Voltage regulator fault driving me MAD

Dimwit100 · Joined: 09 Jan 2007 Posts: 9 Location: Telford UK

I'v got a LG dvd/vcr/surround sound unit with a strange fault in the power supply that is driving me mad.
There are two 7805 regulators wired in parallel all three pins are totally in parallel and they are mounted on identical heatsinks but in use one heatsink gets very hot and I mean very hot if I swap the two regulators over complete with the heat sinks the same one still gets hot even though it is in the position of the other one on the pcb also swapping them over onto each others heat sink has the same effect the same heat sink gets hot. I have also changed the regulators for new ones but the same heat sink gets hot. I have examined the heat sinks closely and the look identical there are no sharp bits high point etc.
Can anyone help?
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If it wasn't broke before I started it soon will be!!

cory · Guest

im not familiar with this unit and maybr your right but seems impossible resoning why they would put 2 regulators in parallel.whats the point, recheck and you will probably find each supplies different loads.find what is loading heated regulator and troubleshoot that section.hope this helps.regs in parallel?

Dimwit100 · Joined: 09 Jan 2007 Posts: 9 Location: Telford UK

I thought as well it seems pointless to put 2 regs in parallel but they are you can clearly see the track on the pcb as they are facing each other about 20mm apart. Like I said above the reg that it fitted to the one particular heatsink is the one getting hot even if you physically swap the 2 regs over on the pcb!!!!!!!!!!!!!!!!!
_________________
If it wasn't broke before I started it soon will be!!

Restoration · Guest

I well belive you are being driven MAD!
Simply put, it's WRONG, there is no way that two 7805 type regulators can work correctly in full parrallel.
Slight variations in the individual ICs will always mean that one will draw more current than the other and cook!
Are you sure that its not supposed to be a complementery Balanced, supply?
+5v 0 +5v or Similar?
That would be normal, and a short between the two + rails would explain the Heat!
Sorry that the only way to trace this is either the old fasioned way, pore over the board to trace the destination or each track and draw a diagram.
or duy a service data sheet/ circuit diagram.
If you were sure that it's exactly as it is meant to be, and the Regulators should be in full Parallel, the insert a Resistor, 1 ohm or half ohm in series with the output terminal of each regulator, 1 watt or so, to balance the loading.
Since the Design is wrong a mod would be appropiate, but see that you are sure of this before you start Hacking!!
Melvyn
Restoration Or Repair.
Northern Ireland

Guest · Posted: Fri May 30, 2008 2:15 pm Post subject:

Elect. tech: Had the problem many times. One 7805 has gone out of spec. Replace both with exact same batch numbers.

Guest · Posted: Fri May 30, 2008 7:46 pm Post subject:

I ended up modding the board and got rid of the two 7805's and fitted a larger 5 volt regulator complete with heatsink remote to the board. But it has been so long ago now I can't remember the type but it worked.