Power Supply Design

"One of the most effective methods for troubleshooting electronics is unplugging power."

The most effective troubleshooting/repair tool available for computers or microprocessor-controlled systems is turning power off, waiting a period of time, and turning power back on.

Here’s why it works:

Computers and microprocessors are control systems which are generally not fully controllable. This means that either the hardware or software can put them into a state where normal control inputs have no effect on the system. This topic is called “Controllability” in formal Control Theory jargon.

An analogy would be an Interstate off-ramp with no returning on-ramp. Once you get off, you have to do something abnormal, like back-track several miles on surface roads, to get back on.

From now on, I will use the term “computer” to mean computer or any microprocessor-controlled system. Your microwave, VCR, and fancy coffee pot are non-computer examples.

One state that all functioning computers can recover from is the power-off state. Hardware and software engineers work diligently to make sure a computer can turn on into a known controllable state. Imagine how upset customers would be if, regularly, they took their electronic devices out of the box, flipped the on switch, and nothing happened. You can be sure the manufacturer would hear from the customer, and that the hardware or software engineer at fault would also hear about it in no uncertain terms.

So, how does a computer get into an uncontrollable state?

In hardware, there are many causes for what is called a Single Event Upset (SEU). A power glitch, a cosmic ray passing through an integrated circuit (IC), or an alpha ray from the plastic IC package, can all cause an SEU, possibly changing a logic state (1 to 0 or vice versa), or triggering latchup in the pnpn layer most ICs have. In software, the computer can get caught in an infinite loop.

How do you turn your computer off, and how long do you keep it off?

Using the off/on switch or normal software shutdown will cure more than 90 percent of the problems, but not all of them. After turning off the computer, you need to pull the plug from the wall and make sure anything the computer interfaces with (modem, printer, etc.) is also turned off and unplugged. (Power strips are great for this.) If your computer has a battery, such as a laptop does, or a built-in UPS battery, you also need to remove this power source. The reason is that even if you turn off your computer, it still draws vampire power to keep certain monitoring and startup circuits alive -- which may be causing the problem.

Now that you’ve turned it off, how long do you keep it off?

Usually, but not always, 30 seconds is enough. This is because bleeder resistors across capacitors used to be designed to discharge logic, memory, and interface voltages to less than five percent of normal voltage in about this amount of time. It was considered good design practice. These discharge paths were also often included in ICs to remove charge from junctions and internal nodes when un-powered.

Today, the discharge resistor is often not included in the design. Cost-savings is one reason, but also because real estate in ICs is so valuable. Active pull-down and pull-up devices, which take less real estate than resistors, are used instead. These work when there is power, but they can be high impedance with power removed. What this means is that a charge on system components and IC capacitance can keep the computer in an uncontrollable state for a longer time. As computers get "better" you need to leave them off longer.

What’s my personal approach?

I turn the computer off (and remove the battery in a laptop), wait 30 seconds, replace the battery if necessary, and turn it on. If this does not cure the problem, I turn it off, unplug everything, remove battery backup, wait several minutes (up to 30 minutes), and try again. If this does not work, then I leave it off overnight.

If these steps aren’t successful, re-cycling power will not solve the problem.

As those of you who have visited my website know, I include a personal anecdote for every problem/solution description I provide. Here is my anecdote for this problem:

When I was newly married, my wife would ask me to fix this or that electronic device that went out. Confidently, I would instruct her to pull the plug, reverse it (before plugs were polarized), wait 30 seconds, and plug it back in. She would first complain that this would not help, and then was amazed when it solved the problem.

Now, 47 years later, she just reported that the microwave went out. Without a word from me, she reached back behind the cabinet, pulled the plug for 30 seconds, plugged it back in, and reset the clock. She now accepts this as the best way to trouble-shoot/repair any electronic equipment. From her own practical experience, she knows it works most of the time.

The bottom line: One of the most effective methods for troubleshooting electronics is unplugging power. The time-tested technique is to turn it off, pull the plug, remove the battery, wait, replace the battery, plug it back in, and turn it back on. And best of all…this powerful method of troubleshooting electronics is free!

Acknowledgement: My daughter-in-law, Jonelle Foutz, is a freelance copywriter and editor. Sometimes, when she is not too busy, I ask her to edit something I have written (so I can learn to write better from a professional). She was kind enough to edit this blog. If you are looking for a writer or editor, you can check out her website at www.writemindonline.com.