# Advantages of Negative Power Supplies

#### dl324

Joined Mar 30, 2015
11,262
I completely fail to see the purpose of this circuit:
Same here...

Furthermore, what's the point of drawing a schematic with no component values?

#### JC13

Joined Jul 5, 2015
16
maybe I am boring, but I just want to get some answer....

I have drawn a schematic from the board and really, drain and source are connected to the ground, I checked 100 times.

Component values are important but not in this case. I just want to know how this circuit works and what is his purpose.

I have tested this circuit on breadboard and first I have connected 100VDC without any voltage between drain and source and then I had Vgs=18V. But when I connected voltage between D and S (30V) I lost the voltage Vgs.

And maybe it is really negative voltage on input, because drain and source are connected to the ground. And this circuit is connected EXACTLY on the input from the board. It must have some purpose.

Second thing....rectifier diodes (D9, D10) are connected also to the ground and from there to input from voltage regulator ?....sorry but I have seen first time circuit like this one.

I really hope that someone has an answer and can explain the purpose of this circuit.

Best regards

Jovan

#### dl324

Joined Mar 30, 2015
11,262
Component values are important but not in this case. I just want to know how this circuit works and what is his purpose.

I have tested this circuit on breadboard and first I have connected 100VDC without any voltage between drain and source and then I had Vgs=18V. But when I connected voltage between D and S (30V) I lost the voltage Vgs.
You need to provide component values. R2-4 form a voltage divider. Without knowing their values and the part number for D1, I can't determine the voltage across R1. As connected, T1 is nothing more than a capacitor.
Second thing....rectifier diodes (D9, D10) are connected also to the ground and from there to input from voltage regulator ?....sorry but I have seen first time circuit like this one.
It's your negative voltage regulator. Can't say more because you didn't provide part numbers for the regulators or values for the resistors.

#### JC13

Joined Jul 5, 2015
16
I am not at home now....I will send you the values on Monday...

Thanks a lot

Cheers

Jovan

#### JC13

Joined Jul 5, 2015
16
Hi dl324, hi everybody....

I couldn't post schematic and values before.

I forgot to write which MOSFET is on second schematic (T2=IRF740).

1. I don't know what is purpose of circuit (with T8) on input, my opinion is that, that is some kind of current
protection. And a explanation how does this circuit works.

2. In second schematic what is the purpose of elements connected between Gate and Source. And also
explanation how does this circuit works.

I am almost on beginning with electronics and every start is hard.
So I want to thank for your help.

Have a nice day

Jovan

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#### JC13

Joined Jul 5, 2015
16
You need to provide component values. R2-4 form a voltage divider. Without knowing their values and the part number for D1, I can't determine the voltage across R1. As connected, T1 is nothing more than a capacitor.
It's your negative voltage regulator. Can't say more because you didn't provide part numbers for the regulators or values for the resistors.
Hi dl324,

I posted 2 schematics with values from the board. If you can see and help me to understand the function.

I hope I am not boring.

Jovan

#### dl324

Joined Mar 30, 2015
11,262
The first schematic is an inrush limiter; perhaps because C3 is large or the load draws high current at startup. T8 seems to be wired incorrectly because the body diode will be forward biased with just a few 10's of mA of current at startup.

The second schematic is incomplete.

#### joeyd999

Joined Jun 6, 2011
4,425
I'm late to the party:

A negative power supply is a good source of electrons.

What does a positive power supply source?

#### cmartinez

Joined Jan 17, 2007
7,062
I'm late to the party:

A negative power supply is a good source of electrons.

What does a positive power supply source?
Mhhh... I'm only willing to play this game with you because I think I'm going to learn something valuable.
My first guess is: "holes", or more specific, a drain for electrons.

#### JC13

Joined Jul 5, 2015
16
The first schematic is an inrush limiter; perhaps because C3 is large or the load draws high current at startup. T8 seems to be wired incorrectly because the body diode will be forward biased with just a few 10's of mA of current at startup.

The second schematic is incomplete.
Thank you very much dl324...

your answer helped me, because I know now in which direction should I go with learning, for example "inrush limiter". It is incorrect from me to ask you about that, I will find and read about it by my self.

I decided to start with learning about power supplies, and after, my wish is to go further with electronics.
I took existing power supply, I took of all parts from the board and I made the schematic. Maybe it is the hard way, but I learned many things about this topic.

I know that one thing is theory and other thing is practice, where man can see and try everything what he learned from the book.
My big wish is to learn.

Second schematic I made and checked 100 times. I can not understand. What do you think, where have I make a mistake. There are no other connections whit a gate or source of mosfet T2 (maybe I missed something but I believe I didn't because I checked really many times). And RLC circuit between gate and source of mosfet?
What is missing?

Thank you one more time, and through your answer I see that you know a lot about electronics and I will appreciate when you can help me.

Cheers
Jovan

#### dl324

Joined Mar 30, 2015
11,262
Second schematic I made and checked 100 times. I can not understand. What do you think, where have I make a mistake. There are no other connections whit a gate or source of mosfet T2 (maybe I missed something but I believe I didn't because I checked really many times). And RLC circuit between gate and source of mosfet?
What is missing?
There are no gate connections to T3 and T4. I assume the component labeled Trafo is a transformer; if that's the case, it would help to know how the other windings are connected. Are L1 and L2 really inductors or are they part of a transformer? The part number for D11 doesn't help identify it's purpose (a catch diode?).

My guess is that the circuit is a DC-DC converter.

It's okay to ask questions on this forum. Even people who know, or seem to know, a lot can learn more. Someone asked for help with a waveform slope detector. I had never designed or analyzed a circuit for that and I learned something in the process.

#### JC13

Joined Jul 5, 2015
16
There are no gate connections to T3 and T4. I assume the component labeled Trafo is a transformer; if that's the case, it would help to know how the other windings are connected. Are L1 and L2 really inductors or are they part of a transformer? The part number for D11 doesn't help identify it's purpose (a catch diode?).

My guess is that the circuit is a DC-DC converter.

It's okay to ask questions on this forum. Even people who know, or seem to know, a lot can learn more. Someone asked for help with a waveform slope detector. I had never designed or analyzed a circuit for that and I learned something in the process.
Hi dl324,

Thank you again....

Sorry, but maybe I didn't mention it before, this is just one part of schematic. The whole circuit is DC-DC converter.
When I finish whole schematic, I will post it here, than you can see.
I wanted to ask how this circuit around Power MOSFET T2 (IRF 740) works. It produce voltage for transformer, but I don't know how it works and how much voltage does circuit gives to the transformer.
T3 and T4 are also power MOSFETs and they are driven by pwm chip. L1 and L2 are coils.

I posted some other part of schematic of DC-DC converter (inverted input for PWM chip), maybe you can check this also.

thanks one more time and have a nice evening

Cheers
Jovan