LM317T Voltage Regulator Input Resistor Cremation.

[Darky8]

Re: LM317T Voltage Regulator Input Resistor Cremation.

So you test the input resistances of the working one in the circuit, but how about on the one that the resistor keep burning up?

The LM317 is the one that burns the resistor.

[budm]

Re: LM317T Voltage Regulator Input Resistor Cremation.

'I tested the LM377 (the working one) and got 1.39-1.44M/ohms stable at input pin.'

'The LM317 is the one that burns the resistor.'

But as you said. you test the resistance at the input of the WORKING LM377 one, so what is the resistance reading at the in put OF THE LM317 which is fed by the 10 Ohms resistor that burnt up?
Other words, why test the working one (LM377) instead of the one (LM317) that keep burning up the 10 Ohms resistor, just read the resistance reading on the output side of the 10 ohms resistor (against GND) that burnt up to see if it feeding very Low resistance which causes the 10 Ohms resistor to burn up.
I do not understand what you are trying to testing the working LM377 when the problem you are having is in the LM317 section that has input resistor burnung up.

[Darky8]

Re: LM317T Voltage Regulator Input Resistor Cremation.

I tested the LM317 input pin which started at a low reading, but grew rapidly until I removed my meter at around 22M.

Sorry, but I'm struggling to make this any clearer. I was attempting to highlight the different ways in which each regulator responded, the working one compared to the non-working one. (oh and by low, I meant comparatively, ~1.85M)

[budm]

Re: LM317T Voltage Regulator Input Resistor Cremation.

It was not clear to me because in your post 1 you indicated that you have 4 LM317 so I have no idea which one (the one that has burnt resistor at the input pin) of the four that you tested.
So the bottom line is that all 4 of the LM317 input pin do not show anything lower than 100 Ohms.

[Darky8]

Re: LM317T Voltage Regulator Input Resistor Cremation.

Problem now resolved, the capacitor between the adjust pin and GND was reverse polarity.

[eccerr0r]

Re: LM317T Voltage Regulator Input Resistor Cremation.

I was also wondering why all the caps looked like they were swapped along with many other components, Did someone do a re-cap on that board? Which is weird, those look like tantalums but I thought tantalums usually last forever "unless they don't." (then again, maybe they were aluminum to start with.)

[budm]

Re: LM317T Voltage Regulator Input Resistor Cremation.

https://www.badcaps.net/forum/attach...3&d=1476094786

So which one in the picture is installed backward? I wonder if there are more than one that are installed wrong when the caps were replaced.

[eccerr0r]

Re: LM317T Voltage Regulator Input Resistor Cremation.

I also wonder what the burn mechanism is, since a reverse capacitor tends to look like a short (as it will leak very badly), it will pull ADJ to REF. This would reduce the voltage of the output and that will tend to reduce current flow.

Maybe I'm thinking too much into this. Dunno.

[redwire]

Re: LM317T Voltage Regulator Input Resistor Cremation.

I've seen a common failure with +/- vreg IC's powering audio op-amps.

During power-up/down or an overload between rails, one Vreg loses and it's output gets forced to the opposite polarity. Then the tantalums short.
A good design has reverse diodes on the Vreg outputs. Example D4/D6 here
This DAC probably is missing them. Otherwise there's no way to troubleshoot what is loading down the rail without the caps or vreg failing.

If the relays are sealed types, I'd give the PCB a wash with spray flux remover. Or Q-tips and isopropyl alcohol and gentle patience.

R35 is chipped, check its value

[eccerr0r]

Re: LM317T Voltage Regulator Input Resistor Cremation.

A shorted cap on the output of the LM317 would indeed easily explain the frying resistor however. Whether it was installed backwards or a transient causes it to fail, either way could explain it. The ADJ one I don't get.

[budm]

Re: LM317T Voltage Regulator Input Resistor Cremation.

A shorted cap on the output of the LM317 would indeed easily explain the frying resistor however. Whether it was installed backwards or a transient causes it to fail, either way could explain it. The ADJ one I don't get.

Agree about not making sense due to the reverse polarity of the ADJ pin to cause excessive current draw through the input of the Reg and caused the 10 Ohms resistor to burn up. I need to setup an experiment to find out for myself. I think it may be the cap on the input pin that was installed wrong.
I am looking at this picture (before the cause of the problem was found): https://www.badcaps.net/forum/attach...3&d=1476094786 to see which cap was installed wrong, I hope the OP will come back and tell us.

[waqqas.qayyum1]

Re: LM317T Voltage Regulator Input Resistor Cremation.

Hi Guys,

I am going to use lm317 sot223 for one of my project. I need to get information regarding the pins. One is Vin, 2nd Vout, 3rd Vadj and 4th one i need to know that it would be connected with ground or what?

[waqqas.qayyum1]

Re: LM317T Voltage Regulator Input Resistor Cremation.

Hi Guys,

I am going to use lm317 sot223 for one of my project. I need to get information regarding the pins. One is Vin, 2nd Vout, 3rd Vadj and 4th one i need to know that it would be connected with ground or what?

[waqqas.qayyum1]

Re: LM317T Voltage Regulator Input Resistor Cremation.

Need information regarding pin configuration of 317MB SOT-223. Please help out me to complete my project. I am confused regarding the top pin. Where to connect that one???

[waqqas.qayyum1]

Re: LM317T Voltage Regulator Input Resistor Cremation.

please guide

[cpt.charlie]

Re: LM317T Voltage Regulator Input Resistor Cremation.

It's the same as pin 2. if you use pin 2 as output and don't want to use the tab at all, just connect the tab to a proper sized section of the PCB for cooling reasons and keep the area electrically isolated.

BTW don't hijack the OP's thread.

[waqqas.qayyum1]

Re: LM317T Voltage Regulator Input Resistor Cremation.

Sir thanks a lot for your reply so can I assemble the below circuit on PCB? It will work??? Sir one thing i m still confused that there is written OUTPUT on the tab so pin 2 and tab will not be connected??? or if we dont connect the tab and connect it to pcb for cooling purposes so it will work?
Sir what is difference & purpose of these two types of tab connections?

[budm]

Re: LM317T Voltage Regulator Input Resistor Cremation.

The big tab is electrically internally connected to the center pin, the big tab is solder so the copper pad for cooing purpose and since it is connected to the center pin then you can connect the load to the tab or the center pin.
Hook up the circuit and find out, that is how you learn, that is what the experiment is for.

[waqqas.qayyum1]

Re: LM317T Voltage Regulator Input Resistor Cremation.

Thank you sooooooooo much sir. Bundle of thanks.