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Pull up resistor value for low power application

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mayuriv

New Member
I am developing a battery powered device. In my circuit, there is a dip switch which is used as an input to micro controller pin. The supply to the switch is 3.3 v. Can I use a high value pull up resistor to minimize current in the circuit? Right now, the resistor value is 200k. Can I increase it to 1 Mega ohm.

Regards,
Mayuri
 

mayuriv

New Member
I am using pic16f1787 as my controller. These switches are configured as an input to the micro controller. will the switch pick up noise if I use 1 mega ohm resistor?
 

Pommie

Well-Known Member
Most Helpful Member
Many μC have built in pullups and these are typically 20k but I have used 200k without any problems. I'm unsure of 1M as you may pickup noise. Why worry about 20k when the input impedance is many times higher and so limits the current. If you're really worried tie all pullups to an I/O and only set it high during reading.

Mike.
 

spec

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Most Helpful Member
Many μC have built in pullups and these are typically 20k but I have used 200k without any problems. I'm unsure of 1M as you may pickup noise. Why worry about 20k when the input impedance is many times higher and so limits the current. If you're really worried tie all pullups to an I/O and only set it high during reading.

Mike.
Another brilliant suggestion from Oz. I would not have thought of that.:)

spec
 

spec

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Most Helpful Member
I am developing a battery powered device. In my circuit, there is a dip switch which is used as an input to micro controller pin. The supply to the switch is 3.3 v. Can I use a high value pull up resistor to minimize current in the circuit? Right now, the resistor value is 200k. Can I increase it to 1 Mega ohm.

Regards,
Mayuri
There are a few issues with high value pull up resistors:
(1) You need to take into account the effect of leakage current from the MCU input
(2) You need to ensure that the input rise and fall times are compatible withy the MCU input. Generally speaking, you can assume an input plus stay capacitance of 20P so the time constant with a 1M resistor would be 200uS
(3) A high impedance node on any circuit is prone to capacitive coupling so precautions would be needed to protect from this.
(4) Switching 3.3uA (3.3v system) or 5uA (5V system) requires special contacts because such a low current would not clean the contacts. Gold or carbon should be OK though.

Update: contact bounce is another characteristic that needs to be taken account of.

My experience with high value pull up resistors says to avoid them if possible, but I would not like to quantify that technically.:)

spec
 
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Nigel Goodwin

Super Moderator
Most Helpful Member
From all your suggestions, it seems that using 1 Mega ohm resistor is not a good idea...
Not at all, spec's rather pessimistic post completely ignored your original question - which was the use of a DIP switch.

Again, it would normally be perfectly fine (and common practice) on a PIC - however, you obviously need to take more precautions the higher you go (make sure the leads to the switch are short ones).
 

spec

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Afterthought,

What you could do is to put as large as possible film or ceramic capacitor (low leakage current) across the switch contacts. this would provide the switch cleaning current and convert the node to a low impedance above a few Hz, but without extra circuitry it would mean that the switch would not be functional for a minute or so (assuming 22uF capacitor) after bring made.

spec
 
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