Hello, complete new guy here. No training, except what I've recently gotten from YouTube U.
I'm trying to figure out the best way to limit inrush current when I plug in an amp I'm building. The amp itself works fine, but I get a bit of a spark when I plug it into a wall outlet, and do not want to damage any components. This is the transformer I'm using:
My question is, does an inrush limiting circuit go between the wall outlet and the transformer? Or between the transformer and the other components? Or both?
The 40vDC powers two power amps. The transformer also has ±15vDC which I will use to power a preamp. Kind of hard to see, but here is the ±15v output from the transformer:
The preamp requires ±12v, so I'm building a dual voltage regulator using an LM7812. I have just learned that an LM7912 is used for negative voltage, and was wondering, since I've got -15v coming from the transformer, would I still need an LM7912? Does the LM7912 input +15v and output -12v? Or, since I have -15v coming into my voltage regulator, would an LM7812 keep the polarity the same?
Many thanks!
I'm trying to figure out the best way to limit inrush current when I plug in an amp I'm building. The amp itself works fine, but I get a bit of a spark when I plug it into a wall outlet, and do not want to damage any components. This is the transformer I'm using:
My question is, does an inrush limiting circuit go between the wall outlet and the transformer? Or between the transformer and the other components? Or both?
The 40vDC powers two power amps. The transformer also has ±15vDC which I will use to power a preamp. Kind of hard to see, but here is the ±15v output from the transformer:
The preamp requires ±12v, so I'm building a dual voltage regulator using an LM7812. I have just learned that an LM7912 is used for negative voltage, and was wondering, since I've got -15v coming from the transformer, would I still need an LM7912? Does the LM7912 input +15v and output -12v? Or, since I have -15v coming into my voltage regulator, would an LM7812 keep the polarity the same?
Many thanks!
Fit a PTC in series with the mains supply Wickman fuse, if it concerns you.
10R 5A for 240volt mains and 5R 10A for 110volt.
If you require -12v and +12v use 79*** for -12v and 78*** for +12v.
10R 5A for 240volt mains and 5R 10A for 110volt.
If you require -12v and +12v use 79*** for -12v and 78*** for +12v.
Yes, there is an NTC there, which I had not noticed before. Thank you all for your replies. I obviously have a lot to learn, but gotta start somewhere.
The NTC there is designed to limit inrush only on a cold or 1st plug in. So it's possible to "hot plug" the unit - after it's been operational for a while - into the AC and then the NTC isnt effective anymore.
Because normally no one sits there unplugging and replugging their device (computer, etc) into the AC line, the initial plug-in reduction of inrush is deemed satisfactory.
We used to have a spec on and test for, hot plug inrush. Why? If there's a power outage just long enough to discharge the bulk caps, but not long enough to allow the NTC to cool, there's a large inrush when the AC comes back on. That must be limited, or with banks of these things on a line, it'll blow the breaker. That's bad, because then the computer doesnt come back on line, until someone physically tends to the breaker.
Try it and see - unplug it overnight, then plug it in next day - do you still get the spark?
Thanks, I noticed the spark (well, heard it, actually) after it had been unplugged for a while. Do you know how long it takes an NTC to cool down? It's an NTC-5D-15. The data sheet has a time constant of 76; I assume that is seconds?
If you look at the schematic of the power supply, there is usually a class X capacitor across the mains before the inrush protection. Maybe that is what is causing you a problem. Or that NTC component is in series with the transformer after the main smoothing capacitors.
No idea on the make and model of the power supply, no idea on the schematic either so just guessing in the wind here.
No idea on the make and model of the power supply, no idea on the schematic either so just guessing in the wind here.
If an NTC was not an option, there are other possibilities.
https://www.powerctc.com/en/node/4564
If the power supply deals with voltages that are substantially higher than the gate of the MOSFET can withstand, one could limit the gate voltage with a Zener diode. Otherwise very simple. No feedback required. The gate circuit is simply slow enough.
Computer simulations (PSpice, LTSpice, or some other) may be used to quickly find out which component values are optimal.
https://www.powerctc.com/en/node/4564
If the power supply deals with voltages that are substantially higher than the gate of the MOSFET can withstand, one could limit the gate voltage with a Zener diode. Otherwise very simple. No feedback required. The gate circuit is simply slow enough.
Computer simulations (PSpice, LTSpice, or some other) may be used to quickly find out which component values are optimal.
That is a VERY small transformer and power board.
Unless there is something you are not showing us (another transformer in the same box?) then if this were a problem we could not plug-in a clock-radio without "inrush limiting". And my well-pump's 44 Amp startup (on a 20A breaker) would collapse the power utility. Bah. Wires are tougher than you think.
Unless there is something you are not showing us (another transformer in the same box?) then if this were a problem we could not plug-in a clock-radio without "inrush limiting". And my well-pump's 44 Amp startup (on a 20A breaker) would collapse the power utility. Bah. Wires are tougher than you think.
Yes, it is quite a small transformer. I wasn't so much worried about the drain on my power supply, but rather to avoid damage to the components downstream. And there's no schematic that came with it. Since I'm just starting out building audio amps, all of which are from kits, I'm really just following YouTube videos, and this one seemed to work very well.
Thank you all for your answers. I'm learning, bit by bit.
Thank you all for your answers. I'm learning, bit by bit.