Using relays versus MOSFETs - Page 1
Forum section: electronics / beginners

author: MikeK
author_rank: Super Contributor
timestamp: June 23, 2012, 09:30:28 pm
content: In the July issue of Everyday Practical Electronics is a digital potentiometer project which uses relays to switch legs of an R/2R ladder (fed by a precision voltage reference).With the specified relays having about 100mOhm resistance, why couldn't MOSFETs have been used, which would have much lower Ron, and would make for a more accurate device?

author: Simon
author_rank: Unknown
timestamp: June 23, 2012, 09:42:44 pm
content: I'm guessing it is the complete isolation between the relay coil and the switch allowing connections that could not happen if you have a comon pin between the switch activation and the switch path

author: MikeK
author_rank: Super Contributor
timestamp: June 23, 2012, 10:45:46 pm
content: Okay, that makes sense. So can a MOSFET be run on two separate supplies?...One for the drain-source and one for the gate? Since the gate doesn't actually pass current.

author: Psi
author_rank: Super Contributor
timestamp: June 23, 2012, 11:42:41 pm
content: You can use two separate supplies but they still need to have a common. The gate capacitance is between the gate pin and source.The gate does actually pass some current, it's just small, it has to charge up the gate capacitance.The voltage of this charge then controls how much the fet turns on.You could make it work with fets if you had some sort of isolated setup, like driving the gate from a photocell+led

author: amspire
author_rank: Super Contributor
timestamp: June 23, 2012, 11:55:53 pm
content: Quote from: MikeK on June 23, 2012, 09:30:28 pmLow RONYou can use a complimentary pair of Mosfets. This is exactly what the analog switch ICs do. Relays do work well, and so if you are not continuously changing the divider, they can do an excellent job. However, most R/2R divider chains do end up using Mosfet switches. It is cheaper, faster, lower power, and there are no contact problems.Issues you have to watch in Mosfet switches are the OFF leakage currents. With mosfet switches and analog switch ICs, you do have to look at the data sheets, and calculate the errors caused by leakage currents with the chosen R/2R resistor values. You do not have to do this step with relays. To get really low leakage current, it can often mean you end up with a high RON, and you then have to look at the variations in Mosfet resistance with temperature and divider voltage, and add this into the error calculations.For an AC divider, the capacitance of a Mosfet switch can be a significant issue.With Mosfet switches, your drive circuit, or the supply rails of analog switch ICs will limit the maximum voltage that can be applied to the R/2R chain, but there are even ways around this. Maxim have an analog switch IC that allows the switched signal to go to +/- 25V regardless of the switch supply voltage.Richard

author: DavidDLC
author_rank: Frequent Contributor
timestamp: June 25, 2012, 06:20:30 pm
content: In one of my recent projects, I received a design made by somebody else using analog switches in parallel to the switches in a digital camera, the idea was to remotely operate the camera using the analog switches.The original design was using the 4066's, with this circuit the camera was operating by itself in a random way, taking pictures, zooming in and out, powering off, etc.We fixed the problem by replacing the analog switches with reed relays.As you can see there are some applications that just don't like analog switches or MOSFETs at all.David.