Author Topic: Lab Bench PSU  (Read 7889 times)

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Offline jeroen79

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Re: Lab Bench PSU
« Reply #25 on: August 31, 2016, 01:05:55 pm »
If that transformer meets your voltage and current requirements: yes
 

Offline Brumby

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Re: Lab Bench PSU
« Reply #26 on: August 31, 2016, 02:25:54 pm »
Could I just rip a transformer from an alarm clock?

If that transformer meets your voltage and current requirements: yes
With that qualification, yes, indeed.

The only thing here is ... an alarm clock does not require much power at all and the transformer will be built accordingly.  You are not going to get much voltage out of it - maybe 6-12V or a bit more if you are lucky - and you certainly aren't going to get much current capability ... a couple of hundred mA or something in that range.

If you want to cannabilise a piece of gear for a transformer, your best bet might be an audio amplifier that pushes out at least 50W RMS per channel.

(The RMS rating is very important.  This gives you the real power.  Don't think any other so-called 'power ratings' like PMPO have any real meaning.  They are just ridiculous numbers that marketing departments cook up so they look good in brochures.  There may be some mathematical process used to get those numbers, but it's more in the realm of fantasy than anything else.)
« Last Edit: August 31, 2016, 02:28:01 pm by Brumby »
 
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Offline Muxr

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Re: Lab Bench PSU
« Reply #27 on: August 31, 2016, 03:11:53 pm »
If you're going to do this, please wear safety glasses. ATX supplies can quite happily dump 200W into your load suddenly on one rail. I really wouldn't use one myself. A little TO220 regulator is going to spread its guts across the room pretty sharpish as are your probes, solder joints and anything conductive that temporarily acts as the load by accident. All it takes is one little mistake.

Yeah, ATX supply without some additional current limiting is a disaster waiting to happen, even though they are protected against short circuit. However, 20A on a 5V rail, no problem! And the same or more on the 12V rail - that's getting into the arc welder territory already.

They also produce a rather dirty output, with a lot of switching noise and not great regulation. For PCs it is good enough, because it is assumed that pretty much everything has local regulation and filtering, but good luck testing some noise sensitive circuits with it ...

I have one, but I am using it only to power high current things like motors or my fume extractor fan, not as a regular bench supply.
Yup.. one of the most useful things about bench power supplies is their current limiting capability. This can not also help you protect your circuits under test (current limiting can often save your accidentally shorted components from letting the magic smoke out), but it's also a safer way to go. It's so easy to miswire a lead on a breadboard or similar.

So if you're rolling your own PSU look at current limiting, or at least include some sort of a fuse/polyfuse.
 
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Offline PotatoBox

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Re: Lab Bench PSU
« Reply #28 on: August 31, 2016, 03:41:21 pm »
Could I just rip a transformer from an alarm clock?

If that transformer meets your voltage and current requirements: yes
With that qualification, yes, indeed.

The only thing here is ... an alarm clock does not require much power at all and the transformer will be built accordingly.  You are not going to get much voltage out of it - maybe 6-12V or a bit more if you are lucky - and you certainly aren't going to get much current capability ... a couple of hundred mA or something in that range.

If you want to cannabilise a piece of gear for a transformer, your best bet might be an audio amplifier that pushes out at least 50W RMS per channel.

(The RMS rating is very important.  This gives you the real power.  Don't think any other so-called 'power ratings' like PMPO have any real meaning.  They are just ridiculous numbers that marketing departments cook up so they look good in brochures.  There may be some mathematical process used to get those numbers, but it's more in the realm of fantasy than anything else.)

How am I going to know if the transformer is meant to be used for a linear psu?
 

Offline Brumby

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Re: Lab Bench PSU
« Reply #29 on: August 31, 2016, 07:13:39 pm »
When cannabilising from other gear, this can be a little bit of a challenge.

The first step is to check that it does, in fact, connect directly to the mains.  This will confirm the primary is designed to handle the voltage.  As such, it will be suitable for a linear power supply - but whether it will be suitable for the power supply you want, is another matter.

Getting the secondary (or secondaries) voltage is not too difficult - just wire it up and measure the voltages.  Finding the current capabilities is a little less straightforward.  If you can get the specifications for the transformer, then these questions are immediately answered.

Usually, there is a correlation between physical size and weight with voltage and current capabilities - the bigger they are, the more grunt they can deliver.  The trick is to have the right voltage(s) on the secondaries, with current capability being equal to or more than you need.

