Feasability of Adding a Second Parallel Darlington?

Hi John, a very good question, and the answer is fairly straight forward

If the Darlington is getting overloaded due to exceeding the current limit of the Darlington then an additional one in parallel would help to fix the problem (or two, or three ), you must limit the base current separately though with its own 3K resistor and the emitter current through its own 1R resistor, this will prevent one Darlington hogging all the glory.The LM317 has more than enough drive to handle it

basically the Darlington's are acting as emitter followers and will be outputting about 2 diode (B-E junctions) drops less than the output of the LM317 (So about 1.5V ish), as long as you don't exceed the output current of the LM317 or the power supply, add as many as you like in the way I described above. The other possibility is the current capability of the Darlington is not being exceeded but the heat sink is inadequate if the additional devices have their own then this should fix it but if not you may still experience issues so check the heat sink before investing in the additional drivers.

The other option if room is an issue and the heat sink is ok, is to replace the current Darlington with a bigger more powerful one

Hope this helps

Peter

Hi Peter, It certainly did. This is exactly the information that I was looking for. I really appreciate your taking the time to give me the details. I can see how important it is to use seperate base and emitter resistors. I ultimately took your secondary advice and searched for a slightly heftier Darlington in the same TO 220 casing and I will use that. Thanks again for your wisdom.

John

More than welcome John

regards

peter

Hi John,

You say that the circuit has no short circuit protection, but the 3k resistor with a hFE of 1000 means some current limit:

The supply is 36×√2 -(1.2V diode bridge drop)  ≅ 50V.

At startup the motor does not generate back-EMF, and the capacitor is empty, so you'll basically just have the 1 Ohm as load.

I did a short calculation here (this will link to an editable version):

The combination of the 1 Ohm resistor and the darlington base resistor limit the current to somewhere around 10.95A. Of course this will heat up the darlington, but it is a current limit. If you will meddle with this circuit, this behaviour will also change. As you can see, changing the output voltage of the LM317 will also change the current limit. If you parallel, double each darlington's base resistor to keep the current limit.

Now for parallelling: this will only work nicely if the darlingtons' conductance has a negative temperature coefficient: if they heat up, they should conduct less. In that way the load will balance evenly over the darlingtons. If the temperature coefficient for conductance is positive, one of them will heat up, conducting better, so getting more amps and heating up more, etcetera.

Succes!

edit: added the Rload of 1 Ohm in the calculation of the current limit

Victor

Hi Victor,

Thank you for the well written analysis of the current limitations in my circuit. I am going to study it closely to help me learn from this situation. Your mention of the change in circuit characteristics when the Darlingtons have a positive or negative temperature coefficient reminds me of number series in mathematics and determining whether they converge or diverge. A positive coefficient will cause the two darlingtons to diverge triggered by small differences in their specs while a negative coefficient will keep them working together as either one that is taking more of the load will heat up and self regulate to a lower current level. One thing that has me concerned is that the rectified voltage 36V * 1.4 = 49.7V should be too high for the LM317. Is this possible because the adjust and output are not at 0V? I have already taken up too much of your time. It was good to hear from you again and thanks again for your insights.

John

John

a quick answer before heading to work.

I'm not surprised the darlington fails.

This is only working because there is a drop across the series 1 ohm resistor.

The transistor is turning on, then its emitter is rising such that it turns off, and its always operating in never really ON or OFF mode so it will heat up.

My suggestion is to look at putting the motor and diode in the collector, and then tweak the regulator to detect the voltage across the 1 ohm, which will be between the emitter and ground.

Adding a second darlington might load share, but the basics are wrong.

Mark

This may help in your understanding John

http://en.wikipedia.org/wiki/Common_collector

this is effectively what you have.

overall the Darlington is providing a bypass boost to the voltage regulator in order to increase its output current capacity, and in fact take over that job entirely

common collector transistors are almost self governing due to the negative feedback provided at the emitter from the load, anyway, have a read of the article and it will help your understanding

in addition to this the series emitter resister de-generates the transistor when used in parallel circuits in order to mitigate the negative temperature co-efficient of the BJT. (FETS of course having positive Temp co do not suffer this problem)

to quote this article http://www.allaboutcircuits.com/vol_3/chpt_4/12.html

Common-collector amplifiers have much negative feedback, due to the placement of the load resistor between emitter and ground. This feedback accounts for the extremely stable voltage gain of the amplifier, as well as its immunity against thermal runaway

Hi Mark, Thanks for taking the time to give me some insights. I will have to explore your suggestion.

John

Hi Peter, Thanks for the followup. I have printed the article on Feedback and will read it tonight. I have also bookmarked the All About Circuits Web site as it looked like it had other good information for a person in learning mode, like me. I also see that I have another one of your Power Supply Tutorials to look forward to. It is great to be retired so I have enough time to look at all this great information.

John

Yup, my videos seem to be too long for currently employed folks to watch and still have a life... , im working on it lol