I wish I had stumbled on this video before i bought the 200 watters ! I'm testing 120w and thought i'd be wasting money on the 100 but come to find out I could have saved a bit. Good video, thanks for the tutorial.
You should be using an isolation transformer to prevent frying your osciloscope and/or frying yourself. The DUT (Device Under Test) should be plugged into the isolation xfrmr. Some folks keep their scopes plugged into an iso and though that's not rthe betst idea, it works. Be careful. A variac does not necessairly provide isolation. I believe some do. Mine does not...Luck
Oh, as an afterthought, there is 'resistance wire' used for heating elements that have a spec of Ohms/foot. They can handle a lot of heat, so you can use them for cheap power resistors, just cut to the length of resistance desired, they can glow red all day with no problem.
I have built audio dummy load using the wire wound resistor and the ceramic resistors. The wire wound are only slightly inductive and this is really negligible at audio frequency. I guess they can still explode but I haven’t suffered such failures but I do under run them.
I have a dummy load using the aluminum style resistors rated at 300 watt overkill for most receivers but I like your ideas I have been doing them for years
Does exist any rule saying what power of resistor should be for amplifier power ? 1:1 so if amp is 50 Watt@8ohm so resistor should be 50watt or more/less ?
Hi! Nice video. Sorry for the question. What dummy load can I use to measure higher power pa amplifiers? For an example: Dynacord L1600. 2X500W 8 Ohm and 2X800W 4 Ohm. Thank you.
i used to just solder a 25ea of 200 ohm 2w carbon composition resistors to 2 pieces of copper clad board (with holes drilled for the resistors) and submerge it all in into a jar of mineral oil. That wicks away any heat very well and those 50w rated composite resistors could handle a lot more power. Back then 2w resistors were very cheap
Hi!! First time viewer, the video is very good, I build audio amplifiers for a hobby, and you said a audio generator may be used for the test tone at 1K Hz. I use an Eico 377 and was just wondering where to set the amplitude(gain) control when feeding the signal into the amplifier? Thank you kindly for any help.... Have a nice day....
They seem perfectly fine. They don't get daily use though, so I can't say how they will hold up in an industrial environment or the like, but I'd put my money on them.
The "floating ground" vs. the other type has me concerned. What is the best way to determine which type of ground our amplifier has? Which type of ground will blow your scope if connected to the source the wrong way?
I want to know that which paint or material they have use over this resistors so that it may not loose its green color after it gets heated, as i want to do on our rheostats
It’s a bad idea to clip your scope probes directly to the resistor. When the resistor heats up it can melt the plastic on the probes. Ask me how I know!
Hi, Very interesting segment! I'm an audiophile myself, and would love to test the output of my Emotiva 50 watt per channel amplifier. I have a scope, should have my waveform generator next week, and will order the load resistors. The only thing I'm not fully understanding is this grounding deal conflicting with the ground of a scope. I'm new to scopes, and have used mine for just small digital breadboard circuits so far. Are you saying that an amp with two separate output's one + one - is safe to check because it's not using a common ground for both channel positive's? If I check the negative output from each channel to the ground lug of the amps power cord I should not see any continuity correct? Please explain. I don't want to blowup my new scope LOL!
You shouldn't see mains earth continuity because the amp will probably have a linear power transformer which isolates it. I've never seen an amp with the speakers grounded, since that would introduce noise. You should still check, but it only takes a second with a multimeter.
Yes, when I use my Fluke 287 between the Negative speaker terminals, and the power cord ground lug there is no continuity. This amp is pure analog with a toroidal power supply transformer, so I should be good to go. Thanks.
So this means we put the ground clip on the chassis or the negative lug on the power resistor and the primary probe clipped to the positive lug on the same power resistor the ground is hooked to the negative? Right? And if the amplifier isnt isolated, then negative speaker output has continuity between the power ground plug…if thats the opposite scenario from the first one above; then what? Thanks.
Maybe I'm wrong but I suspect that with that formula and experiment you are neglecting the power factor where AC Power = V*I*cos(theta). I understand you are using an almost non inductive resistance so the power factor could be = 1 and P = U^2/Z a valid formula but with a speaker coil the inductance is going to induce a diferent phase in the current lowering the power factor (which could account for that lower power in the specs). Wouldn't it be better to meassure with a speaker coil that can withstand that power and use a current clamp for your oscilloscope to meassure V and I simultaneously as well as the phase between them? Then plugging those values into P = V*I*cos(theta) I'm no expert though, maybe P=U^2/Z is valid regardless of phase?
Great video, but don't think that you are safe just because you only have a two prong ac cable, the only way to be sure is use an isolation transformer. the neutral and ground are tied together in the mains panel, and so are the probe ground to neutral and ground on your scope.
I have never tried this so I'm by no means an expert but, this method may not be as accurate as you would hope due to the resistor heating up when under load. maybe you could try measuring the resistance while under load to get a more accurate reading?
+Luke Attubato You are correct in that the resistance will change, but how much depends on the resistors temperature coefficient. These are specced at 350ppm/C, so at 85C, the resistance will increase by only about 0.18 ohms. Not really big enough for me to care. Besides, we can't measure the resistance when it is under load anyways.
+Mark Furneaux Oh, I didn't realise that they could be that accurate! I stand corrected. I assumed from my own tests that they would go up significantly more than that.
TheOrdinarySuperhero do you mean mineral oil? I’ve heard of that before... the water cooled ones generally flow the water through the center of the ceramic tube and have been sealed and purpose built like the Vishay WCR series
@@tsloth1390 you could use demineralized water to cool down the resistor. Water is better heat conductor than oil a if you use demineralized one the electric conduction is very low
@@tsloth1390 you could use demineralized water to cool down the resistor. Water is better heat conductor than oil a if you use demineralized one the electric conduction is very low
So much talking for a little procedure. 13-1/2 minutes of just watching your hands shifting two resistors around, before getting to the subject of your video. It is actually astounding that a super competent person like you, can waffle on with continuous repetitive rhetoric. We both know that this subject was a short one and as such, there usually is a tendency to stretch the subject. Please, don't do this. You don't have the style or the "panache" to pull it off. Just be yourself and be natural and stay focused on the topic. I gave you a thumbs up because you achieved what you set out to do very well indeed. So take some time to sit down, have an open mind, and watch this video of yours. Take note of what you do with your hands and two or three resistors, and the length of time you do it. listen to your speech and your fashionable words and phrases that you use which mean nothing other than being fashionable to inject them into speaking. So, be advised or blow a fuse, use it- don't use it.
