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Why is my l298n dragging down the supply voltage

I have a 7805 configured as a adjustable regulator. It supplies 8.1v with no load. I have this connected to my l298n motor driver but when the the motor is trying to move, the voltage on the output of the 7805 drops to around 4v. I can see on my bench PSU it drawing half an amp. The voltage across the motor is only 3v when it's a 6v motor.
I don't get why it doesn't have enough power.
 
Post schematic .... The 7805 is a fixed reg, how are you setting it for adjustable ?

Sure sounds like 7805 got overloaded, put into current limit....

Do you have a DSO you can trigger on current, one shot, say use a .1 ohm R in ground supply lead,
to see whats actually going on ? Set for say 100 mV trigger.....


Take some time to read some of the ap notes on these regs, more than meets the eye.

 
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What is the measured voltage on the 7805 input when the output is 4V?
 
Sorry Circuit lab wouldn't let me open up the circuit to take a screenshot. I downloaded ltsppice to find it doesn't have a model for the 7805. I borrowed multisim off the world wide web.

I do have quite a beefy heatsink I salvaged from a pc psu. The power source is bench psu. I have set the current limit to Very high. According to my multieter the linear actuator draws around 1A under load. Is the 7805 not good for 1A?

google 7805 variable voltage regulator
 

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I wonder how you made 8V from a a7805 when the error voltage regulation is internal. (3V LED in series with ground pin?). Compare the motor resistance with the driver short circuit current limit or driver ESR which should be less than 5% of motor DCR. Ohms Law applies.
 

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