Could someone pls shed some lights on DC Electronic Load?

[powersys]

I'm interested in programmable "DC Electronic Load". Could some please shed some lights the functional blocks insides a programmable DC electronic load? In other words, how can I build one for myself?

Commercial programmable DC electronic load offers the following operation:
Constant current (CC)
Constant resistance (CR)
Constant voltage (CV)
Constant power (CP)

I understand what CC and CV. What is the application of "CR"? I thought "CR" and "CC" are more or less similar. Kindly correct me if I'm wrong.

Thanks.

 

[Kripton2035]

here is a schematic and explainations of an electronic load sold as a kit bit the ELV firma.
it is in german, but you can use google translators to read it

**broken link removed**

 

[ark5230]

While using a load, it will have some characteristics such as offer some resistance or draw some current or dissipate some amount of power. This is what those characteristics represent. In constant resistance mode it works like a reliable constant resistor, in constant current mode it will draw certain definite current and so forth.