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Topic: IC switching like a relay for very small currents  (Read 693 times) previous topic - next topic

ragtap

Hi All,

I have a small domotics project where I open the outside door through the intercom.
The intercom is digital so I need to emulate the pushing of two buttons: enable the intercom and press the open button.

I am doing it right now with a relay module but its too big for place it inside of the intercom, and therefor is hanging outside...

These relays basically set to GND and output for the intercom controller, that is normally at 5v. The currect is therefore very low. 

I am trying to find an IC to replace these two relays.

Made some tests with the 4066, but the resistance in the output sometimes generates issues.
I have seen that maybe the MAX4644 could be an option, but have no chance to test it.

Essentially what I need is an IC that acts as a relay for very low currents. I know that there are other options like the MOSFETs/opto couplers, but I believe that a IC that matched the requirements could be easier to implement.

Thanx much for your comments,

swe-dude

Hello why not just use 2 pins on a 5v mcu, if you need it to be small maybe attiny85?
Just have the pins set to input and when you need to push the buttons just change it to output and it will ground the pin, and then just change back to input to release the button.

Just a thought.
Best of luck!

ragtap

Hello why not just use 2 pins on a 5v mcu, if you need it to be small maybe attiny85?
Just have the pins set to input and when you need to push the buttons just change it to output and it will ground the pin, and then just change back to input to release the button.

Just a thought.
Best of luck!

Hi,

I am using a Esp8266 that has 3,3V in the output...I though about it, but If I connect it to the GPIO, I would burn it...

Thanx!

tadashimori

#3
Dec 03, 2019, 11:09 am Last Edit: Dec 03, 2019, 11:15 am by tadashimori
The output to low just grounds the pin, so the voltage differential is almost 0, you have to worry about the current your system draws and see if it's not too much.

These relays basically set to GND and output for the intercom controller, that is normally at 5v. The currect is therefore very low. 
Just to be clear, aren't you confusing voltage and current? Being 5v doesn't imply the current is low.

ragtap

The output to low just grounds the pin, so the voltage differential is almost 0, you have to worry about the current your system draws and see if it's not too much.

Just to be clear, aren't you confusing voltage and current? Being 5v doesn't imply the current is low.
I know that the current is very low because I read it in the intercom microchip datasheet. I know that current can be hi even with 5 volts.

Additionally, the 4066 supports very low currents, and have never broke any output from the IC.

Thanx! 

tadashimori

#5
Dec 03, 2019, 11:58 am Last Edit: Dec 03, 2019, 12:10 pm by tadashimori
I know that the current is very low because I read it in the intercom microchip datasheet. I know that current can be hi even with 5 volts.

Ok, I just wanted to confirm if that "Therefore" was about the 5v or about the intercom.

If I'm not mistaken, from the datasheet, the 4066 supports only 10ma.
It's good to check the current anyway, just to be sure, but the Esp8266 can support up to 15ma.


Made some tests with the 4066, but the resistance in the output sometimes generates issues.
Come to think about it, you'll face the exact same problem with the Esp8266, but the internal resistance is lower (about 50 ohms) instead of the 180 ohms of the 4066

Why isn't the optocoupler a good idea again? It is basically a switch, has about the same internal resistance, and you protect your Esp8266.

ragtap

Ok, I just wanted to confirm if that "Therefore" was about the 5v or about the intercom.

If I'm not mistaken, from the datasheet, the 4066 supports only 10ma.
It's good to check the current anyway, just to be sure, but the Esp8266 can support up to 15ma.

Come to think about it, you'll face the exact same problem with the Esp8266, but the internal resistance is lower (about 50 ohms) instead of the 180 ohms of the 4066

Why isn't the optocoupler a good idea again? It is basically a switch, has about the same internal resistance, and you protect your Esp8266.
An optocoupler is also of course an option. Is just that is requieres double cabling and it might be more complicated to find (max. current output for 8266 is 12mA) ... any suggestions?

thanx!

wvmarle

5V DC or AC?
What current?

Often you can replace simple buttons by optocouplers - safe (no electrical contract), and will work assuming the current through those buttons is no more than a few mA. If more than that, it's becoming harder - your comment on the use of the 4066 suggests this actually may be the case, but details are missing.

The moment you connect your project to those buttons directly you also need to know the actual voltage level of the poles of the buttons. With that I don't mean the voltage difference when off, but whether the low end of the button is at ground potential, and whether the low sides of both buttons are indeed connected, etc.
Quality of answers is related to the quality of questions. Good questions will get good answers. Useless answers are a sign of a poor question.