Buying a transformer new makes this trivial.


As always, when working with mains power .... SAFETY FIRST.
« Last Edit: August 31, 2016, 07:15:32 pm by Brumby »
 

Offline Dave

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Re: Lab Bench PSU
« Reply #30 on: September 01, 2016, 01:04:29 am »
If you want to cannabilise a piece of gear for a transformer, your best bet might be an audio amplifier that pushes out at least 50W RMS per channel.

(The RMS rating is very important.  This gives you the real power.  Don't think any other so-called 'power ratings' like PMPO have any real meaning.  They are just ridiculous numbers that marketing departments cook up so they look good in brochures.  There may be some mathematical process used to get those numbers, but it's more in the realm of fantasy than anything else.)
Well, RMS power is marketing wankery of its own. There is RMS voltage and RMS current, but there is just mean power.
<fellbuendel> it's arduino, you're not supposed to know anything about what you're doing
<fellbuendel> if you knew, you wouldn't be using it
 

Online Gyro

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Re: Lab Bench PSU
« Reply #31 on: September 01, 2016, 03:48:28 am »
Not sure about 'marketing wankery'  ???  Surely RMS Power is just RMS Voltage * RMS current into a defined load impedance. In the same way as other power measurements, eg. dBm into (edit) 50R.

Edit: The term at least has some reliable meaning. 'Mean Power' could be manipulated by a marketing department to mean more or less anything.
« Last Edit: September 01, 2016, 03:53:04 am by Gyro »
Chris

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Offline Dave

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Re: Lab Bench PSU
« Reply #32 on: September 01, 2016, 04:15:44 am »
Nope. Have you ever wondered why we do the RMS calculations? We do it to find the equivalent DC voltage/current that would produce the same amount of power on a purely resistive load. You don't calculate the RMS value of power, because it makes zero sense (and the calculation would be absolutely pointless).

Mean power couldn't be manipulated in any way, there is no way to make it look bigger on paper than it actually is.
<fellbuendel> it's arduino, you're not supposed to know anything about what you're doing
<fellbuendel> if you knew, you wouldn't be using it
 

Offline Brumby

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Re: Lab Bench PSU
« Reply #33 on: September 01, 2016, 12:59:55 pm »
OK - Now we're just getting a bit picky.  Remember, this is the beginners section and the OP is not going to appreciate the subtleties just yet.

The key point here is that - whatever marketing influence has been expressed, the RMS value has some relevance to the discussion at hand.  Things like PMPO are just completely useless.
 
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Offline PotatoBox

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Re: Lab Bench PSU
« Reply #34 on: September 02, 2016, 04:31:54 pm »
Just wondering but why aren't transformers as readily available as some other components? I don't have a problem salvaging parts but for example, I can go onto aliexpress and search for resistors, they're a dime a dozen. But when I look for transformers, all that comes up are wall warts and other power supplies. Why is that?
 

Offline Brumby

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Re: Lab Bench PSU
« Reply #35 on: September 02, 2016, 05:39:24 pm »
Because some sellers are advertising power supplies as transformers.

It's annoying, but not mysterious and boils down to 'common usage'.  Consumers often do not differentiate between the two.  As an example, when I was younger, I had an HO gauge train set, which was powered by my 'train transformer'.  In reality it was a stepdown transformer with a rectifier and a rheostat.  (Dave did a teardown of the very same unit not that long ago.)

There is no such 'common usage' for resistors or MOSFETs, so it's not a problem with those searches.  (I was going to add transistor ... except in the early days of solid state radio receivers, the transistor(ised) radio was often referred to as a 'transistor'.)

Maybe you could try a search term that includes some of the parameters you are seeking.


Oh good grief.....

Just tried doing a search on "toroidal transformer"    :palm:
 

Offline MosherIV

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Re: Lab Bench PSU
« Reply #36 on: September 02, 2016, 07:22:00 pm »
Quote
Just wondering but why aren't transformers as readily available as some other components?
2 main reasons :
1. they are heavy and note worth much so it is not economical to sell them, the postal costs out weigh the item costs when posting from China
2. they are not used much now, most PSU are switcher, so China based sellers do not have much access to cheap transformers
 

Offline setq

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Re: Lab Bench PSU
« Reply #37 on: September 02, 2016, 11:38:03 pm »
I'm always buying transformers. Not sure why people think they not available. Perhaps from Chinese shit shifters they aren't but mainstream suppliers, no problems at all.
 


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