About UPS - 5 questions about the reason they can be harmful to PSUs.

[Sakee__]

Hello,

 

Considering low cost models, I understand that some use:

 

• A relay to alternate between mains and battery power, possibly representing a possibility of PSU shutdown due to the relay switching time;
• A circuit similar to voltage stabilizers, also causing the aforementioned scenario;
• A DC to AC inverter that will produce a square waveform.

 

1. What are the harms of the square wave?
2. Is it enough just for the PSU to have a good hold-up time to solve the problems related to relay switching time?

 

About the first question, I was searching for information and I found comments indicating that this waveform causes:

 

• High dV/dt in the high voltage signal of the APFC circuit controller;
• High current flow in the circuit;
• Inductive voltage spikes.

 

1. Is this information correct?
2. Could this current flow be ripple current?
3. Would it be enough to use a snubber capacitor after the bridge rectifier as some PSUs do?

 

If someone can elaborate further, I would be grateful.

[Bombastinator]

As a person not well educated in PSUs what I want out of a PSU is to:

-make my machine work and not damage the other components

-do it for a long time (goes to cost and trouble)

-when it does die, to do it in a way that will not damage the rest of the parts.

low price is only above being easy to install and remove. PSUs are not the most expensive part of the system.  They attach to pretty much everything though including stuff that costs multiple times what they do.  It’s all about component safety.

 

If square wave makes ripple, and I am given to understand that ripple can cause problems, then it is bad. 
If it negatively affects any of those other things it is also bad. 

[Sleepycat3]

2 hours ago, Sakee__ said:

Hello,

 

Considering low cost models, I understand that some use:

 

• A relay to alternate between mains and battery power, possibly representing a possibility of PSU shutdown due to the relay switching time;
• A circuit similar to voltage stabilizers, also causing the aforementioned scenario;
• A DC to AC inverter that will produce a square waveform.

 

1. What are the harms of the square wave?
2. Is it enough just for the PSU to have a good hold-up time to solve the problems related to relay switching time?

 

About the first question, I was searching for information and I found comments indicating that this waveform causes:

 

• High dV/dt in the high voltage signal of the APFC circuit controller;
• High current flow in the circuit;
• Inductive voltage spikes.

 

1. Is this information correct?
2. Could this current flow be ripple current?
3. Would it be enough to use a snubber capacitor after the bridge rectifier as some PSUs do?

 

If someone can elaborate further, I would be grateful.

Just get a pure sine wave UPS. They have cheap ones now, which are fine for consumer use, but not mission-critical or enterprise use. 

 

The pure sine wave resolves the issues that you raise around the square wave.

 

The relay switching time is easily covered by the design of the UPS. That's why you get 1200VA and 1500VA UPS models with the same batteries, but each is designed to be able to switch a system with that power draw (remember to convert VA to W with the right power factor) within the timeframe required for the relay to switch. 

[Sakee__]

22 minutes ago, Sleepycat3 said:

The relay switching time is easily covered by the design of the UPS. That's why you get 1200VA and 1500VA UPS models with the same batteries, but each is designed to be able to switch a system with that power draw (remember to convert VA to W with the right power factor) within the timeframe required for the relay to switch. 

If during the relay's switching time the power distribution to the connected equipment is interrupted, the higher reactive power capacity would perform what function?

22 minutes ago, Sleepycat3 said:

The pure sine wave resolves the issues that you raise around the square wave.

What issues are caused by the square waveform?

22 minutes ago, Sleepycat3 said:

Just get a pure sine wave UPS. They have cheap ones now, which are fine for consumer use, but not mission-critical or enterprise use.

The intent of this topic is not a purchase recommendation, but taking advantage of your comment, I must use an example. I have a friend who lives in an underdeveloped country, where good quality uninterruptible power supplies are not easily found. Most of the time, the only products to the average citizen have the characteristics described in the topic. For this reason, I want to understand their actual consequences.

[Sleepycat3]

3 hours ago, Sakee__ said:

The intent of this topic is not a purchase recommendation, but taking advantage of your comment, I must use an example. I have a friend who lives in an underdeveloped country, where good quality uninterruptible power supplies are not easily found. Most of the time, the only products to the average citizen have the characteristics described in the topic. For this reason, I want to understand their actual consequences.

There are many articles explaining the differences between a square wave and sine wave for a DC/AC inverter, and the types of equipment where a square wave can and cannot be used. I recommend doing a search of "square wave vs sine wave" and reading those articles first. I don't feel like copying and pasting information that is easily found here. My approach is that the more complex the piece of equipment that is designed to run on AC, then the more important a sine wave is to have.

[Bombastinator]

6 hours ago, Sakee__ said:

If during the relay's switching time the power distribution to the connected equipment is interrupted, the higher reactive power capacity would perform what function?

What issues are caused by the square waveform?

The intent of this topic is not a purchase recommendation, but taking advantage of your comment, I must use an example. I have a friend who lives in an underdeveloped country, where good quality uninterruptible power supplies are not easily found. Most of the time, the only products to the average citizen have the characteristics described in the topic. For this reason, I want to understand their actual consequences.

My understanding is that a square wave is on off on off while a sine wave is up and down.  My understanding is not super solid in this matter though

[Caroline]

On 5/22/2022 at 4:32 PM, Bombastinator said:

My understanding is that a square wave is on off on off while a sine wave is up and down.  My understanding is not super solid in this matter though

A square wave is technically not a wave but a series of concatenated pulses cycling at 50-60Hz to resemble a proper waveform. The difference is that these cycles is that the amplitude is harsh, there are two states involved but they're not on/off as the source is always feeding power to the circuit.

Another difference is that a square wave is incomplete as there are no peaks involved. 

 

On 5/22/2022 at 10:19 AM, Sakee__ said:

The intent of this topic is not a purchase recommendation, but taking advantage of your comment, I must use an example. I have a friend who lives in an underdeveloped country, where good quality uninterruptible power supplies are not easily found. Most of the time, the only products to the average citizen have the characteristics described in the topic. For this reason, I want to understand their actual consequences.

I'm that friend. 100% of the UPS being sold in my country at premium prices are crap, but tbh the same happens with everything, most things that involve the use of electricity are fire hazards in one way or another.

 

I trust a fuse more than those things to protect a computer.

 

You must already know inductive loads and pulsating sources don't like each other so take that to power supplies and shit tier UPS and you have an answer, filter caps and PFC mitigate the effects but... if you were to live in a country where only the worst UPS are being sold, what are the odds of you having a proper power supply and not some "1000W" gamemax unit? I know, slim to none.