Op-Amp Subsitiution

Overclocked · Dec 31, 2005

I am building a Linear 12v 14 Amp power Supply (as some of you may recall from another thread too) I am using a LM338K Pos Adj regulator.

Consider the following circuit (taken from the data sheet)

**broken link removed**

It says I need a LM107, But thats obsolete, and the company I am ording it from doesnt have it. I assumed I can use LM741 opamp. But I only have the 18V version. But I also have TL082 and LM1458. Could I use either of those instead of a LM741?

chemelec · Jan 1, 2006

A 741 Op-amp should work.
Besides that 18 volts is both Plus and Minus.
So in this circuit it could allow for up to 36 volts.

If I understood you correctly.

Overclocked · Jan 1, 2006

Im talking about internal circitry of the opamp. I know that LM1458 is the same as 741, but has 2 opamps and doesnt need a v neg (Im not sure on that part though) But the TL082 has Jfet tech for inputs

But Im assuming the job of the op amp here is to make sure both regulators get an equal amount of workload, so it shouldnt matter which op amp I use, Correct?

ljcox · Jan 1, 2006

Overclocked said:
Im talking about internal circitry of the opamp. The internal circuitry is not critical. I know that LM1458 is the same as 741, but has 2 opamps and doesnt need a v neg (Im not sure on that part though) But the TL082 has Jfet tech for inputs. JFET inputs are suitable.

But Im assuming the job of the op amp here is to make sure both regulators get an equal amount of workload, so it shouldnt matter which op amp I use, Correct? Correct

Op amps have a very high gain. So (without feedback) a small voltage difference across the + / - input pins causes a large voltage at the output.

So in the circuit shown, there is feedback due to the currents through the 0.1 ohm resistors. The result is that the output voltage of the op amp is set such that the currents through the 0.1 ohm resistors are approximately equal.

audioguru · Jan 1, 2006

The old LM107 was chosen because its inputs still work when they are at its positive supply voltage. The inputs of a 741 or 1458 won't.
The inputs of a TL081 work at its positive supply voltage.

Overclocked · Jan 1, 2006

So TL081 Will work?

ljcox · Jan 2, 2006

Overclocked said:
So TL081 Will work?

Audio answered that question in his last sentence. The TL081 will operate with its inputs at the + supply rail as the LM 107 did. So the answer is yes.

Someone Electro · Jan 2, 2006

Just to remind you you are only gona get 10A out for this thing when the voltage is thruned up to the max.Even if the output is 2 V less you are meking 20W of heat.At 10 V difereance the regs wod instalntly blow.

Oh and you need an realy realy realy big heatsink and an fan to get rid of all that heat.

audioguru · Jan 2, 2006

Someone Electro said:
Just to remind you you are only gona get 10A out for this thing when the voltage is thruned up to the max.Even if the output is 2 V less you are meking 20W of heat.At 10 V difereance the regs would instalntly blow.

Oh and you need an realy realy realy big heatsink and an fan to get rid of all that heat.

The regulators are smart and won't blow. They just reduce their output current or shutdown to protect themselves. With 32V input and a low adjusted output voltage, each regulator will provide way less than 2A, only 1A is guaranteed.

The K package is a T-03, has a thermal resistance 1.5 times a 2N3055 and has a max temp 50 degrees C cooler. With a whopping huge heatsink, no insulator and a fan then each should be able to dissipate 30W.

The project should be able to provide 2A at 5V continuously.

Overclocked · Jan 2, 2006

I have a Huge Heatsink I salvaged from a power Supply and can mount 3 TO-3 Transistors. Ive calculated it roughly to be around 1.5C/W(that was a rough Calculation)

Im planning on having A couple fans in there too to help cool off the regulators.

