Hi,
I'm a enthusiast of electronics, I always liked but never pursued this career.
A few months ago, I brought a small water pump for a 5 gallon vessel. To take water out without too much effort.
All the suddenly it stops work. I think it is the lithium battery. I'm sure it´s a chinese appliance, and I don't trust those batteries too much.
I would like to eliminate the battery at once and let it plugged in all the time.

I found a instructable where the guy uses a LM-317T voltage regulator and 2 resistors (a 5k on the adjust pin and a 250 on the out pin) to eliminate 4 type D batteries. I'm assuming that my battery is a 3.7v and the guy total voltage is 6. 4x1.5v.
Can anyone help me on which resistor should I use in my case? Or at least how do I calculate the apropriate one. I don't even know how to search for this topic.

Thanks a lot.

 

Welcome to AAC.
First, you shouldn't need a resistor when powering a motor (pump). Second, if the unit has quit working I'd check for the obvious things first:
1) Is the pump impeller obstructed with something? A twig or maybe some string has choked it.
2) If the motor runs but not pumping, has the impeller become loose?
3) If the motor runs but not pumping, is there something blocking the discharge tube.

Those are the first three things I'd check IF the motor IS running but not pumping.

4) Can you run the pump with new batteries directly connected to the motor? If the motor runs - - -
5) If the motor runs then are the original four D cell batteries dead? If not - - -
6) If the original batteries are not dead, has the switch malfunctioned? Broken?
7) If the switch is not broken are there any broken wires?

You may find the pump obstructed in some way, or you have dead batteries, or the switch or some wire(s) is(are) broken.

If you can run the motor from a separate power source, since the original setup was to run on 6V then try wiring up a lantern battery (6V). If the motor pumps water then you can go ahead with finding a low voltage power supply capable of supplying the necessary amperage. I'll get back to you in a sec - gotta check something. Watch for an edit.

[edit]
Reference: how many amps can a d cell deliver
OK, looks like an average D cell battery can deliver 6 or more amps when fresh. To know what amperage you need to drive the pump motor you need to know how many amps it draws at startup. If it draws (for example) 4 amps to start up and then runs on 1 amp (numbers made up) then you'll need a power supply capable of pushing at least 4 amps. I'd recommend 5 amps minimum. Since the motor draws heavy current at startup but doesn't require nearly as much amperage to continue running I think a 5 amp 6 volt power source would do nicely. But that's assuming my "example" numbers are correct. Since we don't know anything about your pump we really can't get specific.

I'd watch for the issues mentioned and I'd check the batteries, the switch and wires. If the batteries are good and the motor runs then there's either a failure in the switch or the wiring.
[end edit]

 

There are on-line calculator for using the LM317 and circuits.
But assuming that is a common DC brushed motor, you do not really need a regulator.
DC motors are quite flexible, up to a point, on voltage.

 

The LM317 is a linear voltage regulator which can give you up to 1.5A output:

The output voltage is determined by the potential divider R1 and R2. The value of Vout is determined by Vout - Vadj (where R1 and R2 meet) being equal to 2.5V. Vadj = Vout x R2/(R1 + R2). Vin needs to be about 2 volts higher than Vout.

But, if the pump motor is designed for 3.7V as you describe, if it still functions it should at least spin with two batteries, i.e. 3V and it will probably survive for quite a while with 6V - just to check the motor still works. It would help if you have a multimeter to test with, but the easy way to reduce the volatge across the motor with a battery which has too high a voltage is to put a resistor in series with the motor. Better would be to use a potentiometer across the battery like 10k ohms with the motor being driven in series with the collector/emitter pins of a transistor - the centre tap of the pot into the base would allow you to adjust the speed of the motor.

 

@Tonyr1084 Thank you for the input. I din't thought to test the motor fisrt. I assume that the battery may had died out.
I did a simple test, using a 3v adapter, and it did run for a sec. Not sure what failed this time, but I'll follow this path first.

@MaxHeadRoom Nice to hear that. Will try with a little diference adapter. Thanks.

@Jerry-Hat-Trick I saw a formula similar but not quite the same. Thanks for clarifying that for me.