good morning to all,

I am new to this forum, studying third year college.

for past 2 months i was involved in studying power supply design and now I started building Lab power supply with LM317 and 337.
Positive supply was adjustable from 1.2 to 28V, but negative was starting from -8 to -24v.

Initially when i soldered and connected the circuit, my adjust pin of 337 was grounded and IC was heated, i have corrected that now and output was adjustable from -8 to -24V.

My question,

1) I have 7805 IC for powering peripheral circuit, after capacitive filtering the voltage was 35V.the output of 7805 is 10V instead of 5V, why?
2) why my negative supply was from -8 to 24V?
3) how to check LM337IC regulator?

Thanks you all

 

You may have cooked the LM337.
Connect the LM337 ADJ to GND and the -ve out should go to -1.25V.
If not, try disconnecting the 2N3055 Collectors and try again.
If still faulty, replace the LM337.
Also, the 7805 needs bypass caps on the input and output. Maybe add an LED with a 220R on the 7805 output for a small load too. That will give you an ON light and dump any small leakage through the 7805.

 

Also check the maximum input voltage rating of the version of 7805 you are using.

 

Also check the maximum input voltage rating of the version of 7805 you are using.

35V sir

 

after capacitive filtering the voltage was 35V

NEVER run components at their maximum rated values (35V in the case of the 7805) if you want them to have a long and happy life.

 

good morning to all,

I am new to this forum, studying third year college.

for past 2 months i was involved in studying power supply design and now I started building Lab power supply with LM317 and 337.
Positive supply was adjustable from 1.2 to 28V, but negative was starting from -8 to -24v.

Initially when i soldered and connected the circuit, my adjust pin of 337 was grounded and IC was heated, i have corrected that now and output was adjustable from -8 to -24V.

My question,

1) I have 7805 IC for powering peripheral circuit, after capacitive filtering the voltage was 35V.the output of 7805 is 10V instead of 5V, why?
2) why my negative supply was from -8 to 24V?
3) how to check LM337IC regulator?

Thanks you all

Hello there,

Looks like the classic floating ground syndrome. You do not have an actual ground the circuit depends on a perfect impedance balance for the positive and negative regulator sections. You need to use plus and minus input source voltages that already have a common ground. You can not develop the ground with the circuit, at least not like that.

To test the LM317 or the LM337 simply wire them up temporarily as the data sheet shows for a simple regulator. Make sure you get the pins right or they burn out.

Here's a cleaned up version of your schematic...
The other drawing shows the correct power input connections.

 

CERTAINLY a 7805 is able to oscillate and deliver a higher than believable output voltage, if those 0.1 bypass capacitors are not in place correctly. As for the regulator not providing the correct adjustable negative voltage, the previous comments are valid, and also it may be oscillating, and that will lead to incorrect performance.

 

hello,

as @MrAl said, you need a ground connection.
how are vin+ and vin- generated?
that is not given in your schematics.

bertus

 

LM317 or the LM337

Do you have the pin out right on the LM337? It is not the same as the LM317.

 

Your output voltage isn't the same because the negative supply on your schematic is missing the pre-regulator (negative version of a 7805 for example). I'm also not sure what the parallel transistors are doing.

Try this simplified circuit. A transformer and full bridge rectifier is preferred over trying to drop excess DC voltage (such as what your 7805 is appearing to do even though it's output is not connected to anything).

Keep in mind LM317/337 requires headroom of about 3V and operates in the safe region with a 40V differential voltage.

For 1.25 Vout, Vin should not exceed:

40V (differential) - 3V (headroom) - 1.25V (Vout) = 36V • 75% (derated) = 27V.

 

Your output voltage isn't the same because the negative supply on your schematic is missing the pre-regulator (negative version of a 7805 for example). I'm also not sure what the parallel transistors are doing.

The 7805 is not used as a pre-regulator. It is just another fixed +5V supply. Nothing to do with the LM3xx regs.
The extra transistors are current boosters. Once the current via the regs gets high enough to forward bias the transistors via the voltage drop across the 22R resistors, the transistors turn on and supply the extra output current. This is a common way to increase the regulators current capacity.

 

3) how to check LM337IC regulator?

Use the regulator without any of the external pass transistors. Use the example circuit from the datasheet.

Make sure you're using the correct pinout. LM317 and LM337 have the input and output terminals swapped.

 

As has been said, your input voltages must have a common ground, eg: typically one of the following...

 

The 7805 is not used as a pre-regulator. It is just another fixed +5V supply. Nothing to do with the LM3xx regs.
The extra transistors are current boosters. Once the current via the regs gets high enough to forward bias the transistors via the voltage drop across the 22R resistors, the transistors turn on and supply the extra output current. This is a common way to increase the regulators current capacity.

I reviewed his description and I still struggle to make sense of his problem. He mentions the 7805 in #1 as "powering peripheral circuit", whatever that means. This doesn't seem to relate to #2 either. As for the pass transistors, I haven't seen a paralleled arrangement on the input.

I've subjected my LM317s to extended periods of abuse and they operate good as new. Not sure what is going on here.

 

Here is an nice version I found on this site a few years ago. The series diodes and resistor on each input allow the outputs to reach 0V. It was great for helping me learn about the direction of currents and symmetrical circuits.

 

Do you have the pin out right on the LM337? It is not the same as the LM317.

Hi,

What do you mean, I never illustrated the pinout of the LM337 nor the LM317, did I?
All I did was clean up the schematic I did not change any connection except in the second drawing which shows the proper way to connect the input source voltages and that does not alter any pinout.
Maybe your reply was for the original thread starter? That's what I was thinking too because if the pins are swapped it can burn out the part which means it would have to be replaced by a new part.

 

Here is an nice version I found on this site a few years ago. The series diodes and resistor on each input allow the outputs to reach 0V. It was great for helping me learn about the direction of currents and symmetrical circuits.

View attachment 317578

Much cleaner drawing too to make things more clear. Now just add the extra transistors to make it complete.

I've used diodes on single supply circuits like this too in order to get a 0v output. I think they reduce the thermal stability of the voltage regulation, but not by much. We could look into that.

 

Much cleaner drawing too to make things more clear. Now just add the extra transistors to make it complete.

I've used diodes on single supply circuits like this too in order to get a 0v output. I think they reduce the thermal stability of the voltage regulation, but not by much. We could look into that.

I have replaced a new lm337, the circuit was working fine.

I have connected a load of 12v60 watt automobile lamp, lamp was fine glowing for less than 10 second. After that ic output came to-32V and 120ohm 1 watt was releasing smoke.

I could not predict what is happening, may be Lm337 could be counterfeit.
Since lm317 works fine with same lamp.

 

Does the LM337 have an adequate heat sink properly installed. You are dropping a lot of voltage across the device and every bit of that drop becomes heat. 60 watts at 12 volts is 5 amps and if the supply voltage is 30 volts then you have 5A x 18 volts=90 watts of heat and so the heat sink must remove that much heat which I very much doubt it could do. In addition, running 5 amps thru a one amp regulator will cook it in just a few seconds.

 

Does the LM337 have an adequate heat sink properly installed. You are dropping a lot of voltage across the device and every bit of that drop becomes heat. 60 watts at 12 volts is 5 amps and if the supply voltage is 30 volts then you have 5A x 18 volts=90 watts of heat and so the heat sink must remove that much heat which I very much doubt it could do. In addition, running 5 amps thru a one amp regulator will cook it in just a few seconds.

My lm337 may be loaded by 100ma by current boosting transistors by their base terminal typically.

These current boosting transistors will bare the heat and these were fine after that.