Author Topic: Linear power supply mains transformer  (Read 1499 times)

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Offline BelrmarTopic starter

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Linear power supply mains transformer
« on: September 09, 2017, 10:45:45 am »
I want to build my own linear  power supply , but choosing a mains transformer is getting a bit too complicated, and it gets heavy and expensive. So... is it a good idea to use a smps as a transformer andthen feeding that to the linear power circuitry?

Tx for the help and i apologize for my bad english
 

Offline IanMacdonald

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Re: Linear power supply mains transformer
« Reply #1 on: September 09, 2017, 11:14:24 am »
There is no reason not to do that, and it does eliminate some issues. The main objection is that SMPS tend to generate a fair bit of RF noise and so are less suitable  for general bench testing.

Other point is that linear PSUs tend to use transformer tappings to reduce heat dissipation. Unless you have a variable SMPS (and they are less common)  then you have to put up with one input voltage to your linear reg. For a 30V 5A max output, that would mean dissipating 125W when operating at 5V, 5A. Which would call for a fairly beefy series pass. 

Also, use a SMPS with a decent safety approval such as TUV. Many of the no-brand  ones are substandard in this respect, and you definitely don't want a short to mains on the test bench.
« Last Edit: September 09, 2017, 11:18:39 am by IanMacdonald »
 
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Offline tooki

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Re: Linear power supply mains transformer
« Reply #2 on: September 09, 2017, 12:29:55 pm »
Couldn't you theoretically use multiple SMPSs in series to achieve the same thing as transformer taps?
 
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Offline Cliff Matthews

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Re: Linear power supply mains transformer
« Reply #3 on: September 09, 2017, 02:28:04 pm »
Theoretically yes, multiple SMPS can be stacked to do the same but use caution, some have mains coupled outputs. Also, the quality of galvanic isolation on some cheap SMPS transformers is at best questionable, so if you're going to stack them, your odds of failure may increase.
 
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Online Kleinstein

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Re: Linear power supply mains transformer
« Reply #4 on: September 09, 2017, 03:24:19 pm »
In addition some SMPS (especially those without a PE ground) have class Y caps. Having several of these in parallel will at some point give to much capacitance / leakage.

There are variable SMPS and also a few with multiply outputs, just like transformers with multiply taps.
 

Offline not1xor1

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Re: Linear power supply mains transformer
« Reply #5 on: September 10, 2017, 05:53:55 am »
Couldn't you theoretically use multiple SMPSs in series to achieve the same thing as transformer taps?

you could use a single AC-DC module plus a few buck or boost converters to get multiple fixed DC outputs referred to the same ground
then design the linear regulator as a sort of asymmetrical (unless you need current sink as well) class G audio amplifier

BTW with the aid of a small negative voltage module you could build a real 2 quadrant PSU, able to sink current down to 0V
 
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