
94
Iadj is from 5uA to 15uA. Typically closer to 10uA.

Example R=6.04k and 1.25meg to get 250V if there was no Iadj problem.
Roughly speaking, you will get a error of 1.25volts for every 1uA of current. Typically 10uA=12.5V of error. That makes your output voltage not very predictable.
If you used 604 and 125k resistors the effect would be 1/10 as much. 10uA will cause 1.25V of error. Now the 125k resistor will burn 500mW and the regulator will see approx., 200mW more heat.
Play with the numbers and see what happens. Maybe R1=1200 ohms then R2=250mW.
Just remember the current could be as low as 5uA or as high as 15uA.
Thank you sir.. I'll target for 10uA

#### crutschow

Joined Mar 14, 2008
31,091
My calculation:
For 250V output, the current through R1 is 1.2V/6.04k = 198.68uA for the nominal 1.2V internal reference voltage.
The nominal current through R2 is then 198.68μA +10uA =208.68μA.
For 250V output, the value of R1 is the (250-1.2V) / 208.68μA =1.19meg ohm.

Since the internal reference has a ±5% tolerance, and the Adj current can vary from 5μA to 15μA, you may will need to add a pot in series with R1 to tweak the voltage, if you need an unadjusted output voltage tolerance better than about ±15V.
I would suggest a 200k ohm pot in series with a 1.1meg ohm resistor for R1

#### Dynaman

Joined Jan 17, 2008
94
My calculation:
For 250V output, the current through R1 is 1.2V/6.04k = 198.68uA for the nominal 1.2V internal reference voltage.
The nominal current through R2 is then 198.68μA +10uA =208.68μA.
For 250V output, the value of R1 is the (250-1.2V) / 208.68μA =1.19meg ohm.

Since the internal reference has a ±5% tolerance, and the Adj current can vary from 5μA to 15μA, you may will need to add a pot in series with R1 to tweak the voltage, if you need an unadjusted output voltage tolerance better than about ±15V.
I would suggest a 200k ohm pot in series with a 1.1meg ohm resistor for R1
Crutschow, did you get your R1's and 2's backwards maybe? I thin R2 is supposed to be 1.19M, and R1 remain 6040 ohms. The same for adding a trimmer. You mean R2 and not R1?

#### Dynaman

Joined Jan 17, 2008
94

SO I built this, and it does not regulate. Boo. C3 too high maybe?

#### AlbertHall

Joined Jun 4, 2014
12,186
What are the voltages on the three pins of the LR8?

#### ronsimpson

Joined Oct 7, 2019
2,515
it does not regulate
D1 or D2 soldered in backwards?
What is the voltage rating on C2? Will have 250V or more in it.

#### crutschow

Joined Mar 14, 2008
31,091
Crutschow, did you get your R1's and 2's backwards maybe? I thin R2 is supposed to be 1.19M, and R1 remain 6040 ohms. The same for adding a trimmer. You mean R2 and not R1?
Yes, I meant R2.

#### Dynaman

Joined Jan 17, 2008
94
Well, I am not in front of the bench at this moment, but I slowly crank the AC in up to about 120VAC. The output tracks at about 4 volts less. Not sure if I should feed the full 250VAC in yet. trimmer doesn't really do anything. Maybe it needs the full voltage. I am new at this part. Shall I go for the full monty?

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#### Dynaman

Joined Jan 17, 2008
94
Perhaps the question is: At what input voltage shall I expect the output to start regulating?

#### AlbertHall

Joined Jun 4, 2014
12,186
It will only regulate once the input voltage is greater than the expected output and until that point the pot won't do anything.
While the input is low the output will be close the input as the chip tries to increas the output.

If you are expecting the regulated output to be 250V then you are going to need something like 260V input to get there.

#### Dynaman

Joined Jan 17, 2008
94
Thank you sir.. I am being very cautions due to the high voltage. Looks like I gotta go for the full monty. I'll report back.