Electronic Switch with Load Before and After
simulate this circuit &ndash; Schematic created using CircuitLab


Here we have a regulated "voltage source (low) power supply with a known drop in voltage due to a rise in current due to internal resistance for example,  shown as 0.01 Ohm.



  We call this the effective series resistance (ESR) or the output impedance Zout.



An ideal switch with a 1 Ohm load will drop 1% due to resistor divider.
If the RdsOn of the switch is equal to,  or higher than the Zout from the supply, then Voltage drop will increase even more.



  We call this Load Regulation error and it equal to the ratio of source to (load+source) impedance, like any voltage divider.  Therefore it is a good idea to choose the switch according to the % load regulation you need.


One must know the switched and steady currents of the load to determine the effect R values of the load.  The FET or relay contact resistance can be selected based on load current rating.  

Normally a much higher current rating in FETs is chosen to minimize the resistance if you want good load regulation.


A "High Side" switch is shown needs a low Vg to switch on thus a P-ch MOSFET is used.
If using  "Low-side" switch on Gnd to load then V+ , we use an N-ch switch when there is only a single supply.
Distributors carry both for automotive and industrial use in high power switch applications.
For 24V,20A  you may choose a 30A relay or a 60A MOSFET depending on load inductance, it may need further margin derating.


http://www.digikey.com/product-detail/en/stmicroelectronics/STL62P3LLH6/497-15481-1-ND/5244758


specs: 62A 30V , 10.5 mOhm @ 7A, 10V   ($0.91/1pc)
SMD only