Why is my l298n dragging down the supply voltage

[amateur_24]

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.

 

[danadak]

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.....

https://www.ti.com/lit/an/snoa660a/snoa660a.pdf?ts=1749043387607&ref_url=https%253A%252F%252Fwww.ti.com%252Fproduct%252FLM340

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

LM340 data sheet, product information and support | TI.com

TI’s LM340 is a 1.5-A, 35-V, linear voltage regulator. Find parameters, ordering and quality information

www.ti.com

 

[crutschow]

What is the measured voltage on the 7805 input when the output is 4V?

 

[schmitt trigger]

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....

LM340 data sheet, product information and support | TI.com

TI’s LM340 is a 1.5-A, 35-V, linear voltage regulator. Find parameters, ordering and quality information

www.ti.com

My exact same questions.

 

[amateur_24]

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

 

[danadak]

No cap on output to manage transient I draw ?

Any scope shots using the actuator ?

Why adjustable when you can get a LM7808 fixed reg..... ?

 

[ronsimpson]

12V into the regulator 8V out. 1A current 4 watts. Is the regulator very hot?

 

[amateur_24]

No it's stone cold.

No cap on output to manage transient I draw ?

Any scope shots using the actuator ?

Why adjustable when you can get a LM7808 fixed reg..... ?

Yeah I only had a 7805 to hand since my experiments tend not to be above 5v.

I think I might just use a 7808. I have bought myself a kit of regulators.

 

[schmitt trigger]

Amongst your regulator kit include a LM317. It is a proper adjustable regulator, required for any nonstandard voltages.

 

[Tony Stewart]

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.

 

[danadak]

He did it this way (from datasheet) :

 

[ahsrabrifat]

​

The 7805 is not designed for high current at higher output voltages.

Source: https://www.theengineeringprojects.com/2018/05/introduction-to-7805.html

The 7805 is a linear voltage regulator, and it’s designed for 5V output, usually with a maximum current of ~1A, but that:
Requires a large heatsink
Decreases with increased input/output voltage difference
You’ve adjusted it to output 8.1V, which puts even more thermal stress on the regulator.
Linear regulators dissipate power as heat:
Pdissipated=(Vin−Vout)×Iload

If you're supplying 12V in and getting 8.1V out at 0.5A:



P=(12−8.1)×0.5=1.95 W

That’s a lot for a TO-220 regulator without proper cooling.

L298N is inefficient. The L298N has a voltage drop (typically 1.8V–2.5V) across its internal Darlington transistors.

• If your input is already dropping to 4V, the motor only sees:

Vmotor≈4V−2V=2V

Which matches your observations.