Power supply can't get zero volts

[agent86]

Hi All, thanks for reading my post.
I had an old HO train transformer (power supply), that had failed. I decided to modernize the circuit components to make it safer, and more reliable. I used a solderless breadboard to make the circuit below. As designed, the operator would use the 10k Ω potentiometer as a throttle to control the voltage to the track (Vout). Ideally, I want to be able to control the voltage from 0-16 volts with a max current of 1 amp.

The circuit as shown works well from 2 volts to 16 volts, and the current is adequate. The problem is that when the potentiometer wiper is set to "0" volts, the output of the op amp pin 6 remains at about 2 volts. And the output of the transistor 2N3055 pin 2 (emitter) remains at about 1.9 volts.

The input at the left-hand side of the circuit is about 28 volts dc from a step-down transformer and the output of the L7818 is 18 volts.

Question: What is the simplest modification to make the circuit work as desired? I was considering using a "push-pull" transistor pair instead of just one as shown, but I'd also like to understand what is wrong with the circuit I've designed. I thought the op amp used as a follower would yield 0 volts when the input pin 3 was drawn to 0 volts or the common pin of the regulator voltage.

 

[Bluejets]

Op amp operation needs in many instances to operate from a split supply in order for the output to get down to 0 volts. Many years since I did the explanation on this so others will be able to give you the exact reasons why.

 

[AnalogKid]

On the 741 data sheet it states in tables and graphs that the output cannot swing to within about 1.5 V or 2 V of the rails and the input doesn't function when outside of the same range (working from memory). This is a common issue with general purpose opamps. The LM358 was one of the first devices to address this, with an input range that includes the negative rail and an output that swings very close to the negative rail. It is a dual opamp so you would use only 1/2 of it.

If you want to stick with a single opamp that has the 741 pinout, search for devices with "rail-to-rail" input *and* output stages. Also, the 3055 does not have much gain, so you will get better regulation with less stress on the opamp output stage if you change to a power darlington transistor, like something in the TIP1xx series.

ak

 

[duke37]

A Zener diode between the 741 and 3055 will solve the op-amp output problem but I have not studied the input requirements of the 741.

 

[AnalogKid]

A Zener diode between the 741 and 3055 will solve the op-amp output problem.

Could you sketch that?

ak

 

[AnalogKid]

Here is what I think you are talking about::

http://www.discovercircuits.com/DJ-Circuits/dcindicator6.htm

ak

 

[duke37]

Could you sketch that?

ak

I do not have the tackle at present to load a diagram.
All you need to do is to place a zener between the 741 output and the base of the 3055. So if the minimum output of the 741 is 2V and you place a 3.3V zener in the feed to the 3055 then it will not be turned on.

The disadvantage is that the maximum output voltage is reduced by the zener voltage.
I have not looked at the 741 input, this will need to work down to zero volts for control.

Use an appropriate op-amp which can do input and output down to zero volts.

 

[agent86]

Thank you all for your advise.
Special thanks to AnalogKid, your first note was really helpful. The datasheets I'm using do not have any graphs (TI and Nat Semi), but they do imply what you noted with the "Output Voltage Swing" parameter specification. I just didn't understand that point but your note was perfect, and saved me hours of frustration.

So the "fix" i came up with was pretty simple. I took an old phone charger that supplies 5 volts, and simply connected the plus 5 side of the charger to the circuit ground of my existing circuit, thereby giving me a negative 5 volts with the neg side of the charger. I connected the neg 5 volts to pin 4 of the 741 pulling the rail more than the required 2 volts below the desired operating point. The 5 volt charger supplied more than enough current since I only used it to pull the V minus rail of the 741 down below zero. As shown in the circuit, I added the charger beside my circuit and mounted it inside the project box. The thing works great! Thanks again, to all.

 

[duke37]

Excellent result.
One suggestion I would make is to connect the 2N3055 collector to the reservoir capacitor. This will reduce dissipation in the regulator and give a slightly higher maximum voltage.