LM338 ouput voltage is decreasing with an increasing laod - why?

[bg]

You are trying to dissapate 90 watts in the regulator. Thats alot of power
even for a TO3 case. I don't have the specs but my gut feel is that you have
to lower the 32 volt input to about 20 volts otherwise the regulator will go
into a protection mode.
bg

 

[bg]

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

 

[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

 

[Thomas C. Sefranek]

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.

Ahhh, It FAR more important to have bypass capacitors shown in the
spec.sheet
on the INPUT and OUTPUT than any "ripple" cap on the adjustment pin!
I NEVER bypass the adjustment, it slows the response of the regulator!

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??

1. Oscillations! (lack of proper bypassing.)
2. Improper pin connections.
3. Not enough heat sink.

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

thanks in advance
jamie

--
*
| __O Thomas C. Sefranek [email protected]
|_-\<,_ Amateur Radio Operator: WA1RHP
(*)/ (*) Bicycle mobile on 145.41, 448.625 MHz

http://hamradio.cmcorp.com/inventory/Inventory.html
http://www.harvardrepeater.org

 

[Thomas C. Sefranek]

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.

Ahhh, It FAR more important to have bypass capacitors shown in the
spec.sheet
on the INPUT and OUTPUT than any "ripple" cap on the adjustment pin!
I NEVER bypass the adjustment, it slows the response of the regulator!

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??

1. Oscillations! (lack of proper bypassing.)
2. Improper pin connections.
3. Not enough heat sink.

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

thanks in advance
jamie

--
*
| __O Thomas C. Sefranek [email protected]
|_-\<,_ Amateur Radio Operator: WA1RHP
(*)/ (*) Bicycle mobile on 145.41, 448.625 MHz

http://hamradio.cmcorp.com/inventory/Inventory.html
http://www.harvardrepeater.org

 

[Thomas C. Sefranek]

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.

Ahhh, It FAR more important to have bypass capacitors shown in the
spec.sheet
on the INPUT and OUTPUT than any "ripple" cap on the adjustment pin!
I NEVER bypass the adjustment, it slows the response of the regulator!

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??

1. Oscillations! (lack of proper bypassing.)
2. Improper pin connections.
3. Not enough heat sink.

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

thanks in advance
jamie

--
*
| __O Thomas C. Sefranek [email protected]
|_-\<,_ Amateur Radio Operator: WA1RHP
(*)/ (*) Bicycle mobile on 145.41, 448.625 MHz

http://hamradio.cmcorp.com/inventory/Inventory.html
http://www.harvardrepeater.org

 

[Jamie]

Thanks for the info,

The pin connections are fine, I can draw 500 - 800ma and its no problem, but
with a 2 amp load ,the output voltage is reduced.
I have 2200uf caps on the input and on the output, and also 0.1uf on the
input, that should be sufficient?

I have tried lower input voltages (15vac into the diode bridge instead of
24vac) and the same occurs...

 

[John Popelish]

Thanks for the info,

The pin connections are fine, I can draw 500 - 800ma and its no problem, but
with a 2 amp load ,the output voltage is reduced.
I have 2200uf caps on the input and on the output, and also 0.1uf on the
input, that should be sufficient?

I have tried lower input voltages (15vac into the diode bridge instead of
24vac) and the same occurs...

The input capacitor may be part of the problem. With a 2 amp load, it
is going to have a pretty large ripple voltage, especially if it is
not in particularly good condition. Do you have another similar cap
you could parallel on it, to see if the symptoms change?