LM317 Current Limiting problem with switching power supply

[Lyncos]

Hi,
I have a really simple circuit. the l317 Current limiting with 1 L317 and 1 resistor. It's the basic circuit you can find everywhere on the net.

The LM317 Current limiting circuit is set to about 1 amp with 1ohm resistor (I use 2x 0.5 ohm 2watts resistors in serie).

When I plug it to a 12volt wall-wart power supply it works well .. when I short the 2 leads I get something around 1amp...

But when I plug this to my 24 Astrodyne Power supply (SP-320-24) I get nothing more than 0.010amp ... even If I short the leads I get no more .. The power supply is rated at 13A and It gives 13A trust me.

Is this because the astrodyne is a switching power supply (Fixed switching frequency at 100KHz)


If someone can help me with this I will appreciate .. I hope I'm clear enough.

 

[3v0]

Do you have a scope to look at the power from the switcher.

 

[Lyncos]

Do you have a scope to look at the power from the switcher.

No, I don't have a Scope... But I have a multimeter with Hz setting on it... I don't know what it worth. I tried it on my Astrodyne power (unloaded) supply and it says 0Hz

 

[3v0]

Check it with the PS connected to the LM317.

 

[Lyncos]

Check it with the PS connected to the LM317.

- With no load at PS output: 0Hz
- With only the lm317 at PS output: 0Hz
- with only the lm317 at lm317 output 0
- With only the lm317, lm317 output shorted to ground at the PS output: 0hz

Anything else I can check ?



EDIT: Over the resistor ... I have Variable frequency..... Its unstable it can go from 0 to Overload (my meter max is in KHz)

 

[Lyncos]

With a 13 amp 24Volt power supply.. I can light a 3mm cheap led with no resistor with my lm317 set to 1amp.... the led dosent explode ...



EDIT: If I unplug the power supply and plug it back again... it blow the led It dosent even light the led and the led is blowed right away... it seems at the startup it can deliver full power....

If I use the power supply on a 24 volt motor it draws 10 amps and the motor run just fine ( No LM317)

 

[Mr RB]

Your SMPS is shutting down, probably due to driving a high impedance load like a smart regulator (LM317) returning some weird signal back to the SMPS.

Try putting a big cap directly on the output of the SMPS, like 2200uF, this is before the LM317 current regulator. It's also good to put a small cap on the LM317 output, maybe 10uF.

 

[Lyncos]

I only have a 25v 2200uF, I'm not sure if 25v is enough for testing I don't want it to blow in my face. I will go buy a 50 volt one and see what happens later today

 

[Lyncos]

Even with both Caps 2200uF at power supply output and 10uF at lm317 output .. I get the same 0.011A from the lm317 ...

 

[carmusic]

what voltage do you have at power supply??

 

[Lyncos]

With the lm317 shorted to ground (0.011A) I get 24Volts
Without any load I get 24 volts too

 

[carmusic]

then the power supply is ok, do you have a big heatsink on the lm317? it will need to dissipate 24W at 1A current. 24W in to-220 will overheat it in few milliseconds and the chip will shutdown.

 

[Lyncos]

Yes I have a good heatsink on it ... it become hot with my 12volt power supply.. but it stay cool with the 24volt one

 

[Lyncos]

Maybe I hit the thermal shutdown right away ? I will try with let say 500mA and see what happens

 

[Lyncos]

I tried higher and lower values... same results

 

[carmusic]

well maybe the lm317 is defective it should work ok at 24v

 

[Lyncos]

I tried 3 different lm317 ... and with my 12Volt power supply it works well...

 

[ronv]

What package do you have for the regulator? It also has SOA protection and the current goes down as the voltage goes up. If it is To220 it should be marginally ok. Make the resistor 1.5 ohms and see what happens.

 

[Lyncos]

Yes i'm using the To220, I Tried with many different resistors values ... even to drive a led a 20mA and I still get 10mA or less ... I really don't know what hapenning.. when I use only a resistor to limit the current without the lm317 it works well I can get up to 13 Amp from my power sup.... It seems that it dosen't like the lm317 circuit ... I don't know why... I also tried with couple of big cap .. still no luck. It's crap because I buy this 24volt power supply for my stepper motors and I pay near 70$-80$ and I can't make it work... a stupid 12volt wall adapter do better than that..

 

[KeepItSimpleStupid]

Are you using any bypass caps on the LM317.

For instance a 47 uf cap designed for switching supplies in parallel with a 0.1 uf ceramic cap. Put these close to the LM317 input.