LM317 not getting results I expected

Thread Starter

mopar

Joined Oct 21, 2015
16
If you're familiar with radio controlled vehicles it's the receiver that I'm getting power from which receives 6v from the electronic speed controller. The device I'm powering is a transponder (we call them houses because they were always supplied by the track you are racing at) used to count the vehicles laps (most forms of motorsports racing use transponders from the same company). When I had everything on my breadboard I was using 4AA batteries as my 6v power source. Once I verified the transponder was emitting a signal I soldered the regulator, resistors and receiver plug to the transponder itself. Then when I plugged the unit into the receiver and powered the receiver I got nothing. The company that made these battery powered units also make them in a smaller package (we call them personals) that is powered from the receiver only. I do not have load specs as the company will not release the info on these units (they haven't made house transponders for 5yrs and offer no support for them, they won't even replace the batteries in them any more, they just tell you to trade them in towards new personals). The current load is very small due to the fact that the original batteries in these were 80maH coin cell NiMH batteries and they would last for 12-16 hours on a charge which gives you roughly 7maH load at 12 hrs which isn't enough for the LM317 hence the reason why I had to use lower value resistors to create the load required. Like I said on my breadboard I had 3.15 on Vout and it worked perfectly.

Thanks guys.


What is the source of the 6V? If it is another battery then the LM317 is the wrong way to go. Very inefficient regulator.
 

spinnaker

Joined Oct 29, 2009
7,837
If you are using 4AA batteries then the LM317 is the wrong way to go. First the LM317 is a linear regulator. You drop that voltage through heat. Heat means a waste of energy. You might as well just use a 5V zener or drop the one volt with a couple of diodes. Second the LM317 is going to have a voltage drop. I am not sure if a 5V linear regulator would work with only a 6V supply.

The right way to do this is use a buck regulator. You can hack one from those USB chargers from a car. They typically use a 34068 which is an older regulator and not the most efficient out there but very obtainable and well supported. And still more efficient than the LM317. There are also buck regulators on eBay. Again I am not sure if it will work with a 6V input. I have one here I designed. I will hook it up later to 6V and see what I get.
 

spinnaker

Joined Oct 29, 2009
7,837
Ah you need 3V out?? Thought you needed 5v My 34068 works at 7v in with 5V out, so I am sure it would work 6V to 3V. I would need to change out the sensor resistors to get 3v out.

If you want 3v, why not get a 2 cell battery pack?
 

Thread Starter

mopar

Joined Oct 21, 2015
16
These transponders are for general use, so they have to be able to work in everybodies vehicles, and some vehicles don't have room for extra batteries, hence the reason why I'm trying to power it from a device that everybody has in the vehicles. Plus 2AA batteries would be a lot of weight to adjust for. The AA's are only used to give me 6v (same voltage provided by the receiver) for testing on my breadboard.

I will look into using a 34068.


Ah you need 3V out?? Thought you needed 5v My 34068 works at 7v in with 5V out, so I am sure it would work 6V to 3V. I would need to change out the sensor resistors to get 3v out.

If you want 3v, why not get a 2 cell battery pack?
 

Thread Starter

mopar

Joined Oct 21, 2015
16
After some tinkering last night I was able to get everything to work (not sure what it was as I just un-soldered everything and then re-soldered). I then changed R2 resistor to a 220 to get me a starting voltage of 3.5 (need at least 3v to bring the device out of sleep mode and with the 180 I was barely getting that), and the best part is now when everything is 1st powered on there is 3.5v which is more than enough to wake the transponder, and when the load of the transponder is applied to the circuit it drops the voltage to 2.45. It's higher than a 1cell NiMH battery fully charged, but I think it's well within what the device can handle (little extra voltage will increase signal strength as well).

This is a prototype, but I have several to do so I have plenty to experiment with.

More efficient, and easier to implement ideas are welcome.

I'll look into a buck regulator as well spinnaker, if you could let me know if your buck works to bring 6v down to 3v and what you needed to get it to work I'll give that a try as well. It just has to be small as the board is only 3/4" x 3/4" and I'd like to keep all the components within that area.

Thanks again.
 

MikeML

Joined Oct 2, 2009
5,444
...I then changed R2 resistor to a 220 to get me a starting voltage of 3.5 (need at least 3v to bring the device out of sleep mode and with the 180 I was barely getting that), and the best part is now when everything is 1st powered on there is 3.5v which is more than enough to wake the transponder, and when the load of the transponder is applied to the circuit it drops the voltage to 2.45. ....
You obviously did not read or heed the advice I gave you in post #20!

I gave you the reason why the output of the LM317 drops when you connect a load to it. If you had used the correct resistors, that doesn't happen.
 

