Bench Power Supply Circuit Problem

[TotalMadness]

Hi,

I found the attached circuit for building a simple bench power supply.

I am using all the components as in the schematic, but I cannot change the voltage down to zero. I have tested this with a LED (2.4v) as the supply output load and a mobile phone charger as the input source (5.3v 500mA). The description that came with the circuit is:

"The two transistors and three resistors form an adjustable, stabilised output. The FET transistor has a high output power handling capacity and a very low input power requirement and so is good for controlling the output voltage. Resistor ‘VR1’ is padded with the 4K7 resistor solely to reduce the voltage across the variable resistor. VR1 is adjusted to control the output voltage. If the current draw is increased and the output voltage is pulled down slightly, then the voltage on the base of the BC109 transistor is reduced. This starts to turn the transistor off, raising the voltage at point ‘A’, which in turn, raises the output voltage, opposing the variation caused by the load."

while testing, the readings I get are 3.1v and 8mA even when turning VR1. I checked VR1 and it does give readings of 0 to 5K ohms when turned. The voltages I get on the base of transistor BC109 range from 0 to 315mV. In fact, I can remove the BC109 transistor and I still get the same readings for the output.

I am using multimeters as shown in the schematic.

Any ideas?

 

[Nigel Goodwin]

It's not supposed to vary down to zero, it's far more complicated to make a PSU that does go to zero, and it's hardly ever needed anyway.

I would expect that (extremely crude) circuit to only go down to about 1.5V, but it's an absolutely useless circuit, and it's not stabilised to any reasonable degree.

You also don't feed LED's with voltage, you feed them with current (series resistor as minimum), plus your input voltage sounds rather low.

 

[TotalMadness]

Ok thanks. I'm waiting for some 12v batteries to try out. This would then be a project for the future to try and make a better one. It's actually important for me to be able to reduce the voltage down to zero while I am learning to see what happens in circuits to try and understand them more.

 

[Nigel Goodwin]

Ok thanks. I'm waiting for some 12v batteries to try out. This would then be a project for the future to try and make a better one. It's actually important for me to be able to reduce the voltage down to zero while I am learning to see what happens in circuits to try and understand them more.

I can't imagine why been able to reduce the voltage to zero would help you learn at all? - it doesn't seem to be of any educational use?.

 

[TotalMadness]

Because I would like to see what waveforms, voltages, and frequencies there might be around components in a circuit at different levels. It may well be a waste of time, but is something that interests me. Today is a good example. Lets assume the above circuit does work correctly at 12v, if I had not used 5v I would never have discovered that it doesn't work at 5v and that removing the transistor made no difference at that voltage. But who know, I certainly don't at this time, maybe it works at 0.8v, but I will find out.

 

[TotalMadness]

Maybe you could tell me what I should be looking for in a commercially bought power supply?

 

[ronv]

How much current do you need?
Could you use a 15 volt wall wort as the bulk power supply?
It will be a bit more complicated... Is that ok?

 

[TotalMadness]

I decided to order one of these:

**broken link removed**

Maybe I can figure out why the above circuit doesn't work correctly now.

 

[hannobisschoff@gmail.com]

Basically you need to check the output ripple, the Vout and Current-out