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### Topic: using a transistor or mosfet to power sim800 module (Read 707 times)previous topic - next topic

#### robbideloose

##### Sep 25, 2018, 08:48 pm
Hi Guys

I'm really beginning to notice my lack of electronics background..

I'm building a low power beehive monitor using Arduino pro mini 3.3v. everything is powered from li-ion battery at 3.7 - 4.2v

sim800L module likes its voltage between 3.7 and 4.2v and takes about 2 amps peak.

To reduce the power consumption I would like to be able to cut the power to the sim800L module.
I have been reading up on transistors but I'm quite lost as I don't understand what I'm reading in the datasheets of different types of transistors.

I was trying my luck with a TIP120, but that seems to give me a voltage drop of 0,7v = 3,5v, causing the sim800L to go into undervoltage protection.

I see people talking about p-channel mosfets, but I think I might face the same issue there.

Can somebody help me out, or point me in the right direction?

#### blh64

#1
##### Sep 27, 2018, 06:21 am
Search for a MOSFET with low RDSon resistance (milli Ohms) and then the voltage drop will not be as great

#### Grumpy_Mike

#2
##### Sep 27, 2018, 07:20 am
Quote
I was trying my luck with a TIP120, but that seems to give me a voltage drop of 0,7v = 3,5v
You sure of that? The TIP120 is a Darling transistor pair and according to the data sheet at 3A collector current you can waste 2V from collector to emitter. At the verey least I would expect 1.4V drop.

Quote
I see people talking about p-channel mosfets, but I think I might face the same issue there.
No you don't.

A FET does not have the equivalent of a Collector to Emitter saturation voltage. Instead it has a turn on resistance. Therefore the voltage you drop depends on this resistance times the current you are drawing. Typically this resistance is much less than one ohm, and more likely in the tens of milli ohms or smaller. So the voltage you loose is going to be very small.

#### septillion

#3
##### Sep 27, 2018, 09:55 am
But why do all this to save so little? The sleep current is 0,7mA max. Okay, it's not nothing but a single led draws at least ten times that.
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#### robbideloose

#4
##### Sep 27, 2018, 11:47 am

Search for a MOSFET with low RDSon resistance (milli Ohms) and then the voltage drop will not be as great
A FET does not have the equivalent of a Collector to Emitter saturation voltage. Instead it has a turn on resistance. Therefore the voltage you drop depends on this resistance times the current you are drawing. Typically this resistance is much less than one ohm, and more likely in the tens of milli ohms or smaller. So the voltage you loose is going to be very small.
Thx for the tips guys! I'll look into the mosfets

You sure of that? The TIP120 is a Darling transistor pair and according to the data sheet at 3A collector current you can waste 2V from collector to emitter. At the verey least I would expect 1.4V drop.
I think the sim800 will only use +2A when connencting to/sending over gprs, so amps will be much lower when starting up and conecting to network.

#### robbideloose

#5
##### Sep 27, 2018, 11:59 am
But why do all this to save so little? The sleep current is 0,7mA max. Okay, it's not nothing but a single led draws at least ten times that.
It is not just the sim800 that will get it's power from the transistor, but a second arduino and a couple of other sensors as well (HX700's, htu21 DS18B20's, ...). Power is provided by 2 18660 batteries. primary arduino is a pro mini, modified for low power(wich will turn into a breadboard style arduino on a pcb if I got all the periferals sorted out.

Te device will have to remain outside for the entire year and report the readings every 15 to 30 minutes. that a lot of time doing absolutely nothing. I'll add a small solar pannel, but in winter I won't be able to rely on that if it would snow for example.

#### septillion

#6
##### Sep 27, 2018, 01:27 pm
Okay, I can only go with what you tell us But still, most sensors have a sleep function. But if you really switch them off, pay attention to phantom power and startup stuff!

And to go to the sim800L, how long does it take from power up to be able to send a SMS? If that's 30 seconds at 20mA that's the same amount of power as the module in sleep for 15 minutes...

PS, I hope you removed the regulators on the Arduino's?
Use fricking code tags!!!!
I want x => I would like x, I need help => I would like help, Need fast => Go and pay someone to do the job...

NEW Library to make fading leds a piece of cake

#### MarkT

#7
##### Sep 27, 2018, 03:18 pmLast Edit: Sep 27, 2018, 03:18 pm by MarkT
But why do all this to save so little? The sleep current is 0,7mA max. Okay, it's not nothing but a single led draws at least ten times that.
700uA means you can't run something off batteries unattended for more than weeks
(unless pretty large batteries)
~10uA standby means you can run something unattended, battery powered, for years...
(assuming the standby current dominates and battery chemistry is low self-discharge)

There's a qualitative difference in something you only have to maintain once a year rather than
once a week...
[ I will NOT respond to personal messages, I WILL delete them, use the forum please ]

#### MarkT

#8
##### Sep 27, 2018, 03:22 pmLast Edit: Sep 27, 2018, 03:23 pm by MarkT
Thx for the tips guys! I'll look into the mosfets

I think the sim800 will only use +2A when connencting to/sending over gprs, so amps will be much lower when starting up and conecting to network.
No, that's a bogus assumption.  It may require 2+ amps at any time it chooses, that is something
negotiated on the fly between the module and the base station depending on reception conditions.

You need a _logic level_ pMOSFET, note, that's essential.  0.1 ohms or less is good or you'll need a
heatsink.  Remember that if you power down the module there must be no Arduino OUTPUT pins connected to
it still sitting at a HIGH level, or you'll burn something out.
[ I will NOT respond to personal messages, I WILL delete them, use the forum please ]

#### septillion

#9
##### Sep 27, 2018, 03:24 pm
I know, but I'm still afraid OP is pissing away power
a) with other components
b) getting up and running once every 15min
Use fricking code tags!!!!
I want x => I would like x, I need help => I would like help, Need fast => Go and pay someone to do the job...

NEW Library to make fading leds a piece of cake

#### robbideloose

#10
##### Sep 27, 2018, 05:56 pm
No, that's a bogus assumption.  It may require 2+ amps at any time it chooses, that is something
negotiated on the fly between the module and the base station depending on reception conditions.
I was assuming it would not even try to connect to the network when undervolted. Didn't send any AT command to it in that state, so I'm not 100% sure, but according to the leds it is not even searching for network. It does not realy matter anyway

You need a _logic level_ pMOSFET, note, that's essential.  0.1 ohms or less is good or you'll need a
heatsink.  Remember that if you power down the module there must be no Arduino OUTPUT pins connected to
it still sitting at a HIGH level, or you'll burn something out.
I was not aware of that, thx for the heads-up

#### robbideloose

#11
##### Sep 27, 2018, 08:08 pm
I know, but I'm still afraid OP is pissing away power
a) with other components
b) getting up and running once every 15min
yes, everithing is powered directly from battery, no power regulators.

For now I'm using a pro mini (sparkfun) with the low power jumper soldered. But In the end it should be just the chip, few capacitors and oscilator.

#### Paul__B

#12
##### Sep 28, 2018, 12:38 pm
For now I'm using a pro mini (sparkfun) with the low power jumper soldered. But In the end it should be just the chip, few capacitors and oscillator.
Which is in fact, a Pro Mini with the regulator removed, which is the cheapest and generally most compact approach.

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