Selecting resistor
ThePenguinMaster
Posts: 89
ok so heres ythe problem...
im wrking on a power supply that outputs both 5 and 12 volts. im working with 18 volts to start. 18 volts and 2.5 amps.
what i have is two power regulators and two caps, the two regulators are a 12 volt regulator and a 5 volt regulator. i have 18 going into the 12 volt regulator and the 12 comming out checks out. so im running 12 volts into the 5 volt regulator and the 5 volt regulator maintains a steady 5 volts so everyting is working well..
however, after a few minutes, both regulators begin to over heat. the power supply is to drive a hard drive, a basic ata hard disk. after it over heats the power goes on and off. theyre both heat sinked and im still having the problem.
i tryed to place a resistor between the 12 volt regulator and the 5 volt regulator to decrease the voltage going into it. when i do this, it stops overheating and there is a steady 6.5 volts going into the 5 volt regulator but my hard drive shuts off ant wont turn on, as if maybe the voltage is correct but the amprage is off?
so if adding a resistor shuts off the drive, and adding resistance lowers the amprage and shuts off the drive.. how do i balance this? am i selecting the wrong resistor?
any help would be much appreciated. i know there is a basic conccept im missing and i would appreciate if someone could clue me in.
im wrking on a power supply that outputs both 5 and 12 volts. im working with 18 volts to start. 18 volts and 2.5 amps.
what i have is two power regulators and two caps, the two regulators are a 12 volt regulator and a 5 volt regulator. i have 18 going into the 12 volt regulator and the 12 comming out checks out. so im running 12 volts into the 5 volt regulator and the 5 volt regulator maintains a steady 5 volts so everyting is working well..
however, after a few minutes, both regulators begin to over heat. the power supply is to drive a hard drive, a basic ata hard disk. after it over heats the power goes on and off. theyre both heat sinked and im still having the problem.
i tryed to place a resistor between the 12 volt regulator and the 5 volt regulator to decrease the voltage going into it. when i do this, it stops overheating and there is a steady 6.5 volts going into the 5 volt regulator but my hard drive shuts off ant wont turn on, as if maybe the voltage is correct but the amprage is off?
so if adding a resistor shuts off the drive, and adding resistance lowers the amprage and shuts off the drive.. how do i balance this? am i selecting the wrong resistor?
any help would be much appreciated. i know there is a basic conccept im missing and i would appreciate if someone could clue me in.
Comments
Also, unless the 12 volt regulator is capable of significant current, it will overheat once you get the 5 volt regulator fixed up.
Other than the useful experience of designing a power supply, I'd use a surplus computer power supply or purchase a new switchmode supply.
Cheers,
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Tom Sisk
http://www.siskconsult.com
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so is it possiple im drawing more than the regulators can handle, and its not because theyre working with an input voltage that is too high? is that an issue(having a vin too high for the regulator?)?
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Think Inside the box first and if that doesn't work..
Re-arrange what's inside the box then...
Think outside the BOX!
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- Stephen
motor trying to accelerate the drive to running speed. This can
be 1 to 2 amps for 5 to 10 seconds. If you haven't mounted the
regulators on adequate heat sinks, they can easily overheat.
Some regulators employ an over temperature sensor to
prevent destruction. When the over temperature protection
kicks in, the +12 v regulator shuts down. Since you are
feeding the + 5v regulator from the + 12v regulator, it
shuts down also.
Solution: Use heat sinks large enough to accommodate the
expected thermal load. I wouldn't try to use resistors.
phil
These regulators only draw as must current as the load. Phil is right. The drive spindle motor may be drawing a lots of amps until it settles at its run speed. Heat sinks help, but I suspect the voltage reulator is not rated at 1 amp or better. If the regulator over heats, it will indeed shut down. Try the LM7812 and the LM7805 one amp regulators. With a good heat sink, these regulator should not overheat at 1 amp.
so it is possiple that it is drawing more than that?
i pulled the heat sink out of an old broken computer power supply and it is about 4inches wide and 3 inches tall with a fanned top. im using heatsink compound and the whole heat sink gets way too hot while the hard disk is in idle mode.
you said most regulators are rated at 1 amp? i have 2.5 amps comming in. and lets sy im using 800ma that means im burning off 1.7ma?
i guess it is a possibility i could have a short somewhere. that could do it.
how can i check how many amps im pulling. i used a multi meter, however its a cheap walmart meter. i put the posistve terminal from the meter onto a hot wire and ground out my negitive. i read 5v running stable and when i switch it to DcMa it reads only .01mA which dosnt seem right. this is while im idleing.
is it possible that when the hard disk is inactive i have too much spare current and its burning up my regulators? or is it more likely im just shorting out?
i looked into how they use the zener diodes for voltage regulation. i think its an interesting concept. i might try to incorprate that concept.
also i will try to run both regulators off of the 18v 2.5 mA supply, that will be safe right?
and i have thought about using a pre made power supplu.. trust me.. its tempting. but i want to figure out some more intermediate uses for analog components. i guess its a learning experience lol.
The LM7812 would run considerably cooler with a mere 15 volt supply. 18 volts may be too high.
If the regulators are top quality, they have thermal shut down that protects them.
As point out before the 12volts drives the motor of the hard disk and is most of your power consumption. The 5 volts is quite minimal.
Nonetheless, going from 12volts to 5volts is again a big heat dump.
It might be easier to get a dual powered switching supply that provides +12 and +5 as that was what they were designed for.
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"Everything in the world is purchased by labour; and our passions are the only causes of labor." -- David·Hume (1711-76)········
Can you connect just the LM7812 to the 18V with the bypass caps and no load without it getting hot? Try the same with the LM7805.
