Can this dual secondary transformer be used to make PSU with two isolated outputs?

[king.oslo]

Hello there,

I've bought a transformer. It has dual secondary windings. I want to make a bench power supply with two isolated outputs.

However, whilst reading the manual, I found this:

BOTH PRIMARY AND SECONDARY COILS MAY BE CONNECTED AS EITHER SERIES OR PARALLEL, BUT BOTH MUST BE USED SIMULTANEOUSLY

It is the "BUT BOTH MUST BE USED SIMULTANEOUSLY"-part that worries me.

The datasheet can be found via a link here: http://search.digikey.com/scripts/DkSearch/dksus.dll?WT.z_header=search_go&lang=en&site=us&keywords=VPS36-3600&x=10&y=21

Can the transformer be used to make two PSU with isolated outputs? A lot of the time I will only use one of the outputs.

What are your thoughts?

Thanks!M

 

[alec_t]

My thoughts are that it will be fine for two isolated outputs. IMHO (I stand to be corrected) there's no reason why both windings must be used simultaneously. The voltage between the terminals of an unloaded winding will be higher than for a loaded winding, because of voltage drop due to the winding resistance, but I don't believe the unloaded voltage would ever be high enough to be a problem.

 

[king.oslo]

Thanks Alec, this makes me really down to hear :/

How much current do you reckon the insulation can take before it starts to burn away?

It also says this in the datasheet, but I cannot make out what it means: "5. Temperature Rise: 30C TYP (45C MAX allowed)".

Thanks.M

 

[alec_t]

How much current do you reckon the insulation can take before it starts to burn away?

If it's taking any current then it isn't insulation! The important part of the spec is the maximum current the windings can take (determined from the VA rating).

I cannot make out what it means: "5. Temperature Rise: 30C TYP (45C MAX allowed)".

To me it means that at the specified maximum continuous output power the temperature of the transformer will typically rise to 30C above ambient, but the absolute maximum (even for a short period) is 45C.

 

[Blueteeth]

For your application, as long as you meet the required current limits - I do not think you need to worry.

Transformer output voltage will vary, it will be a bit higher than rated when open, and lower than rated when you draw current from it. So it has two output windings, at 18VAC @ 3.6A max. Providing you draw <3.6A from either winding (independantly, or at the same time) I'm sure ti'll be fine.

Also remember, if you're using each secondary output winding for a seperate supply, ALL regulation circuitry uses up at least some current. So even if you are not using one of the two supplies, it will still draw *some* current from the transformer for the regulation circuit - it will have at least some form of load, albeit very small.

As for the primary (mains) side, I believe you must use either series or parallel for the voltage you wish to run it off, this is so it can deliver its rated 130VA. If you only connect one primary winding to 115VAC, you could only draw half the power form the secondary side.

I'm not sure where you got that 'quote' from, the datasheet doesn't mention it - although it does specify that the windings are 'not intended to be used seperately'. To me that just means the voltage regulation spec will be out, giving you a higher or lower output voltage should you just use one winding.

I don't see anything special aobut that transformer that requires different design from any other mains transformer, just abide by the ratings and you'll be fine

 

[king.oslo]

Thanks guys! M

 

[cachehiker]

It's probably related to its agency certifications from UL, CSA, and TUV.

The isolation between the secondary windings is probably not as robust as the 4000V isolation between primary and secondary and between the windings and the core. Therefore, you are violating the agency conditions of acceptability if you connect the secondaries in a manner where there is a significant potential between them.

As an example, suppose you have one secondary connected to accessible parts like a terminal block for speakers and the other secondary sharing a common connection with rectified 230VAC. TUV would consider that a shock hazard and refuse to grant you a certification that your product is safe. If you later ran into a problem with giving some consumer hair like Yahoo Serious, the manufacturer would not be liable and your lack of a safety certification would put you in deep Kimchi as a defendant.

On the other hand, if it's just homebrew stuff and the potential difference is less than 100V or so, I wouldn't be terribly concerned.