Thread Starter

mopar

Joined Oct 21, 2015
16
Mike, I did read your reply and that's why I changed the resistors. If you noticed in the 2nd line of your quote I said "and the best part is". Now if you look on the 1st page half way down you'll see where I was looking for a way to give the transponder a momentary burst of 3v (to wake the transponder up), and then have it run off a voltage closer to the 1.5v it usually runs on. One guy mentioned a momentary contact switch, and another mentioned using a transistor. Changing the R2 resistor did just what I needed, without the load of the transponder I had 3.5v and as soon as the transponder receives the 3.5v it wakes up and starts functioning and creates enough of a load to drop the volts down to 2.5 which I'm comfortable running these at.

Atferrari, my current version is I have the Vout leg of the LM317 soldered to the positive side of the battery connector in the transponder. I have a 120ohm resistor for R1 soldered directly to the legs of the LM317. I have a 220ohm resistor for R2 soldered from the adj leg to the negative side of the original battery connector. I have 1 wire of a harness soldered to the negative pin of transponder and the other wire of the harness soldered to the Vin leg of the LM317 that will plug into my receiver to get power from. Everything works when powered off 4AA's. I didn't have a vehicle with a receiver in it last night to actually try it how it will be used, but I'm confident it will work just the same.

I appreciate all the help everyone has given me! I don't do a lot of this kind of electrical work so I'm learning as I go and research. I'm just getting into using an Arduino UNO to make simple project so this will help me there as well.
 

atferrari

Joined Jan 6, 2004
3,489
Atferrari, my current version is I have the Vout leg of the LM317 soldered to the positive side of the battery connector in the transponder. I have a 120ohm resistor for R1 soldered directly to the legs of the LM317. I have a 220ohm resistor for R2 soldered from the adj leg to the negative side of the original battery connector. I have 1 wire of a harness soldered to the negative pin of transponder and the other wire of the harness soldered to the Vin leg of the LM317 that will plug into my receiver to get power from. Everything works when powered off 4AA's. I didn't have a vehicle with a receiver in it last night to actually try it how it will be used, but I'm confident it will work just the same.
Why not a simple schematic instead of the so many words as above? Believe me, it is better for anyone discussing the same thing.
 

Dodgydave

Joined Jun 22, 2012
8,477
Draw it out on paper with a pen or pencil, take a photo with your phone or camera, upload it.

A picture paints a thousand words!
 

Thread Starter

mopar

Joined Oct 21, 2015
16
Hopefully this explains what I'm doing better.

All joints are soldered. Right now without the transponder I show 3.5v out, when the transponder wakes up the voltage drops to a reasonable 2.45. I think will work, but if someone has a better or more efficient method of accomplishing the same thing I'm all ears.

The power source usually has 3A max. available to it, but every setup is slightly different.
 

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spinnaker

Joined Oct 29, 2009
7,837
I fail to see why all of this energy is being wasted on the LM317 when it is the wrong way to go. A linear regulator is simply not the best solution for this application.

OP,

Take a look at one of these

http://www.ebay.com/itm/1pc-Buck-Convert-Step-Down-adjustable-DC-regulator-LM2596S-40V-In-3A-US-Seller-/261372754088?hash=item3cdb07b8a8:m:m6d7bbXlK87smCwKgY3Pq7A

This one is adjustable. You might be able to find one with set 3V out but all I could find is 3,3V. But keep searching maybe one is out there. If not then you have the adjustable solution.

This will be your easiest and most efficient solution to this problem.
 

Thread Starter

mopar

Joined Oct 21, 2015
16
Dave, I didn't have a lot of different resistors available when I did this the other night. I'm heading to Radio Shack after work to pick up a variety pack to give me more to work with to reduce the adjust current. The transponders only get used for roughly 20min a couple days a week and we run 5000mAH packs and charge between every use so it's not much in the scheme of things, but I do want to reduce waste and heat and to make things last longer.
 

Thread Starter

mopar

Joined Oct 21, 2015
16
spin, I agree the LM317 isn't the most efficient, and would like to have a step down buck (if I can find one smaller than the ones in your link), but I'm working with limited supplies/resources right now just to see if I can make this work. Once I'm convinced that these units can run on 3v or maybe a little higher without damage I'm going to investigate better methods like the drop down buck.

I do appreciate the help, and I'm writing things down for future reference as well.

I fail to see why all of this energy is being wasted on the LM317 when it is the wrong way to go. A linear regulator is simply not the best solution for this application.

OP,

Take a look at one of these

http://www.ebay.com/itm/1pc-Buck-Convert-Step-Down-adjustable-DC-regulator-LM2596S-40V-In-3A-US-Seller-/261372754088?hash=item3cdb07b8a8:m:m6d7bbXlK87smCwKgY3Pq7A

This one is adjustable. You might be able to find one with set 3V out but all I could find is 3,3V. But keep searching maybe one is out there. If not then you have the adjustable solution.

This will be your easiest and most efficient solution to this problem.
 
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