skn133229
Posts: 1
Joined: Sat Mar 02, 2019 12:30 am

Running Raspberry PI with tip120

Sat Mar 02, 2019 12:36 am

Hi,

I have a problem I can't seem to figure out and wonder if I am fundamentally missing something. I am trying to turn on a Raspberry PI (RPI) 3 periodically to perform a task and then shutdown after the task is complete. A TPL5110 timer from Adafruit (https://www.adafruit.com/product/3435) is used to set a logic line high when it is time to turn on the RPI. The logic line from the TPL5110 connects to the Base of a darlington transistor TIP120 via a 2.2k resistor. The RPI 5V connects to POWER 5V and the ground connects to the Collector of the TIP120. The Emitter of the TIP120 connects to ground.

The problem I am having is that when the logic line goes high, the RPI turns on but seems severely under-powered. The green LED light on the RPI would flicker, then there will be a few flashes of the screen and the device seems to shutdown. The red LED never turns on. With my TIP120 arrangement, the current limit on the Collector - Emitter should be around 1.7A which should be largely sufficient to run the RPI. Why am I having this problem?

Any thought would be appreciated.

Thanks,

Andyroo

Re: Running Raspberry PI with tip120

Sat Mar 02, 2019 2:42 pm

Best guess is the Pi is pulling more current at start up than the 1.7Amps you can give it with the official supply is 5.1v at 2.5A with decent wires to allow for start up (all coresrunning) and the odd hat or two.

A couple of thoughts:
1) Just use a basic mains timerswitch on the supply - just make sure the Pi shuts down before powering off
2) Use one of the many HATs that can control the on/off cycle with an RTC

Not as much fun though :D

fruitoftheloom
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Location: Delightful Dorset

Re: Running Raspberry PI with tip120

Sat Mar 02, 2019 2:45 pm

skn133229 wrote:
Sat Mar 02, 2019 12:36 am
Hi,

I have a problem I can't seem to figure out and wonder if I am fundamentally missing something. I am trying to turn on a Raspberry PI (RPI) 3 periodically to perform a task and then shutdown after the task is complete. A TPL5110 timer from Adafruit (https://www.adafruit.com/product/3435) is used to set a logic line high when it is time to turn on the RPI. The logic line from the TPL5110 connects to the Base of a darlington transistor TIP120 via a 2.2k resistor. The RPI 5V connects to POWER 5V and the ground connects to the Collector of the TIP120. The Emitter of the TIP120 connects to ground.

The problem I am having is that when the logic line goes high, the RPI turns on but seems severely under-powered. The green LED light on the RPI would flicker, then there will be a few flashes of the screen and the device seems to shutdown. The red LED never turns on. With my TIP120 arrangement, the current limit on the Collector - Emitter should be around 1.7A which should be largely sufficient to run the RPI. Why am I having this problem?

Any thought would be appreciated.

Thanks,

Maybe ??

http://www.uugear.com/product/wittypi2/
Retired disgracefully.....

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Burngate
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Location: Berkshire UK Tralfamadore
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Re: Running Raspberry PI with tip120

Sat Mar 02, 2019 6:51 pm

skn133229 wrote:
Sat Mar 02, 2019 12:36 am
... The logic line from the TPL5110 connects to the Base of a darlington transistor TIP120 via a 2.2k resistor. The RPI 5V connects to POWER 5V and the ground connects to the Collector of the TIP120. The Emitter of the TIP120 connects to ground.
I think your problem is that the collector-emitter voltage of the TIP120 is unlikely to be less than a couple of volts, leaving far too little for the Pi.

I couldn't find any details of what current Adafruit's device can supply, but it appears it uses a DMG3415, which should be able to supply the current the Pi needs, without adding your own transistor to the mix.

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tlfong01
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Re: Running Raspberry PI with tip120

Sun Mar 03, 2019 2:13 am

Burngate wrote:
Sat Mar 02, 2019 6:51 pm
skn133229 wrote:
Sat Mar 02, 2019 12:36 am
... The logic line from the TPL5110 connects to the Base of a darlington transistor TIP120 via a 2.2k resistor. The RPI 5V connects to POWER 5V and the ground connects to the Collector of the TIP120. The Emitter of the TIP120 connects to ground.
I think your problem is that the collector-emitter voltage of the TIP120 is unlikely to be less than a couple of volts, leaving far too little for the Pi.
I couldn't find any details of what current Adafruit's device can supply, but it appears it uses a DMG3415, which should be able to supply the current the Pi needs, without adding your own transistor to the mix.

TIP120 Vce(sat) vs base current
The datasheet seems to say that increasing base current decreases Vce(sat)
Or better use a logical level (3/5V) triggered Power MOSFET with very low Rds(on), such as IRL540N.


TLP5110 Controlling DC-DC Boost or any other PSU
TLP5110 datasheet Fig 13 shows controling MOSFET, and Fig 15 controlling DC-DC boost. So I guess you can use TLP5110 to control any power source, say a PSU with a enable input.

Running Raspberry PI with TIP120
https://forum.allaboutcircuits.com/thre ... 20.157328/

TIP120/122 Datasheet
https://www.onsemi.com/pub/Collateral/TIP120-D.PDF

TLP5110 Datasheet - TI
http://www.ti.com/lit/ds/symlink/tpl5110.pdf
Attachments
tip122_spec_2019mar0301.jpg
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I am an electronics and smart home hobbyist.

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Burngate
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Re: Running Raspberry PI with tip120

Sun Mar 03, 2019 12:15 pm

tlfong01 wrote:
Sun Mar 03, 2019 2:13 am
The datasheet seems to say that increasing base current decreases Vce(sat)
Indeed - that's true of just about any bipolar transistor - that's how they work.
TIP120eq.png
TIP120eq.png (4.28 KiB) Viewed 354 times
The problem with Darlingtons such as TIP120 is that the collector of the second transistor cannot go lower than its base, so there'll always be at least one diode-drop across it.
Remarkably similar question - it would have been nice of him to come back to us with a response.

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