LM317T - where have I gone wrong?

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tim_ber

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I want to drop 9V to 3.8V to run a small camera.

Where have I gone wrong with my circuit?

I thought that whatever the input voltage, the output stayed constant if the resistors were correct (used an online calculator), yet I am not getting a constant output with variable input BUT most importantly i am not getting 3.8V out with 9V input.

Maybe my diagrams attached wiil help

Thanks in advance.
 
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tim_ber said:
I want to drop 9V to 3.8V to run a small camera.

Where have I gone wrong with my circuit?

I thought that whatever the input voltage, the output stayed constant if the resistors were correct (used an online calculator), yet I am not getting a constant output with variable input BUT most importantly i am not getting 3.8V out with 9V input.

Maybe my diagrams attached wiil help

Thanks in advance.
Click to expand...

I think you have the out and adjust wiring muddled up

See DATASHEET

Nigel Mercier will be along shortly to sort you out.
 
tim_ber said:
I want to drop 9V to 3.8V to run a small camera.

Where have I gone wrong with my circuit?

I thought that whatever the input voltage, the output stayed constant if the resistors were correct (used an online calculator), yet I am not getting a constant output with variable input BUT most importantly i am not getting 3.8V out with 9V input.

Maybe my diagrams attached wiil help

Thanks in advance.
Click to expand...


You have it wired wrong.

The input goes to input pin with a capacitor fitted to ground, this is missing.

The out put goes to the load you are supplying, again capacitor is missing.

There is a resistor that goes from output pin to the adjust pin, this sets current limit as well, then a second resistor from adjust terminal to ground.

Brian
 
Thanks.

One of the websites I used was this one:

http://www.reuk.co.uk/LM317-Voltage-Calculator.htm

hence I didn't use capacitors.

I did find wbesites that used capacitors, but as this website didn;t use them, neither did I. I obvioulsy don't know what they are used for and whether I need them or not.

Just going back to see where my wiring went wrong.
 
halcyon said:
You have it wired wrong.

The input goes to input pin with a capacitor fitted to ground, this is missing.

The out put goes to the load you are supplying, again capacitor is missing.

There is a resistor that goes from output pin to the adjust pin, this sets current limit as well, then a second resistor from adjust terminal to ground.

Brian
Click to expand...

Thanks for that.

Did I get my wiring correct except for the capacitors? If not, where did I go wrong?
Is this website incorrect:
http://www.reuk.co.uk/LM317-Voltage-Calculator.htm

Do I need the capacitors?
I've obviously done something wrong, so is it:
incorrectly wired,
incorrectly wired and lacking capacitors,
correctly wired except for capacitors,
or something else?
 
tim_ber said:
Do I need the capacitors?
I've obviously done something wrong, so is it:
incorrectly wired,?
Click to expand...

The adjust pin is wired to wrong point, you have the output going through the resistors, the output should go straight to the load.

You should have the resistors wired between output pin and ground.

The adjust pin is wired to the junction between the two resistors.

Yes you need the capacitors for a stable voltage.

Brian
 
Thanks.

Can you point out where my wiring has gone wrong?

Is the diagram on paper correct?

I have source of 9V red wire, positive to right leg (heat sink is away from me and I am looking at the black plastic side).

Negative of supply goes to left leg as per my diagram on the originally attached photo.

Thanks.
 
halcyon said:
The adjust pin is wired to wrong point, you have the output going through the resistors, the output should go straight to the load.

You should have the resistors wired between output pin and ground.

The adjust pin is wired to the junction between the two resistors.

Yes you need the capacitors for a stable voltage.

Brian
Click to expand...

Thank you, the above post wasn't there when I typed my question.

Back to the breadboard.
 
Offer to OP:

I've a few of those devices here. If you've broken it or even want a couple more, send me a pm with your address. Or are they 2p each anyway these days?
 
Thanks for the offer,

but thanks to the advise I now have the circuit working (I think I had a spell of circuit diagram dyslexia there) and lots of spare LM37s. In fact I have spare parts all over the place and keeping track of these tiny little things takes some organising.
 
The capacitors are normally used to damp down high frequency ripple on the output, something like about 1MHz at a few millivolts from memory when I measured it on a scope many years ago. This is of very low level and for most practical purposes can be ignored, however I always fit them anyway when using any type of regulator as it is good practice to do so, exact values as specified in the data sheet are not critical.
 
Dabhand said:
The capacitors are normally used to damp down high frequency ripple on the output, something like about 1MHz at a few millivolts from memory when I measured it on a scope many years ago. This is of very low level and for most practical purposes can be ignored, however I always fit them anyway when using any type of regulator as it is good practice to do so, exact values as specified in the data sheet are not critical.
Click to expand...
I agree. There's no point in adding even more mush to the radio spectrum on the boat, and certainly no point in adding mush unnecessarily onto the DC wiring.

I have a problem with RF interference on Marine HF from the multiple devices that generate MF and HF interference on our boat. Adding suppression to various areas is on my list of jobs to do. Adding the capacitors to the voltage regulator circuit is pence and well worth it.
 
Conachair said:
:cool::cool::cool:

How much does it loose on way through? Can you use it for regulated 12v/
Click to expand...

If you mean how efficient is it, then it will be more efficient than the LM317. I think the Ebay ad talks about 90% efficiency.

The one I posted was just an example. There are lots of simlar devices on Ebay, (search on "step down" for starters) mostly from China and Hong Kong and I've managed to find something suitable for all sorts of little projects. FWIW delivery is usually two or three weeks but, despite initial reservations about buying things from half way round the globe, I've never had a problem.

I'm not sure what you mean by "using it for regulated 12v". To source from 12v or to supply 12v? In any event the answer is probably yes to both but the one I posted will have a voltage drop (2volts IIRC) so, from a 12v battery it could supply up to 10volts. The maximum current will also limit it for some applications........but there are other types which could be used.

If you wanted to supply a 12volt regulated supply from a 12 volt battery (I don't know why though?) then you would need to use one of the step up units.

The arguable problem with the buck/switch mode devices is electrical interference but I've never had a problem.
 
VicS said:
Nigel Mercier will be along shortly to sort you out.
Click to expand...

A bit late now, sorry.

My first thought us that you should download the data sheet from the manufacturer of the IC. Not only will they tell you what capacitors you need (and you will probably need 3), but will also give you the correct pinout for their part.

The capacitors are not normally optional, if the device goes into oscillation it can destroy itself.
 
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