Currently I have a 28v (design constraint) DC power supply that is being cut down to 9v by a 78L09 regulator. I have tested the output voltage and it reads 9v, but when I connect the output to Vin it drops to an unstable 5v and the regulator becomes very hot. The current going through it is within the design specs for the regulator(~40mA), so I am lost as to why the regulator does not output 9v when connected to Vin.
The regulator becomes very hot.
That's why. It's getting too hot and shutting down or not working properly as it is too hot.
Does it have a heat sink (I suspect not) and does it have at least a 0μ1 capacitor between input and ground and output and ground?
Are you familiar with how to calculate power? Are you familiar with how to calculate the expected temperature rise of the device?
TBH, if I were you I'd forget the linear regulator and use a buck converter with a 5V output connected to the 5V pin.
Regulator voltage changing when put into Vin (Arduino Mega R3)
The current going through it is within the design specs for the regulator(~40mA)
I have just tried my own Mega fed from 5V and it draws 80mA, I doubt yours is only drawing 40mA. That's 80mA with nothing else connected to it.
I changed the dc power supply to 5v and had it go right into the Mega Vin, which drew 40mA. I don't understand why my regulator would be overheating then if the current max and vcc max are within range. Ill take a look at buck converters. Thanks
Update: I fashioned a heatsink and its working fine. Thanks for the input!
In practice the 78xx regulators are usually thermally limited. It can't put-out the rated current without a heatsink. 40mA would be fine if you're only dropping a couple of volts across it. But, you've got 19V x 40mA = 0.76 Watts. That's going to need a heatsink.
78L09 is a TO92 case regulator, rated for 100mA.
It has no heatsink capability, which you need when dropping 28V to 9V.
Power dissipated in regulator
P = V * I
P = (28-9) * 0.1 = 21 * .1 = 2.1W, which is too much for a small plastic case.
You need a TO220 case LM7809.
Can you please post a copy of your circuit, in CAD or a picture of a hand drawn circuit in jpg, png?
Do you have the proper bypass capacitors near the pins of the regulator as per the datasheet?
Bizarre approach overall.
Use a switchmode "buck" converter to regulate down to 5 V, because that is what the Arduino actually uses - and power it via the "5V" pin.