This question might fly close to the rule about asking to select parts. What I'm really asking is what parameters of a PTC Fuse I should be looking for. Or even should I be using one. [Software head, so not classicaly trained] I'm not sure where I got the "inspiration" for the below circuit, I'm using it as a high side switch to power a USB Host connection. As a result I've got fairly strict 5V output requirments. Now there's two diodes which both drop 0.3V so the output from my power supply has to be raised to 5.6V and that's taken care of. (Maybe I don't need the one after the P Channel Fet, possibly belt and braces)

My problem is that the PTC fuse, (MULTICOMP MC33189) which is meant to stop more then 500mA going to the USB Device connected. It is dropping voltage as a function of the current passing through it. As a result I can't simple add on voltage to compensate for it.


simulate this circuit – Schematic created using CircuitLab

So my question is should, or even can I use a PTC Fuse in this application or are there selection parameters I should be looking for? When the fuse I'm using gets close to the 500mA it starts heating up and gradually dropping the voltage more and more well out of spec. About 1.2 V up at 500mA and raising.

  • \$\begingroup\$ LM3526 IC is for dual channel USB port power control. A similar IC can be used. I still appreciate realising using discrete... \$\endgroup\$
    – User323693
    Commented Jun 26, 2015 at 2:09
  • \$\begingroup\$ Pick a PTC with a higher trip current. PTCs don't have particularly sharp elbows, and running within 100mA of the trip current will indeed lead to a large voltage drop. \$\endgroup\$ Commented Jun 26, 2015 at 7:04
  • \$\begingroup\$ Thanks a million Umar! It never even crossed my mind to use an IC and there are a load of dedicated High Side Switchs out there with Current limiting included. Thanks a million for the hint. \$\endgroup\$
    – jwhitmore
    Commented Jun 28, 2015 at 16:17


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