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Weird LM317 question

Devilz Rej3ct

Devilz Rej3ct

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I have a really weird thing happening with my Lm317 driver. I built it for my A140 445 diode. When I connect my DMM in Amp mode to the output side I get 0.99 amps which is what I was aiming for in terms of current. But when I connect the DMM in series with the diode, I am reading only 500 mA. Why is this? So how much current is my diode really getting? I should mention I am using a 9 volt battery as the input source. Thanks.
 





9V batteries aren't very good at supplying much current. What's happening is that your battery can only provide ~ 7.5 volts @ 500mA. Under those conditions the 317 is operating wide open but there isn't enough voltage to cram an amp through the diode. Use a different power source. I built a 405 using 3 CR123s in series for my 9V source. These batteries won't last long if you're pulling an amp from them but it'll be vastly better than the 9V battery.
 
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Yes thank you! That was it I connected a 12 volt PSU supply and now my diode is getting what it needs lol.:san:
 
DO NOT connect the DMM in series with the diode after the driver's capacitor (if you have one)!

Diode's connection must be permanent, because a split second of non-secure contact, and capacitor charges up with full driver's output (which changed in voltage due to change in load's resistance), and when reconnected, destroys your diode!

Always use a test load to measure the driver's output.

And yeah those small 9V bricks absolutely suck. I'm amazed you were able to get 500mA out of it.
 
Eudaimonium said:
DO NOT connect the DMM in series with the diode after the driver's capacitor (if you have one)!

Diode's connection must be permanent, because a split second of non-secure contact, and capacitor charges up with full driver's output (which changed in voltage due to change in load's resistance), and when reconnected, destroys your diode!

Always use a test load to measure the driver's output.

And yeah those small 9V bricks absolutely suck. I'm amazed you were able to get 500mA out of it.
Click to expand...

Great advice, many diodes have died this way:angel:
 
Well I appreciate that advice. However I have always connected it this way. Maybe I am just lucky or dumb.:crackup: But nonetheless you are right. Now I think about it no load would charge the cap up beyond what we want for the diodes. +REP to you good sir!
 
Devilz Rej3ct said:
Well I appreciate that advice. However I have always connected it this way. Maybe I am just lucky or dumb.:crackup: But nonetheless you are right. Now I think about it no load would charge the cap up beyond what we want for the diodes. +REP to you good sir!
Click to expand...

Possibly lucky beyond compare :D

Like Ehgemus said, many diodes died this way.

That would include mine.
 
ionlaser555 said:
As its a Linear regulator , just measure the current on the input side as Input current = output current .
Click to expand...

That is also true, and good advice. Using DMM in Amp meter mode to connect the driver to a battery is a good way to measure current with accurate results.
 
Devilz Rej3ct said:
The input side of the LM317? Series or parallel?
Click to expand...

DMM in current mode in series with the positive line on the input side to the LM317 CC circuit .

All you are doing is placing a low value resistor ( current shunt inside DMM ) in series with the regulator input and as current is the same thought out the whole LM317 CC circuit it will show diode current , while keeping the diode safe

Eudaimonium said:
That is also true, and good advice. Using DMM in Amp meter mode to connect the driver to a battery is a good way to measure current with accurate results.
Click to expand...

Thanks , Ive done it a few times when using LiPo's
 
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