lm741 in power supply

[Gaston]

could i drive the base of a tip140 directly off of pin 6 on an lm741? or would i have to put another transister in between? if so what would i use? an 2n2222, and how would i bias it

 

[ericgibbs]

hi gaston,
if you have a diagram on how you want to connect your op amp to the
transistor please post it.

A lot depends on the circuit is powered and what the transistor is going
to do.

Regards
EricG

 

[Gaston]

i'll have to figure out how to get a schematic on here. i guess i can design it in multisim then take a screenshot and post it as a jpeg?

 

[Hero999]

Yes, do that.

What are you using as a voltage reference?

How much current are you planning to draw?

Have you tested it off load?

 

[Gaston]

im useing a zener diode and capacitor as a voltage referance,i'm panning on drawing ten amps through 4 "to3"output transistors. it works without a load but when a load is put on it the voltage drops and the voltage at pin six on the op amp goes down to almost nothing. im going to try to post a schematic. i haven't doen it before so we will see what happens

 

[Cabwood]

There's nothing wrong with you connecting the op-amp output directly to the darlington base in THIS configuration. My guess is that the load causes the transistor emitter to drop to nearly zero volts, and the two bases will follow it down. This would happen if the load (R2) is extremely heavy. Also, have you checked Vcc to see if that drops also under heavy load?

D1 should be a resistor, not a diode. Either the zener or D1 will smoke as you've connected it here.

 

[Nigel Goodwin]

Either the zener or D1 will smoke as you've connected it here.

Both - as the zener is connected the wrong way round.

 

[audioguru]

The zener diode is upside down with a voltage of only +0.7V.
The opamp doesn't have a power supply. It probably needs a negative supply in addition to a positive supply to allow low output voltages.

 

[Gaston]

vcc stays nice and steady. i do have a resister between the two diodes i forgot to draw it. sorry. i did realize after studying the data sheet for the lm741 that the supply voltage is only good for 18 volts. i had 35 volts going to it. i added a lm7812 in the supply line to the op amp it worked for a while but i could only get about 15 volts out of the power supply and after a load it only puts out about 5 volts. i had this thing working when i built it a few years ago. i tested it but never realy used it because i didn't need that much current and i used my smaller store bought supply. i finaly needed the current and it quit working.

 

[Cabwood]

Good call Nigel. You've saved us from the looming "I bought a 9.6V zener which only gives me 0.7V" post.

 

[Gaston]

sorry i just drew it wrong. its been a while since i used the multisim and i was just trying to explain the layoput of the transistors. like i said it used to work fine. i could push 10 amps at about 25 volts into a modified toaster oven and the elements would glow. i let ir run for a while like that too to do a burn in test. i can jump 35 volts to the base of the output transistor, and the darlington and it will put out so i know the problem is before the darlington. i tested the voltage geting to the input of the opamp and it goes up and down with the pot and the invertiong input follows the output. the output of the amp just isn't correct. i don't know if its drawing too much current and cant keep up or what. now i did have a nec d381 in between the opamp and the darlington originally when it was working. i cn't find the spec sheet on it and don't remember what it is. i figured i should be able to bypass it. but mabey not? anyone know what the d381 is? my guess is a plain npn transistor but im not sure

 

[audioguru]

Four output transistors in parallel with a 10A load each have a load of 2.5A. The minimum current gain at 2.5A for a 2N3055 power transistor is about 30 so the max current from the TIP140 is only 10A/30= 333mA. The minimum current gain of the TIP140 at 333mA is about 250 so the max output current from the 741 opamp is only 333mA/250= 1.3mA. The minimum output current from a 741 opamp is about 15mA so the circuit should work fine if all parts have minimum spec's. Replace the opamp.

 

[audioguru]

I don't think you are using the correct pin numbers for the opamp.

 

[Gaston]

i finally got it to work. the elements in my toaster oven are 2.3 ohms. it maxes out at 23.2 volts. i let it run maxed out for about twenty minutes. the heat sink was hardly warm. i had the oven on the floor and it was makeing my feet uncomfortable so i shut it off. then i went to resume my project that i should have been working on all day instead of this. i am running a fair sized stepper with a pic and four tip142s. i turned it on and it was drawing max current and the stepper wouldn't turn but kept shaking. then the smoke came out of my power supply. it was the op amp.what could be causing this. the power supply has the tip142s in between the stepper and itself so no transients should get through. is it a harmonic of the stepping frequncy destroying the op amp? any thoughts on this?

 

[kchriste]

then the smoke came out of my power supply. it was the op amp.what could be causing this. the power supply has the tip142s in between the stepper and itself so no transients should get through.

If the transients are high enough they'll cause the junction of the tip142s to go into zener breakover and feed right into pin 2 of the poor little 741.... Put some clamping diodes accross the stepper coils.

 

[Gaston]

do you think there would be an advantage if i put a TL071 in there instead? thats the only other op amp i stock.

 

[audioguru]

A TL071 has the same pins layout and the same max output current of about 15mA as an741 opamp. But if the stepper motor makes unarrested voltage spikes then the TL071 will also be destroyed.

 

[Hero999]

vcc stays nice and steady. i do have a resister between the two diodes i forgot to draw it. sorry. i did realize after studying the data sheet for the lm741 that the supply voltage is only good for 18 volts. i had 35 volts going to it.

The LM741 can take a maximum of +/-22V, i.e when it's running from a bipolar power supply, with a single supply it can take 44V.

The maximum differential input voltage is +/-30V, i.e. the +input can be +15V and the -input can be -15V or vice versa.

The maximum input voltage when run off a bipolar supply is +/-15V.

Your circuit sould be fine working from 35V, providing that zenner is a high enough voltage to keep the -input high enough above 0V.

 

[Gaston]

could the voltage spikes get all the way back to the op amp though? it would have to get through the tip142s which have suppression diodes in them, then the output diodes of the power supply, the output transistors, the tip140 driving the output,the d381, then a 2n2222. i guess i could put my scope on the op amp and see what happens. i just don't want to keep destroying parts because it might take out something else next time.

 

[kchriste]

could the voltage spikes get all the way back to the op amp though?

According to the diagram you posted (https://www.electro-tech-online.com/attachments/opamp-and-darlington-with-text-jpg.10977/), pin2 of the op amp is connected directly to the load so it is easy for the spikes to get into the op amp.