I have a dehumidifying cabinet which requires 5V 900mA, and I have a universal adapter that is rated at 1000mA 12V, however the slider doesn't have a 5V marking, only 4.5 and 6V. Is it okay to point the slider to somewhere between 4.5 and 6 in hope of getting around 5V?
If the slider is a switch, then you won't be able to set the output to an intermediate value (5.0V). The only way that you would be able to get a 5.0V output is if the slider is a linear potentiometer, although I find that unlikely.
It's more likely to be a switch because switches are cheaper and don't require calibration for specific positions (4.5V, 6.0V, etc...). It may still work at the 4.5V setting, depending on the electronics since 4.5V is only -10% from the specified voltage.
NO, depending on the type of switch and the design of the circuit - the output could be either much lower, 4.5v, 6v or a lot higher.
There are different ways of controlling a regulator with a switch
and there are 4 possible answers... A) Both 4.5v and 6V Inputs are selected at once B) 4.5V Input Selected C) 6V Input Selected D) No Input Selected
B and C are harmless. - Just the same a normal use.
A and D cause problems - if both are selected or neither are selected - depending on the design this will cause either a higher or lower voltage - it is impossible to guess without more information... and could cause damage to the unit.