# npn darlington TR

Discussion in 'General Electronics Chat' started by mah, Dec 30, 2014.

1. ### mah Thread Starter Active Member

Mar 15, 2010
276
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if i used TP121 npn transistor to control a load that will hold 2Amp . will it need a heatsink?

2. ### tcmtech Well-Known Member

Nov 4, 2013
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What does the specs sheet say about it?

3. ### ian field Distinguished Member

Oct 27, 2012
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791
A Darlington can never saturate - its worth remembering this when calculating the dissipation at maximum collector current.

The output transistor of a darlington pair would be capable of a VCEsat of around 0.4V - but it never quite gets there because it clamps the collector voltage of its own driver transistor.

4. ### WBahn Moderator

Mar 31, 2012
18,087
4,917
Maybe. Maybe not.

What will the voltage across the transistor be? That will determine the power (coupled with the 2A load current). The power, in conjunction with the case style and the ambient conditions, will then determine if you need a heat sink.

5. ### mah Thread Starter Active Member

Mar 15, 2010
276
2
the voltage across transistor is +24 v (Vc).i will use it in linear mode ,the base voltage will vary from 0-24 v

6. ### ErnieM AAC Fanatic!

Apr 24, 2011
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So you have 24 volts at 2 amps. That means the power is 24*2=48 watts.

Every put your hand on a 50 watt light bulb?

7. ### crutschow Expert

Mar 14, 2008
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3,372
If the load is a linear resistance than the maximum power dissipated in the transistor will occur at 1A with 12V across the transistor, giving 12W dissipated. So you will definitely need to mount the transistor on a healthy sized heat-sink.

8. ### mah Thread Starter Active Member

Mar 15, 2010
276
2
what is the size of heat sink ?اif i used high frequency PWM , will that solve it?. this type is used in linear power so , i am on the right way.

9. ### crutschow Expert

Mar 14, 2008
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3,372
Yes PMW will reduce the power dissipation in the transistor to just switching losses. The frequency does not have to be very high if the load has significant thermal inertia.

10. ### mah Thread Starter Active Member

Mar 15, 2010
276
2
the load is a proportional valve . the input signal will range from 0-24v . could i use UC3842 PWM controller.?
but if i used PWM i could replace this npn with a mosfet to hold more current as the operation will be converted to switching , won't it?

Last edited: Dec 31, 2014
11. ### mah Thread Starter Active Member

Mar 15, 2010
276
2
if i used dc-dc converter it will take space on the board .any suggestions?

12. ### ian field Distinguished Member

Oct 27, 2012
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That chip is for mains operated SMPSUs so you have to watch out for the UVLO, but with 24V you should be OK. There's an internal 30V zener on the Vcc pin so you have to protect it from any spikes on the rail that might exceed that - you'll probably have to use a MOSFET because that's what its designed to drive.

13. ### crutschow Expert

Mar 14, 2008
13,492
3,372
Yes, you should use a MOSFET for PWM, but not because it "hold (carries?) more current", it's because a MOSFET has no saturation switching delay, has a lower ON voltage, and requires no steady gate current (just what is required to charge/discharge the gate capacitance), so is generally more efficient as a switch.

14. ### mah Thread Starter Active Member

Mar 15, 2010
276
2
but Mosfet is not designed for linear loads. how to use it with PWM will it do the job?

15. ### ian field Distinguished Member

Oct 27, 2012
4,447
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PWM gives you the net effect of linear drive while switching the MOSFET either fully on or fully off, so dissipation in the MOSFET is minimal.

If your PWM driven MOSFET was in a PSU, you'd need some form of LPF (LC network) to get smooth DC. You probably don't need that for your application, but you haven't made the details all that clear.

mah likes this.

Mar 15, 2010
276
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17. ### mah Thread Starter Active Member

Mar 15, 2010
276
2
i want to control proportional valve using op-amp .the output from the opamp is 0-24V .this voltage will be used to switch on transistor which will open the valve. that's all

18. ### ian field Distinguished Member

Oct 27, 2012
4,447
791
That's under voltage lock out - the chip you mentioned needs Vcc to reach 16V for it to start, once its running - taking Vcc down to about 10V will cause it to shut down.

2 of the SMPSU chips in that series are for automotive applications and have lower UVLO thresholds.

19. ### mah Thread Starter Active Member

Mar 15, 2010
276
2
i propose the attached sketch . using a comparator will control the current of the mosfet by controlling the width of the duty cycle ,when the input signal changed from 0-24V. i put R1 to control max voltage consequently max current in the valve . i think now we don't need heatsink.

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20. ### ian field Distinguished Member

Oct 27, 2012
4,447
791
What are you feeding into the comparator negative input?

PWM is easy with a 555, all you need is the track of a pot making separate charge and discharge paths - a couple of steering diodes determine which path is charge or discharge. The same idea can be applied to a much simpler CMOS inverter oscillator - the left over gates can be used as a buffer to drive the MOSFET.