Unknown Voltage drop on a simple circuit with LM317T

[Corky]

Hello guys,

I'm building a simple voltage regulating circuit with LM317T and two reistors (330r and 68r), im using a breadboard

when i build the circuit i have a 1.5V output (middle Pin) from it, which is what i want but when i put a load from the output to 0v the load only has a Vd of 0.6v

it makes me think there is a resistance im missing somewhere between the output and 0v but the load is the only resistance i can see, does the breadboard have a high resistance?

i have a load between the middle pin and the negative channel on the left (the same one as the resistor)

 

[JimB]

What is your input voltage?
In the LM317 datasheet is graph showing the minimum input voltage for various load currents, you will need something like 1.5 to 2.5 volts more on the input than the output.

JimB

 

[Corky]

the input is 6v, so should be plenty i think, ill have a look at the datasheet again see if im missing something

Cheers

 

[Cicero]

What kind of current does your load pull? And does your input voltage stay at 6V when loaded?

 

[Corky]

im not sure, but i think this is where the problem lies, it has a very small resistance so the current is going to be way to high for the LM, ill have a mess when im home and let you know how i get on

Cheers

 

[Cicero]

im not sure, but i think this is where the problem lies, it has a very small resistance so the current is going to be way to high for the LM, ill have a mess when im home and let you know how i get on

Cheers

Probably also temperature issues. Keep in mind, you can't draw anywhere near the max current without a heatsink. And if you try, it has built in thermal shutdown so you gotta do your calcs.

Lets just say you're trying to pull 1A. Means the power dissipation in the regulator is (6-1.5)*1=4.5W
Thermal resistance junc-ambient is 80degC/W. So that means at 1A, you're heating to 360degC. Which will obviously trigger the thermal shutdown.

The current without a heatsink you can actually draw is more around 260mA, or 6Ω. That'll keep your junction temp at max 100degC (which is still a lot!).
Calcs:
P=(6-1.5)*260mA)=1.17W
JuncTemp=1.17*80=93.6degC

 

[Corky]

It is attached to a huge piece of Alu so i dont think that is a problem atm i think the current is to blame,

the resistance is around 0.15 on the glow plug so the current will be alot, i didnt even thin about this until i posted this then i realised

cheers

 

[Corky]

im trying to drop 6v down to 1.5 for a load of about .15 ohms but i cant think of a way this is possible now im stumped again

 

[JimB]

1.5Volt
0.15Ohm
= 10 Amp (a bit too much for a 317)

JimB

 

[Corky]

ye :/ didnt even think of this, im thinking of using a power resistor instead between the 6v and the load but now im thinking is the battery going to last with 10A being drawn :/

the load will only be used for about 30 seconds but its running from 4 AA alkaline Bats

 

[ClydeCrashKop]

How about running your 4 AA batteries in parallel? Still may not last long.

 

[Cicero]

ye :/ didnt even think of this, im thinking of using a power resistor instead between the 6v and the load but now im thinking is the battery going to last with 10A being drawn :/

the load will only be used for about 30 seconds but its running from 4 AA alkaline Bats

Drop the current using a resistor, but drop it to a current which you know the lamp work at. Its that simple.

 

[Corky]

hi,

my problem is i need my circuit to run on 6v but one branch of it can only work at 1.5v, the load in this branch has a very low resistance so the current will be very high about 10A i was looking for a way to drop the 6v to 1.5v

i cant add aother resistor as this would drop some of the voltage and my load wouldnt have the 1.5v it needs,

 

[Cicero]

hi,

my problem is i need my circuit to run on 6v but one branch of it can only work at 1.5v, the load in this branch has a very low resistance so the current will be very high about 10A i was looking for a way to drop the 6v to 1.5v

i cant add aother resistor as this would drop some of the voltage and my load wouldnt have the 1.5v it needs,

No, hold on, I'm not following.

You have a glow plug as part of the load you need to power, among other things. Does that NEED 10A? How did you find that out?

Also, is the lamp in series with the load you still need to give 1.5V? Why would a resistor drop the voltage to that?

 

[Corky]

I think were getting tangled in ideas here haha,

i have a 6v power supply on a circuit, in a different circuit i have a glow plug rated at 1.5v, the glow plug has a resitance of 0.15ohms, i want to use the 6v power supply to power my glow plug, how do i get to the needed 1.5v from the 6v source. if i use a resistor to drop 4.5v in series with my glow plug then the total resistance is 0.6 ohms, giving me a current of about 10Amps

does this help?

 

[Cicero]

I think were getting tangled in ideas here haha,

i have a 6v power supply on a circuit, in a different circuit i have a glow plug rated at 1.5v, the glow plug has a resitance of 0.15ohms, i want to use the 6v power supply to power my glow plug, how do i get to the needed 1.5v from the 6v source. if i use a resistor to drop 4.5v in series with my glow plug then the total resistance is 0.6 ohms, giving me a current of about 10Amps

does this help?

Yeah, I'm also being a quite naiive about what a glowplug actually is, haha. But man, 10A, are you sure thats what it needs? I know its rated at 1.5V and a resistance of 0.15, but are you sure it wont work at a lower voltage/current? 10A is obscene.

 

[Corky]

i did think this myself but after doing a bit of research apparently thats pretty normal, a glow plug is a a coil of wire in an engine to warm it up before the 'burning' of the fuel so it does get pretty hot, i know it needs 1.5v as the manual states it and i measured the resistance myself so i assume it needs the 10A, after looking online again i saw some glow plugs need up to 50 amps :O pretty unbelieveable really in a car (side tracking here) haha

 

[Nigel Goodwin]

i have a 6v power supply on a circuit, in a different circuit i have a glow plug rated at 1.5v, the glow plug has a resitance of 0.15ohms, i want to use the 6v power supply to power my glow plug, how do i get to the needed 1.5v from the 6v source. if i use a resistor to drop 4.5v in series with my glow plug then the total resistance is 0.6 ohms, giving me a current of about 10Amps

That's because the glowplug takes 10 amps (according to it's voltage and resistance ratings) , use the correct 1.5V battery - or make toast if you're adding a resistor to 6V.

 

[Corky]

can i not use a power resistor of about 50w, i could go out and buy a 1.5v battery to do this seperately or even a starter but i wanted to try a few things, learn a few things aswell, if its impossible ill admit defeat but i would of thaught there was a way somehow

 

[Nigel Goodwin]

ye :/ didnt even think of this, im thinking of using a power resistor instead between the 6v and the load but now im thinking is the battery going to last with 10A being drawn :/

the load will only be used for about 30 seconds but its running from 4 AA alkaline Bats

You can't get 10A from an AA anyway - so no hope of that.

As already suggested, place the four in parallel - or better still use the correct type of battery.