339VDC -> ~15DC - Without TRANSFORMER

[truenoteno]

1st thing that comes to mind is a step down SMPS, but my pulse width modulator (LM3524) can' t handle 339v input voltage. I have the TL494 if that's a better chip

 

[Nigel Goodwin]

1st thing that comes to mind is a step down SMPS, but my pulse width modulator (LM3524) can' t handle 339v input voltage. I have the TL494 if that's a better chip

For a start you don't give any indication of the current you require, if it's low you could use a simple dropper resistor - a large one!.

Assuming you need more than that, as you say an SMPS would do the job. 339V is pretty well what you get from recified mains, so there are LOTS of suitable designs already available, as used in TV's etc. Try searching for TDA4605 or UC3842, both are commonly used in TV SMPS's.

Neither of the two chips you mention seem to used much in domestic equipment, although the TL494 is used in PC PSU's quite a lot.

 

[truenoteno]

1st thing that comes to mind is a step down SMPS, but my pulse width modulator (LM3524) can' t handle 339v input voltage. I have the TL494 if that's a better chip

For a start you don't give any indication of the current you require, if it's low you could use a simple dropper resistor - a large one!.

Assuming you need more than that, as you say an SMPS would do the job. 339V is pretty well what you get from recified mains, so there are LOTS of suitable designs already available, as used in TV's etc. Try searching for TDA4605 or UC3842, both are commonly used in TV SMPS's.

Neither of the two chips you mention seem to used much in domestic equipment, although the TL494 is used in PC PSU's quite a lot.

i don't think a voltage divider network would be suitable.

Just say enough current to run a hub, none of my transformers would fit into the hub casing so i gotta go smps

 

[Styx]

do not use resistors!!!

you will loose power in them and it is not the best way.
YOu say you cannot fit in a XFMR due to space, as long as you are happy with the loss of safety/isolation:

check out a simple "auto-transformers". Basically an inductive voltage divider, you dont get anywhere as near the losses that you would get from a resistive divider.

All you got to do is make sure you dont use too high inductor values otherwise you wont get much power transfer. You dont need that high inductor values.

It is a easy/small solutionthat you can build. All you will need is:

2 inductors
4 diodes (or a rectifier module - prob better)
electrolytic cap

 

[Exo]

Even a smps requires a tranformer (altough a much smaller one).
I don't think there is any chip that can be run off mains directly...

Try measuring the hub's current requirements, if they stay relatively steady and if it doesn't use too much current (only few mA's) then a power supply with resistor and zener might be possible...

neverteless, it isn't very safe for you and your hub, and anything connected to the hub...

 

[Styx]

err "auto-transformer" !!!!!

check it out it is yr solution - see above post

 

[Nigel Goodwin]

err "auto-transformer" !!!!!

check it out it is yr solution - see above post

You appear to have missed the original specification? - the supply it needs to work from is DC!.

Also, two inductors don't make an auto-transformer, it's just a 'proper' transformer without isolated windings.

 

[Styx]

err "auto-transformer" !!!!!

check it out it is yr solution - see above post

You appear to have missed the original specification? - the supply it needs to work from is DC!.

Also, two inductors don't make an auto-transformer, it's just a 'proper' transformer without isolated windings.

ok true I did miss the DC bit for the voltage - I had just designed it tho. 3W
so the question is how are you generating the 300DC volts?


and 2 inductors DO make an autotrasformer. An autotransformer is an unisolated transformer.


Viroc do very small DCC converters that run form high Dc levels down to low DC levels.

the Vi-J60-EY runs from 100-300V DC in downto 28V out

 

[Nigel Goodwin]

and 2 inductors DO make an autotrasformer. An autotransformer is an unisolated transformer.

Yes, it's a transformer, and works by transformer action between the two windings - two inductors won't do that, unless they are wound on the same core - in which case it's a transformer, and not two inductors.

You can't just connect two inductors together and create an auto-transformer - but feel free to try it?.

 

[Styx]

and 2 inductors DO make an autotrasformer. An autotransformer is an unisolated transformer.

Yes, it's a transformer, and works by transformer action between the two windings - two inductors won't do that, unless they are wound on the same core - in which case it's a transformer, and not two inductors.

You can't just connect two inductors together and create an auto-transformer - but feel free to try it?.

for a autotrasformer to step-up the voltage yes it has to be on the same core to ensure that the flux-linkage is as high as possible.

For step-down also it can be on the same core, but I was stressing a simple solution to use the auto-trasformer priciple but in a voltage-divider style.

It does work with just inductors ONLY to step-down, they tend to be wastful because of teh flux-leakage is high, but the point is it works!


back onto topic:

how is this 339Vdc generated?

 

[pebe]

..... It does work with just inductors ONLY to step-down, they tend to be wastful because of teh flux-leakage is high, but the point is it works!...

It *cannot* work. There must be mutual inductance between the two windings.

 

[Styx]

..... It does work with just inductors ONLY to step-down, they tend to be wastful because of teh flux-leakage is high, but the point is it works!...

It *cannot* work. There must be mutual inductance between the two windings.

as I said as a step-down it will not work as well if it does not have any flux.

