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How to Catch Loading Down in DC Power Supplies Before It Catches You


Son of Samurai

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Loading down is a failure scenario unique to DC power supplies, and if you don't know what to look for, it can leave you scratching your head and eating some expensive parts. How do you identify that you have a potential loading down scenario, and what steps can you take to verify that? That's exactly what we cover in this Master Samurai Tech Workshop.

Topics covered include:

  • How DC power supplies work
  • How they affect the board's operation
  • What loading down is and how it manifests
  • How to test for loading down

...and more!

You can view this workshop in its entirety right here for free. Want to see more like it? We have over 100+ hours of technical workshop videos like this available here -- only for premium tech members at Appliantology.

 

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Thanks for covering this topic! Question: what's to keep the unit from throwing an error code when it doesn't sense the load is connected, then possibly locking down the unit? For example, disconnecting a condenser fan, the board doesn't sense feedback and throws the CF error code, making it troublesome to further investigate...

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Excellent work! Never able to catch these live, but wanted to let you guys know how much these visual aids are appreciated and help a fellow appliance nerd hone his skills. Keep pumping them out!

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  • Team Samurai
Samurai Appliance Repair Man

Posted

3 hours ago, KaveMan said:

Question: what's to keep the unit from throwing an error code when it doesn't sense the load is connected, then possibly locking down the unit? For example, disconnecting a condenser fan, the board doesn't sense feedback and throws the CF error code, making it troublesome to further investigate...

Great question! This is where we have to apply tech mojo and understand the technology we're dealing with. Specifically, BLDC motor technology. In a two-wire BLDC motor configuration, such as the example in the video, the only wires are for DC voltage and DC Ground-- there is no RPM feedback signal generated by the motor and sent back the board. So the board never knows whether the motor is running or not. 

In three or four wire configurations, you'll have either an RPM feedback (motor to board) or a PWM speed control signal (board to motor) or both. You'll have to discern the particulars from the schematic and board pinout diagram (especially like the ones Samsung often includes in their Fast Tracks, which, in my opinion, should be industry standard). 

Sam has a good blog post here that explains BLDC motor configurations.

In cases where the computer is monitoring RPM and detects 0 RPM when the motor should be running, the board can be programmed to take several actions, such as remove power and try again in 15 minutes, repeat three times and then lock out-- this is the Samsung algorithm. Other manufacturers may do it differently. Algorithms are something we cannot determine from schematics or pinout diagrams; the manufacturer has to explicitly tell us about any algorithmic quirks that could affect our troubleshooting. 

In most cases where the computer does lockout a defective load, such as an evap fan motor that's not rotating, placing the control into force mode overrides this and forces the computer to supply power to the evap fan. You can do force mode with the evap fan harness both connected and disconnected, checking for your DC voltages at the harness connector on the board. 

It should be noted that typically, if you're really dealing with a loading down problem of the DC power supply on the main board, multiple DC loads may be affected and even the ability to enter service mode or force mode may be affected until the low impedance load is disconnected from the circuit. 

Loading down is caused by loads with an input impedance that is below the input impedance specified by the manufacturer. I have a video that explains the physics of loading down here

Did that answer your question? Lemme know! 

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The note about relays being controlled by DC voltage was awesome, I hadn't considered that and that part has definitely messed with my head several times with loading down scenarios. Thank you!

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Samurai Appliance Repair Man

Posted

5 hours ago, MarcusF said:

The note about relays being controlled by DC voltage was awesome,

This is a key concept of power supply control in circuits and it’s deployed all over the place in appliances.  Relays, triacs, and other devices are used to allow a tiny DC voltage and current (ie, a DC power supply) to control a big mondo AC voltage and current. That DC control is often directly controlled by a microprocessor.

This also means that AC loads cannot cause loading down of DC power supplies because the DC supply is never directly connected to the AC supply. It’s always done via isolating switches like relays or triacs. 

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