posted
Originally posted in Alternators, Batteries, Interconnects
Hi All,
I built a DC power supply so I could use my car stereo amp in my room, but I'm having some problems with it. Any adivce on what to try is appreciated, but keep in mind that I'm a college student living off savings.
Some background... I'm on a tight budget while I go to school, and I wanted bass for my room. Since I didn't have the funds to buy a new sub and amp, I decided to use the parts I already had. I'm using a Blueprint 1001 in a vented enclosure tuned to 29hz, powered by a Phase Linear PL300 amp(class ab).
Running the amp off my car battery, I measured 12.5 volts at the battery, and 26 volts at the speaker outputs. This is about 170 watts rms, and plenty of power to push the sub to it's limit in the lower frequencies.
I built my DC power suppy using a transformer rated for 25 amperes at 12.5v, two rectifiers rated for 20 amperes at up to 60v, three 15v voltage regulaters rated for 3 amperes each, and a single 2,200uf 50v capacitor. Everything is attached to a heat sink and wired in parallel. I can provide a schematic if someone thinks it's necessary.
With this power supply I was expecting to get about 50 watts out of my amplifier, or about 14 volts on the speaker terminals. Instead the amplifier shuts off if I try to push it past 6.5 volts. I measured the voltage at the power supply and it was dropping below 11v when the amp turned off. That is a whopping 10 watts rms!!!! Assuming the amplifier is about 50% efficient, my power supply is dropping below 11v with a measly 2 ampere load [Frown]
So my question is, does anyone know why? Any advice on how I could fix this? I haven't tried bypassing the voltage regulators which should be the bottleneck.. can I bypass them safely?
posted
I have the schematic written down on a piece of paper. Unfortunately I don't have access to a scanner or digital camera, so no pics
Everything is wiried in parallel. Transformer -> Rectifiers -> Voltage Requlators -> Capacitor. If it will help, I'll draw up the schematic.
I've been thinking about adding a small battery to this setup. The power supply should be enough to charge it, and it will prevent the voltage from dropping below 12v. What do you think? I could probably find one for less than $30.
posted
What I would be concerned with is the ripple on the power supply. With only a 2200uF cap the voltage ripple would be pretty high under load. With the amp turned down the voltage never drops low enough to cut the amp off, but when the load is higher the ripple is much worse and discharges the cap down during each cycle to where the amp probably shuts off. A battery would smooth it out, but without really looking at the voltage over time I can't say that you still won't have some problems.
Posts: 2575 | From: GA | Registered: May 1999
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posted
mmmm...sounds fun working on a DC-DC converter for my senior design project...interesting stuff (WAY more involved than i thought too).
jc2 beat me to it...i was gonna mention ripple voltage. Can you draw up a schematic for us?
-------------------- "Those who give up liberty for the sake of security deserve neither liberty nor security" - Benjamin Franklin
DiRtY SoUtH DEAF SqUaD
Elemental Designs
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--------------------------------- Loud...it's when you can see the ground move, it's when you have to force yourself to breath, it's when the rest of the world ceases to exist. Posts: 4136 | From: Lafayette, LA | Registered: Sep 1999
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posted
You may want to try buying a cheap no-name capacitator, like a farad, or even half a farad, to see if that helps with the ripple. E-Bay perhaps?
-------------------- (1) PPI AMP PCX-2200 (800 Watts @4ohms Bridged) PCX-480 for my highs (2) DD 3512a's in a 4.4cube box with 50sq. inches of port tuned to 33Hz Street 1-2 [TEAM DD] 147.7dB My Sound Domain page Posts: 1655 | From: Tucson, AZ | Registered: Jul 2001
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posted
That would be a great idea if cheap no-name 1 farad caps were rated at higher than 16V.
Posts: 768 | From: Ottawa, On, Canada | Registered: May 2000
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posted
I think a battery would be much better and more cost effective.
