LM338, output voltage drops with incresing load

[Jamie]

Hi all,

I have just made a regulator using LM338 TO-3 Reg.
I have 32V DC 5 amps as the input to the reg, and the reg is reducing this
to around 14 volts.
I have a 240 ohm resistor from the adj to the out of the reg, and a 5k 10
turn trimpot from the adj to the circuit ground.
The 240 ohm is connected directly to the reg case.

Protection diodes are installed as per the lm338 datasheet, and an 0.1uf
ripple cap, from the adj pin to ground parallel with the 5k variable res.

The problem is if i test the unit by connecting a 6 ohm 10 watt wirewound
resistor from the output of the reg to ground, my output voltage drops from
14v down to around 6-7 volts. The 6 ohm resistor represents a ~2 amp load,
The reg should be stable up to 5 amps??

The input voltage does not drop when the load is applied, only the output
voltage.- any ideas why??

If anyone has any suggestions, please email me
[email protected]

thanks in advance
jamie

 

[mike]

Hi all,

I have just made a regulator using LM338 TO-3 Reg.
I have 32V DC 5 amps as the input to the reg, and the reg is reducing this
to around 14 volts.
I have a 240 ohm resistor from the adj to the out of the reg, and a 5k 10
turn trimpot from the adj to the circuit ground.
The 240 ohm is connected directly to the reg case.

Protection diodes are installed as per the lm338 datasheet, and an 0.1uf
ripple cap, from the adj pin to ground parallel with the 5k variable res.

The problem is if i test the unit by connecting a 6 ohm 10 watt wirewound
resistor from the output of the reg to ground, my output voltage drops from
14v down to around 6-7 volts. The 6 ohm resistor represents a ~2 amp load,
The reg should be stable up to 5 amps??

The input voltage does not drop when the load is applied, only the output
voltage.- any ideas why??

If anyone has any suggestions, please email me
[email protected]

thanks in advance
jamie

Did you set the output voltage with a load exceeding the miminum load spec?
Is it oscillating?
Is it adequately heat sinked/sunk. It will go into thermal shutdown
when it gets hot. You're running it near max voltage rating. Won't
take long to heat up.
mike

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[Winfield Hill]

Jamie wrote...

I have just made a regulator using LM338 TO-3 Reg.
I have 32V DC 5 amps as the input to the reg, and
the reg is reducing this to around 14 volts. [snip]

The problem is if i test the unit by connecting a 6 ohm
10 watt wirewound resistor from the output of the reg to
ground, my output voltage drops from 14v down to around
6-7 volts. The 6 ohm resistor represents a ~2 amp load,
The reg should be stable up to 5 amps??

Oh dear Jamie, Jamie, Jamie. You'll have to learn about
reading data sheets and engineering specification: they
apply only under stated conditions. And these conditions
include the fine print. Look on page two. NSC states the
Current Limit will be 5A minimum if VIN − VOUT < 10V, and
if Tj = 25C. (The latter is in the fine print at the top
of the spec section. The former spec relates to Note 2,
the 50W limit.) This means that if the junction temp is
25C and if Vout > 22V, you'll get a short moment of 5A
load capability, before the junction starts to heat up.

That's the current limit, you also need to worry about
the Thermal Limit, which can occur whenever the junction
temperature exceeds about 150C. I assume you know about
the thermal resistance specs and how to use them?

What's going on with your 2-amp 7-volt scene? 32-7 = 25V
Vin - Vout difference times 2A is the dissipation within
the LM338, which is 50W. That's apparently all your case-
to-insulator-to-heat sink setup can handle. It implies a
thermal resistance of around 2 C/W, which is reasonable.
Sorry, back to the drawing board!

Thanks,
- Win

whill_at_picovolt-dot-com

 

[Tim Shoppa]

Hi all,

I have just made a regulator using LM338 TO-3 Reg.
I have 32V DC 5 amps as the input to the reg, and the reg is reducing this
to around 14 volts.
I have a 240 ohm resistor from the adj to the out of the reg, and a 5k 10
turn trimpot from the adj to the circuit ground.
The 240 ohm is connected directly to the reg case.

Protection diodes are installed as per the lm338 datasheet, and an 0.1uf
ripple cap, from the adj pin to ground parallel with the 5k variable res.

The problem is if i test the unit by connecting a 6 ohm 10 watt wirewound
resistor from the output of the reg to ground, my output voltage drops from
14v down to around 6-7 volts. The 6 ohm resistor represents a ~2 amp load,
The reg should be stable up to 5 amps??

A 2 amp load means that the TO-3 package is being asked to dissipate
(32-14)*2 = 36 Watts. If the TO-3 isn't very well heatsinked, it will be
very incapable of doing that.

If you read the spec sheet carefully, you will find that you only get 5
Amps out *if* the package is adequately heatsinked *AND* Vin-Vout is less
than 10 volts. You have Vin-Vout = 18 volts.

Tim.

 

[Winfield Hill]

Winfield Hill wrote...

Current Limit will be 5A minimum if VIN − VOUT < 10V

.. ^^^^^^
.. A minus sign copied from the data sheet...

Thanks,
- Win

whill_at_picovolt-dot-com

 

[Capoot]

Jamie wrote...

I have 32V DC 5 amps as the input to the reg, and
the reg is reducing this to around 14 volts. [snip]
The reg should be stable up to 5 amps??

Oh dear Jamie, Jamie, Jamie. [snip] The former spec relates to Note 2,
the 50W limit.) [snippity snip snip]
Thanks,
- Win

Jamie,
Win is correct. 32 volts is way too much overhead for 5 Amps @ 14 volts
out. I don't think it would hurt to accomplish part of the voltage drop
with a 2.5-3 ohm, 100 watt resistor in series with the regulator's input.
Incorporated into a cup holder, it would also function as a coffee warmer.
Voltage regulators make great oscillators isolated from bypass capacitors by
only a few inchs of wire. Be sure that input cap is right at the regulator.
Ray