Author Topic: Connecting two bucks in series  (Read 1141 times)

0 Members and 1 Guest are viewing this topic.

Offline mdijkens

  • Regular Contributor
  • *
  • Posts: 138
  • Country: nl
Connecting two bucks in series
« on: March 05, 2017, 04:31:45 pm »
Hi I have two of these units
According to the manufacturer they are not designed to put in series.

I want to power them with two separate SPS from old HP laptops. These SPS seem isolated to me (very high (Megaohm) resistance
If they are really isolated, could there still be a problem putting them in series?
I want to use it to for example to easily create a positive and negative rail on my breadboard projects with only low current circuits ....

I don't want to brick them!
Can I do this? test it in a safe way?
Are there precautions I need or can take?
 

Offline jeroen79

  • Frequent Contributor
  • **
  • Posts: 515
Re: Connecting two bucks in series
« Reply #1 on: March 05, 2017, 06:20:36 pm »
If the supplies to the buckmodules are isolated then the buckmodules will be isolated as well and you can put them in series.
 

Offline mdijkens

  • Regular Contributor
  • *
  • Posts: 138
  • Country: nl
Re: Connecting two bucks in series
« Reply #2 on: March 05, 2017, 10:08:44 pm »
Thanks

As a beginner, what is isolated? If I read 1M ohm between in and out of the SPS, does that count as isolated?
 

Online blueskull

  • Supporter
  • ****
  • Posts: 13552
  • Country: cn
  • Power Electronics Guy
Re: Connecting two bucks in series
« Reply #3 on: March 05, 2017, 10:13:22 pm »
Thanks

As a beginner, what is isolated? If I read 1M ohm between in and out of the SPS, does that count as isolated?

If the negative output is not dead short to ground, then it can be considered isolated.
If it is shorted to input earth, then don't try to float input earth (ground), you may just kill yourself.
« Last Edit: March 06, 2017, 01:24:25 am by blueskull »
 

Offline Gregg

  • Frequent Contributor
  • **
  • Posts: 766
  • Country: us
Re: Connecting two bucks in series
« Reply #4 on: March 05, 2017, 11:26:48 pm »
Thanks

As a beginner, what is isolated? If I read 1M ohm between in and out of the SPS, does that count as isolated?

The buck converters you listed are not isolated in the sense that the input terminals are not totally separated from the output terminals.  You can check with your multimeter in both ohms and diode modes in every possible configuration.  This means that you cannot series the buck converters from the same input source.
However if both of your laptop power supplies are confirmed totally isolated form their inputs, you have a chance of making it work.  Try powering both laptop power supplies and measuring voltage across the outputs between the two; you may get some phantom voltages that go away with a fairly high resistance (like 20K) connected to the terminals you are testing. 
After you have confirmed that the laptop power supplies are indeed isolated, try connecting your buck converters to them and before connecting them in series test voltages between outputs like you did for the laptop power supplies.  You might want to put separate loads on them for testing.
If all seems isolated, before connecting them in series, you probably should put some appropriate diodes on the output of each buck converter to prevent any reverse spikes or currents that will brick them.  This will mean the voltage read on the buck converter meter will be less at the load because of the diode voltage drop but the internal meters will still be a reasonable reference for balancing the two converters. 
Think of all the ways things could go wrong and test / correct them before powering up anything. 
And remember that the current may like the best path but it will find any path and screw up your day.
 

Offline mdijkens

  • Regular Contributor
  • *
  • Posts: 138
  • Country: nl
Re: Connecting two bucks in series
« Reply #5 on: March 06, 2017, 06:19:24 pm »
Thanks Gregg for your tips.

I'll do all tests step by step to see if it is really isolated
 


Share me

Digg  Facebook  SlashDot  Delicious  Technorati  Twitter  Google  Yahoo
Smf