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Do I need to use a sense resistor for the MC34063 Step-Up Circuit?

I would like to step-up my voltage from 5V to 40V. After doing the math, I've found that I will probably need a 0.16 Ohm Sense Resistor. First of all, I can't seem to find any through-hole sense resistors, and second, I can't find any at a low enough value for this.

So my question, again - Do I need to use a sense resistor for this circuit? What does it actually do for this circuit?

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    \$\begingroup\$ My 3W 10mohm resistors would like a few words with your "can't"s. \$\endgroup\$ Jun 25, 2014 at 21:45
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    \$\begingroup\$ Also, digikey.com/product-search/… \$\endgroup\$ Jun 25, 2014 at 21:47
  • \$\begingroup\$ @IgnacioVazquez-Abrams Are those resistors "Sense" resistors? Does it matter if they aren't? What's the difference? I can find plenty of regular resistors, but are these sense resistors something special? \$\endgroup\$
    – ntgCleaner
    Jun 25, 2014 at 21:49
  • \$\begingroup\$ If you really wanted to you could attach Kelvin connections to it, but at 160mohm it may not be worth it. \$\endgroup\$ Jun 25, 2014 at 21:50
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    \$\begingroup\$ @ntgCleaner A Sense resistor is a regular resistor, just with a very very low resistance, as to not draw too much Power, drop too much voltage or produce much heat as to burnout, which are important in current sensing. A 10 milli-Ohm resistor is a sense resistor. \$\endgroup\$
    – Passerby
    Jun 25, 2014 at 22:32

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The sense resistor is used to limit the peak current through the transistor switch and the output in the case of a short circuit on the output or the output inductor saturating.

A sense resistor can be a PCB trace or a length or wire. For example, a 1.8 inch length of 40GA. copper wire will have a resistance of about 0.16 ohms at 25C.

But, it does not have to be used.

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  • \$\begingroup\$ I don't buy the sense resistor being optional. Where does it say it in the datasheet? \$\endgroup\$
    – Matt Young
    Jun 26, 2014 at 3:38
  • \$\begingroup\$ The data sheet spec "Current Limit Sense Voltage" typical = 300mV is used to calculate the resistance value for the over current condition. The shunt resistor ohms is = 0.3V/Imax. The only time this part of the circuit is active is when the current being conducted through the transistor switch exceeds the Imax value. When this happens the current conduction cycles is short cycled to turn off the switch and interrupt the current. This happens when there is a short on the output or very high input voltage. Not using the shut resistor may not be a good idea, but it is not required. \$\endgroup\$
    – waltx
    Jun 26, 2014 at 4:52

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