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LM317 works Bad "SOS"


steveggz
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Hi. Today I bought and assembled parts to make a voltage regulator in reference to couple of web pages but mostly from http://casemods.pointofnoreturn.org/vregtut/tutorial-full.html.
I attached a picture of the schematic.
Alun also showed me a schematic very similar to this one. Im sure that this schematic works because of the many tutorials on the web.
After assembly I measered voltages and they are working great. I can get to the 9.6vdc I was looking for. But there is a big problem, the amprege or current is very very low. Its at a constant 235mA.
The input voltage is 12vdc and 1.5A. I measured this before and after, and the power is accurate. But my ouput Amprege after the circuits is 235mA.
Can anyone figure why? By looking at the schematic, is there anything wrong? I checked my circuits many times and it looks accurate with the schematic.
Could it be the resistor? Or the pot?

post-13145-14279142451539_thumb.jpg

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Hi Steve,
Usually an LM317 has 120 ohms for R1,  to hold down its output voltage without a load, but your 1k is fine with your pot and your fan.
Since the output voltage is correct, then obviously your fan doesn't draw more than 235mA. Therefore the LM317 will be able to drive up to 6 of your fans in parallel.

What is the fan's current rated at when operating from 9.6V?
If it is rated at 12V, then its current will be lower with only 9.6V.

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Actually Im using it for a video camera. On the back of the camcorder it mentions the voltage (9.6) but not the current or ampege.
I measured the current on my mutimeter without a load, meaning I didn't connect it to the camcorder and then check the current. Im new to electronics, is that the wrong way to check the current?
Its good to know the resistor is right, im going to check the current again, this time with a load.

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Hi Steve,
You are lucky that you didn't blow the fuse in your multimeter.
Current is measured by connecting the current meter in series with the power source and the load. A current meter is like a dead short to be able to pass current without much resistance. You used your meter to short-circuit your supply.

Connect your current meter in series with one lead on the output of the LM317 circuit and the other lead at the camera's positive power input. Connect the ground of the LM317 circuit to the camera's ground with a piece of wire. Operate the camera and the current meter will show its current draw. If the camera has a rechargable battery then the current will be higher when it is low and the camera is operating. Check that the LM317 doesn't get too hot. If it does get too hot then it will automatically shut-down to protect itself.

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Hi Audioguru, thank you for your help. Im really not familiar with this stuff. Before reading your second post I tried again and it came up around 235ma again but it looks like I am doing it wrong, no wonder it sparked every time I touched the leads to the ciruit. Im going to try again the way you explained it.

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I connected everything like you mentioned and I think I got an accurate measurement.
I was unable to connect it through the little hole on the back of the camera(I dont know what those connections are called) which is 9.6v, so I used the connections for the battery. The input voltage for the battery is labled 10V and 850ma. So I set the voltage to 10V on the voltage regulator and connected it properly this time. The camera, because I had it on "On" turned on by it self when I connected the multimeter probe to the positive. As it started up the current changed a little.
First the current was under 100ma and when it was ready it was about 105ma.
Does that seem right? Its labled 850ma, do you think camera manufacturers have higher current ratings for a buffer.
I would like to see if the voltage regulator can actually go as high as 1A. I guess the only way to find out is to use a device that draws around 1A.

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Change the value of R1 to a lower value and try again.

MP


Although the 1k resistor's value is high, it is a close match to the 10k pot and 9.6V output.
The 1k resistor wil have 1.25V across it and therefore 1,25mA of current. To provide the remaining 8.35V the pot's resistance must be 6.68k which is when the pot is set near halfway.
If you use 240 ohms  like LM117 regulators use instead of the 1k resistor, its current will be 5.2mA and with the 10k pot at halfway the voltage across it will try to be 26V which is much too high. Therefore the pot must be turned way down to about 1.6k for the output voltage to be 9.6V.

As for the low camera's current, I think its motor will draw much more current when it drives a tape that has high friction. Also a fairly high current will be used to charge a dead rechargable battery.

Steve, does the battery say, "850mA/hr"?
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No, it just sais 10v 850ma. I tried powering the camera again at 10v with the camcorders light turned on and it showed a "low battery" sign and turned off.
It does the same thing with the 9.6v input connection on the back. For some weird reason im not getting enough current when it comes out of the circuit.

