DC Inrush current limiting circuit ideas

[Dannyx]

Good day folks. With nothing better to do in the shop and the weather being all cold and rainy, I was digging around the parts bin and found an audio amp module presumably out of a TV or something...smells like a Vestel TBH, since it's an 18AMP...something (can't remember) and I could've sworn I've seen this board before somewhere.

Without going into too much detail, I wired it up and powered it with 24v from a laptop power brick and it works pretty good...great for driving speakers and stuff (you can never have too many speakers) But I noticed one thing: when I connect the power to it there's quite a considerable spark at the connector - enough to make me jump the first time, though I should've expected it with so many electrolytic caps on there. I dug around the internet to devise the simplest possible inrush limit circuit for this thing and I believe I found something here, towards the bottom with the N-channel MOSFET in series with the negative line.


Looks simple enough to build....no specs on those components though, so that's why I'm hesitant and requires some guessing. I don't have access to some super fancy measuring equipment, nor do I expect any laboratory-grade result...just something simple to allow this thing to turn on without creating a loud pop....I was gonna put this in an enclosure and make a desktop amp for a friend (don't judge ), but it's got this slight issue which would ruin any power connector pretty quickly, plus it would cause jump-scares The device itself is not relevant - it could be any DC capacitive load.

[budm]

Re: DC Inrush current limiting circuit ideas

How about using delay turned on relay to bypass the inrush current limiter resistor?

[Dannyx]

Re: DC Inrush current limiting circuit ideas

I was thinking of a relay too, but wasn't sure how to control it...there has to be some very basic and primitive current sensing to tell it when to turn on and bypass the resistor....or a time delay in this case...not sure how "bulletproof" that would be.

[budm]

Re: DC Inrush current limiting circuit ideas

You just need a very short delayed on of the relay when power is applied to the circuit.
I.E. https://www.electronicshub.org/time-...relay-circuit/
https://www.electroschematics.com/48...ay-on-circuit/
http://www.theorycircuit.com/time-delay-relay/

[Dannyx]

Re: DC Inrush current limiting circuit ideas

I like that. I also like that it can operate over a relatively large voltage scale...say the amp box can be powered from 12v to 24v and it would still do its job.

[AdrianM]

Re: DC Inrush current limiting circuit ideas

I far prefer the Mosfet solution
If you've got any N-channel parts kicking around in a TO-220 pack they'd probably do. So long as the Ron resistance is below an Ohm or so. The Zener wants to be between 7.5 to 15V and the R C divider set up to give the time delay you want. I reckon you could probably pull all those parts out of a duff PC PSU.

[Dannyx]

Re: DC Inrush current limiting circuit ideas

I've got plenty of parts laying about- I'm a repair shop I even got some pretty decent FETs with low R-On that I got a while back for a different project but never got to using...it looks like it's their time to shine.

[budm]

Re: DC Inrush current limiting circuit ideas

You need to add discharge circuit for C so that it will always delayed on if you momentary lose power so the delay will always be present.

[Dannyx]

Re: DC Inrush current limiting circuit ideas

Correct. I was thinking of doing what the 555 timer does internally and somehow hook up a transistor that closes and "drains" the circuit...though come to think of it, with no power, such a transistor wouldn't function. A passive component would be better suited....a cleverly placed diode perhaps ? Like a flyback diode ?

[stj]

Re: DC Inrush current limiting circuit ideas

the usual method in comercial designs is to use a PTC with a relay across it and a timer circuit - usually a 555.
they sell mains limiters like that for big inductive loads.

[Dannyx]

Re: DC Inrush current limiting circuit ideas

A PTC would mean the resistance is high when the device is hot and low when it cools down ? Wouldn't that be a bit counter-productive ? Wouldn't that be an NTC ? Just askin'.

[AdrianM]

Re: DC Inrush current limiting circuit ideas



D2 discharges C1 through the load when supply removed to reset delay for next time. Component values give roughly 0.5 seconds - if that's too short or long adjust C1 x R1. But watch out how long the Mosfet is in the linear region. May dissipate power if too long.

[Dannyx]

Re: DC Inrush current limiting circuit ideas

So I was right on the money with my diode idea...I was just about to drop it where you dropped it too Great ! I'll try it out and see what goes pop
EDIT: how do you calculate the time ? Was it "tau" = r*c ? :|

EDIT2: how'd u make the schematic ? I've been looking for a good piece of software so I wouldn't have to draw them by hand in MS Paint....something that can at least have the symbols and join the traces automatically >_>

[AdrianM]

Re: DC Inrush current limiting circuit ideas

The schematic was created in LTspice which is a free simulator tool - so you can try ccts out without popping anything! Yes, approx r*c gives the time constant. The Mosfet gate takes no current so it's pretty academic what R you use. I was aiming more at getting a couple of mA into the zener. BTW, that's an essential part to clamp the gate to well below its breakdown- usually 20V.