Strange thing is, I hooked the transformer up to 120V(Only 1 Primary) and measured voltages no load, ad I got some where around 13V to 16 or so volts. I Only planned on having it go from 0 to 12V or Just stay at 16 or so volts (Its going to be used to power Pelts, Or just to generate Hydrogen)

Someone Electro · Jan 2, 2006

You would may be way better off whith 3 2N3055 staked togeter on that heatsink.And are used to "amplefy" the regs curent.Just stick wethwen the + input and + output then conect the regulators output to there base trough a resistor.And dont forget the resistors in the transistors so they share the curent equly.

The TO-3 are designed to distapth a LOT of heat and linear regs do exsctly that.But the mounthing on a heatsink is way more complicated than an TO-220.And also dont forget termal paste for better heat transfer.

Roff · Jan 6, 2006

Someone Electro said:
You would may be way better off whith 3 2N3055 staked togeter on that heatsink.And are used to "amplefy" the regs curent.Just stick wethwen the + input and + output then conect the regulators output to there base trough a resistor.And dont forget the resistors in the transistors so they share the curent equly.

The TO-3 are designed to distapth a LOT of heat and linear regs do exsctly that.But the mounthing on a heatsink is way more complicated than an TO-220.And also dont forget termal paste for better heat transfer.

Yeah, but 2N3055s aren't "bulletproof" like the LM338.

Overclocked · Jan 6, 2006

Someone Electro said:
You would may be way better off whith 3 2N3055 staked togeter on that heatsink.And are used to "amplefy" the regs curent.Just stick wethwen the + input and + output then conect the regulators output to there base trough a resistor.And dont forget the resistors in the transistors so they share the curent equly.

The TO-3 are designed to distapth a LOT of heat and linear regs do exsctly that.But the mounthing on a heatsink is way more complicated than an TO-220.And also dont forget termal paste for better heat transfer.

Hmm now that I found out what type of transistors I have in my box, it doesnt sound like a bad Idea. But one thing comes to mind. Cost.

It would be cheaper to build the thing with op-amps than use a bunch of transistors. Sure I have some TIP120's lying around not being used, ( I would have to use 2 for one 12V line)

But I can get .1W Resistors pretty cheap at bgmicro (69 cents,) and Im buying 2 for the opamps anyway...So I'll see..

But as mentioned above, they dont have thermal protection. BUT then again, transistors can operate at a higher temp than regs.

Overclocked · Jan 15, 2006

For the 3 Resistors (not the 2 .1ohm ones) Is it ok to use 1/4 or 1/8W Resistors? The 120ohm and 2K Resistor are only there to control voltage, so I assume they dont need to be high power ones, Correct?

audioguru · Jan 15, 2006

The power in a resistor is caculated by the voltage across it squared, then divided by the resistance. it won't be reliable if operated at its max power rating.

dinofx · Jan 16, 2006

What about building a synchronous buck converter? You could generate a triangle wave for the PWM, and run it through a comparator that is controlled by a voltage regulator carrying only a fraction of the actual load.

I just finished building a circuit that was disappating 4W from the same TO-220 package. The heatsink is approaching 100 C.

You could add some power resistors in parallel around the REGs, and turn them off with mosfets as the voltage decreases.

audioguru · Jan 16, 2006

dinofx said:
The heatsink is approaching 100 C.

Then the heatsink doesn't do its job and the chip's temp is much higher, way over its rating.

dinofx · Jan 16, 2006

Keep in mind I'm measuring temps with my butt. Isn't it rated at 150C? What do you think that corresponds to externally at the heatsink:

radio shack heat sink

audioguru · Jan 16, 2006

dinofx said:
Keep in mind I'm measuring temps with my butt. Isn't it rated at 150C? What do you think that corresponds to externally at the heatsink?

I never tried to measure temp with my butt. :lol:
Your little heatsink is rated for only about 4W. RadioShack doesn't have spec's for it and doesn't even say its small size. Somebody on another forum recently tried it and it was useless.

An LM338 in a TO-220 case has a max temp of 125 degrees C and a max thermal resistance of 4 degrees C per W. Therefore if the power is only 4W and the heatsink is 100 degrees then the chip is 116 degrees C, pretty darn hot.

Op-Amp Subsitiution

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