Do you think it could be the size of the wire that runs through the circuit? What I mean is the wire on the capacitors and resistor are pretty thin, maybe they are causing resistance? For the resistor I got a 1/2 watt but the capacitors did not specify that so I got what it said on the web site. There wires look pretty thin compared to the incoming wires from the 12v 1.5a power supply.
I dont know much about electricity but could that be the problem?
I wish I was able to take a picture and show you how the components look on the circuit, il try to get my friends digital camera.

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Hi audioguru, could the camera use up more that its rated 850ma on the lable?
Il check the current with the light on but there is one problem, every time I turn the light on the camera shuts down. Do you recomend increasing the voltage to keep the camera on to take the measurment?

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Ok, finally things are starting to make sense. I measured the current with the light turned on and it went up to about 125ma from 87ma. So what I did was, before I made the voltage regulator I was using a plug pack labled 9vdc 1A. It would work but when I tried some features on the camera it would shut down with a low battery sign. So I connected the multimeter to check current with that power supply and it is also very low, actually its the same as with the voltage regulator. I guess the camera doesn't use a lot of current.

The weird thing is when I measure voltage with the 9v 1A plug pack it reads at about 12.2V.
Almost all my plug packs that I have measure a higher voltage than labled. Do you think that the camcorders lable 9.6v really needs more voltage?
At 9.6 volts it does not turn on, at 10v it turns on but turns off when I put the light on. At 12.2V with the 9v labled plug pack it stays on a a lot longer.
Do you think the real required voltage is something around 12.5 or a little more?

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Like I said: Change R1 to a lower value. You might also want to change the pot to a different value so that it is more useful, but it will get you by if you want to leave it there. It will just not be making adjustments near the center of it's rotation.
If this does not work, you probably do not have enough current available to the regulator. If all this does not get you there, then you have something wired wrong.

This is not rocket science.  ;)

MP

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Hi MP, id like to thank you and Audioguru for your help. I found the problem, it was me. I feel like an idiot. I never learned how to read a multimeter. I am getting over 1A of current from my voltage regulator, I was reading it wrong. First I was measuring the current wrong but Audioguru showed me how, and then I read the numbers wrong. The decimal point fooled me, I was measuring at 10A on the multimeter.
The video camera pulls around 680ma to 890ma without recording and with the light around 1.2A.

Thank you very much for your help. ;D
I love this forum :-*

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  • 1 month later...

Yes, gogo:
The use of diodes will give you protection. You will be able to short circuit the output of this power supply without causing the project to go up in smoke.
Sure, the regulator has some internal protection, but why stress it?
No one anticipates shorting their supply, but there are those times when it happens. A diode is cheap insurance for just a few cents.
I highly recommend adding them.

MP

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Yes, gogo:
The use of diodes will give you protection. You will be able to short circuit the output of this power supply without causing the project to go up in smoke.
Sure, the regulator has some internal protection, but why stress it?
No one anticipates shorting their supply, but there are those times when it happens. A diode is cheap insurance for just a few cents.
I highly recommend adding them.

MP

The factory shorts the output of every one they make, to make sure it limits its current and shuts-down when it gets too hot, without protection diodes.
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...actually, the factory does NOT test every chip. A select percentage of chips are tested for QA assurance. This is why there is a parts warranty.
No one tests every chip they manufacture. Once a set of specifications are setup and initial testing of a batch have provided statistics, a program is setup to determine how many of each batch will actually be tested. This saves time and manpower but provides a low percentage of drop out that is manageable.

MP

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indulis, you make a very good customer. Some testing is always done. All tests are rarely done.
audioguru: preconditioning is not the same thing. It is interesting that the guy who talks the most here on the forum about chips that do not meet specs is the same guy to say all regulators are 100% tested always. Manufacturing only tests all products for every function when there is a large quantity of returns. If the Company does not lose money on warranty, they do not waste manpower in unneeded testing. That is the reason they offer a warranty. Warranty is like an in-house insurance policy.

MP

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I would disagree, at the die level there is 100% static and some dynamic testing done because it cost too much to finish processing the wafer using a bad "chip". Way back, when I worked as a design engineer at LTX (www.ltx.com), a manufacturer of mainframe semiconductor test equipment, part of what I did was design prober/handler interfaces. The manufacturers want/need a way to decide if a part gets sold to the government or to Radio Shack, and the ONLY way to tell is by testing.

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