Power supply for AMP in CAR

[ante1]

OK, suppose we have four transistors in parallel and we like to switch say10 Amp@12 Volts.
The transistors are not exactly the same (not from the same batch) but we want to use them anyway.
The first has 0.2 V drop the second 0.3 the third 0.25 and the last 0.25.
This does not mean that all the current runs through the first one whit the lowest resistance sins they are all in parallel.
If we add resistors to each one of the transistors we also add further drop (0R1 X 2.5A = 0.25V) if the transistors had been equal.
Can you explain the reason for the resistors by adding info on the following drawings, please?
Sorry for being a pain in your neck, but I think debating things like this is very stimulating.
Just stop me when I become too much! ;D
The (Swedish) mule!

Ante :

View attachment 35156

 

[hotwaterwizard2]

Check out this link!
http://www.williamson-labs.com/480_xtor.htm

 

[MP1]

Ante: I do not think this debate really belongs in this thread. Others are probably tiring from it. I will have to stop now. Please check out the link above for more on transistors.

In answer to the question...since all of your transistors are on at the same time, your resistances are all in parallel and thus follow the rules of parallel resistors. You do not want one of them acting as a direct short because the others will no longer factor into the equation. In order to have 12 volts at 10 amp, you must have a total resistance of your circuit of 1.2 ohms when the transistors are turned on. (12/1.2 =10) otherwise you do not have 10 amps. If a transistor draws your total resistance down to 0.1 ohm, for example, then you are running 12/0.1 = 100 amps through this one transistor. The others will not react because all the current is flowing through the one. Adding small resistances to each leg of the circuit to balance the load will insure that the flow is equal. This is because of the parallel resistance formula.
This is only one small area of the design math. There are many other factors also, like hfe of the transistor, DC current gain, MMP, etc.

I hope that this explanation is helpful in resolving your questions.
Please, back to the inverter. I am interested in the outcome.

MP

 

[ante1]

MP,
OK, there are some things I still not agree on. One example: how can a 10Amp load suddenly pull 100Amps through one transistor even if the transistor should be shorted?? I must have a lot to learn about this. But because this is not the right forum for this I rest my case. I am also interested in the outcome of this inverter but I will not question the construction again. By the way, the link did not contain any info on switching transistors.
Thanks.

Ante :

 

[pigeon_snake]

Hii..

Check this out..

View attachment 35211

 

[hotwaterwizard2]

My whole Idea is to use 2N3055 Transistors.
I don't like MOSFET.
Anything that can blow from touching it with your finger is not relyable in my book.
MOSFET is static sensitive and I don't like the Idea of using something them in a powersupply with magnetic fields and stray electrical currents floating around.

 

[MP1]

Just a quick look at the circuit above...looks like you could use 2N3055 in place of the MosFets if you use a resistor to ground as discussed earlier. I think you could also get around the SG3525 with a 556 timer IC. One would have to compare the data sheets to get the correct pin out and frequency for the clocking.
This is just a quick look at the schematic and I have not looked at any data sheets on this. Any comments welcome.

MP

 

[audioguru2]

Hotwaterwizard,
Your inverter design is coming along, but I have some suggestions:
1) D3 and D4 don't appear to do anything. I suggest replacing them with 100 ohm, 1/4W resistors so that the bases of the output transistors have a pull-down instead of a float-down.
2) Add diodes in series with the multivibrator transistors' bases with the cathode towards the base, and add 1K ohm resistors from the bases to ground. Most silicon transistors have a maximum reverse-bias voltage of only about 7V for the base-emitter junction due to avalanching. But the avalanching causes damage to the transistor's hFE. In your circuit, the bases are driven to almost -12V.
3) Add 2A diodes across the transformer windings with the cathodes to the switched +12V center-tap and the anode to the output transistors' collector. These diodes protect the output transistors from a flyback voltage spike that would occur if the load was removed with the circuit operating.
4) Add 8 ohm, 10W resistors in series between the emitter of the driver transistors and the bases of the ouput transistors. These resistors will reduce the voltage across the driver transistors when conducting, and reduce their unlimited current (less E times less I = less heat).

 

[Kasamiko]

Just a quick look at the circuit above...looks like you could use 2N3055 in place of the MosFets if you use a resistor to ground as discussed earlier. I think you could also get around the SG3525 with a 556 timer IC. One would have to compare the data sheets to get the correct pin out and frequency for the clocking.
This is just a quick look at the schematic and I have not looked at any data sheets on this. Any comments welcome.

