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On the Murata OKR-T/10, the data sheet specifies a 49kohm pulldown resistor but I've seen people using a 1k. Is there a more correct value or does anything 1k or above work just as well?

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  • \$\begingroup\$ Please post a link to the datasheets of the parts in question. It saves time and clicks. Folks out here appreciate that. Welcome to EE.SE, by the way. \$\endgroup\$ – Nick Alexeev Mar 11 '14 at 17:52
  • \$\begingroup\$ Sorry about that. Here's the data sheet: murata-ps.com/data/power/okr-t10-w12.pdf \$\endgroup\$ – Bradart Mar 11 '14 at 18:02
  • \$\begingroup\$ Possibly, the 49kΩ pull-down is built into the converter itself. Here's a discussion about choosing a pull-down resistor. \$\endgroup\$ – Nick Alexeev Mar 11 '14 at 18:25
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I would say that a 49kR is inside the chip and if you want to enable it you need to explicitly drive On/Off High (either by a transistor or a logic signal).

If you use a 1k resistor (pull-up) it will be always-on.

Don't forget input and output caps.

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  • \$\begingroup\$ Perfect. Thank you. My understanding of pull up resistors was totally wrong. Now I think I get it. \$\endgroup\$ – Bradart Mar 11 '14 at 18:40
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Here's what the data sheet says: -

enter image description here

The 49kohm is the internal resistance of the circuit, not an external resistor that has to be supplied - note the circuit diagram is suggesting a switch open or closed because that's the simple way to show it but resistors will do. Just don't go too large.

Also note the error in the specification - the picture says "open = on" and "closed = off" whereas the data in the table is implying the opposite - it says "OFF = open pin" but that is probably a typo.

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  • \$\begingroup\$ Awesome. Thank you. I wasn't fully understanding pull up resistors but I think in getting it now. \$\endgroup\$ – Bradart Mar 11 '14 at 18:41
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    \$\begingroup\$ @andyaka: Note that the data sheet says "Standard Version: open= ON", but "E" version is open = OFF!! Only the "E" version has the 49K pull-down. (as I read it) \$\endgroup\$ – Peter Bennett Mar 11 '14 at 18:52
  • \$\begingroup\$ @PeterBennett you are absolutely right - hung by my own pictures. How painful. \$\endgroup\$ – Andy aka Mar 11 '14 at 20:14
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FWIW I've never seen an 'E' version for sale, PN OKR-T/3-W12E-C, only the Standard version is available, PN OKR-T/3-W12-C.

I mention this because the answers seem based on the wrong version.

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