Alaska, if you have any questions on the DC-DC, let me know. I've done a couple and the last few worked really well right out of the box. The first couple, not so good Posts: 2575 | From: GA | Registered: May 1999
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have you done any using Cuk converter topology? that's the design we gotta use...i think we can do it ok, we just need to find his journal paper on how to analyze the circuit (we need to know the equation for the current waveform to figure out our component values and all). i'll prob email ya about it...still vegetating on xmas break hehe
-------------------- "Those who give up liberty for the sake of security deserve neither liberty nor security" - Benjamin Franklin
DiRtY SoUtH DEAF SqUaD
Elemental Designs
To cheat is to admit defeat
--------------------------------- Loud...it's when you can see the ground move, it's when you have to force yourself to breath, it's when the rest of the world ceases to exist. Posts: 4136 | From: Lafayette, LA | Registered: Sep 1999
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Cuk converter topology doesn't ring any bells for me. So I probably haven't dealt with that one specifically. My first designs were basic transformer based designs, and my latest ones are bigger PWM designs like in car amps. I still don't have a very good idea how to design transformers for switching supplies, so if you have info on that I would love to see it.
Posts: 2575 | From: GA | Registered: May 1999
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I had a 1phd cap in my closet from an old install and hooked it up. Now my amplifier takes an additional 6 seconds to start once I filp on the power supply, and it keeps on going once I turn it off
After hooking up the cap my initial test was good. I managed 12v at the speaker terminals with a 50hz test tone. Thinking the problem fixed I cranked some music, only to see the amp turn itself off. Playing the same test tone again caused the amp to cut out again at 6.5v.
This weekend I'm taking the voltage regulators out of the loop. If that doesn't fix it.... I'll have to go with the battery I guess... Doesn't make sense since the transformer is rated for 25A at 12.5v.
posted
Not hijacking, just a side conversation. We're still here for you
The problem isn't the transformer, it will supply plenty of power at the right voltage to run a car amp. The problem is the rest of the circuit. You limit yourself to 9A with the voltage regulators, and some regulators don't like to be wired in parallel without additional circuitry to prevent them from killing each other. Then there is the problem of the voltage ripple. With no capacitor the voltage output will be the positive portion of a sine wave with the voltage going from 0 to roughly 17 volts (12.5V x 1.414). When you add the capacitor it should prevent the voltage from dropping all the way to 0. This assumes a properly sized cap for the load. With a cap that is too small the voltage will drop below the point that the amp will shut off. As you turn the amp up, the load increases and gets to a point where the voltage drops too far.
Adding the bigger capacitor, allowed you to increase the output using the test tone. Removing the voltage regulators may not be a good idea, because the amp would see a voltage above 17V and might shut off due to high voltage, or it could be damaged.
Posts: 2575 | From: GA | Registered: May 1999
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posted
ok, now I'm confused. I guess I don't understand exactly what a rectifier does. I thought it turned AC into DC. Am I right in assuming that all it does is invert the negative portion of the AC sine wave, so that the voltage is always positive?
That seems to make sense... time to go read some more FAQ's. bbl
posted
I just realized I hooked up my capacitor incorrectly. It's connected parallel between the voltage regulators and the amp. It's a 20v cap, so connecting it between the rectifiers and voltage regulators should allow it to charge up to 17v, instead of 15v like it currently is. It should also lower the stress on the voltage regulators....
posted
Yes, the rectifier just inverts the negative portion of the sine wave, so you have a bunch of humps that are half sine waves, and always positive. Putting the capacitor in front of the regulators may help slightly, but if the cap isn't big enough your voltage can still sag down under load, and you might still have the same problem.
