Feasability of Adding a Second Parallel Darlington?

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

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 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

Hi John,

Today, driving the car, I found back the way to include the load correctly in the calculation of the output current. I updated the calculation above.

John Wiltrout wrote:

 

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?

Probably it is saved by a slowly rising supply in combination with the voltage at the adjust terminal.

important question:

Do you know what the calibration procedure is? Is the output voltage tuned to a certain value, or is it tuned per load device?

more important question:

you have a problem with overload. Do you know what the requirements are for the load? Do you need a constant voltage, or a constant current limit or both? Without this, it's hard to know what the 'right' solution for your problem is. Adding a thermal fuse (polyfuse) might protect the darlington, but cut down the motor power too much.

If you know what current / voltage limit you need, you could design for that purpose. Linear has some nice constant current / constant voltage solutions, actually meant for LEDs, but maybe also for this application.

Hi John,

Today, driving the car, I found back the way to include the load correctly in the calculation of the output current. I updated the calculation above.

John Wiltrout wrote:

 

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?

Probably it is saved by a slowly rising supply in combination with the voltage at the adjust terminal.

important question:

Do you know what the calibration procedure is? Is the output voltage tuned to a certain value, or is it tuned per load device?

more important question:

you have a problem with overload. Do you know what the requirements are for the load? Do you need a constant voltage, or a constant current limit or both? Without this, it's hard to know what the 'right' solution for your problem is. Adding a thermal fuse (polyfuse) might protect the darlington, but cut down the motor power too much.

If you know what current / voltage limit you need, you could design for that purpose. Linear has some nice constant current / constant voltage solutions, actually meant for LEDs, but maybe also for this application.

The LM317 is good for 0 - 37V out according to https://www.fairchildsemi.com/datasheets/LM/LM317.pdf

and it can have a Vin to Vout of up to 40V so as long as the output is never taken to less than about 10V on the LM317 it should be good

regards

Hi Victor, This is a motor unit that the dentists use to trim and polish things like dentures outside the mouth. Each time they use it they pick a setting for VR-1 that they want and then they use the handpiece to grind away plastic or polish a surface. As they apply pressure the motor slows and demands more current due to less counter EMF. I have already repaired and sent the unit back or I would be able to measure DC resistance and operational current which I did not do when it was on my bench.

John