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Old 17th December 2014, 09:00 AM   #1
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Default help on lt1085 dropping voltage on load

Hi,

Anyone can shed light, measuring of load i can adjust the lt1085 with turn pot just fine.

When i set it to say 13v without load, once small load is connected voltage drops to 2v or so

Im using a ebay lm317 board and did put a 10000uf cap at the input

Please anyone can chime in.
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Old 17th December 2014, 01:00 PM   #2
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Schematic?
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Old 17th December 2014, 01:29 PM   #3
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The regulator will drop out if (Vin-Vout)*Iload is too high, and it isn't heat sinked.

The LT1083,4,5 family are quasi "low-drop out regulators" (LDO) and as such needs a capacitor on the output for stability. 100uF aluminum electrolytic should be Ok. See Linear Tech's data sheet.
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Old 17th December 2014, 02:24 PM   #4
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its a basic design with turnpot for adjust and a cap at output.

even connecting to power a 3.3v regulator not connected to any load causes a drop.

it fine without amything connected.

vin 22v load i am powering maybe 13v, 1 - 2 a.

big heatsink but not warm, just drops on anything connected.
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Old 17th December 2014, 02:32 PM   #5
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You probably realise that provided there are no faulty connections or fake chips, this behaiviour is quite impossible.
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Old 18th December 2014, 03:38 AM   #6
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Old 18th December 2014, 04:51 AM   #7
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You need at least 100uf on the output or it drops out
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Old 18th December 2014, 05:36 AM   #8
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DUG,

sorry i purchased the board from ebay (very basis design), it is a lm317 reg so i swapped it out. Basic bridge to a cap to Vin, 1 resistor 22uf cap and a pot.from Vout and V adj.

ticknpop,

Thanks you are right that i studied the schematic for the LT1085 it need a cap at output which the board doesnt have. I have placed a 2200uf cap but frustratingly, it is the same.

The Lt was purchased from element14 so not a fake, could it be damaged. This is the 2nd LT one i replaced. FWIW, may not be the reg.

The adj resistor is a 1/4w, could this be causing the problem?

Thanks once again.
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Old 18th December 2014, 12:39 PM   #9
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Bought on ebay !

ask the retailer for customer support !
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Old 18th December 2014, 02:09 PM   #10
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Is the single resistor 120R or 240R?

These regulators need a minimum load so 120R is recommended...otherwise the output voltage will rise above what is set and will go back to a regulation point when minimum current requirements are met.

The pot should be as shown, not from Vout to Vadj.

See .jpg (caps not shown)
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