Voltage drop caused by fuse?

I'm wondering how the fuses on the RAMPS board function. The reason for asking is that when the heater for my extruder turns on, I can hear a difference in the pitch of my fan.
I have my bed on a separate 16V power supply because it could not reach 100C on 12Volts. So my 12V/30A supply is a bit overkill for just the extruder heater and the steppers.
When I measure directly on the PS there is no drop in voltage (12.11 V) when the heater turns on or off. So there must be a voltage drop after the fuse (my fan is connected to the 12V aux next to the X-stepper-driver). I'm in the middle of a 30-hour print so I cannot measure the voltage after the fuse, but the fan-sound is changing quite a lot.
So I guess the fuse must have some resistance causing the voltage drop when the current goes up. Every now and again I have a shift on my Y-axis, sometimes on one of the first layers, sometimes after ten hours of printing (%$#$!!). As I tried everything I could find about this I now think maybe when all motors work at the same time and the heater is on maybe the resistance of the fuse is high enough to cause a skipped step on my motor. Not very likely, as the motors are current-driven and rated 3.4 Volts so the voltage would have to drop very much to affect the motors, but I just want to be sure...

Yup.

The PTC fuses are horrible things. I removed mine, replaced them with wire links and put in fuses off-board.

The way the PTC fuses work internally is that they use a chemical that when exposed to current heats up. At a set temperature, this chemical goes from being conductive to non-conductive. There's quite an avalanche in this, but there are a few problems with PTC fuses that can cause issues.

1. At currents below the threshold, there is a variable amount of low resistance that varies with current. The more current, the more this low resistance value changes. It's not much (usually way less than 1 ohm), but it's enough to affect circuits that draw a lot of current. Normal fuses do suffer this but to a much smaller degree that it's really not noticeable.

2. Because the system is heat related, how well the PTC fuses are cooled and the environmental conditions (room temp) really determine what that set point is. So a PTC fuse at 20 Deg C with a fan (airflow) behaves much differently to a PTC fuse at 30 Deg C without a fan. It triggers at a different current, and it recovers from triggering faster.

3. PTC fuses are slow, so something like a brief dead short generally isn't caught by a PTC fuse in time. If the PSU can provide a fair amount of current, then a high current spike can pass through the PTC fuse which can damage your electronics (eg: burning out a track). If the spike is big enough, the PTC fuse itself becomes dangerous. The temp inside the PTC fuse lags behind the current flowing through it, so with a huge current a huge amount of thermal energy gets pushed into a PTC fuse. In extreme cases with a high current PSU, this can lead to the PTC fuse actually catching fire because it's so hot.

In short, I don't like them and will stick to plain ordinary fuses. This can sometimes be inconvenient when one blows. However, this is a small price to pay to avoid the issues listed above.

Thanks, I suspected something like that. I have a couple of fuse-holders left over from another project, I'll follow your lead and remove the PTC's and replace them with good old glass-fuses. Then if I still have skipped step at least i know it's not these things...

I am having the same problem! What size fuse did you use as a replacement? I am using a 12v 30a 360W LED power supply.