Then try a simple LED using a 1K resistor for both voltages. The regulators should not even get warm with these tests. If they do perhaps you dont have them connected correctly.
If you test the current you need to put the positive from the 7812 to the positive lead of the meter and the negative lead of the meter to the load. That way the current flows through the meter, in its positive lead and out its negative lead. Your meter has to be rated for the amps that will flow however. Typically it will be fused and the fuses are inside the meter. Some have separate connectors for low current measuring (typically up to 200 milliamp s) and high current measuring (up to 10A).
Another way to measure the current is to use a large wattage resistor in series with the circuit and measure the voltage across the resistor and calculate the current using OHM's Law.
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Think Inside the box first and if that doesn't work..
Re-arrange what's inside the box then...
Think outside the BOX!
ok so lets say that im burning off too many watts. oi can potentially use a external pass transistor and zener diode to control the wattage?
i know sthease are some basic questions but there concepts that i need to learn. i appreciat your help guys..
oh and on a side note.. when the hard disk is not running the regulators do not overheat.. so i dont think its a short. prehaps i will be unable to use a voltage regulator because it draws more amperage than the regulator can handle.
I hope that clears thing up for you.
better check your math....
Regardless if your supply is 18V or 35V and you are drawing 800mA through the output of a 12V regulator, you are also drawing 800mA from the 18V or 35V supply.
Suppose that your supply is 35V and you are drawing 800mA from the output of the 12V regulator. That means that there is a difference of 23V across the regulator that has to go somewhere. It does this in the form of heat. Using the formula P=I*V ... P = 800mA * 23V = 18.4 Watts of HEAT.
The same could be said for an 18V supply drawing 800mA from the 12V regulator. In this case there is a difference of 6V. Using the same formula P=I*V ... P = 800mA * 6V = 4.8 Watts of HEAT.
Hope this helps.
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Beau Schwabe
IC Layout Engineer
Parallax, Inc.
My apologies!!
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Think Inside the box first and if that doesn't work..
Re-arrange what's inside the box then...
Think outside the BOX!
i also tryed checking how many amps the drive is drawing on startup. what i did is i have two wires going from the adapter to the project board. i disconnected the positive wire comming from the adapter where it connects to the project board.
i then connected the positive wire comming from the adapter to my positive probe on my meter, then connected the negitive probe to where the positive charge from the adapter would enter the project board.
i turned on the board and the amps jumped up and then went back down. now i have a cheap meter and its hard to understand what im reading here, there are two settings, mA and A. wether i set it to amps or mA it still reads 6.1
just the symbol behind it changes from m to ma which makes no sence lol. so should i assume my project board is consuming 6 amps? i think i need a better meter..
i know that there are places i can buy a supply, and i do google a lot of stuff.. google is my friend, but i ask a lot of basic questions, and finding some of thease basic concepts on google can be a little challenging, also its not as good for trouble shooting, so thanks for dealing with me asking my noobie questions lol
EDIT:
oh! ok! i see it now! the parallel power regulators. at first glamnce i thought it was 2 different voltage regulators, like a 5 and a 12, but its a way to distribute the current being drawn across two regulators of the same voltage.. ok i get it. thats interesting.. ill have to try out that idea and see if i can distribute the load.
Post Edited (ThePenguinMaster) : 7/11/2007 2:00:46 PM GMT
Keep in mind that D1, D2, etc for the schematic that metron9 supplied ALSO need to handle the current demand of 1A.
In my opinion, there are better ways to do this. If you look at the datasheet for the regulator.
on Figure 12 and 13 there are already established methods from the manufacturer on how to increase the output current drive.
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Beau Schwabe
IC Layout Engineer
Parallax, Inc.
The previous post was could it be done, I don't think using dual regulators should be done unless you really know what you are doing.
The 90 cent part here and others that range up to the 8 dollar range all have different features. This one has a 4% voltage tol so it is cheap and perhaps not recommended for a hard drive, I din't know.
I added another pdf file on a 5A regulator in a to-220 package that has only 1.2V dropout. Comes in fixed and adjustable versions. Under 3 bucks
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Think Inside the box first and if that doesn't work..
Re-arrange what's inside the box then...
Think outside the BOX!
Post Edited (metron9) : 7/11/2007 3:28:19 PM GMT
Electrical Specifications
Current Requirements
12 VDC
Read/Write 450 mA
Idle 470 mA
Standby 23 mA
Sleep 23 mA
5 VDC
Read/Write 800 mA
Idle 750 mA
Standby 330 mA
Sleep 200 mA
Power Dissipation
Read/Write 12.80 Watts
Idle 9.50 Watts
Standby 1.90 Watts
Sleep 1.30 Watts
i hooked the 5 and the 12 volt regulators directally to the 18 volt supply, instead of having the 5 volt mooch off of the 12 volt regulator.
At 800ma on the 5V regulator it will get hot, too hot to touch so it needs a good heat sink. I like to double the maximum supply so your 12V at 1A max is fine. The 5V 1A is near the top but should still work.
The peak current however may be higher when starting up though. if you use a big capacitor make sure you read the datasheet on the diode needed to protect the regulator, there is a schematic and warning.
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Think Inside the box first and if that doesn't work..
Re-arrange what's inside the box then...
Think outside the BOX!
another way i tryed was to have the 5v feed off of the output of the 12 volts.
they both provided thew same result. When i get home today im going to draw a schematic and take some pics. its something i need to do anyways because i like to document my projects. then ill post them on here. things are rapidally comming into focus on how thease things work. apperentally there is a problem but its not what i thought it was at first.. being the whole adding a resistor thing.. now i realise that wouldnt help lol.
thanks for your help guys.. you helped me understand a lot of different concepts, which can be applied to other projects.·