SMPS component function question

Thread Starter

John Czerwinski

Joined Jun 19, 2017
62
I'm new to the SMPS topic and have the basic understanding, in a block diagram sense. I'm working on repairing a SMPS for an arcade game (it's a typical "switcher") and narrowed the problem to two resistors that have failed open (R2 and R3).

My two questions are:
1. What are the functions of the 2 100K 2W resistors (R2 and R3) below?
2. On the +5 VDC power rail, what's the function of the 50 ohm resistor (R29) between +5 V and GND?


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DickCappels

Joined Aug 21, 2008
10,231
1. What are the functions of the 2 100K 2W resistors (R2 and R3) below?
2. On the +5 VDC power rail, what's the function of the 50 ohm resistor (R29) between +5 V and GND?
1. They are bleeder resistors so that when the input voltage is switched off the capacitors are discharged.

2. It may be left over from an earlier drawing - the resistor would be a nominal load for testing and things like that.
 

Thread Starter

John Czerwinski

Joined Jun 19, 2017
62
Thank you for the response.

on #2, on the actual PCB I have, it has the 50ohm resistor between the +5 and GND. When troubleshooting the dead SMPS, I thought there was a short between the rails but saw it was the 50ohm resistor. To verify, I took it out of circuit and there was no connection between the rails anymore. I think I've seen this before, but have no idea why it's there
 

Juhahoo

Joined Jun 3, 2019
302
Some power supplies require a constant minimum load on them to maintain output stability, specially fly-back converters, 50 ohm load would set constant current flow of 100mA @5V. There is one resistor load on each output... R29, R26, R27
 
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