F5 power amplifier

[bobodioulasso]

See post #5313

[jackies]

Thanks for the reply!
That's exactly the post I was talking about. I guess my question was more on the lines of:
Do I need to build that two transistor contraption or just two resistors would have been enough?
Although I don't see why not.
Thanks anyway, probably gonna just hook up the two transistors and see what happens..

[bobodioulasso]

I have tried this arrangement (resitors only and transistors) with different amplifiers.
I noticed that different resistors values are necessary to obtain equal voltages on the two halves when loaded.(by two modules when stereo).
Equal resistors produce equal voltages at powering on but a slow drift occurs untill an equilibrium.
I found easier to equilibrate using a low power potentiometer and transistors.

[rikkert1978]

My torody transformer just got in, just have to build an inrush protection now.

The transformer is 500va 230volt pre and dual 18v secondaries.

I looked at some of the ntcs needed, and I found a shop that has these:

Display Electronics - Siemens B57364S509M - Siemens NTC 5 Ohm - 5 watt

These should be rated at 8.5 amp. 5ohm resistance (when hot?)

My transformer not having a centertap on the primaries, can I just use two of those thermistors on both the ac taps, and thats it? Or should I put them in series of eachother on 1 ac tap?

I cannot upload a picture right now, so hope you understand.

[jacco vermeulen]

or buy a single B57364S100M at Conrad for €1,95 ?
=> 10 Ohm, 7.5A, 5.1W [CL60 is 10 Ohm, 5A, 4.5W]

Eeh, if your transformer is a 230V type you only have 2 primaries, no ?
Watt-The-Fuse does it matter where you put them, they'll always be in series. The idea is that you have 2 times 5 Ohm in series COLD, aka 10 Ohm, to limit the inrush current.

[AndrewT]

if the pair of series connected primaries are ~1r0 and the two series connected Thermistors are 10r, then that is probably not sufficient resistance to prevent fuse blowing at start up on a 220/240Vac supply, using T2A as the mains fuse.

I think the inductance of the de-energised transformer is virtually the same as an air cored inductor with those turns on it.
That will have a very low impedance at 50Hz.
The peak start up current <220V*1.414/11r <28Apk under worst case conditions.
I think you should be aiming to half that peak current (double the cold series resistance to 22r) to get the transformer to start on a T2A fuse.

[CBRworm]

Could someone that has an F5 simulation running tell me what will happen if one rail of the power supply goes open?



My thought it that nothing bad will happen, but before I test this with working parts I am hoping that someone can just turn off one of the rails in a simulator.

I am planning on putting in rail fuses and I want to make sure that if one of them blows it won't take out the other side of the circuit. If so then I will come up with something different.

Thanks.

[bobodioulasso]

My opinion is : the 1,3 amp bias current will run through the speaker.

[CBRworm]

That was my other thought. If that's the case I will be better off with no rail fuses, or a complex set of protection circuits with relays that will disconnect output if the power supply becomes unbalanced and/or DC appears at the output.

[rikkert1978]

Originally Posted by AndrewT

if the pair of series connected primaries are ~1r0 and the two series connected Thermistors are 10r, then that is probably not sufficient resistance to prevent fuse blowing at start up on a 220/240Vac supply, using T2A as the mains fuse.

I think the inductance of the de-energised transformer is virtually the same as an air cored inductor with those turns on it.
That will have a very low impedance at 50Hz.
The peak start up current <220V*1.414/11r <28Apk under worst case conditions.
I think you should be aiming to half that peak current (double the cold series resistance to 22r) to get the transformer to start on a T2A fuse.

Thanks for the info, I will use 2x 10ohms then to get to around 22r
Should be under 15A so well possible. 230*1.44/22= 14.6A
They are rated for 7.5 amp (epcos that Jacco advised few posts back) , so it should be very close.

Oh and Jacco, you are right. It doesnt matter where I put them... ups..

Still I wonder what the rest of the world does with such a transformer... Can`t possibly line up 3 thermistors just to get this beast started? Sigh.