The budget fot the whole unit would be about £250 or $379 USD
Hello again,
Chris:
Good question
Flyback:
Well, since your load has to be varied so it can be used to test the power supply at various power outputs, then one idea is to connect all the resistors in parallel and switch them in and out as needed.
For example, if you connect twenty-six 4.7k, 50 watt resistors in parallel, each resistor will dissipate 25 watts which is a safe operating point, and you can switch them in or out to vary the load from 0 to 650 watts in steps of 25 watts each. I would think that is sufficient. So that's 26 resistors, each with a rating of 50 watts, and each with a resistance of 4700 ohms.
Now on a site like Mouser for example, they sell "chassis mount" resistors like this for as low as about $3.50 each (USD) so you could purchase 26 of them for under $100 (USD).
If you would like more steps (like 12.5 watts per step) then you'll need more resistors, and of course more switches too.
Another question is what kind of switches did you intend to use with this to switch the resistors in and out. Most switches are rated for 50/60Hz not 50kHz. At that frequency or 50kHz it might be better to turn the power supply OFF before switching the load. After the load is set to the required test load value, then turn on the power supply.
One note here is that if 25 watts per step is too small, then you can go UP to about 35 watts per step using one 50 watt resistor per step, but you shouldnt really go above about 35 watts for a 50 watt resistor. If you wanted say 50 watts per step (13 steps) then you should go to a higher power resistor like 75 watts or 100 watts.
Another point is that you will probably want to mount the resistors on heat sinks inside the cabinet. You dont have to use only one heat sink for all the resistors, you can use panels of heat sinks with each panel having it's own fan for example. Heat sinks are mandatory for these kind of resistors though, unless you run them at less than 1/2 power (like 1/4 power).
One last note:
Make sure the resistors are non inductive. If not, look for ones that are non inductive. Some of these parts have the same part number for non inductive type, except for a few extra letters like "NS" or something like that. So you'll have to make sure you get that type.