Help. LDO, intermitent DC Motor

Thread Starter

Alex Cristian R

Joined Feb 16, 2019
2
Hi!

I have made this simple circuit to power a DC motor following the datasheet of LM317 and i have an issue. When i switch on S2 with the Vo set higher than 8V the motor goes on and off continously and fails to start correctly. At the same time the LED blinks.
If i switch on S2 with a Vo lower than 8V then the motor starts just fine. Then i can increase Vo to maximum an the motor still works. I always swtch it on with no load on the motor.
One more thing to mention is that the LDO and D1 get pretty hot.

What am i doing wrong and how can i improve this circuit to be able to start the motor correctly at any Vo i set?IMG_20220820_100824.jpg
 

Sensacell

Joined Jun 19, 2012
3,098
A DC motor at rest looks like a very low resistance, this causes your circuit to draw enough current to cause your 24 V supply to drop out.
Since it's a linear circuit, all the motor current flows through the regulator and power supply, more than 50% of the power is wasted as heat.

I would re-design this as a PWM motor driver which will have much greater efficiency and can start the motor without overloading the PSU.
 

bertus

Joined Apr 5, 2008
22,120
Hello,

Your R1 has a to high value.
LM317_voltage_regulator_schematic.png
Some datasheets even use 120 Ohms for R1.

What is the supply from the 24 Volts?
When the led is blinking and it is directly connected to the 24 Volts, the powersupply can not handle the startup current.
Also R3 is to large, use a 2k2 to 4k7 in stead.

As others said, a linear regulator will waste a lot of power.

Bertus
 

Jon Chandler

Joined Jun 12, 2008
432
V2, the 24 volt supply – it's a switching power supply? When you increase the voltage to the motor, it will draw more current. If the motor "hiccups" –:starts to spin then stops over and over – it's drawing more current than the 24 volt supply can deliver, causing the 24 volt supply to shut down. Then it tries again and the process repeats.
 

Thread Starter

Alex Cristian R

Joined Feb 16, 2019
2
V2, the 24 volt supply – it's a switching power supply? When you increase the voltage to the motor, it will draw more current. If the motor "hiccups" –:starts to spin then stops over and over – it's drawing more current than the 24 volt supply can deliver, causing the 24 volt supply to shut down. Then it tries again and the process repeats.
I don't know what kind is the 24V supply. This is it's model: XH-2401000-A1 but couldn't find much about it. I also taught that this was the culprit because the LED had no reason to blink. So, how can i improve this circuit so that it starts as it should when the Vo is >8V?
 

Jon Chandler

Joined Jun 12, 2008
432
Find a power supply with a greater current rating. I usually get "wall warts" st the thrift store for a couple bucks. Computer recyclers are another good source too.

A 24 volt supply might be tough to find, but laptop power supplies at about 19 volts should be pretty common, and will be rated at 3 amps or more.

Remember, the motor will only draw the current it needs, so getting a power supply rated for greater output current won't cause any problems.
 
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