Analog switch

Community Projects
1

I want to purchase an analog switch from sparkfun or digikey, but I do not understand the terminology and cannot figure out which one I need.

I need one which will switch up to 16 volts and will be activated by 3.3 volts. I am not particular about the On Resistance.

I have looked at the data sheets for many but I cannot figure out what the various properties mean.

I need two of these so if there is an option for a unit that has them rolled into one that is fine.

Can someone please help me find what I am looking for and perhaps explain why they suggested it.

Thank you for your help!

2

More info needed. What currents will flow through the switches ? Is switching time important ? Is the switch to be btw a power supply +V and a circuit ? Or btw a circuit and “ground” ? Can you give us some hint of the usage or schematic ?

3

I would be glad to add more info.

For one switch I want to turn a voltage divider, two resistors, on and off. They will be between the switch and ground. The current that flows through it can be limited to any amount as all I am trying to do is read the voltage from the divider. The top of the divider will be connected to the switch, the bottom to ground, and the middle to a microcontroller.

For the other I was thinking of having a solenoid between source and the switch. I already have this working with a TIP 120 so I do not need this as bad but I figured I could save myself some board space if I can these two in the same package. If not I can do with out this. The solenoid uses 500 mA.

I hope this helps.

Thanks!

4

Ok, 500mA will not happen with an analog switch. You’re current solution is ok and using a MOSFET could be better.

16V may be a problem with an analog switch. What does the data sheets say about maximum Voltage?

A P-ch MOSFET and a NPN transistor will do the job of switching 16V to a divider.

5

We can forget about the 500mA. Like I said that is already working. All I really care about is the divider. I can use a resistor to limit the amperage if that is the problem. I wanted to use the switch for that because the on resistance was mostly static.

I have tried a series of transistors and MOSFETs to switch the divider on and off but they always seem to add some kind of variable resistance that I cannot account for.

6

Ok, If the MOSFET is not biased fully on and if it has a fairly high Rdson then that can happen.

Post a drawing of the schematic and list the values and part number you used. We can then make suggestions to get this working properly.

If you do try an Analog switch you may find that its resistance is also variable.

7

I will admit this is not first time I tried to solve my problem here on this forum. Here is another post I did when trying to solve it.

https://forum.sparkfun.com/viewtopic.ph … ry#p160828

I tried to use the schematic from dlotton. The fets I used are

The DigiKey part numbers are Zvn4424a-nd and Zvp442a-nd

I tried a few times to get them to work but I could never get any voltage to the divider. I was using the 12 volts dc wall wart I was talking about to test it. I have a voltage regulator that I use to bring the 12 volt system down to 3.3 for the XBee. I used that as input into the system in place of the microcontroller(XBee) into the N Channel Fet.

I had also attempted to use the following information and a TIP 120 and a TIP 125

http://www.w9xt.com/page_microdesign_pt … ching.html

I got that working though like I said the voltage coming out of the PNP was significantly lower than the voltage from the wall wart and delta on this difference as I dropped the voltage was not uniform, but went up and down.

I will spend some time this weekend trying it again. I had hoped the analog switch would make it a little easier.

Thank you for your help. I really want to get this done as I have been working on this for a few months now.

8

The Zvn4424a-nd and Zvp442a-nd MOSFETs have a fairly high Rdson value but less than 1Ohm if the Vgs (source-gate voltage > 4) is high enough and the drain current low enough. Use an NPN transistor is also needed to ensure the MOSFET is fully turned on else the Rds is still higher. Attached is a schematic of this circuit.

What are the values of the resistor in the divider?

9

R3 was suppose to be 4.7 k.

R4 was suppose to be 300.

These values were picked to bring 0 - 20 volts down to 0 - 2.2 volts. So anything in a 20 volts range can be read.

I will try your schematic using the TIP 120 transistor I have and the P-Channel Fet I have.

10

Ok. 16V / (4700 + 300) = 3.2mA. Your MOSFET should have less than 1 Ohm Rds on and shouldn’t affect the divider output Voltage any more than the tolerance of the divider resistors. 100 * (1/5000) = .02%