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Voltage Drop When Loaded LM317T Regulated Power Supply

06/28/2017 5:25 AM

I put together a power supply like this one:

The difference is that I fixed the output voltage to 12V, and has a single output. The transformer I'm using is rated 15V 1A.

The lm317 voltage regulator With no load the output measures 12V, but when I connect a led strip the voltage drops to 10.6V. The current measured then is 510mA.

Why does this happen, and how can I prevent it?

Thanks!

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#1

Re: Voltage drop when loaded LM317T regulated power supply

06/28/2017 5:27 AM

One of the possibilities is that the transformer is only rated at 6VA, then, if it were true, that detail has been withheld from the forum.

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#2

Re: Voltage drop when loaded LM317T regulated power supply

06/28/2017 6:59 AM

The LM317T requires two small capacitors on the input and output close to the part in addition to any filter capacitors.

These small capacitors suppress parasitic oscillations which can casue strange actions like this.

The product monograph should mention this.

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#3

Re: Voltage drop when loaded LM317T regulated power supply

06/28/2017 8:52 AM

Which specific LM317T, did you use? There are a few dozen listed on the data sheet.

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#4

Re: Voltage Drop When Loaded LM317T Regulated Power Supply

06/28/2017 9:59 AM

Shouldn't the voltage be set under load?

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#5

Re: Voltage Drop When Loaded LM317T Regulated Power Supply

06/28/2017 10:26 AM

There are so very many possibilities here. However, all of them should be very easy to identify by measuring the DC and AC voltages at the other nodes than just the output node in your circuit.

Don't forget that if you have a poor connection (cold solder, broken land, undersized land/wire, etc.) then you will have an actual circuit that differs from your expected circuit.

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#6

Re: Voltage Drop When Loaded LM317T Regulated Power Supply

06/28/2017 11:49 AM

You probably don't have enough input voltage to drive the LM317T correctly. With the circuit on load, measure the input voltage to the LM317T.
The data sheet will tell you how much voltage drop the IC requires.
Another possibility is that the LM317T is getting hot and limiting current or voltage. Do some arithmetic based on the DC input to the 317 minus the output voltage multiply this result by the current to see what the chip is trying to dissipate.
Chip power handling is often stated at 20degrees C or some such, and if it is getting how it can't dissipate so much. It will may need a heatsink.
Del

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#7

Re: Voltage Drop When Loaded LM317T Regulated Power Supply

06/28/2017 1:46 PM

Just because the transformer is rated 15 VA (rms), that says very little about the output resistance on the rectified and regulated DC supply out. That little word is impedance, specifally source impedance.

The IM317 may in fact not be able to respond to the load you want it to, and this shows up as primary impedance out (for steady state load).

You may need some power transistor(s) if you need a lower source output impedance.

Vs = 12

Vc = 10.6 @0.510 A

Vs-Vc = 1.4 V @0.510A

Rs = 2.75 Ω

I would think you should aim for Rs value about 0.3 Ω, depending on what you need out of the power supply.

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#8

Re: Voltage Drop When Loaded LM317T Regulated Power Supply

06/28/2017 8:42 PM

Get rid of everything beyond your 1000 microfarad capacitor (rated 35 VDC I'm guessing).

This power supply is overdesigned for your intended purpose....

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#9
In reply to #8

Re: Voltage Drop When Loaded LM317T Regulated Power Supply

06/28/2017 9:09 PM

It seems you understand far better than the OP what the anticipated load will be. Please explain.

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#10
In reply to #9

Re: Voltage Drop When Loaded LM317T Regulated Power Supply

06/28/2017 9:42 PM

True, his intended load may not be a LED strip...he did not state this.

If it is, his power supply is overdesigned.

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#11

Re: Voltage Drop When Loaded LM317T Regulated Power Supply

06/29/2017 7:43 AM

Do you have a heatsink?

If you don not, the chip might be limiting voltage due to high temperature. From the 1st paragraph of the datasheet

"The LM317 is an adjustable 3–terminal positive voltage regulator capable of supplying in excess of 1.5 A over an output voltage range of 1.2 V to 37 V. This voltage regulator is exceptionally easy to use and requires only two external resistors to set the output voltage. Further, it employs internal current limiting, thermal shutdown and safe area compensation, making it essentially blow–out proof."

1.5A is the rating with heatsink. It will probably work with 10% of the nominal current without it, but I could not assure you that it will work with 35% and no heatsink.

I would put a heatsink and try again.

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#12

Re: Voltage Drop When Loaded LM317T Regulated Power Supply

06/29/2017 9:59 AM

Intended purpose of this power supply is what now? LED strip lighting, does not this have several LED's in series? Each diode of the LED strip should have small voltage drop. Make sure you are reading the voltage at the correct place, right upstream of the LED Strip. Your power regulator chip is not working properly (probably cannot dissipate its own heat).

Heat management!

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#14
In reply to #12

Re: Voltage Drop When Loaded LM317T Regulated Power Supply

06/29/2017 4:37 PM

Without affecting the supposition that the power supply is being overloaded, I would mention that all the commercial LED strips I have played with have the LEDs in parallel and can be cut to a chosen length.

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#15
In reply to #14

Re: Voltage Drop When Loaded LM317T Regulated Power Supply

06/30/2017 4:49 PM

OK, thanks, I have not messed with them.

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#13

Re: Voltage Drop When Loaded LM317T Regulated Power Supply

06/29/2017 12:43 PM

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