MP

Have you revised the circuit yet?? A 556 looks interesting..

 

[hotwaterwizard2]

Zapco Used no MOSFET in their old Power supplies.
Here is their Diagram. The problem with this one is it uses PNP and I already have about 150 Transistors 2N3055. The transformer is special made and hard to find.

View attachment 35297

 

[hotwaterwizard2]

And here is a Schematic from a Pyramid PB300 Amplifier.

PB300G_Schematic.pdf

 

[MP1]

audioguru, in your comments to hotwaterwizard you mentioned that he should change the 2 diodes in the first transistor stage to resistors in his design. These transistors are connected as a darlinton pair. Adding such a resistor would disrupt the whole purpose of using a darlington pair. It sounds as if you are changing the design to an audio power amplifier.

MP

 

[ante1]

Hi MP, hotwaterwizard,

Just a comment on the Zapco diagram; there are no emitter resistors here!

Ante :

 

[audioguru2]

Hotwaterwizard,
I am sorry, but I made errors when I suggested modifications to your project:
1) DO NOT add diodes across the transformer's primary.
2) DO NOT add 8 ohm resistors in series with the bases of the output transistors.
My reasons are explained in the other inverter post here:
http://www.electronics-lab.com/forum/index.php?board=2;action=display;threadid=659;start=15

 

[audioguru2]

MP,
I did not see any function for diodes D3 and D4 since they are always reversed-biased. Pull-down resistors having only about 20mA were suggested to replace them for reasons as follows:

1) To turn-off the output transistors, fast. Without pull-down resistors, the bases of the output transistors just float down.
2) To bypass the leakage current of the output transistors and ensure that they actually turn off. The leakage current increases with temperature (thermal runaway).
3) To discharge the capacitance (the miller effect multiplies it) at the bases of the output transistors quickly. Without the pull-down resistors then the output transistors become integraters with their own substantial capacitance.

Darlingtons are emitter-followers with their collectors tied together. The miniscule 20mA of current into the pull-down resistors won't affect the amps of current that are fed to the bases of the output transistors.

Darlingtons are not used for high-speed-switching nor high quality audio because they are too darn slow.

 

[hotwaterwizard2]

The diodes are for spike protection to prevent the transistors from burning when the field colapses in the transformer windings. If you look at a good circuit involving a relay you will see a protection diode across the coil for this reason.

 

[MP1]

looks like the collectors are tied together to me. What schematic are you looking at? Perhaps I am on the wrong page.

MP

 

[MP1]

BTW, I think you are misinformed about darlington pairs. I have attached a pdf document with information regarding them. Darlington pairs are in nearly every IC on the market.

MP

DarlingtonCircuit.pdf

 

[audioguru2]

Hotwaterwizard,
Good point about spike protection!
I was trying to figure out how a positive spike at the collectors of the output transistors can get through their turned-off collector-base junction, and how a reversed-biased diode can arrest this spike. Now I realise that the positive spike produced at the collectors of the output transistors that are turning-off, causes a negative spike at the collectors of the output transistors that have not yet turned-on, due to center-tapped transformer action. Since this negative spike forward-biases the latter's collector-base junction, then it also forward-biases the protection diode, which arrests the spike.
Those diodes are good protection.

Another concern: Perhaps you should change your multivibrator to the 4047 type that is used in the other post:

1) The 4047 has a divide-by-two to ensure that its outputs have exactly a 50-50 mark-space ratio. Yours depends on resistor/capacitor values, parts tolerance and lack of drifting.
2) I have seen it happen: If your values are the same then both transistors may "latch" during power-on, when both transistors turn-on and stay-on. Then they won't oscillate.
3) The 4047 needs only 1 resistor and 1 capacitor, compared to all the parts for your multivibrator, and costs much less.
But if you use a 4047, you will need to add a pre-driver transistor, like the other post, to boost its output current.
These two projects are getting to be more and more identical!

 

[audioguru2]

MP,
Thanks for posting the article about darlingtons.
There it is, in the schematic on the 1st page, a darlington showing pull-down resistors, and exactly the same description for their function that I gave.
Sorry to continue this argument, but ONSemi (Motorola), in their MC34071 opamp, use pull-down current sources on the "darlington" (their wording) input transistors. See the chip's schematic. Most if not all opamps that use darlingtons have this arrangement. The MC34071 data sheet is here:
http://www.onsemi.com/pub/Collateral/MC34071-D.PDF