Voltage Regulation

[MOSFET KILLER]

I am building a power supply and am wondering if there is a method to supply +15V,0V and -10V. I don't have any linear voltage regulators or zener diodes, also any SMPS device is out because of lack of breadboard space. The supply needs to be good for about an ampere. I do have a 24V transformer but it has no centre tap. I can use diodes to simulate the centre tap, but it is only half wave rectified.

 

[ke5frf]

You really need a center tapped transformer and a bridge rectifier, either in IC form or 4 individual diodes, and some good regulators. A 24V transformer will not produce +15 volts in a center-tapped bridge circuit so you need another transformer.

The problem is your power requirement. One way to get a negative voltage from a positive supply is a 555 timer in astable multivibrator configuration with some diodes rectifying the output. But this will not supply stable voltage and only limited current.

 

[ke5frf]

You might look into building a switch mode supply, but you STILL need regulators and special transformers and transistors and ICs for this type of circuit (more complicated)

 

[MOSFET KILLER]

A switch mode supply for this project would be massive. I would need 3 outputs and 1 is for a high side driver which is floating.
I guess that I will just get the right transformers for this project and some voltage regulators.

 

[ke5frf]

I was typing my comment on the way out the door from work, and I thought to amend my comments to remind you that USUALLY a center tapped transformer precedes a full-wave rectifier configuration, which has one polarity for its output. Dual polarity from a center tap requires a bridge circuit.

I read a post on another thread that implied you are a student. I didn't go into detail about why a 24 volt transformer isn't appropriate for both a -10 and +15 volt supply, so in case you haven't learned this yet I will describe it.

Obviously, you need a center tap in a linear power supply to achieve both polarities. remember that in full wave and bridge rectification, the negative pulse gets "flipped" to fill the inbetweens of the positive pulses, then smoothed, filtered, and regulated.

Well, in the dual polarity supply, the mirror image is happening on both sides of the center tap. Thus, the full 24VAC on your secondary is cut in half for each polarity.

The best you can hope for is 12 volts of unregulated, pulsed DC.

But this is unacceptable to drive most devices, so we have to smooth out and regulate the voltage.

The pulsed DC is PEAK 12 volts, but it is sinusoidal. I don't care about doing the math, but you can figure on losing a couple of volts when your regulator takes over. It needs something called HEADROOM to operate properly, or else it really isn't regulating.

So with our 24 volt center tap design, we're doing well to get a -10 and +10 supply out of it with efficient regulation and optimal smoothing.

 

[MOSFET KILLER]

Right, the capacitors filter the sine wave to the RMS voltage, so I need higher voltage transformers and proper voltage regulators.

 

[colin55]

It can be done if the -10v section is taking exactly the same current as the +15v section.

 

[MOSFET KILLER]

colin55, what do you mean.
Also how can I make the -10V and the +15 draw the same current, heard something about current mirrors but I don't know if it would be right for this application.

 

[indulis]

I am building a power supply and am wondering if there is a method to supply +15V,0V and -10V.

A switch mode supply for this project would be massive. I would need 3 outputs and 1 is for a high side driver which is floating.

I'm confused... you say you need +15V and -10V. What is the third voltage?

Also, a DC-DC converter to do this isn't that difficult. How much current do you need for each output, and how well does each ouput have to be regulated? Will the loads be pretty much constant?

 

[Papabravo]

Right, the capacitors filter the sine wave to the RMS voltage, so I need higher voltage transformers and proper voltage regulators.

This is incorrect. The filter capacitors preserve the PEAK voltage of the rectified AC waveform. That is why a transformer with a 12 VRMS output will produce an AC waveform with a PEAK voltage of 16.97 Volts. The bridge rectifier will consume 1.4 Volts leaving a DC output of 15.56 Volts for the capacitors to remove the ripple from.

 

[MOSFET KILLER]

I'm confused... you say you need +15V and -10V. What is the third voltage?

I mean that I will have to build 3 individual power supplies, 1 for the high side driver and one fore the low side driver, and the last one is for other circuits.

I would need:

2 +15V, 0V, -15V supplies
1 +20V, 0V supply

The current should be high enough on the 2 supplies to turn on a pair of IGBTs.
The 20V supply is for driver circuits and should have a pretty much constant current load.