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LM338 floating

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I have rectified & smoothed 150V DC. I want to insert current limiting, I've tried an LM338 with a 1 Ohm 10W resistor connected between Adj & Out, connecting the load to the Adj pin.

As I understand it, the 338 should always maintain 1.25V across the resistor so my current should be limited to 1.25V/1 ohm = 1.25A
I've tried it once & it announced its disapproval by sending smoke signals rather promptly. Before it even had a chance to transfer any heat to the heatsink. So I don't believe the root cause is thermal.

I'm aware that the LM338 has a max of 32V, but I have no ground connection to it, so to my mind, with it floating, it's only exposed to the differential voltage between its input & output.

Would appreciate any guidance. Thanks.
 

crutschow

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So what was the load when it released the magic smoke?
Any capacitors?
 

dknguyen

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Just throwing an idea out: maybe it takes a finite time for the regulator to start up, and during that time the drop across LM338 exceeds 32V. If you really want to test if this is the case, hook the thing up to an autotransformer and slowly crank the input voltage from below 32V to above 32V all the way to 150V and see if it survives.

The datasheet does say in it that no capacitors are required and that it should work for your intended purpose up to hundreds of volts as long ass the voltage differential is not exceeded.
 

audioguru

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150V input and an output that is shorted to ground. The LM338 has 148.75V across it and is destroyed by the voltage being too high. If it survives the voltage then its heating will be 148.75V x 1.5A= 223W and will probably glow red hot!
 
150V input and an output that is shorted to ground. The LM338 has 148.75V across it and is destroyed by the voltage being too high. If it survives the voltage then its heating will be 148.75V x 1.5A= 223W and will probably glow red hot!
There is no ground though, and I don't believe it's a short. It's certainly not at the time of connection anyway
 
Just throwing an idea out: maybe it takes a finite time for the regulator to start up, and during that time the drop across LM338 exceeds 32V. If you really want to test if this is the case, hook the thing up to an autotransformer and slowly crank the input voltage from below 32V to above 32V all the way to 150V and see if it survives.

The datasheet does say in it that no capacitors are required and that it should work for your intended purpose up to hundreds of volts as long ass the voltage differential is not exceeded.
Thanks. I read that piece in the datasheet & pinned my hopes on it! I don't have an autotransformer. It would be great for diagnosis though, but ultimately, if it still blows up I'm in the same position!
 

crutschow

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10,000uF across that in put side. I'm not sure what the load is doing - it has a bit of a mind of its own!
So what is the load?
Is it a secret? o_O
 

GromTag

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If I'm even close....

voltage input = near infinite, that is with no connection on adjust regulators outputs, = Open circuit both Out and GND/ADJ.

Application, constant current.

When applying a load the current demands on what value they may be? Pulls across the regulator through the load as the GND/ADJ pin has no where to go other than through the load, the device at those voltages acts as an underrated in term of Wattage handling resistor and develops a voltage drop on the regulators output during load draw that is within the possibility to develop a drop greater than Input-Output relation and thermal loading for devices total rating.

:Edited: Ohh and the Adjustment pin would be more forward with the regulators out resisted, that Adjustment pin would pass more effort via at that voltage with less capability to do so than the out pin. Generating heat with that alone.
 
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So what is the load?
Is it a secret? o_O
Yep, it's a secret! (if I put it in brackets/parentheses nobody will see - it's a solar inverter. Don't tell anyone though)
The reason I can't ascertain its behaviour is because it does its own pre-flight checks. One possibility is that it may be shorting the 'panels' during its tests in which case my current limiter I guess would would be doing its best to maintain 1A, but having 149V across it.
I described it as smoking. I should have added that there was a small bang in advance of the smoke..
 

crutschow

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it's a solar inverter
It's likely that the charging of the its input capacitors (which look initially like a very low impedance), is what zapped the regulator.
At the instant of turn-on there would be the full voltage across the regulator. :eek:
Not surprising it got zapped.
You need a current limiter that can tolerate the full voltage.

A circuit like this with a high voltage MOSFET or BJT transistor for T1 should work (T2 must be a BJT).
The current limit is approximately 0.7V / Rsense.
upload_2017-7-28_9-29-30.png
 
A circuit like this with a high voltage MOSFET or BJT transistor for T1 should work (T2 must be a BJT).
The current limit is approximately 0.7V / Rsense.
View attachment 107290
Many thanks for taking the time to draw this up. I'm pretty stale on this stuff. Would be grateful if anyone could cast an eye over my proposed component choices & calcs:-

So if I'm after a 10A limit, Rsense = 0.07 Ohm 10*10*0.07=7.0 Watts max. (assumes bias voltage = 0.7V)
T1 = SPP20N60C3 Expected dissipation 10*0.19 = 1.9W
T2 = 2N3904
R1 = 10k ??

Manufacturer Data:
Infineon SPP20N60C3 N-channel MOSFET, 21 A, 600 V CoolMOS C3, 3-Pin TO-220AB
Channel type N
Maximum Continuous Drain Current 21 A
Maximum Drain Source Voltage 600 V
Maximum Drain Source Resistance 190 mΩ
Maximum Gate Threshold Voltage 3.9V
Minimum Gate Threshold Voltage 2.1V
Maximum Gate Source Voltage ±20 V
Package Type TO-220AB
Mounting Type Through Hole
Pin Count 3
Transistor Configuration Single
Channel Mode Enhancement
Category Power MOSFET
Maximum Power Dissipation 208 W
 
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crutschow

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if anyone could cast an eye over my proposed component choices & calcs
How did you go from a 1A limit to 10A?

I did forget one thing.
With a MOSFET, the minimum drop across it will be its threshold voltage which will cause a dissipation of up to [email protected] for that MOSFET.

So you likely either want a MOSFET with a maximum threshold of a volt or so, or use a BJT, which will have an ON voltage of about a volt
Due to the high current it needs to be a Darlington stage.

Question: If this is feeding a solar inverter, why do you need a current limit at the input? :confused:
They normally just take the current they need.
 
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Many thanks again. 1A was really just a starting point that I could build up from without blowing anything up - supposedly! Unlikely to want 10A, but happy to use that as the design goal & sit on the safe side.
I can't seem to find a Darlington BJT that'll do that voltage & that current and even for MOSFETs I can't find anything down to 1V threshold, but there's plenty starting around 1.5V and I can live with 15W dissipation.

Why current limit? - It's feeding the mains, so the demand is 'infinite' & it'll supply all available power. Normally the panels would be sized such they cannot exceed the inverter input limits. I don't know what mechanisms are in place to prevent it going into meltdown.
 
230-110 transformer, 3kVa non-continuous, centre earth tap removed, so isolated. bridge rectified, 10,000uF electrolytic. So the voltage would start lowering with increasing current.
 
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