Hello,
I used an LM7805 voltage regulator to power up 3 relay modules, which draws 0.200mA when all three are turned ON.
But after just a 30 seconds or more, my regulator get very hot and I cant hold my thumb on it even for second.
I tried 3 regulators and all are the same.
After I measured the output Voltage of my 12v adaptor, it was about 13 so I added a 7810 befor the 7805 to regulate the input voltage, but its still the same, the 7810 heats up too but in more time and very lower temperature.
My circuit is the simple one with two 330uF capacitors on input and output.
I know i have to use heat sinks but this much of heat for this low current is not ok, right?
What should i do?
You should use heatsinks, simple as that. Those linear regulators on their own are not capable of passing more than 100mA or so. Although I make extensive use of them, if low noise is not important to you I suggest you get a buck converter which will be far more efficient.
For the 7805, try bolting it to a decent metal plate, or equipment housing. Even a stout crocodile clip or similar will help.
Output capacitor is 100nF, input is 330nF. You may need a 100uF depending on how good your PSU is. However that will not resolve your problem which is purely due to dissipation of the heat. Bear in mind that at 13V and 200mA, you are needing to get rid of 2.6 Watts of energy.
So you mean the heat is normal? I used these a lot before but this time, it was just too much, I wanted to make sure if everything is ok or not.
And, is there a problem with capacitors that big? or its just not necessary?
So you mean the heat is normal?
Yes, the heat is normal. Linear regulators like the 78xx series convert the 'excess' voltage to heat. Buck converters, which I suggest you use, 'swap' voltage for current.
You need to learn that power in Watts equals:
voltage * current
voltage squared / resistance
current squared * resistance
Those equations are important.
Homework:
Calculate the power being dissipated in your 7805.
From the 7805 data sheet find its thermal resistance.
From the calculated dissipation and the thermal resistance calculate the expected temperature rise.
Hi,
Look at the regulator datasheet and see what they recommend as bypass capacitors at the terminals of the regulators.
If you have only 330uF then you are not providing the needed bypass.
Add 0.1uF at least to the input and output terminals of the regulators.
Tom... 
This is my regulator setup. Notice the individual finned heatsinks as well the fact that I have bolted them to the baseplate. At 300mA, the 7805 is just slightly warm.
You must have the correct bypass capacitors. Notice that I have added them right next to the bodies of the device using SMD capacitors. Also note the 100uF at the input. The ceramics are required to stop the device "hooting" as we say (breaking into oscillation) which can destroy the device.
Hi,
Can you please post a picture of your project so we can see your component layout?
Can you please post a copy of your circuit, in CAD or a picture of a hand drawn circuit in jpg, png?
Thanks.. Tom...
Most builders do not realize that a member the 78XX family of voltage regulators
is not a simple transistor but it has an op amp built into its feedback network.
The voltage gain of the op amp is well over 1000, so it can react to very small
disturbances on the regulator output. The bypass capacitors are required so
that unwanted signals do not influence the operation of the IC. It can oscillate
at over 20 KHz and your DMM will not respond well to that. Use the recommended
bypass caps installed close to the input, ground and output pins.
Herb
TomGeorge:
Can you please post a picture of your project so we can see your component layout?
Can you please post a copy of your circuit, in CAD or a picture of a hand drawn circuit in jpg, png?
emm... I actually dont know how to get a URL to copy here, do you know any website to upload my pictures on?
groundFungus:
How to post images.
Thank you!
TomGeorge:
Can you please post a picture of your project so we can see your component layout?
Can you please post a copy of your circuit, in CAD or a picture of a hand drawn circuit in jpg, png?
Heres my breadboard connections:
And heres the circuit:
***SORRY SORRY i forget to connect the 7810 GND in schematic, dont see that
TomGeorge:
If you have only 330uF then you are not providing the needed bypass.
Add 0.1uF at least to the input and output terminals of the regulators.
I dont have 0.33uF caps, So I did as you said, just used 0.1uF caps.
Hi,
Thanks for the pics.
Have you actually measured the current out of the LM7805?
Tom....
TomGeorge:
Have you actually measured the current out of the LM7805?
Yes, when all three relay modules are connected and turned ON, its 0.190mA
And the signal pins are connected to Arduino Nano which is powered by 7810, I didn't measured 7810 current because some other things are powered by Arduino.
Hi,
What is your 12V adapter?
Tom....
190mA is input or output current (to/from 7805). It may have a large ground current due to a fault.
TomGeorge:
What is your 12V adapter?
12V 3A, I dont know what do mean by what it is, but its not a switching PSU.
Try to connect it between 7810 out and 7805 in.
Setrik_aZ:
12V 3A, I dont know what do mean by what it is, but its not a switching PSU.
What has it got written on it?
Please post a picture of the power supply?
Thanks.. Tom...
If you have 5volt relays, then why not simply use a 5volt power supply.
An old 5volt cellphone charger will do.
Or get a 12volt relay board.
Leo..