5V regulated power supply oscillates with load

[juanf]

Hi everybody,

I really appreciate if someone can help me with this problem:

I'm designing a power supply for an small embedded computer. The computer needs a 5v - 1.2A to work.

I'm using a 6V - 2A transformer, a bridge rectifier, a 3300 uF condenser as a filter and then a L7805 as a regulator.

The circuit works fine without any load. I'm measuring 5.03V on the output. When I connect the computer everything goes fine for a few seconds During the booting process the output voltage drops suddenly to 4.6 causing the computer to restart. When the computer restarts, the output voltage goes to 5 but seconds later, drops again to 4.6 causing the computer to restart again. The current the computer is driving from the power supply at the moment of the voltage dropping is about 850 mA. What could be the cause of this voltage dropping?

Thanks in advance.

 

[kchriste]

Well, for starters, the typical 7805 is rated for 1A so you are overloading it and it may be going into current and/or thermal limiting. Also, you are operating it very close to it's minimum input voltage of 7V, so that might be a problem also.
I would change the 7805 to a low dropout 2A or more version. What parts suppliers are available to you?

 

[blueroomelectronics]

The LM78S05 is a 1.5A version of the 7805. It's going to need a good sized heatsink.

Better yet a 2A 5V switch mode wall adapter should work.

 

[picbits]

What sized heatsink are you using for the 7805 ?

 

[Hero999]

Use a 9V transformer, 6V is too low, the input voltage needs to be >7V for it to regulate properly.

6√2 = 8.5 - 1.4 = 7.1.

This leaves no headroom for the ripple on the filter capacitor.

It isn't oscillating, you're just seeing mains ripple on the output at twice the mains frequency.

 

[juanf]

Thanks. I guessed that could be the problem so I already changed the transformer and the regulator. I'm using now a 12V transformer and a 5V - 2A regulator (NTE1934). The problem persists. I can't get 5V on the output when I connect the computer.

 

[juanf]

I'm not using heatsink. Could that be the problem?

 

[blueroomelectronics]

Could be, you also need a capacitor on both the in & out of the LM7805 (read the datasheet), plus as noted before your 6VAC supply may be too low.

 

[MikeMl]

I'm not using heatsink. Could that be the problem?

Almost for sure. Put your finger on the regulator as the computer boots. If it burns your finger it needs a heatsink

 

[solis365]

I havent looked up the specifics of your new regulator, but if its 2A and youre running it at 1.5A it will probably need heatsinking. It could be thermally limiting itself and causing some ripple.

 

[OutToLunch]

so now you are regulating 5V from 12V with a linear. With just 1A the LDO will be dissipating 7W of power. If you are using a wallwart anyway, why not just get a 5V one with enough rated current?

 

[Hero999]

It certainly will need heatsinking, especially with 12VAC in.

You'll need quite a large heatsink.

I recommend and old CPU heatsink from a PC.

 

[picbits]

It certainly will need heatsinking, especially with 12VAC in.

12VAC into a 78xx aint a good idea at the best of times

But yes - you will need a bloody good heatsink before you do anything else (which is why I asked above if you were heatsinking the 7805)

 

[Hero999]

You should use 9V as I suggested previously.

By the way I haven't explained to you what was causing the oscillation.

On light loads the LM7805 was providing a high enough voltage to power the computer. Once the hard drive got going and the CPU started working harder the computer needed more power. The LM7805 wasn't on a heatsink so it got too hot, overheated and started to shutdown. The CPU's watchdog sensed the under voltage causing it to reboot. Before the system starts, less current was required causing the LM7805 to switch back on again for the cycle to repeate.