Cunning interface reads dc voltage through a transformer

0-10vinterface-293.jpgHidden at the bottom of an LED lighting circuit from iWatt is a rather neat idea for sensing a 0-10V (and 1-10V) control voltage via an isolation transformer.

Looking much like a dc-dc converter with the switch on the secondary side, it allows the chip to sense the control voltage while the transformer blocks any errors due to dc offset or damaging transients on the control line. Nor can the converter stuff mains back up the control lines.

I am guessing it imposes a read signal from the Tdrv pin.
And I am assuming there is a resistor involved on the primary side or the primary winding resistance is high enough to stop it shorting out the source.

As far as I can see, the downside is that the circuit imposes a variable impedance onto the 0-10V control line which might cause ripple that upsets other devices sensing the same control line.

The company told me that it is no worse that shorting the transformer output, and that the 0-10V spec requires the far-end dc source to be able to both source and sink current.

The rest of the chip – the iW3630 – is a 45W two-stage off-line ac-dc switcher with primary side control to avoid the need for an opto-coupler.
Power factor is over 0.95, and it is also PWM dimmable.

Frustratingly, the firm is following an annoying trend by not putting the data sheet up on its website – at least not when I wrote this.
 

‘Alice’

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2 Comments

  1. 'Alice'
    February 22, 2013 09:46

    Hi Mike.
    Your mention of saturable core reactors remainds me of mag amps, and how I never understood them, however hard I tried.
    I am going to email iWatt to try to find out how this thing works, and why it doesn’t it short out the 0-10V source.
    ‘Alice’

  2. Mike M
    February 22, 2013 09:31

    Hi,
    This circuit reminds me of a very old fashioned method of dimming stage lights based on a saturable core reactor ?
    I recall that this used a varying current in the primary of a transformer to control the inductance of the secondary – or something like that ?
    A bit of Googling on this topic lead me to this very simple oscillator
    http://www.tradeofic.com/Circuit/12052-SATURABLE_REACTOR_OSCILLATOR.html
    Mike M

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