Hook up a big dummy load of about 4 ohms. Many amps actually dissipate maximum power at a bit less then full output. Drive it with a sine wave at 3/4 power output and let it cook. Monitor heat sink temperature plus the temperature of any driver transistors that are likely to heat up. Can it run continuously, or is it heat sink limited? If you really want to blow it up, just set near full power at 1 kHz then raise the frequency until the output stage can't switch fast enough and it crowbars the supply and blows the outputs. A good amp design won't do that, but many popular circuits will.
If you want to be cruel, make the load vary. This is a circuit that I use; the input is driven by a square or rectangle wave, and varying the frequency and duty cycle will definitely make your amp sweat. The components should be rated to take the amp's output power. If you want to be very cruel, substitute some reactive components for the load resistors.
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A couple of methods...
1. Hook up four PA speakers (with over-rated HF horn drivers that you won't blow) in parallel to each channel and crank it up until you can't tell what song is playing anymore. That's the method used by DJs. Let simmer for 4 hours or until the police arrive.
2. Connect a step-up transformer to the output so that full power translates to 120V RMS (or 230, depending on where you live). Put in a 25-50 Hz sinewave, and plug in your skil saw. Attempt to cut plywood. If it doesn't bog down and shut the amp down, you could try to build a pair of speakers
3. 4.7 mH air core choke at 100 Hz sine wave. This is a VERY GOOD worst-case simulated loudspeaker load test.
I have used all 3 methods (didn't run the saw very long, though).
1. Hook up four PA speakers (with over-rated HF horn drivers that you won't blow) in parallel to each channel and crank it up until you can't tell what song is playing anymore. That's the method used by DJs. Let simmer for 4 hours or until the police arrive.
2. Connect a step-up transformer to the output so that full power translates to 120V RMS (or 230, depending on where you live). Put in a 25-50 Hz sinewave, and plug in your skil saw. Attempt to cut plywood. If it doesn't bog down and shut the amp down, you could try to build a pair of speakers
3. 4.7 mH air core choke at 100 Hz sine wave. This is a VERY GOOD worst-case simulated loudspeaker load test.
I have used all 3 methods (didn't run the saw very long, though).
sakis said:
Saki
The first thing that i do always when i finished an amp. it is to check if the iddle current remains stable after 1 hour of operation with the input grounded and without load in output. Let us to say, you place an iddle current of 25mA per output device. This measured as a voltage drop accross an emitter resistor with a voltmeter. Also we place a thermometer in contact with heatsink. After one hour about, the iddle current will be increased as well the temp. of heatsink significantly - which is about the same with Tj of output transistors for good heatsinks - and the temp usually touches the 55deg C. Then you readjust the iddle current in its original value by decreasing it, and after you must observe again if the iddle current remains stable and the temp of heatsink drops in 45deg C about. You must spend at least 1/2 hour for the second process. If all remain stable after this, then you have obtained a nice circuit with great stability under all conditions.
After this, you can proceed in the further ABUSH tests as proposed from all the other friends here. All methods are good.
Fotis
wg_ski said:A couple of methods...
1. Hook up four PA speakers (with over-rated HF horn drivers that you won't blow) in parallel to each channel and crank it up until you can't tell what song is playing anymore. That's the method used by DJs. Let simmer for 4 hours or until the police arrive.
2. Connect a step-up transformer to the output so that full power translates to 120V RMS (or 230, depending on where you live). Put in a 25-50 Hz sinewave, and plug in your skil saw. Attempt to cut plywood. If it doesn't bog down and shut the amp down, you could try to build a pair of speakers
3. 4.7 mH air core choke at 100 Hz sine wave. This is a VERY GOOD worst-case simulated loudspeaker load test.
I have used all 3 methods (didn't run the saw very long, though).
Those tests are pretty evil and will blow up an amp with indadequate protection pretty fast.
I believe you can blow up the QSC amps which use the collapsing opamp-supply method of current limiting by connecting a transformer on the output and let it saturate for a while during each cycle. The RMX amps and most of the older ones use this I believe. For the right (wrong?) conditions this load condition will charge the supply enough before the saturation that the current limit won't decrease enough to protect the output stage when the transformer saturates near the end of each half cycle.
That's a pretty evil test
QSC Output-Averaging works well for overload conditions that are either resistive or of short duration - like a shorted speaker cable. Even the PL700 would pass this. When the overload is reactive, you get lots of volts and amps at the same time. If the overload is persistent, you drive up Tj. The combination causes secondary breakdown. If you overload a QSC, its output drops until the op-amps bootstrap caps are drained - then it clamps the output to 15 volts. At these low output voltages, the op-amp is biased directly and the only real limit is Hfe. A resistive overload (say, 1 ohm) will draw little current. A reactive load draws full current when the output voltage goes low (and VCE is high!) , and the current limiting is ineffective.
Re: Re: looking for methods to abuse amps
To not misinterpret me, all above mentioned they can applied in a project such as this because except the non presented protection computer (it is in another pcb) there is on the board presented the bias restoration circuit, the input output filters (except the coil) and the VI limitter circuit.
fotios said:
To not misinterpret me, all above mentioned they can applied in a project such as this because except the non presented protection computer (it is in another pcb) there is on the board presented the bias restoration circuit, the input output filters (except the coil) and the VI limitter circuit.
An externally hosted image should be here but it was not working when we last tested it.
wg_ski said:
Looks like the newer model QSC:s has changed to a VI-limiter like circuit which drains a cap actively which controls current limit instead... The old method relying on hfe and stuff also has the problem of requiring manual trimming for correct operation.
Amps with conventional VI-limiters should be safe even into a transformer if designed conservatively enough. Which might not be the case always, seems hard to get adequate protection without the circuit activating when it doesn't need to.
megajocke said:
The PLX amps use conventional VI limiting. They have to, because the channels share a common power supply and they use a more conventional CFP-with-gain output circuit and drive the "high" side of the load rather than the "low" side. The "output averaging" circuit would be impossible to implement without the floating power supplies. I've analyzed the 3402 circuit - it limits at around 120A peak and about 50A into a short circuit. The switchmode supply provides additional long term limiting. RMX amps use their "standard" topology and current limit that they've been using since the dawn of time. Although it appears they've added a trimmable max in addition to their op-amp supply cap based circuit. Going class H on their bigger amps helps a lot too - because zero output voltage is a lot lower VCE. Reactive load handling is much much better, and I would expect the RMX5050 to have no trouble driving a saturated transformer, even in a 1450 would go kabang.
I don't see why it would not work with non-floating rails: Resistors from rails provide a small current and then the extra resistor and diodes are connected to the output of the amplifier to charge the caps more when there is an output signal.
The only difference will be that the resistors from the rails won't provide extra current during the swing but this should be possible to compensate for by using a smaller resistor from the output.
But I guess the VI-limiter has too many benefeits like:
No caps, resistors and so on required for opamp supply
Better protection for reactive loads
Allows darlingtons to be used for the output so that
smaller devices can be used to level translate the opamp output
More well-defined limit
Current limit tracks rail switching in class H-amps.
The only difference will be that the resistors from the rails won't provide extra current during the swing but this should be possible to compensate for by using a smaller resistor from the output.
But I guess the VI-limiter has too many benefeits like:
No caps, resistors and so on required for opamp supply
Better protection for reactive loads
Allows darlingtons to be used for the output so that
smaller devices can be used to level translate the opamp output
More well-defined limit
Current limit tracks rail switching in class H-amps.
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