LM338 Current Dropping

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Suraj143

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I'm powering SIM800L module using LM338 regulators. For the regulators voltage is supplied from a center tapped 15VAC/2A transformer.The 1st regulator is for charge 12V Sealed Lead Acid battery & the second regulator is making 4.1V for module.

The problem is on workbench its working but in site condition the SIM800L is not registering to GSM network.When I bypass the 1st regulator then the SIM800L is registering.

Some type of current limit is happening in this arrangement. Any ideas?
Note: Battery is not included when checking.


 
Could it be that the regulators are oscillating?

Have a look at the datasheet, it recommends an 0.1uF capacitor on the input to the regulator if it is located some distance from the supply reservoir capacitor.

JimB
 
All the components are in a PCB including reservoir capacitor.Only the transformer is coming from externally.The transformer secondary wire is only 3 inches length to the PCB.
 
Why not just try it and see what happens?

Or, the first LM338 could be faulty?

On further reading of the datasheet, the LM338 requires a minimum load of 5mA, try adding a load to the output of the 1st LM338.

JimB
 
Last edited:
The feedback resistors (2k5) will load the first output with 5mA. Does that count?

Mike.
 
If the first lm338 is charging a lead acid battery, it may be shutting down because of over current protection built into the chip so there's no power for the second regulator.

why not connect them in parallel? Why are you cascading the LM338 devices?

AND add the capacitors that the datasheet recommends on the output of each LM338. Oscillations can be created by the LM338 if they are not there.
 
Cascading to reduce heat on regulators when SIM800L module searching network, receiving calls etc....

I want battery backup when 230V fails battery will power up.

But in my tests never connected a battery yet.
 
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