# assignment

Thread Starter

#### dascspl

Joined Mar 19, 2006
1
A power supply in a controller was working erratically, its display was dimmer than usual and flickery (strobing on and off at 100Hz rate).
Its power supply is nominally 15 Volts DC drawwing approximately 0.5 amp continuously. The transformer AC secondary is 12V, being fed into a bridge rectifierand a 2200uF electrolytic capacitor filter. The AC input to thr bridge is measured at 12V, al four diodes in the bridge check ok and the DC from the rectifier is measured with a multimeter to be 10.4V. What is likely to be the fault?
What further measurement(s) could you do to affirm your answer?
Give reasons to support what you suspect thefult to be.

#### beenthere

Joined Apr 20, 2004
15,819
Hi,

If there are three components in a system and two of them are working, the third might reasonably be the source of the problem.

#### Gadget

Joined Jan 10, 2006
614
Suggest you check keywords "Smoothing" and "Filtering" and the role of Smoothing capacitors in a full wave rectified Power supply.

#### windoze killa

Joined Feb 23, 2006
605
Originally posted by beenthere@Mar 20 2006, 02:07 PM
Hi,

If there are three components in a system and two of them are working, the third might reasonably be the source of the problem.
[post=15194]Quoted post[/post]​
I would certainly be looking at the capacitor. They are renowned as being the first thing to go in a power supply. A lot of the time you can see them starting to squeeze out of their plastic covering. Always a good sign to change it before it pops and blows the lot up.

#### thingmaker3

Joined May 16, 2005
5,083
Originally posted by dascspl@Mar 19 2006, 06:59 PM
and flickery (strobing on and off at 100Hz rate).
[post=15193]Quoted post[/post]​
How many pulses per second does one get when one rectifies a 50Hz sine wave?

Also: what is the approximate mathmatic relationship between 10.4 and 15?

#### windoze killa

Joined Feb 23, 2006
605
Originally posted by thingmaker3@Mar 21 2006, 09:40 AM
How many pulses per second does one get when one rectifies a 50Hz sine wave?

Also: what is the approximate mathmatic relationship between 10.4 and 15?

[post=15234]Quoted post[/post]​
If the strobing is at 100Hz you wouldn't see it. The human eye can really only see strobing up to about 30hz.

#### thingmaker3

Joined May 16, 2005
5,083
Indeed. A dead givaway "hint" built into the question!

I am reminded of one schizophrenic fellow who can percieve the 120Hz flickering in his facility. He has to be kept away from AC lighting lest he become agitated. DC lighting doesn't bother him. The human brain is truly astounding.

#### windoze killa

Joined Feb 23, 2006
605
Originally posted by thingmaker3@Mar 22 2006, 11:50 AM
Indeed. A dead givaway "hint" built into the question!

I am reminded of one schizophrenic fellow who can percieve the 120Hz flickering in his facility. He has to be kept away from AC lighting lest he become agitated. DC lighting doesn't bother him. The human brain is truly astounding.
[post=15271]Quoted post[/post]​
Are you sure you mean schizophrenic? It is normally epileptics that can see and be affected by flashing lights.

#### thingmaker3

Joined May 16, 2005
5,083
I didn't meet the fellow myself. Nor am I familiar with DSM-IV techniques. (I leave the head-shrinking to my wife.) The pro from the facility said "schizophrenic." I have no cause to doubt the pro's choice of words.

Myself, I can't distinguish 25 flickers per second. I know the difference between 15 and 10.4, though. What I can't figure out is why the autor spent extra money on Shottkey diodes for the bridge rectifier. What benifit would Shottkeys have in this application?