Paul__B

Note that when referring to a "4066", we must be talking about a 74HC4066, not a CD4066 which has much higher "on" resistance.

If pulling to ground, just use a transistor.  A bipolar would do, a logic level FET is better and nowadays, almost as cheap.

If you really want proper advice, you need to provide the circuit relating to the intercom device and the information you have on its microcontroller.

ragtap

5V DC or AC?
What current?

Often you can replace simple buttons by optocouplers - safe (no electrical contract), and will work assuming the current through those buttons is no more than a few mA. If more than that, it's becoming harder - your comment on the use of the 4066 suggests this actually may be the case, but details are missing.

The moment you connect your project to those buttons directly you also need to know the actual voltage level of the poles of the buttons. With that I don't mean the voltage difference when off, but whether the low end of the button is at ground potential, and whether the low sides of both buttons are indeed connected, etc.
Note that when referring to a "4066", we must be talking about a 74HC4066, not a CD4066 which has much higher "on" resistance.

If pulling to ground, just use a transistor.  A bipolar would do, a logic level FET is better and nowadays, almost as cheap.

If you really want proper advice, you need to provide the circuit relating to the intercom device and the information you have on its microcontroller.
I was able to measure the exact current that passes trough the points I want to emulate:14 μA and 5 Volts DC.

Ideally, would need 0 Ω on resistance, but it could be slightly more.

Any suggestions will be welcome.

Thanx!

Wawa

Note that all electronic switches (like the 74HC4066) can only switch things within their own supply range.
If the chip is powered with 3.3volt, then the load must also be <3.3volt and >ground.
Can't switch something 'floating' (no ground shared with the chip).
If you want to switch something not galvanically connected, then use an opto coupler (or opto fet).
Leo..

ragtap

Thanx!!
Note that all electronic switches (like the 74HC4066) can only switch things within their own supply range.
If the chip is powered with 3.3volt, then the load must also be <3.3volt and >ground.
Can't switch something 'floating' (no ground shared with the chip).
If you want to switch something not galvanically connected, then use an opto coupler (or opto fet).
Leo..
The 74HC4066 is connected to a 5v source. Grounds are now connected. What I essentially need is a relay inside of a IC that would be able to have very low "on" resistance, accepts maximum 14 micro amps and able to switch the status with 3,3 volts.

MarkT

An optocoupler is also of course an option. Is just that is requieres double cabling and it might be more complicated to find (max. current output for 8266 is 12mA) ... any suggestions?

thanx!
You can use a dual opto coupler, run the inputs in series and the outputs in parallel, assuming there's enough voltage headroom to drive the two in series from 3.3V.
[ I will NOT respond to personal messages, I WILL delete them, use the forum please ]

gilshultz

Try this is a medical part from Ablic the HDL6M06503A/B which has a 2.7 Ohm resistance when turned on. Some of the Key Features are:
High-integration facilitates downsizing and cost reduction
32-channel integration provides downdizing and cost reduction of your systems.
Signal voltage-independent low on-resistance for better image quality
Stable 8Ω on-resistance in the range of ±100V ultrasound signal achieves better image quality.
Single 5V power supply for very low power dissipation
A single 5V power supply achieves 14mW at 50kHz all 32-channel switching, and provides a safety design to your systems. Especially suitable for in-probe applications.
Unique pin configuration for efficient PCB wiring
The unique pin-out of poles on one side and throws on the other side reduces the number of board layers, and consequently contributes to compact, low-cost, and light-weight systems. The link is: https://www.ablic.com/en/semicon/healthcare/analog-switch/  Another solution is Maxum, try this link: https://www.maximintegrated.com/en/products/analog/analog-switches-multiplexers/MAX4968B.html
This response is to help you get started in solving your problem, not solve it for you.
Good Luck & Have Fun!
Gil


Wawa

I am using a Esp8266 that has 3,3V in the output...
The 74HC4066 is connected to a 5v source.
Can't do that.
The ESP8266 usues 3.3volt logic. All I/O must also be powered from the 3.3volt supply of the ESP if you don't want to fry the ESP.
Also means you can't switch anything that's outside 3.3volt/GND.
What I essentially need is a relay inside of a IC that would be able to have very low "on" resistance, accepts maximum 14 micro amps and able to switch the status with 3,3 volts.
A 5volt DIL reed relay, powered from 5volt, and switched with an NPN signal transistor.
That will give you full isolation between the ESP and the device, and the common reed switch should have a very low problem-free contact resistance.
Leo..

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