Hi,
for my first THD measurements with my QA401 I need a dummy load. I am thinking of 16x 50W 8,2Ohm Widap WD resistors to get a Power rating of:
8Ohm 200W (4x)
4Ohm 400W (8x)
2Ohm 800W (16x)
Unfortunately, these resistors are inductive and its value is not mentioned in the data sheet. With up to 16 resistors, I can imagine it sums up to 100-200uH. I cannot measure it because I haven‘t bought these parts yet.
But my actual question: Should I care for such inductivity during THD measurements at all? What is the inductivity limit which can be regarded as negligible?
The alternative, if pure resistive load is advisable, is to make an own resistor with resistance wire in a bifilar fashion or to buy expensive non-inductive resistors.
How are you dealing with that?
for my first THD measurements with my QA401 I need a dummy load. I am thinking of 16x 50W 8,2Ohm Widap WD resistors to get a Power rating of:
8Ohm 200W (4x)
4Ohm 400W (8x)
2Ohm 800W (16x)
Unfortunately, these resistors are inductive and its value is not mentioned in the data sheet. With up to 16 resistors, I can imagine it sums up to 100-200uH. I cannot measure it because I haven‘t bought these parts yet.
But my actual question: Should I care for such inductivity during THD measurements at all? What is the inductivity limit which can be regarded as negligible?
The alternative, if pure resistive load is advisable, is to make an own resistor with resistance wire in a bifilar fashion or to buy expensive non-inductive resistors.
How are you dealing with that?
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If after you have the parts couldn't you just nutralize any inductance with a zobel ? Or you could just use parts like these . 5pcs 100W 50ohm Dummy Load RF Resistor 100Watt 50R HF Power terminator 1109 | eBay
You might be interested in this:
Building my own noninductive 8R 150W load using wire wound resistors
Building my own noninductive 8R 150W load using wire wound resistors
Zymorg, I built dummy loads using the exact same approach you have proposed.
Building my own noninductive 8R 150W load using wire wound resistors
The key result was: wirewound resistors don't all have the same inductance, it varies quite a lot from unit to unit. Therefore, plan to test and tune the inductance-neutralizer on every final resistor-assembly. I did the testing and tuning using a signal generator, oscilloscope, and a PCB layout that let me add more and more neutralizer capacitance, very gradually. Don't lay out one PCB footprint for a capacitor; lay out three or four capacitors in parallel.
Building my own noninductive 8R 150W load using wire wound resistors
The key result was: wirewound resistors don't all have the same inductance, it varies quite a lot from unit to unit. Therefore, plan to test and tune the inductance-neutralizer on every final resistor-assembly. I did the testing and tuning using a signal generator, oscilloscope, and a PCB layout that let me add more and more neutralizer capacitance, very gradually. Don't lay out one PCB footprint for a capacitor; lay out three or four capacitors in parallel.
Way too expensive, the non-non-inductive ones cost 2,50 each. If the Zobel has no drawback which I don‘t see then its perfectly fine.
It's a reasonably straightforward problem to simulate in LTSPICE.
First make a conservative overestimate of the inductance to be neutralized, e.g., 3 microhenries.
Next, calculate the correct value of compensation capacitor for that inductance and resistance. Build this into a simulation .circuit
Apply a ±65 volt, 50 kHz square wave in simulation, and monitor the transient current through the compensation capacitor. Also monitor the power dissipation in the compensation resistor. Use these simulation results to choose a compensation capacitor with plenty of safety margin on the ripple current, and to choose a compensation resistor with plenty of safety margin on the power dissipation.
(also double check that your compensation capacitor value was calculated correctly. Plot (V/I) for the assembly and verify that it's 8 ohms before, during, and after an edge of the square wave.)
First make a conservative overestimate of the inductance to be neutralized, e.g., 3 microhenries.
Next, calculate the correct value of compensation capacitor for that inductance and resistance. Build this into a simulation .circuit
Apply a ±65 volt, 50 kHz square wave in simulation, and monitor the transient current through the compensation capacitor. Also monitor the power dissipation in the compensation resistor. Use these simulation results to choose a compensation capacitor with plenty of safety margin on the ripple current, and to choose a compensation resistor with plenty of safety margin on the power dissipation.
(also double check that your compensation capacitor value was calculated correctly. Plot (V/I) for the assembly and verify that it's 8 ohms before, during, and after an edge of the square wave.)
Inductance combines like resistance, so only adds up for resistors in series. In parallel the inductance goes down, just like the resistance. 100's of uH are unlikely.Zymorg said:
A bit of inductance probably does little harm. Real speakers are not pure resistances. Anyway, you can compensate using capacitors - although watch the bandwidth.
I was rather thinking of trial and error using my capacitor box. But the lowest value is probably already too high. However, I have placed the order for the parts and I will hopefully find the right capacitors in my shelves. Why I should need anything more than trial and error with a scope and signal generator?
Those resistors need either lots of heatsinking, or else severe derating.
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I'm looking for dummy load resistor,too.This thread very nice thank you all.I learned noninductive issue at here.This resistor is useful for amplifier load ?
MP9100-12.0-1% Caddock Electronics Inc. | Resistors | DigiKey
MP9100-12.0-1% Caddock Electronics Inc. | Resistors | DigiKey
For those in the U.S. or willing to pay international shipping: load resistor - Parts Express Ships Fast and Ships Free.
I have a set of these heatsinked to some big aluminum.
I also have some electric hot water heater elements configured to make 8 ohms in a 5 gallon bucket of water that is sealed and vented.
I have a set of these heatsinked to some big aluminum.
I also have some electric hot water heater elements configured to make 8 ohms in a 5 gallon bucket of water that is sealed and vented.
I mounted the resistors today on a surplus heatsink. This should be capable of dissipating hundreds of watts.
It was a bit difficult to find the right cap for the Zobel with my cap box. My Picoscope signal generator has a output impedance of 600Ohm and an audio amp as buffer has far too low slew rate. So I made the best of the Pico-signal generator. I would go for 15nF, this seems to me a reasonable value, what do you think?
It was a bit difficult to find the right cap for the Zobel with my cap box. My Picoscope signal generator has a output impedance of 600Ohm and an audio amp as buffer has far too low slew rate. So I made the best of the Pico-signal generator. I would go for 15nF, this seems to me a reasonable value, what do you think?
A zobel for high frequency compensation would preferably use a form of resistor that has minimal self-inductance. It also only needs a minor wattage rating unless you aim to test amplifiers that oscillate uncontrollably or aim to do frequency testing out to circa 1MHz.
Some power resistors provide impedance plots, eg:
http://info.incomp.hu/NETPDF/PR0102.pdf
PS. Zymorg, if you downsize your photo then that would be a benefit for general viewing.
Some power resistors provide impedance plots, eg:
http://info.incomp.hu/NETPDF/PR0102.pdf
PS. Zymorg, if you downsize your photo then that would be a benefit for general viewing.
Inductance is independant from frequency. My resistors have a measured inductance of around 60uH.
The overshoot is not caused by the Pico, just tried it with a metal film resistor as you proposed. No ringing at all. But the Visaton resistor shows ringing, so not complety free of inductivity. I will order some metal film resistors to replace them in the next step to be sure to really have non-inductive resistors.
I am also looking for a buffer IC to buffer the Pico signal generator. Any advice?
The overshoot is not caused by the Pico, just tried it with a metal film resistor as you proposed. No ringing at all. But the Visaton resistor shows ringing, so not complety free of inductivity. I will order some metal film resistors to replace them in the next step to be sure to really have non-inductive resistors.
I am also looking for a buffer IC to buffer the Pico signal generator. Any advice?
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