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Electronics => Beginners => Topic started by: ikrel on February 08, 2014, 03:12:22 am

Title: Voltage Reg dropping out
Post by: ikrel on February 08, 2014, 03:12:22 am
I'm trying to setup a voltage regulator and every time I put a load on it the voltage drops out. It goes from 24v to 15v with 100mA of load, and I don't know why. I eventually want this to handle 6A of current. Can anyone explain this?

This is what I have going: http://imgur.com/bS05bqL (http://imgur.com/bS05bqL)
Title: Re: Voltage Reg dropping out
Post by: T3sl4co1l on February 08, 2014, 03:25:05 am
It's regulating to a ratio of VCC, which assumes VCC is regulated to begin with.  :clap:

It may be oscillating too.  1N4007s and 2 x 470uF aren't even in the ballpark for 6A, of course.

Read up on: power supply design, source impedance, voltage reference, loop stability and so on.  :-+

Tim
Title: Re: Voltage Reg dropping out
Post by: ikrel on February 08, 2014, 03:52:33 am
This is just the "get it to work stage." Once I have it working I'll work on getting the actual output current i want.

I've also just figured it out. The value of the 1K resistor on the base of the transistor was too high and was not allowing the current needed to keep the transistor turned on.
Title: Re: Voltage Reg dropping out
Post by: c4757p on February 08, 2014, 03:55:47 am
Quote from: ikrel on February 08, 2014, 03:52:33 am
This is just the "get it to work stage." Once I have it working I'll work on getting the actual output current i want.

Dude, when you want to build a car, you don't start by throwing a lawn chair on a wheelbarrow, then fix the problems. Start with the end specs in mind.
Title: Re: Voltage Reg dropping out
Post by: ikrel on February 08, 2014, 05:30:31 am
The diodes fit in the bread board(I have higher current ones that I will use for the PCB), the TIP41C is good for 6A(I've now switched to a FRP50N6 MOSFET).
I still have to Research how to size the filter caps properly(suggestions?)

Also any suggestions on reducing/filtering the noise on the output? I'm currently getting 200mVp-p of noise and would like to reduce this.

EDIT: no worries about the noise, just need to connect the ground lead the ground. do'h
Title: Re: Voltage Reg dropping out
Post by: Tac Eht Xilef on February 09, 2014, 12:51:44 am
The point is that your circuit isn't a voltage regulator. At best, you've got a slightly complicated voltage follower.

What happens to the 16.2v 'reference' on the wiper of the pot when the unregulated Vcc drops (e.g. when you load up the supply)? How does that affect your output voltage?