Description

This is a picture of a dummy load I built. It uses 94 resistors in parallel. Each resistor is 4.7KOhm and is capable of handling 5W of power.

Going on the theory that when you stick resistors in parallel the total resistance goes down, we can work out the maths as to how many Ohms this will yield:

Determine total resistance:

R(total) = R(individual)/R(number)

R(total) = 4700/94

R(total) = 50 Ohms.

And to calculate how much power it can handle (in theory):

P(total) = P(individual) * Number of resistors

P(total) = 5W * 94

P(total) = 470W

Builder's Comments

This was one of my very first projects. The idea was to build a dummy load that would handle HF at power levels that would be good for when I upgraded to my advanced amateur license. I built this when I was an F-call and I have learned a couple of important things since then.

1. A metal chassis with a ground lead would have provided more shielding to other devices around the load
2. A metal chassis would have better thermal dissipation
3. Wire wound resistances have a thing call inductances. When you push RF through a wire coil, it creates a magnetic field. The 94 magnetic fields interact with each other and present a different impedance (a type of resistance). The good thing is that with each resistor added, the magnetic field is smaller and so the net effect is lowered.
4. I should not have spaced so many resistors so close to each other.
5. I should have used better wiring from the N-type connector to the vero-boards. I honestly don't want to consider putting any more than 50W into this.
6. Plan your chassis before you start building! I didn't drill the holes for the plastic spacers until after I had laid the components.