LM 317 POWER Supply dropout voltage

Hello all.

I built a psu with LM317 and 3 BD902 bypass transistors. As you can see in the circuit diagram. I tested with a jigsaw from proxxon which normaly runs at 12V max 2amps. The problem is that when i start the tool the output voltage drops from 12v to 10v and from 15V to 11V.
Can you tell me please what is happening?

Thanks in advance!

A motor that runs at 2A might start with 20A. Then the value of C1 is much too low causing the input voltage of the LM317 circuit to be too low.

Check input voltage dropdown. Power source have enough power to handle this, is it battery?


Best regards,
Peter

motors can have an in-rush current of up to [or exceeding] 10 times their operating current, so maybe your transformer can not handle the current

what's the transformer or input rated at?

i don't know why you have bypass transistors. Why aren't they driven by the lm317 ??

kam1787 said:

i don't know why you have bypass transistors. Why aren't they driven by the lm317 ??

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The bypass transistors are driven by the voltage drop across R2 caused by the current in the LM317. It is a standard circuit as shown in the datasheet for the LM317 and in the datasheet for 78xx regulators.

I think i figured out what is happening.
The transformer is 13v ac. After rectifing and filtering is around 17v dc. I have a 1.4 voltge drop across bridge so the output dc voltage is 15.6v.
Now the voltage drop across R1 assuming a 100 mAmps current for LM 317 is 2.2 v. So the voltage at the input of the ic is 13.4 v. In the data sheet of LM 317 it is said that the input must be 3v greater than the output, so 13.3. -3=10.3V.
So please correct me if i am wrong.
I think i will use lm723.what do you think about that.

As mentioned before, voltage drop of input voltage can be source of problem, try LDO regulators or look to use other power source with higher input voltage. You should ensure at least 3V difference (Vin-Vout) even on load.

That voltage what you get after rectification 15-17V rappidly drops on load, you need different transformer.


Best regards,
Peter

I use a big transformer from an ups. I think it can deliver a lot of power.
PS i dont want to use this with the jigsaw, i only tested the cicuit with it. I also found that seting the output at 10.5v and using as load a 10 ohm res the voltagge drpos to 10.34 v. So, i think that 10.5v is the max output

Scolioza said:

I use a big transformer from an ups. I think it can deliver a lot of power.

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You mention that transformer gives 13V AC, its hard to get 12V DC from that with standard voltage regulator (non LDO) in full power without voltage boosting or similar tech. Transformers from UPS usually have many primary windings for voltage regulation, check other wires, maybe you can get more than 13V on output.

UPS transformers are often with bad efficiency, lots of power over small transformer square area and heat is there. They are constructed to work some time (backup time) and turn off.

Its ok to drain 2A from it, but you need to ensure enough voltage for stabilisation, and you need to include inrush current of motor what you plan to use.

Did you check DC motor voltage range?


Best regards,
Peter

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Scolioza said:

I use a big transformer from an ups. I think it can deliver a lot of power.
PS i dont want to use this with the jigsaw, i only tested the cicuit with it. I also found that seting the output at 10.5v and using as load a 10 ohm res the voltagge drpos to 10.34 v. So, i think that 10.5v is the max output

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Voltage of 10,5V is too low for your needs.

Check motor datasheet if possible.

It's not the best idea to use a jigsaw as a test load[back EMF, high inrush current, non-constant load]! What do you wish to use?

Hi again.
The story goes like this. For a pic project I need two separate psu (no common ground) . The budget was limited but i had 2 identical ups transformers, so i start building 2 voltage regulators using LM317. I was thinking that it was a waste of power not to use the transformer capabilities, so I added 3 bypass transistors. I need no more than 3 amps so I thought that even using UPS transformes (which generates lot of heat due the small section) it will be ok. I used 3 transistors beacuse space inside the case was limited so there was not enough room for pasive cooling. In this conditions you think that the transformers will become to hot?
Assuming to use LM723 with a driver transistor and 2 power transistors in parallel the max output will be :15.6-1.4=14.2V? If I am not correct please let me know.
Kind regards.

A cheap transformer has thin wire so its resistance creates heat and makes it hot when a current goes through it. Its voltage drops a lot when loaded.
A high quality transformer has thick wire with low resistance so it remains only warm, not hot when current goes through it. Its voltage is fairly stable when loaded.

An overloaded transformer might saturate its core then it gets extremely hot.

Scolioza said:

I used 3 transistors beacuse space inside the case was limited so there was not enough room for pasive cooling. In this conditions you think that the transformers will become to hot?
Assuming to use LM723 with a driver transistor and 2 power transistors in parallel the max output will be :15.6-1.4=14.2V? If I am not correct please let me know.

Click to expand...

It is up to you - use one high power transistor with sufficient heat-sinking or multiple medium-power transistors [with adequate heat-sinking]. The difference is not really a factor on the transformer. If you using a smps then that might reduce the load the transformer by a small margin [as you are not dropping that much voltage].