Sharing 18-0-18 with LT1963A

[akamika]

I am trying to power es9028q2m using one transformer. It has separate power inputs 12V for Digital and 18-0-18 for opamps (LM337+LM317) , image below. Digital section is using LT1963A to supply 3.3 to DAC.

I think the circuit is regular full bridge followed by capacitor and regulator. The capacitor is 4300uF and 25V. Datasheet for LT1963A says it is rated for 20V

My question is can i use the 18-0-18V to power digital section as well. I tried, plugging one end and center pin to DC, got sparks and smokes under the capacitor. Is it 4300uF and 25V capacitor too less for the 18V after bridge. Or it has to do with the partial use of the central tap power supply.

Thanks

TRANSFORMER

OLD USE OF TRANSFORMER IN AVR-40, TOP SECTION IS NOT USED (40-0-40V)

 

[DF96]

The quick answer is no. This is because two parts of a circuit cannot be both separate and connected.

 

[akamika]

My assumption was power supply separation was made to isolate noises from one part to another.

When you said separate and connected, did you mean assuming separation was required. Or something relating to electronics principals and elimination of high enough noise or other complex reason that separation is not a choice but needed for its functioning.

Thanks

 

[DF96]

It is very simple. If two PSU circuits are designed to be fed from two different secondaries then almost always it is impossible to feed them from one secondary. Attempting to do so almost always leads to something getting short-circuited, as you found. They were designed to be separate; therefore they cannot be joined.

 

[eziitis]

of course, you need 2 secondaries for this board, but if you have only 1 transformer, you can mod it and feed LT1963 from the positive shoulder after LM317. not optimal but it will work, and some ebay designs use such tricks.