When using these dummyloads for testing is it critical that the loads are exact to the OT's output.The amp will be fine. Just remember what the actual resistance is if you intend to do accurate power measurements/calculations.
Can you convert drawing to PDF?
These are very cheap if you are still sourcing your resistors. :icon_biggrin:
http://www.ebay.com/itm/Golden-Mini-8-Ohm-100W-Watt-Power-Wirewound-Resistor-Metal-Aluminum-Shell-Case/291670336157 (http://www.ebay.com/itm/Golden-Mini-8-Ohm-100W-Watt-Power-Wirewound-Resistor-Metal-Aluminum-Shell-Case/291670336157)
I used two 8Ω 50 watt. But you can get any wattage you want on eBay. Search for "8 ohm resistor". Here are a couple pics of my cheap D/L...I am looking for a dummyload for 2,4,8 and 16 ohms for up to a 100 watt output amp. Read enough about the chassis mounted resistors to get confused. One thread mentioned that it is not automatic that 100 watt dissipation will be obtained with a 100 watt chassis mounted resistor. From what I read factors like using an aluminum plate to install the resistor on, thermal paste on both the resistor and finned looking heatsink and ventilation to help dissipate the heat contribute to the wattage dissipated. These methods are all present in your design.
...or two 100 watt resistors in series (for example two one ohm 100 watt resistors in series to give two ohms). Would these two resistors be able when combined dissipate the 100 watts?Two 1Ω/100W resistors in series would be equal to one 2Ω/200W resistor.
Personally, I'd just use a barrier terminal block. More reliable connections.
I found this chart from Arcol resistor company on their aluminum housed power resistors. Notice the power
de-rating when not using a heat sink.
Here are some pics of an Arcol power R, 100w and the 100w power R's I bought on ebay (China) right next to each other, Arcol is ~ ~ double the mass in size.
Arcol; 2-9/16" x 1-7/8" x 15/16" with 1/8" thick base plate, ebay/China; 2-3/8" x 1-1/8" x 5/8" with 1/16" base plate.
Here's that video I mentioned, my build:
OK Rube! :icon_biggrin:
Here's that video I mentioned, my build:
It looks nice in that box but with the lid closed it will build up heat instead of letting the heat dissipate into free air space and de-rate the wattage value.
Mr. GoldbergI couldn't graduate college till I built one of His devices, mine started with a toilet brush and ended with a nerf ball threw a hoop :icon_biggrin:
So a very handy addition to a dummy load is a wattmeter.A full-on wattmeter is quite a device, measuring voltage and current and accounting for phase differences for accurate power measurement. There is also a device called a Bolometer that measures RF power by heating effect.[/size] But if you are dealing with a fixed, resistive, load as we are with an amplifer, then a square-law voltmeter can be calibrated to read directly in watts on a normal linear-scale meter. As it happens, for low voltages the turn-on characteristic of a germanium diode is a square-law. Initially, and up to a point, the current through the diode rises as the square of the applied voltage across it. As simple as it is, this makes a surprisingly accurate linear-scale wattmeter. (http://www.ozvalveamps.org/techsite/dummyload/dummyl.gif) Connect and bring out the mid-points for 4 ohm work[/size]Calibrated at 50 watts mine is still better than 5% at 10 and 100 watts. I don't take it too literally in the threshold below 10 watts, but it's still pretty good. The rectifier bridge is four OA81 germanium diodes but it shouldn't be critical and you might find that the e-b junction of germanium transistors salvaged from an old radio could be pressed into service. I was also lucky enough to score a 270-degree scale 1mA meter which makes accurate readings easy.“Are these supposed to glow red-hot?” asked Al as I got a bit carried away with a 400-watt rack amp. An inviting addition would be a cooling fan driven by a rectifier and regulator connected across just one of the load resistors. A possible alternative indicator is to use a logarithmic Line-of-Light LED display driver LM3915 powered like the fan. If I built it again I'd make these changes;
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