800 power supply board voltage regulators

[dukes909]

For the 7805 and 7812 regulators on the 800 PS board, I do not get 5V and 12V on pin 3 of these unless the PS board is connected to the motherboard via J201/J115.
 

Is this normal? If so I am not clear how that works; is a load needed to get the proper output voltage which would be something new to me.

Cheers,

Dukester

[reifsnyderb]

Hello,

 

I am not sure which pin 3 you are referring to.  

 

While I have not verified this with a meter, I would expect this behavior for the +5VDC supply.  On an 800, Atari has resistor R203 between the input and output of the 7805 voltage regulator.  So you won't get the proper voltage reading without a load on the circuit.

 

This isn't the only place I've found such a resistor.  Atari installed a similar resistor on the 400 power supply and the 1090 80 column board for it's +5 VDC power.

 

I tried finding a reference online to using a voltage regulator in such a fashion and didn't find anything.  Admittedly, I spent maybe 5 minutes searching for it.  My theory is that this resistor takes a lot of the load off of the voltage regulator by supplying a lower voltage to the circuit.  The voltage regulator then makes up the difference by bringing the voltage up to the desired voltage.  Without the resistor I believe the voltage regulator would be overloaded.  Without a load (i.e.  connecting it to the motherboard) you should read a higher voltage as the resistor won't function properly without that load.

 

Best Regards,

 

Brian

 

[dukes909]

12 minutes ago, reifsnyderb said:

 

I am not sure which pin 3 you are referring to.  

Pin 3 on the regulator. It is the output pin and where I would have expected to (and have in all other use cases until now) see 5V on the 7805 and 12V on the 7812..

[reifsnyderb]

7 minutes ago, dukes909 said:

Pin 3 on the regulator. It is the output pin and where I would have expected to (and have in all other use cases until now) see 5V on the 7805 and 12V on the 7812..

Ok.  Yes, the output pin on the 5VDC regulator has that resistor coming from pin 1 on the regulator.  So, without a load, you'll have a higher voltage on pin 3.

[_The Doctor__]

Yes it is normal, there is a resistor that firms things up that allows the voltage to climb slightly when not under load. When properly under load the voltage will be correct.