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InvisibletrendalM
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Question for any electronics guys, about a power supply
    #4326094 - 06/22/05 08:18 PM (11 years, 5 months ago)

I recently obtained the power supply from a dead DVD player, and would like to use it to power a small laser I will be receiving shortly.

It puts out three nice rails of +12v -12v and +5v. The problem I am running into so far is that without a load (the rest of the DVD circuitry) on ALL three rails the voltage on all three is VERY unstable and there is an audible clicking from the power supply.

Could I just use a few resistors to place a load on the -12v and +5v rails? The only one I need to use for the laser is the +12v.

Here's a pic of the board in question:


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OfflineBigGameHunter
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Re: Question for any electronics guys, about a power supply [Re: trendal]
    #4326250 - 06/22/05 09:13 PM (11 years, 5 months ago)

The power supply in your picture is called a pulse power supply or switching regulator power supply. I think simply using resistors to provide a load may not work properly with this type of power supply. In basic terms for this power supply, the line voltage (house electric) comes into the power supply & gets rectified then is coupled across the transformer via two transistors usually FETs turning on & off very fast & then rectified again on the secondary side of the transformer for each voltage line. To prevent this type of power supply from running wide open, it is regulated via feedback from the secondary side of the power supply through a device called an opto-isolator. As the current requirement changes on the secondary, the frequency the transistors (on the primary) switches on & off changes to compensate. If the feedback line(s) to the opto-isolator is tied directly to the secondary voltage lines & nothing else then you may be able to get away with it. If there are other line(s) tied to the opto-isolator such as protection lines or control lines that come from the microprocessor from the DVD player then it will probably not work. Also the ground on the primary side of the power supply is a hot ground (has voltage on it) so be careful.


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InvisibletrendalM
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Re: Question for any electronics guys, about a power supply [Re: BigGameHunter]
    #4326432 - 06/22/05 10:12 PM (11 years, 5 months ago)

There are no feedback lines coming from the DVD circuitry.

The connector you see at the far left side of the pic has 6 pins markec NC, G, -12v, 9, +12v, +5v.

NC is not used.
The G and 9 pins are both ground and connected to eachother (why is one marked "9" though?).
The other are the voltages.

What would this opto-isolator look like?

Oh, and there appears to be only one FET on the board?


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InvisibleDiploidM
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Re: Question for any electronics guys, about a power supply [Re: trendal]
    #4327489 - 06/23/05 04:06 AM (11 years, 5 months ago)

Power supplies of this type usually don't regulate the +12v/-12v; instead, they regulate only the +5v and track it to control the other voltages. This saves on costs and is usually sufficient depending on the application.

The problem you're seeing is probably related to he fact that most switching power supplies need a load to regulate the voltage. Singe you're leaving the regulated +5v line floating and since this line is used as a reference to regulate the other voltages, it's probably the source of the instability.

Tie the +5v line to ground across a resistor calculated to pass about 10% of the power supply's rated current, and it will probably resolve the problem. Use Ohm's Law to figure the value and make sure it's rated to dissipate the power at whatever current 10% turns out to be.


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2) You can't have an abortion no matter how much you don't want a child.
3) You can't have a certain plant in your possession or you'll get locked up with a rapist and a murderer.

4) We need a smaller, less-intrusive government.


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InvisibletrendalM
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Re: Question for any electronics guys, about a power supply [Re: Diploid]
    #4327739 - 06/23/05 09:17 AM (11 years, 5 months ago)

Well I have no idea what the rated current is for this supply, as it came out of a broken DVD player which had no service information :rolleyes:

A 25 ohm 2W resistor should do OK on the +5v line. That would pull down 200mA, right? I doubt the supply is rated much over 1A...


--------------------
You're here because you know something.
What you know you can't explain,
But you feel it;
You've felt it your entire life.
That there's something wrong with the world.
You don't know what it is, but it's there....
Like a splinter in your mind...
Driving you mad.


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OfflineSeussA
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Re: Question for any electronics guys, about a power supply [Re: trendal]
    #4327836 - 06/23/05 10:22 AM (11 years, 5 months ago)

You should only need to supply a load around 20ma to 60ma... there is nothing wrong with 200ma (assuming the supply can deliver that much), but it is probably overkill. I am basing this off of what I know of DC to DC converters, which are more or less switching power supplies.


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InvisibletrendalM
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Re: Question for any electronics guys, about a power supply [Re: Seuss]
    #4329763 - 06/23/05 08:38 PM (11 years, 5 months ago)

I ended up using a 33ohm resistor (for around 150mA) I had laying around, between the +5v line and ground. All power lines are stable at rated voltage.

I tried using a 50ohm resistor (~100mA) but the power supply had an audible high-pitch whine to it, which didn't sound too good.

The supply is happily powering my new laser tube:


--------------------
You're here because you know something.
What you know you can't explain,
But you feel it;
You've felt it your entire life.
That there's something wrong with the world.
You don't know what it is, but it's there....
Like a splinter in your mind...
Driving you mad.


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InvisibletrendalM
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Re: Question for any electronics guys, about a power supply [Re: Seuss]
    #4329770 - 06/23/05 08:39 PM (11 years, 5 months ago)

I ended up using a 33ohm resistor (for around 150mA) I had laying around, between the +5v line and ground. All power lines are stable at rated voltage.

