Regulator issue

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haxan

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Hi, I have a controller board i made for PIC18F4520. The regulator i used is 7805. The problem is that it gets really hot. I dont know why this happens. Can anyone suggest me how to debug this issue. Also what are the values suitable for the Caps for 7805.

I am giving in 9V and getting out 5V.
My PIC is running on HSPLL mode (40MHz)
 
Are you sure it is connected the right way round? That is one of the most common causes of a 7805 getting too hot.

Post your circuit so we can see what you are (apparently) doing, then we can help more.
 
One of the things I witnessed: the reason it's too hot it's because some Grounded the GND of the the 7805 at the input/output pin in which the corresponding pin bit is programmed with the port indicating it as "0".When the PIC's running, the Vout immediately is shorted to GND via the output pin (which behaves as a GND)... and gets hot.

That is IF you've mistakenly grounded it at the I/O pins, which is just beside the Vcc and GND.
 
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The regulator may be self-oscillating - if you don't provide capacitors on input and output - and close to it.
 
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Hi, sorry for not being so very responsive. I have attached the PCB file ( i dont have schematic ). There is a MAX487, Nokia 6100 screen, a Demux IC and the PIC controller. The regulator gets hot even if everything is detached other than the controller.

Now to answer the questions:
The circuit actually works fine so i am assuming its connected alright. I get 5V output from the regulator's 3rd pin.
I have not connected any traces from regulator with my PIC controller IC other than the two VDD's and VSS's and attached a 0.01uF cap on one side.
I have caps on the regulator (both 10uF)
 

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You also need a 100n to stop it oscillating and it must be very close.
Is the 9v dirty? Is it AC or DC? Is the micro getting hot? Are there any other components or lines or wires or resistors in the circuit?
 
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i have a 0.1uF cap close to the the controller. The 9V is DC. Sorry i do not know what dirty means
There are a few resistors, 10K, 470,120 and one resistor pack for 10K to pulldown portB. The controller it self is not getting hot nor any other components.

EDIT: If you are talking of placing 100nF close to regulator, i have an polar 10uF cap there on both Input and output.
 
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Check your current drain, that 7805 is dropping 4V by whatever current you are pulling. If you are pulling 100mA the Power converted to heat is 4 * .1A to give .4W etc. So it will develop some heat. If u want cool operation use a switching regulator.
 
10u will do nothing to prevent 1MHz oscillations. 10n is not high enough. The cap has to be right up against the regulator lead. Dirtly 9v means the input may be half-wave or poorly smoothed or way over 9v on no-load. It may be 15v on no-load and drop to 9v when current is taken. These are all the possibilities that causes teh regulator to heat up. The regulator may not be heatsinked fully.
 
ok i will try replacing it with 10n caps. I used the circuit described here Microcontroller power supply notes which states that 100uF 25V should be used for input and 10uF 6V or greater volts should be used at output.

I have a smooth regulated 9V supply coming from a Power supply not an adapter.
I do not have a heatsink attacked.
 
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The heatsink is your problem. The regulator will get hot, no matter what the current, if it not heatsinked. The caps must be 100n not 10n.
 
haxan said:
ok i will try replacing it with 10n caps. I used the circuit described here Microcontroller power supply notes which states that 100uF 25V should be used for input and 10uF 6V or greater volts should be used at output.
Click to expand...

These capacitors are not for filtering, they are recommended to ensure you get a smooth output with no ripple if there are fluctuations in your input supply.

As colin55 said, you also need to remove any oscillation using 100nF (1uF) capacitors. Multilayer ceramics are the best for this, like these little ones:

**broken link removed**

If you've already made your board and you don't have enough holes, you could solder it straight to the leads or try and negotiate it into the same hole as the regulator.

haxan said:
I do not have a heatsink attacked.
Click to expand...

Depending on your load you might not need one. Below 1A a 7805 really shouldn't be getting hot. I'd keep looking for other issues if you fix the capacitor problem above and you still have trouble.

Your board is very neat. I like it.
 
Below 1A a 7805 really shouldn't be getting hot
Click to expand...
edeca Don't say things you know absolutely nothing about.
You have obviously never used a 7805 in your life.
At 1 watt dissipation, you can hardly hold your fingers on the regulator. At 2 watts you burn your fingers and at 4 watts the regulator shuts down.
The 7805 regulator drops 2-4 volts as the lowest voltage drop and at 1 amp it dissipates up to 4 watts. The regulator can be dropping 5 - 10 volts depending on the input voltage so saying anything about the current flow is absolutely and completely unrelated to the dissipation.
Before you put anything in writing, make sure you have carried out exactly what you are describing and not producing "armchair electronics spiel."




​
 
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Don't say things you know absolutely nothing about.
Click to expand...
You have obviously never used a 7805 in your life.
Click to expand...
...saying anything about the current flow is absolutely and completely unrelated to the dissipation.
Click to expand...
Before you put anything in writing, make sure you have carried out exactly what you are describing and not producing "armchair electronics spiel."
Click to expand...
Current actually has a great deal to do with power dissipation!

You're no where near the expert you think you are and you have no right coming down on people with statements like those.
 
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colin55 said:
At 1 watt dissipation, you can hardly hold your fingers on the regulator. At 2 watts you burn your fingers and at 4 watts the regulator shuts down.
Click to expand...

My comments were based on the original post in this thread, which stated:

"I am giving in 9V and getting out 5V."

You are of course correct that at higher input voltages and lower output currents, a lot more is dissipated as heat.

I don't take any offense from the internet but please: be a little kinder with you responses. We're all here to help and where people need correcting (as I often do) they should be able to learn constructively from it.
 
Small problems like regulators heating up shouldn't invite too much trouble, and certainly doesn't warrant any name calling.
 
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