Here's why it is difficult: If you just connect your meter across the output terminals of the power supply, you are shorting it out. The voltage will drop to pratically zero, but it will crank out a lot of current. Problem is, 12 A at 0 Volts is zero watts (deliverd to the load).
The current rating of a transformer comes from two things: it's ability to maintain its output voltage while delivering a lot of current, and how much it heats up while doing that. Obviously, it has to stay reasonably cool, and the voltage has to stay reasonably high too. So the current rating isn't the maximum amount of current that it can deliver, it is the amount of current that it can deliver while keeping the voltage above X% of the rated voltage, and without overheating.
Here's what you can do: Get some power resistors for a dummy load. Let's figure out what you need. You have a (approx) +-24V supply and 4 ohm load, so the amp can deliver at absolute maximum, 23.5/4 = 5.88 A of current. This is the peak current; RMS (think: average for a sine wave) current is about 70% of that, or 4.1 A (Power is I*I*R = 68 W, how about that!). So let's gets some resistors that will (try to) draw up to 4 A out of your supply, and see how well it can maintain that voltage. Buy four (or more) 22 Ohm 10 W resistors.
Since you need to measure the voltage, you can't use your meter to measure current (you could if you had two meters of course). That's no problem at all; just use Ohm's law to calculate current from the resistance and voltage: I=V/R. R is just the resistance of the resistors you are using as a load.
Be careful: each resistor will dissipate over 20 W of heat, and will get hot fast. Also, since you are using 10 W resistors, they will get hot enough to be damaged if you leave them connected too long. Only connect for a couple seconds; long enough to take a voltage reading, then disconnect.
Connect one resistor from V+ to ground and measure the voltage across it. Use Ohm's law to calculate the current. Now connect two (in parallel) to draw more current. Measure the voltage. The R is now 22/2=11 Ohms. For three, it is 22/3 = 7.33 Ohm, etc. When in doubt, measure R using your meter (make sure to disconnect from the cicuit first!). Write down all the voltage and current for each load.
By the time that you connect four of these resistors, the voltage that you will have measured will probably be several volts lower than without any load. You could use some mathematics to determine to 'optimum' load for this transformer (to draw the most possible power from it), but with the help of the datasheets, you should also just be able to 'see' where the voltage has dropped too low to be useful for a 4 ohm speaker. You may also notice that a 8 ohm speaker would be a better match for this transformer.
I'll second the idea of the resistors...but get some higher power ones...so they dun burn out...of course it would be slightly more expensive...they can be of any type...so go for wirewound...
Oh, and any ideas on the popping? It seems to only happen with certain frequencies, not just certain volumes. When you lower the volume, they go away, but some songs, depending on the frequency of the bass can go much louder without popping. I would really like to fix this, but don't know the cause. I am going to try that onkyo amp again with the same song and see if it is the speaker or not. Also, like I said, the pops aren't as loud as with the other transformer, but still appear.
I have checked my solder connections before, and found no problems. Are you saying that possibly there is a loose connection that when the speaker vibrates it shakes the connection loose?
Could it be the speaker?
Could it be the speaker?
Now, I went back and re-soldered the connections to everything, and it is still there. It isn't very noticeable, nowhere near as much as with the other transformer, but still there. I tried the Onkyo amp again with the same song, no popping.
Could the small caps be the cause?
Could the small caps be the cause?
Just wondering,
I know (or should say, am pretty sure) a subwoofer is not a very efficiant speaker, especially not a car one. Could the popping be the amp's current limiting kicking in? If so, would making the exact amp (from page 1 of the datasheet, and connecting the inputs and the outputs of each together, anotherwords making a parallel mode amp, which would have a higher current output stop this? Also, 4-ohm is the lowest load, so it would make sense that the problem is the curent limit is the cause.
I know (or should say, am pretty sure) a subwoofer is not a very efficiant speaker, especially not a car one. Could the popping be the amp's current limiting kicking in? If so, would making the exact amp (from page 1 of the datasheet, and connecting the inputs and the outputs of each together, anotherwords making a parallel mode amp, which would have a higher current output stop this? Also, 4-ohm is the lowest load, so it would make sense that the problem is the curent limit is the cause.
Is the popping do you think not good for the speaker? Because, if so, I don't think I will be trying it with my expensive B&W speakers. I know it doesn't affect a sub, but I also know a sub can handle more power.
it might not damage your speaker...it will depend on the power that is put out during popping...like how bad it is...might be the protection kicking in...relay disengaging...maybe the amp cannot handle 4ohms??
Measuring current
What's the Maximum current measurement capability (the maximum range) of your DMM? Assuming it's adequate, switch it to it's highest range.
Then disconnect one of the supply rails from your amp, and connect it to one terminal of the DMM. Connect the other terminal back to the amp. Now the current flowing to your amp goes through the DMM. It will read the current directly. It's changing rapidly if you're listening to music, so switch the DMM to measure AC current.
Hope this helps.
Cheers, BC
What's the Maximum current measurement capability (the maximum range) of your DMM? Assuming it's adequate, switch it to it's highest range.
Then disconnect one of the supply rails from your amp, and connect it to one terminal of the DMM. Connect the other terminal back to the amp. Now the current flowing to your amp goes through the DMM. It will read the current directly. It's changing rapidly if you're listening to music, so switch the DMM to measure AC current.
Hope this helps.
Cheers, BC
No it isn't bad at all, no where near as loud as when I used to have too small of a heatsink which would cause it to overheat, and the popping was caused by too much heat. So, I should be fine with my other speaker. The speaker I would try it with is an 8-ohm speaker.
What do you mean by "Relay Disengaging"? I have no relay on my circuit.
My DMM measures up to 10ADC unfused, but I have a 20ADC analog gauge. So, how would I test it? Disconnect the positive lead from the power supply cap, then connect the meter in line with the + wire of the amp and the power supply's + output?
What do you mean by "Relay Disengaging"? I have no relay on my circuit.
My DMM measures up to 10ADC unfused, but I have a 20ADC analog gauge. So, how would I test it? Disconnect the positive lead from the power supply cap, then connect the meter in line with the + wire of the amp and the power supply's + output?
sorry...after the rectifier and before the caps 9in between the bridge rectifier and the smoothoing caps...) Must've been my blain not wrking well...it's 12 midnight here...
Uh, don't you want to measure AFTER the caps, where you will see the average current drawn by the amp, rather than the periodic charging pulses?
Jan Didden
Jan Didden
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