Isolated switch.

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I need to make a isolated switch.
The problem is that I don’t have any power on the switch side, and they can’t be grounded together.

The idea I had was to have a tiny AVR to switch a small transformer with a serial resistor. And then put the other side of the resistor to an ADC.
So now if I short the sec. side of the transformer (my switch) ADC readings will change. (to avoid making noise I will probably rectify my switch side.)

Are there a better way to do this ?

I know that I need some protection diodes etc.

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Quote:
The problem is that I don’t have any power on the switch side, and they can’t be grounded together.
A small isolated dc to dc converter could provide power for your switch which could operate an optocoupler.

John Samperi

Ampertronics Pty. Ltd.

www.ampertronics.com.au

* Electronic Design * Custom Products * Contract Assembly

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The problem is that a Isolated DC-DC cost more !
Or what ? (a fast look at digikey it's more than US $5)
The transformer can be just a ferrite bead (This is for a HF radio so we use a lot of ferrite).

Jens

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I did not know how things were being used. :-)

John Samperi

Ampertronics Pty. Ltd.

www.ampertronics.com.au

* Electronic Design * Custom Products * Contract Assembly

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Thanks for the fast response.
In an old produckt it was made with a 4093 two optocouplers about 20 other parts caps,res,diodes !!!

Jens

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Sometimes it's feasable to use some LEDs from the "power's available" side to illuminate a photovoltaic on the "control side", at least enough to get things running..

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What are you switching ? AC, DC, Voltage, current requirements, switching speed ?

A better description will result in a better response.

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As others already said optocoupler is a good option and depends your needs is cheap and easy.
Capacitive isolators chips could be other option, do you look at it?

Brunomusw

Regards,

Bruno Muswieck

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I have 24V power at all time.
I need to start a 24 to 12 V DC-DC converter.
Where I have the switch is on the 12V side that is dead when the radio is off.
The two power planes is floating.

I looked into "Capacitive isolators" but they don't give any power to the switch side.

Jens

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Let me understand. You have 24V on one side of the barrier (lets call this the primary side). You have a radio on the other side - the 12V Side (lets call this the secondary side). This side has no power at all ? No battery ?

Since you want control from the secondary then an optocoupler suitable for your level of isolation is needed. A simple transistor output opto work fine. Current for the diode of the opto would be supplied from the control source on the secondary side (yes you do need some sort of power for this). The transistor side of the opto would have power from the 24V supply. If your DC-DC converter is turned on with a "low" level, then a pull-up on the opto transistor may be all that's needed. If you need a "high" to turn on the DC-DC, you could add another PNP to invert the opto output. You might also need to level shift the inverted output to the appropriate level for the DC-DC converter turn-on signal.

Another way is to run a DC-DC from the 24V side all of the time. Then, a simple switch, electronic or mechanical could be used to supply this available power to your secondary device on demand.

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1. Why not run the wires from the switch over to the converter side?

2. Why do you need isolation for a radio?

3. How much power are we talking about?

If you think education is expensive, try ignorance.

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First to emuler
1: This is reuse of an existing cable where I only have 1 wire for this (our normal way we only use 12V so this is normaly pulled to 12V and the switch put it to gnd).

2: That's tha law in EU (here in us no problem).

3: It's a 150W HF radio, but the switch just need power so noise can't trigger a signal on a 1000 feet cable.

To gahelton
The isolated DC-DC is the problem (cost) not the opto.
My way will just be a tiny13 + 5 volt reg 3 caps, 1 res 1 diode and a transformer, about $2

Jens

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Radio transmitter/transceiver. Why didn't you say so in the first place? I thought you wanted to run a small radio receiver. :lol:

Let's see if I have this straight.
1. You are trying to modify an existing design for use in the EU.
2. Existing design uses one wire - pulled up on supply side and pulled low by switch to tell the supply to deliver power.
3. New design must have isolated power supply for the radio.

Is that right?

A quick and easy way is to run your existing system using an isolation transformer between the mains power and the power supply. But that would be too easy, wouldn't it? ;)

Your small transformer approach will need to be well shielded from stray RF and magnetic fields. I would also prefer to use the primary of the transformer in an LC oscillator - shorting the secondary will cause the oscillator to stop (or at least change frequency) which you can detect.

If you think education is expensive, try ignorance.

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No RF problems this only have to run when the radio is off.
When it is on I have my 12V on the switch side.

Quote:

A quick and easy way is to run your existing system using an isolation transformer between the mains power and the power supply. But that would be too easy, wouldn't it?

I'm not sure what you mean. My power is 24V DC there is no AC involved.

Jens

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Quote:

My power is 24V DC there is no AC involved.

Yes, but where is that DC coming from? If it is from a battery (no charger connected while radio is working) then where does the need for isolation come from? Let the battery ground float with the radio ground.

If you think education is expensive, try ignorance.

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This is for a product not a installation.(so I can't asume any AC)
And my 12V gnd and 24V gnd can't be the same.

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Jens what is the current that you need?
Check this opto - CPC1017NTR, this solution maybe is cheaper than your tiny13 + 5 volt reg 3 caps, 1 res 1 diode and a transformer.

Brunomusw

Regards,

Bruno Muswieck

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He can't use an opto because there is no power on the other side to drive the opto.

OP: just go with your original idea of using a transformer, based on the principle that the impedance of one winding will change drastically if the other winding is shorted. How you implement it is up to you.
1. You could have just a simple RL voltage divider with the primary of the transformer comprising the L part. When the switch shorts the secondary, the value of L will drop. Drive the RL divider with a known AC signal and sense the change in output. If you use a square wave drive from one of the AVR pins, see that the waveform doesn't get spiky.

2. You could have the transformer primary as the L in an LC oscillator. A bit more complicated to set the values so that the oscillator stops when the secondary is shorted.

If you think education is expensive, try ignorance.