Welcome to our site!

Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.

  • Welcome to our site! Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.

LM317 mystery (at least to me hehe)

Status
Not open for further replies.

jjaweh

New Member
Here's the circuit that I tried assembling:

Regulated Power Supply, Variable

I removed the last node consisting of R3 P2 and the V. I guessed I could make the circuit work without them.

This is my first time using the LM317 IC so I really don't know how it works. After completing the circuit, I tested the output voltage several times and it worked fine. The output was 1.2-32v. I tested it on my 12v 500mA mini-drill. I set the voltage to 12v using my multimeter and connected the drill. It worked but I noticed the reading on my multimeter jumped from 12v to 16v. I quickly turned off the circuit, afraid that my drill would be toasted. After that, I again tested the circuit without any load. Mysteriously, the output changed its range from 1.2-32v to 16v-32v. Tested it several times. Same thing happens. Tried changing my pot. Same thing.

What could be the problem? Please help.

I'm kinda new here. Please forgive me if violated any rules.
 

Mr RB

Well-Known Member
It should feed a 12v 500mA mini-drill, those things use a little tiny toy motor. I use exactly one of those stuck in ta tiny drill press for drilling PCBs and control it with my bench variable power supply.

LM317 take a lot of punishment, they are overcurrent and overheat proof.

It's most likely he's got a wiring fault.
 

jjaweh

New Member
I don't think the IC blew up because of the load. That's only 500mA drill. The max output current of the circuit is about 1.5A. Shouldn't be a problem.

Yeah can also be wiring. But it defeats the idea considering it worked perfectly fine without the load.

I'm really confused here.
 

Boncuk

New Member
Did you consider any back EMF protection for the regulator?

Even small motors are likely to produce nasty back EMF pulses which no IC likes very much.

Boncuk
 

HATHA

New Member
can't that Diode D1 (1N4001 across LmM317) absorb back EMF which produce by the motor?
 

jjaweh

New Member
I don't know what EMF stands for but I'm guessing its the feedback current.

Yeah shouldn't that 1n4001 diode prevent that, like what HATHA said?
 

Boncuk

New Member
Hi both of you, HATHA and jjaweh,

D1 doesn't do anything preventing the output of the regulator from getting back EMF.

EMF = Electro Motoric Force is a voltage with reverse polarity and variable level (depending on the inductive load) which will hit the regulator's output when a motor is switched off.

It can be "killed" by connecting a fast recovery diode antiparallel with the motor.

Boncuk
 
Last edited:

Nigel Goodwin

Super Moderator
Most Helpful Member
EMF = Electro Motoric Force is a voltage with reverse polarity and variable level (depending on the inductive load) which will hit the regulator's output when a motor is switched off.

EMF is 'Electro Motive Force' - I suspect it's a lack of English as your first language?.

Also EMF isn't what you describe above, that's 'Back-EMF' - EMF is just normal voltage.

BTW - as I'm commenting on language, and you're from Germany (originally) - my daughter started Uni last Sunday (still freshers week), and the first person she met in the accomadation was a German called Philip. He appears to spend all his time wearing a leopard print dressing gown, is this normal behaviour in Germany? :D
 

colin55

Well-Known Member
Did you put a diode across the regulator, from output to input. Put a short-circuit across the pot and let me know the output voltage.
 
Last edited:

jjaweh

New Member
@Nigel Goodwin

How'd you know I'm from Germany? I'm not really haha. I'm from Philippines.

I'm just new to building electronic circuits. Too many things to learn. I'm glad I found this forum.

Anyways, so what should I do to prevent that back-emf?

@colin555

Yes I put a 1n4001 diode across the regulator.

Alright, I'll try that.

@Boncuk

Hello.

But the description says that diode is for blocking those feedbacks. If not, what is it for?
 

Nigel Goodwin

Super Moderator
Most Helpful Member
@Nigel Goodwin

How'd you know I'm from Germany? I'm not really haha. I'm from Philippines.

Not you, Boncuk :D

I'm just new to building electronic circuits. Too many things to learn. I'm glad I found this forum.

Anyways, so what should I do to prevent that back-emf?

@colin555

Yes I put a 1n4001 diode across the regulator.

Alright, I'll try that.

@Boncuk

Hello.

But the description says that diode is for blocking those feedbacks. If not, what is it for?

The diode across the regulator is to prevent a higher voltage at the output than the input, which would damage the IC.

To bypass the back EMF, connect a diode directly across the output of the supply - connected so it doesn't normally conduct.
 

HATHA

New Member
Not you, Boncuk :D



The diode across the regulator is to prevent a higher voltage at the output than the input, which would damage the IC.

To bypass the back EMF, connect a diode directly across the output of the supply - connected so it doesn't normally conduct.

is back emf sort of surge voltage? then what sort of abilities the bypass diode should have? can we use normal rectifier diodes for bypass back emf?
 

MrAl

Well-Known Member
Most Helpful Member
Hi there,


The 'back emf' of a motor has the same polarity as the applied voltage, so a reverse
diode on the output probably will have no effect unless something else is wrong too.

The only thing that might have happened is that the motor generated higher voltage
spikes as someone already mentioned and for this you would use a bigger
cap on the output. If the spike is big enough, the 1N4001 diode should shunt
it around the regulator which is good, but if it is too big it will also then over power
the input to the regulator. 2200uf seems good enough though so i dont see how
this could happen, but if the 2200uf is connected too far from the physical package
of the LM317 it could happen anyway. That means wire the 2200uf close to the
LM317 package, and test that diode to make sure it is not blown too.
Another cap, 0.1uf, is also a good idea right across the input of the LM317 and
maybe one across the output too.

Just to note, i have run motors using this very circuit (without 1N4001) and
have had no problems at all with output adjusted to maybe 5vdc.

There is another problem too though, even once the circuit is back up and running
normally. That is, when the output voltage is low enough and the current draw
from the output is high enough the LM317 will go into thermal cutback. A really
good heatsink is required to keep it running cool, and maybe a little fan too.
You can get little fans pretty cheap that run on 12v dc, at low current like 100ma,
so you could power that from the input dc by using another regulator.
 
Status
Not open for further replies.

Latest threads

EE World Online Articles

Loading
Top