Power Supply Help

Discussion in 'General Electronics Chat' started by meijaz, Mar 29, 2011.

Nov 20, 2009
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I have recently made a regulated power supply using LM317T. Output voltage is 1.5 to 18 VDC and input is 220 VAC. I am using a 1 A transformer.

As I need to drive a motor from this supply which draws more than 1.5 A, the power supply gets heated and smells like burning. I want to know if it is possible to use another transformer parallel with the previous one? or make two separate power supplies and couple the output at DC levels? or Do anyone have any idea about how can I use two transformers to increase the current?

2. JDT Well-Known Member

Feb 12, 2009
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Theoretically, you can use 2 transformers in parallel. But only if they are identical. Otherwise if one gives out more voltage than the other it will also supply most of the current. Best just to find a bigger transformer.

How much current will this motor draw? Make sure that the rest of the components in your power supply can also handle this current. Also think of the power dissipation in the LM317T. It will probably need a large heatsink.

A DC motor will take a much larger current at start-up than it will when running at normal speed. This zero-speed or stall current is only limited by the winding resistance which may be a fraction of an ohm.

Once the motor is running, it functions as a generator, the generated voltage opposing the supplied voltage. This reduces the supplied current. This is not "back EMF" by the way - that's something different altogether.

Can you connect your motor to something without an electronic power supply - like a car battery - and measure the current it actually draws? Then design your power supply.

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3. Wendy Moderator

Mar 24, 2008
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LM317 is rated for 1.5A, but a general rule of electronics is never use a component to 100% of its rating. If you must do this, make sure it is well heatsinked, and a fan never hurts. The datasheet also has several ideas to increase the current, which may include transistors as mentioned. Since the 317 is made by several different vendors look at some other datasheets for ideas.

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4. studiot AAC Fanatic!

Nov 9, 2007
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Don't try to parallel power supplies, either at the transformer or any later stage.
In general the added circuit complexity required to ensure proper load sharing in more expensive/bulky than using the appropriately rated component.
If you do not use this load sharing circuitry one of the parallel components will hog more and more of the load which in turn can lead to a runaway situation where it first destroys itself, leaving the other parallel component to supply everything which destroys it in turn.

There is one 'exception'. It is economic to use a pass power transistor, in parallel with the regulator, to supply some of the load current, ensuring the regulator 'takes up the slack.

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Nov 20, 2009
37
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Thank you for your help. I shall go for as you advised and then redesign it.

Why is this generated voltage not "back EMF"? I thought that it was called as so. So what is it anyway?

Nov 20, 2009
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Understood!

But if I use a fan, then I have to take care of it's current and voltage too.

Nov 20, 2009
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I understand but can you explain why would any one component hog more current? Moreover it means that I can not use two regulators in parallel, right?

8. strantor AAC Fanatic!

Oct 3, 2010
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I also am confused by this. I just got through reading a bit that this is referred to as "back EMF". Check this out, 2nd section "Shunt Motor Operation"

edit:
Actually, this is probably more applicable: "You should remember from the basic theories of magnetism that anytime a magnetic field passes a coil of wire, a current will be produced. The stronger the magnetic field is or the faster the coil passes the flux lines, the more current will be generated. When the armature begins to rotate, it will produce a voltage that is of opposite polarity to that of the power supply. This voltage is called back voltage, back EMF (electromotive force), or counter EMF. The overall effect of this voltage is that it will be subtracted from the supply voltage so that the motor windings will see a smaller voltage potential."

Last edited: Mar 30, 2011
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9. Jaguarjoe Active Member

Apr 7, 2010
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Look at the applications section of the LM317 data sheet. Near the end there is a scheme for a higher power regulator that shows 3- LM317's in parallel:

http://www.national.com/ds/LM/LM117.pdf

Each regulator has a small ohm ballast resistor in series with it. These help spread the current somewhat equally through each 317. Without them, because no two semiconductors are identical, one of these regulators would pass more current than the other two. This causes it to heat up more causing even more current to flow through it which creates more heat etc. This is thermal runaway.

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10. K7GUH Member

Jan 28, 2011
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How about a practical approach? First of all, use a transformer (one and only one) which can deliver 3 amp. Then add a pass transistor stage controlled by the LM317. The usual component is a 2N3055, but there may be "better" choices. This will pretty much cover your requirement and not burn up any hardware. Both the LM317 and the pass transistor should be heatsinked.
I'm sure there are plenty of schematics that can guide you, but if all else fails, I'll post one for you. Do, please, let us know how you proceed and the results.

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11. eblc1388 AAC Fanatic!

Nov 28, 2008
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Or use a higher current variable output regulator to replace LM317T, like the LT1038 or the LM396. Both of these can give up to 10A current output.

However, the max. output of LM396 is 15V only.

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12. studiot AAC Fanatic!

Nov 9, 2007
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13. JDT Well-Known Member

Feb 12, 2009
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Back EMF is what you get when you switch off the current in a relay coil. It is the opposite polarity to the energising voltage. It is caused by the collapsing current inducing a voltage in its inductive coil (attempting to maintain the current flow, in fact).

A rotating motor acts as a generator. The voltage is the same polarity as the supply. Caused by the coils moving through the stationary magnetic field. The generated voltage reduces the supply current.

14. Wendy Moderator

Mar 24, 2008
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A major component in why transformers are so efficient too.

Dec 26, 2010
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No, the term "back emf " is quite widely used to denote the induced voltage in a rotating motor. It was so used in the texts I read 30 odd years ago in England, and seems still to be in use - e.g. http://www.animations.physics.unsw.edu.au/jw/electricmotors.html#back

http://en.wikipedia.org/wiki/Brushed_DC_electric_motor
The term "counter emf" is also seen, but the distinction is clearly not universal. Bear in mind that this is an international discussion forum: usage may vary between different English-speaking countries.

16. sheetzke New Member

Mar 31, 2011
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Nov 20, 2009
37
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Your power supply is ATX type. These types are mostly used in desktop pc's. I don't know for what purpose you are using. Usually they have a 12V DC brushless fan (no separate motor).

First try to oil and clean that fan. If still not working check wires' continuity and connections. If none works then all you have to do is to replace that fan. They are easily available from a pc repairing shop.

Nov 20, 2009
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I have found this:
http://www.circuitpowersupply.com/circuitblog/high-current-regulated-supply-by-lm317-and-2n3055x2/

But there is a problem that What is the coil under the transformer? Is it a part of the transformer? Over here only center-tapped transformers are available. If you have a better idea, please let me know.

Thank you for guiding me

19. studiot AAC Fanatic!

Nov 9, 2007
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Yes it is part of the transformer as stated at the beginning of the article.
"The transformer uses a second winding to power the LM317"

This circuit is not what we have suggested and is designed to produce 15 amps.

You stated you required 3 amps.

What was wrong with the circuit I linked to that offers up to 4.5?

20. sheetzke New Member

Mar 31, 2011
2
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Be careful. Don't post too many to the same website address. It might be considered spamming.