I tried using a 50ohm resistor (~100mA) but the power supply had an audible high-pitch whine to it, which didn't sound too good.

The supply is happily powering my new laser tube:


--------------------
You're here because you know something.
What you know you can't explain,
But you feel it;
You've felt it your entire life.
That there's something wrong with the world.
You don't know what it is, but it's there....
Like a splinter in your mind...
Driving you mad.


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InvisibletrendalM
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Re: Question for any electronics guys, about a power supply [Re: trendal]
    #4331425 - 06/24/05 07:59 AM (11 years, 5 months ago)

Well it turns out my little DVD power supply may not be able to handle the laser :frown:

It worked fine for quite a while. Then all of a sudden it started acting very strange! If I touched the laser case near the ballast resistor, the laser would dim to about 50% power and begin making a fast repetitive ticking noise (almost like it was trying to start repeatedly, while already started). I also noticed at this point that the power supply was making a high-pitch whine again. Almost a hissing noise.

Could the laser just be sucking too much juice from the +12v line? I think what was happening is the capacitance of my body was enough to drop the voltage below the running voltage of the laser...which would then have to restart (the tube itself is in excellent condition and starts instantly so it never really visibly extinguishes during this).

I switched over to an old AT power supply and the problem with touching near the ballast resistor disappeard immediately.


--------------------
You're here because you know something.
What you know you can't explain,
But you feel it;
You've felt it your entire life.
That there's something wrong with the world.
You don't know what it is, but it's there....
Like a splinter in your mind...
Driving you mad.


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OfflineSeussA
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Re: Question for any electronics guys, about a power supply [Re: trendal]
    #4332044 - 06/24/05 12:54 PM (11 years, 5 months ago)

> Could the laser just be sucking too much juice from the +12v line?

Perhaps... you might also have a bad DVD powersupply as well. *shrug*

I haven't played with lasers in years. I used to have a laser powersupply that I built back in college. It was _huge_... just a bit smaller than a washing machine. I had four high energy capacitors in there... 50,000V at 0.625 uF (I may be off on the uF... they could hold 750J each) that took up most of the space. I used the supply to fire a pulsed nitrogen ultraviolet laser I built which was used to pump a tunable dye laser. Watching a bowl full of jello vaporize as it lases with over 3000J coming into it was amazing. (The jello was used to suspend the dye... but it had to be replaced after each pulse.)

My best laser story... Denny's resturant was just down the road from the physics building where I went to school. They ran a promotion one time where you would get a game card after you meal. Each card was a potential winner, but you had to scrape off the right boxes without scraping off the 'your a loser' box. They also watched as you did it... if you brought in a card that was already played, they didn't accept it.

I still had the loosing ticket when we went back to the phys lab. While we were supposed to be working, I started playing with the card and the laser and noticed that you could read the symbols on the card through the coating using the laser. It was difficult, but doable.

So we go back to Dennys for dinner and everybody keeps their card instead of scraping it off for a chance to win. We head back to the physics building, look at the cards under the laser light, and memorize the winning pattern. Next day, back to Dennies for the free meal. It lasted about a week before they finally decided to end the promotion early. We all ate very well that week. The employees would be holding the cards up to the lights, holding a ligher behind it, shining flashlights through it... and couldn't figure out how we were cheating... it was obvious that we were cheating... four guys winning every single meal for a week... "Wow, look at that... I won again!"

I never tried with a lottery card, but I suspect the same holds true. The scrape off lottery cards and the free meal cards were very much alike.

(Hmm... how was the ground between the laser and the dvd supply? If the laser was floating with respect to the supply, odd things could happen when you touched either... you might also consider putting diodes in line between the supply and the laser... just remember to take into account the voltage drop accross the diodes.)


--------------------
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Offlinedebianlinux
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Re: Question for any electronics guys, about a power supply [Re: trendal]
    #4332075 - 06/24/05 01:03 PM (11 years, 5 months ago)

Quote:

trendal said:
Well I have no idea what the rated current is for this supply, as it came out of a broken DVD player...




if you have/had the external casing then the current requirements will be plainly stamped near AC cord strain relief or stickered on the bottom plate.


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InvisibletrendalM
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Re: Question for any electronics guys, about a power supply [Re: Seuss]
    #4332092 - 06/24/05 01:08 PM (11 years, 5 months ago)

Ground line for the laser was connected directly to the ground return on the power supply.

The problem with the laser ticking didn't start right away, only after an hour or so of use (not continuous) and disappeared as soon as I switched to the PC power supply.


--------------------
You're here because you know something.
What you know you can't explain,
But you feel it;
You've felt it your entire life.
That there's something wrong with the world.
You don't know what it is, but it's there....
Like a splinter in your mind...
Driving you mad.


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InvisibletrendalM
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Re: Question for any electronics guys, about a power supply [Re: debianlinux]
    #4332104 - 06/24/05 01:11 PM (11 years, 5 months ago)

if you have/had the external casing then the current requirements will be plainly stamped near AC cord strain relief or stickered on the bottom plate.

Those are the input current ratings. I'm not sure what the output ratings (or limits) are for the supply, and it doesn't list them on the back of the player's case.


--------------------
You're here because you know something.
What you know you can't explain,
But you feel it;
You've felt it your entire life.
That there's something wrong with the world.
You don't know what it is, but it's there....
Like a splinter in your mind...
Driving you mad.


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