# 0-30VDC Power Supply (Help)

Discussion in 'The Projects Forum' started by choonfeng, Aug 1, 2009.

1. ### choonfeng Thread Starter New Member

Mar 3, 2009
2
0
Can somebody help me?
I construct the circuit exactly the same with the circuit diagram that shown in attachment. The output voltage of the op amp should be able to vary from 0v to 30v right? How come the actual result that i get is 6v to 30v? Is this circuit problem?

This circuit is able to work on negative supply but it is not working for positive supply!!!

Why ?!?! Can somebody explain to me?

Help T.T

****C19 is 10uF/50V

Thanks

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2. ### yourownfree Active Member

Jul 16, 2008
89
0
am confused too! you said it outputs 6-30 vdc which is positive but then you said it works on the negative supply but not the positive? Also what is the voltage at R7 input side? looks like whatever you put into it, it outputs 3 times as much, so a gain of three for the op amp I guess. You put in 2 volts 6 volts is outputted.

3. ### choonfeng Thread Starter New Member

Mar 3, 2009
2
0
The circuit that i am using right now is positive supply for the op amp, which means the supply that supplied to pin7 and pin4 is 36vdc and 0vdc.
The circuit that works on the negative supply can be varied from 0vdc to -30vdc. The supply that supplied to pin7 and pin4 is 0vdc and -36vdc.
This circuit is a gain of three for the op amp. The input voltage at R7 side is from 0v to 10v, the output should be able to get the voltage from 0v to 30v. However, if i put in 0 to 2 volts but the output that i get is 6volts.

4. ### k7elp60 Active Member

Nov 4, 2008
469
69
1. The output voltage on pin 6 of the OP amp will never be the same as the voltage on pin 7 or pin 4. This is not a rail to rail op amp. Typically the output voltage will be about 2.5V less than the voltage on pin 7 and typically 2.5 volts more + than the voltage on pin 4.
2. There is going to be a voltage drop across the base/emitter junction of the TIP110 transistor, as well as small voltage drop across the 68Ω resistor on the base of the transistor.
3. There is going to be some error as you donot have the offset null circuitry on pins 1 & 5 of the OP amp.

5. ### SgtWookie Expert

Jul 17, 2007
22,182
1,728
The common-mode input voltage of the TL07x opamps has a limit of around Vee+2.5v, give or take some. Since you have Vee grounded, the low limit is around +2.5v; since your gain is three, the lowest it'll output is around 7.5v. Your TIP110 Darlington will drop around 1.3v from that, giving you a minimum output voltage of around 6.2v.

I see you have a 68 Ohm resistor on the output of the TL071. That is not a realistic current limit. 2k Ohms would be more realistic. Even with a 2k load, you'd be lucky to get within 2v of Vcc, and the TIP110 will drop another 1.3v or so from base to emitter.
Therefore, your output range would be from around 6.2v to around 26.3v.

6. ### Audioguru New Member

Dec 20, 2007
9,411
895
In addition to the common-mode input voltage limits, the TL071 and TL081 opamps have The Phase Inversion problem. When an input voltage gets too close to the negative supply then the output suddenly goes positive.

You can use an MC34071 or TLE2141 opamp that has inputs that work properly at the negative supply voltage (ground in your circuit). They have a max allowed supply of 44V so you can increase the supply high enough for a +30V output.

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