Variable power supply

[cardoso115]

Hello,

I'm trying to build a variable DC power supply, I'm using TL494 for PWM and a NPN transistor as the switch.

I'm using a Buck topology, and my problem is that my voltage is fine without any load applied to it, but when I connect a simple led with a resistor the voltage drops a lot.

I'm not using any feedback on the TL494.

Thanx in advance, Carlos Cardoso

 

[maged541946]

Hi, Normally this happens for one of two reasones
either the drive current to the pass transistor is not enough to turn it on or
feedback is less than needed.
You said you do not use feedback, A regulated power supply mainly is a FEEDBACK circuit, it compairs output sample agains a reference voltage then control a pass element.
How this control is done gives the circuit its name.
The data sheet has few nice examples up to 1 amps and a thorough explanation of how it works

http://www.alldatasheet.com,

then type your chip part number
you are then directed to this link

http://www.alldatasheet.com/datasheet-pdf/view/MOTOROLA/TL494.html

I am typing both links because some sites will only allow access to files through its main home page, even they are free...
Any more help I will be glad

 

[felixc]

Depending on your current and input/output voltage requirements, you may want to consider those single-chip converters like the National LM2576 family. Variable-output versions are available so you can control the voltage output.

 

[cardoso115]

Thanx, I've came up with the LM338 from Linear Technologies, that allows me to connect these devices in parallel in order to achieve higher current output at variable voltage.

And without any additional components.

 

[OperaHouse]

Depending on how you drive that NPN, there may be a problem when the input and output voltage is close. You can also look at SG/LM/TL3524 circuits for examples (just different pin out), but the TI lit is pretty good. I suspect you have overlooked something or have a bad part.

 

[benta]

I've designed with the TL494.
In a buck configuration, it will not work with an NPN. Use a PNP instead with base resistor, the output of the TL494 pulling the base to ground when on. You'll need a base-emitter resistor to help turn off the switch transistor.

Benta.

 

[vultan]

I'm attempting to control the power output of a resistive heater using PWM. I intend to use a TL494 IC and hope to use a varistor to alter the output but am not sure how to achieve it. My original thought is to use the varistor to vary the signal input to the deadtime control pin, not sure if this is the best way.

Any ideas would be greatly appreciated.

Vultan

 

[cardoso115]

It is!

If you want to control the heat, you must do some circuit with a temperature sensor.

 

[OperaHouse]

Just take the reference voltage through a 2K, 2K voltage divider to the - (2) input. Also take a 100-470K feedback resistor from (3) to (2) to make it a little more linear ad not an instant snap. Then just use a pot thermisistor voltage divider (13) & (14) to pin(1)+. Drive your NPN from pin (9). Play around with values, you cant do any damage