Posts: 2575 | From: GA | Registered: May 1999
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Cuk converter topology doesn't ring any bells for me. So I probably haven't dealt with that one specifically. My first designs were basic transformer based designs, and my latest ones are bigger PWM designs like in car amps. I still don't have a very good idea how to design transformers for switching supplies, so if you have info on that I would love to see it.
cuk is based on PWM...but unlike buck or boost it can do both simply by altering the pulse width.
i have a bunch of journal papers but nothing in electronic format have you checked out this site before?
and i'd post something on topic but again, jc2 has it under control lol
-------------------- "Those who give up liberty for the sake of security deserve neither liberty nor security" - Benjamin Franklin
DiRtY SoUtH DEAF SqUaD
Elemental Designs
To cheat is to admit defeat
--------------------------------- Loud...it's when you can see the ground move, it's when you have to force yourself to breath, it's when the rest of the world ceases to exist. Posts: 4136 | From: Lafayette, LA | Registered: Sep 1999
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posted
A few questions, What exactly is the transformer rated at? If you measure the voltage under no load conditions it will be much higher than 12.5V. 17V sounds about right. Now once you connect that transformer to something that draws current, the voltage will decrease as a function of current. That’s why it’s rated at a specific voltage and current. My guess is that the 12.5V rating at 20A is AC peak to peak. So that right there cuts your voltage down to .707*12.5 = 8.8V. Now you have a rectifier, that another 0.7V drop giving you 8.1V. Then you have the voltage regulators. They will have a voltage drop as well. Im guessing about .7 to 1.2V, but if you bought it at radio shack or something, it might be worse. Do you have the regulators in parallel? Im not sure if you can do that. It depends on the internal circuitry.....Take some voltage measurements at various places. The transformer (AC), after the rectifier at the Cap, And then after the regulators.
Posts: 1259 | From: Fullerton. CA ,USA | Registered: Oct 1999
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I took my 1,000,000uf cap and connected it between the rectifiers and voltage regulators instead of after the voltage regulators. Problem solved, I can easily do 15v at the speaker terminals now and the voltage at the amp only drops to 13.5 !!!!! Hell yeah!
-------------------- "Those who give up liberty for the sake of security deserve neither liberty nor security" - Benjamin Franklin
DiRtY SoUtH DEAF SqUaD
Elemental Designs
To cheat is to admit defeat
--------------------------------- Loud...it's when you can see the ground move, it's when you have to force yourself to breath, it's when the rest of the world ceases to exist. Posts: 4136 | From: Lafayette, LA | Registered: Sep 1999
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posted
lol.. half wave.. i sure hope he has 4 rectifiers in there
Posts: 1854 | From: freezing in FAIRBANKS ALASKA | Registered: Aug 2000
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quote:Originally posted by timmi: lol.. half wave.. i sure hope he has 4 rectifiers in there
nope, I have two wired in parallel.
It should be pretty obvious that I'm not exactly an expert when it comes to electronics. Everything I know I learned from reading articles on the internet. If there is something I'm missing, please point it out to me. I don't mind you having a laugh at my expense, so long as you contribute something.
Doing some more tests it looks like the amplifier turns off at around 18v at the speaker terminals. This is more than I expected considering the three 3A voltage regulators, because the amp is drawing about 13A at this point. Then again, I don't really know what happens when you try to move more than 3A of current through one of these regulators.
quote:Originally posted by Mr.Dank: A few questions, What exactly is the transformer rated at? If you measure the voltage under no load conditions it will be much higher than 12.5V. 17V sounds about right. Now once you connect that transformer to something that draws current, the voltage will decrease as a function of current. That’s why it’s rated at a specific voltage and current. My guess is that the 12.5V rating at 20A is AC peak to peak. So that right there cuts your voltage down to .707*12.5 = 8.8V. Now you have a rectifier, that another 0.7V drop giving you 8.1V. Then you have the voltage regulators. They will have a voltage drop as well. Im guessing about .7 to 1.2V, but if you bought it at radio shack or something, it might be worse. Do you have the regulators in parallel? Im not sure if you can do that. It depends on the internal circuitry.....Take some voltage measurements at various places. The transformer (AC), after the rectifier at the Cap, And then after the regulators.
Between transformer and cap I measure 17v. After the regulators I measure 15.2v. Playing a 32hz test tone with voltage at the speakter terminals at 15v results in 13.5 volts at the power terminals on the amp. I didn't measure what the voltage was at the cap, but I'm guessing it should be around 14v.
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