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and as I stated it is acting MORE like a voltage divider rather than a "true" autotrasfomer

can we get back on topic or drop this

 

[Nigel Goodwin]

and as I stated it is acting MORE like a voltage divider rather than a "true" autotrasfomer

NO! - it acts solely as a potential divider, not at all like an autotransformer, as such the inductors will dissipate just as much heat as a resistive divider.

can we get back on topic or drop this

We're still waiting for a response about the source of the 339V DC, which I agree would probably be useful to know!.

 

[Styx]

and as I stated it is acting MORE like a voltage divider rather than a "true" autotrasfomer

NO! - it acts solely as a potential divider, not at all like an autotransformer, as such the inductors will dissipate just as much heat as a resistive divider.

can we get back on topic or drop this

We're still waiting for a response about the source of the 339V DC, which I agree would probably be useful to know!.

Electric Machines 2nd Edition; Steady-State Theory and Dynamic Performance; Mulukutla S.Sarma; PWS Publishing Comany; ISBN 0-534-93843-4; p137

Topic: 4.6 Autotransformers

An extract
______________________________________________________________________
1 + N1/N2 = 1 + a N1 and N2 are inductor windings

Thus, the autotransformer with a transformer ratio of (1+a) can be treated as an equivalent two-winding trasformer with a turn ratio of a. The ratio of the autotrasformer configuation is much higher than the two-winding trasformer ration as shown below

Autotrasformer ratio/Two-winding trasformer ratio = 1 + 1/a

This ratio becomes much higher than unity, since for many autotrasformers applications a is rather small and the ratio of teh terminal voltages (1+a) is very nearly unity. This is because most of the power in an autotrasformer is delivered to the secondary through direct conduction and only a small portion is delivered through transformation or induction as it is in a two-winding trasformer
_______________________________________________________________________

Dont believe me? get out the book I have quoted above and check it out. The creation of a single phase coreless autotrasformer is a crude hack that is aeospace approved.

Also your comparrison of this "inductive divider" just like a resistive divider is completly wrong. If you were to connect a resistor straight across the AC no matter what frequecy is chosen current will flow based on V = I*R.

However, connect an inductor across an AC supply and increase its frequency then the current (and thus power trasfer) will decrease - standard inductor theory. No extra power is lossed, the inductor will just "oppose" the flow of current. It is this effect that this crude autotrasformer works on.

If resistors were used you would have conduction loss based on the current flowing, the only loss in this autotransformer arrangement is due to the equivent-series-resistance of the inductors.

 

[Roff]

I ran your simulation and found that the 0.1 ohm resistor dissipates 485 kilowatts. Keep in mind that the inductive reactance of 90uH is only 0.028 ohms.

 

[truenoteno]

back onto topic:

how is this 339Vdc generated?

i'm in australia so 240VAC mains = 339VDC, thru bridge rectifier.

 

[Nigel Goodwin]

back onto topic:

how is this 339Vdc generated?

i'm in australia so 240VAC mains = 339VDC, thru bridge rectifier.

So if you've got AC, simply use a small mains transformer! - it can't really get any simpler. If you don't have room for it, you don't have room for anything else either.

 

[Styx]

Thats what I thought you were getting the voltage from, 339 was setting far to many alarm bells for me, so yep as Nigel said use a small trasformer. However 240Vac will not rectify to 339V - that is the peak of the AC waveform. The average vlue will be closer to 0.9*Vp so abt 305V so make sure you get a suitable turn-ratio.

It might be an idea to generate 18V DC from recitfication and then use a linear reg to ensure you have constant 15V

and "Ron H" that 0.1 resistor was placed in to help the simulation - Jacobian loops and all. In practice it would not be put in circuit - it would serve no purpose but to dissipate power. Reran with a 10uOhm resistor (for the simulation to be happy) and reduced the secondary coil downto 10uH and get the same result

 

[Nigel Goodwin]

and "Ron H" that 0.1 resistor was placed in to help the simulation - Jacobian loops and all. In practice it would not be put in circuit - it would serve no purpose but to dissipate power. Reran with a 10uOhm resistor (for the simulation to be happy) and reduced the secondary coil downto 10uH and get the same result

Try working out the reactance of 10uH at 50Hz, then apply ohms law and work out the current it will draw! - also calculate the dissipation in the inductors from the same values.

 

[pebe]

Dont believe me? get out the book I have quoted above and check it out. The creation of a single phase coreless autotrasformer is a crude hack that is aeospace approved.

Also your comparrison of this "inductive divider" just like a resistive divider is completly wrong. If you were to connect a resistor straight across the AC no matter what frequecy is chosen current will flow based on V = I*R.

However, connect an inductor across an AC supply and increase its frequency then the current (and thus power trasfer) will decrease - standard inductor theory. No extra power is lossed, the inductor will just "oppose" the flow of current. It is this effect that this crude autotrasformer works on.

If resistors were used you would have conduction loss based on the current flowing, the only loss in this autotransformer arrangement is due to the equivent-series-resistance of the inductors.

The section of the article you have quoted refers to an autotransformer or a transformer. In both those cases the primary and secondary windings have mutual inductance because the windings are on the same bobbin.

Your earlier posting referred to discrete inductors with no flux linkage shown. In that case you can get *NO* transformer action. The top inductor will be in series with the load and the bottom one will be in parallel with it. There will also be a potential divider formed by the resistances of the two components, as Nigel has said. The magnitude of voltage across the load will depend on the current it draws, so it will be variable. So one thing is certain - it is *NOT* an autotransformer!