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# Efficient method for converting 230V AC (Germany) to 30V AC with only 5-10mA needed?

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#### raxon

##### New Member
Hello, -

so far I use the good old "capacitor + resistor divider" method. It is easy to calculate that this mainly generates loss.
Therefore, I thought about winding my own transformer (first coil 3-4 turns, second 800+).
Before I go for that folly, I wanted to ask whether there might not be a better (integrated) solution. Is there any building block that could help me here?

15 - 0 - 15 volts transformers are faily common so that is one solution. **broken link removed** . If you did decide that is suitable check to confirm it is 15 - 0 - 15 as the picture shows a 9 - 0 9 transformer. (By the way your tuns ratio is wrong 800 turn primary and 4 turn secondary would give you 1.15 volts. )

Les.

15 - 0 - 15 volts transformers are faily common[/QUOTE said:
I might even have one of those flying around, but how do I get 30V AC out of there without vandalizing the coils?

By the way your tuns ratio is wrong 800 turn primary and 4 turn secondary would give you 1.15 volts. )[/QUOTE said:
It's the other way round, unless I miscalculated.

I might even have one of those flying around, but how do I get 30V AC out of there without vandalizing the coils?

It's already there - just ignore the centre 0 tap, and use the 15 at either end, 15 + 15 = 30 - transformers are designed that way on purpose, even better are ones that give two separate windings (0-15 and 0-15). That gives even more options (15-0-15, 0-15, 0-30, or 2x0-15) - but a 15-0-15 is all you need.

Hello, -

so far I use the good old "capacitor + resistor divider" method. It is easy to calculate that this mainly generates loss.
Therefore, I thought about winding my own transformer (first coil 3-4 turns, second 800+).
Before I go for that folly, I wanted to ask whether there might not be a better (integrated) solution. Is there any building block that could help me here?

Winding your own transformer is not a s easy as it sounds. As Les jones has said, a 15-0-15 transformer is easy to find on stores. You can know more about center tapped transformers here.

Hello, -

so far I use the good old "capacitor + resistor divider" method. It is easy to calculate that this mainly generates loss.
Therefore, I thought about winding my own transformer (first coil 3-4 turns, second 800+).
Before I go for that folly, I wanted to ask whether there might not be a better (integrated) solution. Is there any building block that could help me here?

Why a Capacitor + Resistor method? Why not a Capacitor + Capacitor method? 0.33 uF and 2.4 uF should be the approximate values.

Because it's easier to use a potentiometer and regulate it to 30V, plus I don't have capacitors in all sizes at hand. ;-)

The capacitor method does not provide mains isolation, so any part of the circuit could be at mains potential under fault conditions. For safety's sake use a transformer.

It's already there - just ignore the centre 0 tap, and use the 15 at either end, 15 + 15 = 30 - transformers are designed that way on purpose, even better are ones that give two separate windings (0-15 and 0-15). That gives even more options (15-0-15, 0-15, 0-30, or 2x0-15) - but a 15-0-15 is all you need.
Yes. Unfortunately, they are over-dimensioned. They have the tendency to get warm, even when nothing is attached. This means essentially that there will be more losses than when using the old divider method.

The capacitor method does not provide mains isolation, so any part of the circuit could be at mains potential under fault conditions. For safety's sake use a transformer.
I got two fuses, which is already quite paranoid.

Winding your own transformer is not a s easy as it sounds.
I would agree completely. But, in this case - I can likely manage to do 3-4 turns and find a matching coil for the other side (unwinding is easier).

I got two fuses, which is already quite paranoid.

That's not the point, the point is that it's dangerous to people - and fuses won't help that. If you're using dangerous live supplies in this way the entire circuit MUST be double insulated, and it must be impossible to touch any connected metal.

This isn't going to be for "live supplies" and it won't be "dangerous to people".
However, if I go for the "build your own coils"-method, then there will be the insulation on top of the two fuses. ;-)

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This isn't going to be for "live supplies" and it won't be "dangerous to people".

If you're using capacitive or resistive power supplies from the mains then the entire circuit is live to the mains, and seriously dangerous to people. The reason behind using a transformer is that it isolates the mains completely, and makes the output safe.

If you don't fully understand such power supplies, with their serious limitations and dangers, then you really shouldn't be attempting to use them.

I'll file that under 'rant'.

I'll file that under 'rant'.

I would suggest filing it under "don't kill people" - there's a never ending stream of new members here who always want to build these highly dangerous power supplies, with no understanding of the dangers and limitations of them.

Can you explain what you are using this for so that we can try to understand how it can be safe ? When you say "This isn't going to be for "live supplies" " what do you mean ?. If you do wind you own transformer I think you will need to learn how to calculate the turns per volt from the cross sectional area of the core and it's magnetic properties. As you are talking about 3 to 4 turns for the 30 volt winding this is a value of about 0.1 turns per volt. transformers I have played with (Around 50 VA rating ) have about 5 turns per volt. So my feeling is that a transformer wiith 0.1 turns per volt will be very large (By my standards.) I think it would be a few KVA rating. This does not make sense as your quoted VA requirement is 0.3 VA.

Les.

Can you explain what you are using this for so that we can try to understand how it can be safe ? When you say "This isn't going to be for "live supplies" " what do you mean ?. If you do wind you own transformer I think you will need to learn how to calculate the turns per volt from the cross sectional area of the core and it's magnetic properties. As you are talking about 3 to 4 turns for the 30 volt winding this is a value of about 0.1 turns per volt. transformers I have played with (Around 50 VA rating ) have about 5 turns per volt. So my feeling is that a transformer wiith 0.1 turns per volt will be very large (By my standards.) I think it would be a few KVA rating. This does not make sense as your quoted VA requirement is 0.3 VA.

Les.
I'm getting this:

Primary: 230V, 3.3 turns, 0.1mm diam.
Secondary (for 0.01A): 30V , 810 turns, 0.05mm

This isn't going to be for "live supplies"
This thing you said does not make sense.

"Live supplies" isn't a term that means anything. Typically when we say something is "live", that means that thing is unpowered. If it is not live, then it is not powered. So are you saying your circuit is never powered? That doesn't seem likely.

Unless by "live" do you mean "life"? As in medical equipment? That doesn't matter. Other high-voltage to low-voltage equipment still needs isolation. The only difference is medical equipment has even MORE isolation than normal because it's always connected into someone's body. Not being medical equipment is irrelevant to the situation about whether there needs to be isolation for more safety

and it won't be "dangerous to people".
This just is not true.

A circuit with 230V always has the potential to be dangerous if there is no isolation. It doesn't matter if the circuit converts 230V to 1V. You don't judge whether a circuit is dangerous by saying it only has 1V on the output when it's working properly. You judge whether the circuit is dangerous by asking if that 230V can end up in places where it normally isn't expected to be if the circuit fails. Capacitors and resistors fail all the time and put 230V everywhere in the circuit.

You need to take this more seriously instead of trying to ignore it with winky faces. You also need to be clear that fuses protect the equipment from melting or catching fire, NOT the person from shock.

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Primary: 230V, 3.3 turns, 0.1mm diam.
Secondary (for 0.01A): 30V , 810 turns, 0.05mm
????
Primary should be 230V and many turns, Secondary should be less turns and 30V.
example: using volts per turn
P=230V 460turns = 0.5V/T
S=30V 60 turns = 0.5V/T
230/460=30/60

Example using ratio:
230/30=460/60

By the way your tuns ratio is wrong 800 turn primary and 4 turn secondary would give you 1.15 volts.

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