Electronics > Beginners

Understanding circuit for inrush current limiting

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AngraMelo:

--- Quote from: rstofer on December 19, 2018, 10:34:31 pm ---No, it wasn't clear.  You linked the TI paper and embedded the schematic in your first post.  Your specific questions in the OP were all related to the TI diagram.

Then, in your Reply #1, you talk about the resistor in terms of the secondary voltage as though that is the voltage seen by the resistor.  Are you just trying to throttle the capacitor charging current?  Or the transformer inrush?

--- End quote ---

You are absolutely right, my post was a mess. Sorry about that. At first it seems to me that the 30mF capacitor bank is a much bigger problem than the transformer inrush current, I could be totally wrong so please point that out if it is the case.
I dont see a lot of inrush current limiting resistors in simple project schematics that use a transformer. Are they always needed?
if we consider that a load wont be connected to the supply as it turns on then it doesnt really matter the power rating of the transformer, right? The transformer in that case would pull the same current be it a 1W transformer or a 1000W transformer, correct?

Now, if we consider a capacitor bank that size as a load then I think it might be usefull to limit the transformer inrush current, but Im kinda talking out of my ass now. I think my question is this.
Given that Im building a linear PS and the capacitor bank is rather large, what is the considerations about inrush current?
I did some research and it seems that limiting it with a power resistor and then bypassing the resistor is the way to go on PSs with the power rating Im working on. Other sources suggest using a NTC but it gets reaaaally specific and I dont have a large selection to choose from where I live.

AngraMelo:

--- Quote from: rstofer on December 19, 2018, 10:45:32 pm ---As to charging the capacitors (if that's what you are limiting), Tau = R*C and it takes 5*Tau to get close to the final voltage.
Tau = 82 * (30,000 *10-6) = 2.46 seconds
5*Tau = 12.3 seconds.

--- End quote ---

I did not know about that! that is awesome! thank you!
So do I need to wait for 5*Tau or can I have a minimum charge to trigger a bypass relay?
I guess it is like you said on reply#5, that the resistor value isnt magic. So what could be a common sense approach to that?
If Im expecting them to charge up to around 30V, should I let the relay go around 25V?

rstofer:
http://hyperphysics.phy-astr.gsu.edu/hbase/electric/capchg.html


The charging current falls off pretty fast.  Again, Tau = R*C or 2.4 seconds

I = V/R*(e-t/Tau) so it will be reduced to 37% in 2.4 seconds.  The maximum current occurs at t=0 and is equal to V/R.

You know the maximum inrush, V/R, now all you need to decide is how long should you wait.  I would think 1 Tau is about right.

Assuming you are just limiting the capacitor current.

AngraMelo:

--- Quote from: rstofer on December 20, 2018, 12:49:24 am ---http://hyperphysics.phy-astr.gsu.edu/hbase/electric/capchg.html


The charging current falls off pretty fast.  Again, Tau = R*C or 2.4 seconds

I = V/R*(e-t/Tau) so it will be reduced to 37% in 2.4 seconds.  The maximum current occurs at t=0 and is equal to V/R.

You know the maximum inrush, V/R, now all you need to decide is how long should you wait.  I would think 1 Tau is about right.

Assuming you are just limiting the capacitor current.

--- End quote ---
One last question,
If I use the resistor and relay circuit on the primary side of the transformer will I achieve inrush current limiting for both capacitor bank and transformer?

rstofer:

--- Quote from: AngraMelo on December 21, 2018, 12:22:52 am ---One last question,
If I use the resistor and relay circuit on the primary side of the transformer will I achieve inrush current limiting for both capacitor bank and transformer?

--- End quote ---

Yes...  It's hard to say, without seeing your circuit, just how effective the current limiting will be.  Your transformer is rated about 500 VA and at 120V that is about 4A full load.  Starting inrush MIGHT be as high as 10x or 40A.  But that would only last a couple of cycles.  Still, the entire line voltage and the inrush current are working on the resistor.  Think in terms of P = E2/R = 1440 Watts.  Not for long but it just seems like the wrong way to do things.

You could model your design in LTSpice.  You'll have to search the web to find a transformer model, there isn't one in the default setup.

Another approach:

https://www.ametherm.com/inrush-current/ptc-thermistors-for-inrush-current-limiting

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