Binary weighted AC current load

[Jester]

My patience for soldering multiple power resistors to create a high power ac load has run out, I always need one more resistor than required. So I’m going to build a binary weighted load. I welcome comments from anyone that has built one because as I’m considering myriad design trade offs, some compromise is going to be required.

Ideal specifications:
1) Voltage: prefer 0-250Vrms
2) Current: 0-20A
3) Closed loop
4) 12 bits
5) Power 500W
6) Inexpensive ( stop laughing)

Thoughts..........
100W eBay power resistors are dirt cheap @ about $1-2 a piece.

Switches:
Initial thought was no or minimal # of relays, however triacs are going to be less than ideal with low input voltages, for example an audio amplifier or ac current source.

MOSFET SSR’s are inexpensive for low currents (100ma@400Vp), above that relays are cheap, I’m just cringing at the thought of all that relay chatter when the current is adjusted. Realistically will rarely use at 240V, so hopefully 400V SSR is adequate.

I will use an Arm with 12 bit A/D to measure V, I and calculate power dissipated to limit current when V is high.

Cooling will use a couple of heat sinks fins to fins to create a tunnel with fan forcing air through the tunnel and out the case.

Comments welcome.

Sent from my iPad

[soldar]

A series of about nine should cover what you need: 0.25, 0.50, 1.00, 2.00, 4.00, 8.00, 16, 32, 64

You can build the resistors for close to nothing using nichrome wire from junked heaters. 

Why do you need relays and not use just manual switches? You need some kind of remote control?

[capt bullshot]

Have you considered using a brigde rectifier in front of an electronic DC load set to R mode?
Indeed, there are AC electronic loads available (but for sure rare on the used market).

I've used resistors as AC loads, e.g. I have a set of four 30 Ohm resistors that I can connect in series or parallel (manually by rearranging the wires, still planning to make something up using relays), resulting in a 30 / 4 Ohm to 30 * 4 Ohm range. The resistors are "large open air" style wire wound resistors, so no need for a heatsink.

See RHS at this picture:
http://wunderkis.de/pwramp3/pic/test1.jpg

[Jester]

A series of about nine should cover what you need: 0.25, 0.50, 1.00, 2.00, 4.00, 8.00, 16, 32, 64

You can build the resistors for close to nothing using nichrome wire from junked heaters. 

Why do you need relays and not use just manual switches? You need some kind of remote control?

---> 3) Closed loop

[Jester]

Have you considered using a brigde rectifier in front of an electronic DC load set to R mode?
Indeed, there are AC electronic loads available (but for sure rare on the used market).

I've used resistors as AC loads, e.g. I have a set of four 30 Ohm resistors that I can connect in series or parallel (manually by rearranging the wires, still planning to make something up using relays), resulting in a 30 / 4 Ohm to 30 * 4 Ohm range. The resistors are "large open air" style wire wound resistors, so no need for a heatsink.

See RHS at this picture:
http://wunderkis.de/pwramp3/pic/test1.jpg

I should give the rectifier into the DC Load a try, however, my DC loads are lower voltage in the 60 -100V range, so that rules out --> 1) Voltage: prefer 0-250Vrms
I'm not sure how well this method works with the diode drops and testing with low input voltage?