[budm]

Re: DC Inrush current limiting circuit ideas

Based on 4K7 Ohms and 100uf, so at 1 st time constant, the Voltage on the cap will be charged up to about 63% of the supply Voltage, in this case it will be limited to Vz of the zener.
https://www.electronics-tutorials.ws/rc/rc_1.html

[Dannyx]

Re: DC Inrush current limiting circuit ideas

I thought I'd continue the discussion here rather than opening a new thread. Still on the subject of the TDA8932 this amp module uses, I experimented with it some more today and I ran into an issue: I'm trying to hack it from a stereo amp to a mono bridge amp for a single subwoofer, with a switch to toggle between the two modes just in case I feel like switching back - don't laugh I got one of those large rotary knobs that came out of a parallel printer switch, which offers a ton of switches, so that was perfect for this project, since there's a lot of things to switch around between mono and stereo. Position A denotes stereo mode and position B would be bridge. C is a couple of resistors that need to be "engaged" to the HVP terminals when in bridge mode and disconnected when in stereo mode. I wired it up like in the schematic I posted which I adapted from the original Hitachi TV it came out of. It's just a quick reference schematic that I doodled so I wouldn't forget which wire goes where, so don't judge the quality too harshly.

It works fine in stereo mode (amazingly with so many mechanical switches) but when I try to go "bridge" like in the example circuit in the datasheet on page 36, it sounds rather muddy...it's hard to explain, but it's got no bass and some of the sound gets cut off randomly, like you're rapidly turning the volume knob up and down....something like that.
I'm not sure I got the stereo-to-mono idea right, which is as follows and could be flawed: with the switches in position A, the signal goes through the 4.7k resistors, through the 470nF caps and into each individual input - this works fine and I have no doubts about it since it's the same as in the original schematic, just wired through the switches. When in position B, the idea is the same 4.7k resistors are still in place but a 1uF cap is now selected (as the example on page 36 suggests for bridge mode) and the points labeled B1 get connected together (same for B2) to form a mono sum like this....for some reason it's not working (properly) and I'm not sure why and where to begin. Now, I know bupkis about audio stuff and amps. Ok, maybe I know some basics since I DO work with electronics, I just don't know/care about the physics and "extreme" calculations involved (see friggin' page 23 to see what I mean :O) the distortion, THD, stuff like that can get ridiculously difficult. I just use my ears and go "yep, sounds good" So this is probably going to offend audiophiles out there....nah. This is just me messing around in the shop - it not meant to replace a Hi-fi amp

I'm not sure whether the problem is on the input side or the output side (where the coils and the speakers are on the right side of the page).

Possible causes that I can think of, if on the input:
-the resistor value is too high (4.7k)
-the "summing" resistors shouldn't be used on the GND line as well
-the 1uf caps are the wrong value
-any combination of these

If on the output:
-the coils and caps are not "tuned" for the "BTL mode", as they call it in the datasheet (which they clearly are not - they were designed for the stereo setup, though to my mind it sounds WAY too bad for a coil value to be the culprit)
-input voltage is too high (running on 24v - page 25 talks about clipping and how it can occur if you push it too high in BTL mode)

Any audio gurus are welcome to comment Thank you.

[Dannyx]

Re: DC Inrush current limiting circuit ideas

Files didn't get attached for some reason....try again...

[budm]

Re: DC Inrush current limiting circuit ideas

Loo at the DATA sheet page 11, 12, and 36 (see how 1N1P, 1N1N, 1N2P, 1N2N are connected in bridge mode) then look at your schematic you can see there is no phase reversal at your input.
BTW so when you are in bridge mode it will become mono amplifier which means you need to combine L and R signals first before feeding that mono signal to the amplifier.

[Dannyx]

Re: DC Inrush current limiting circuit ideas

BTW so when you are in bridge mode it will become mono amplifier which means you need to combine L and R signals first before feeding that mono signal to the amplifier.

I hoped I did that via the switch to avoid a separate resistor network. When connecting B1 points together and B2 points together it's the same thing as this....or at least I think so...

[Dannyx]

Re: DC Inrush current limiting circuit ideas

I got it to sound relatively decent by lowering the value of the resistors to 470ohms. Still some distortion going on especially at low volumes, though this could be because of the speaker itself.
The modified schematic would look like this then: in stereo mode, we just keep the 4.7k resistors (A position). When toggled to bridge mode, the 470ohm resistors get selected (B position), which also combines the L and R channels, since B1 and B2 are connected together.