"Lossless" solid state replasement for a mechanical relay? - Page 1
Forum section: electronics / beginners

author: FriedMule
author_rank: Frequent Contributor
timestamp: March 31, 2018, 07:55:55 pm
content: I have spend little over a week to read about SSR, Triac, p / n channel jfet, bjt, ldr and a lot more and i learned a lot, but not enough:-)My goal is to build a replacement for a mechanical relay, a solid state relay. To switch on and off a audio signal from i.e. a cd-player or Mp3 player and so on, but "lossless" (minimal voltage og current drop, no noise and so on.I found a circuit diagram on google images.Will it work as described above?

author: Benta
author_rank: Super Contributor
timestamp: March 31, 2018, 08:00:48 pm
content: Completely useless for audio.Search for "analog switch" instead, there are lots of manufacturers. An on-resistance of, say, 50 ohms will not disturb your audio signal.

author: ogden
author_rank: Super Contributor
timestamp: March 31, 2018, 08:04:39 pm
content: Quote from: FriedMule on March 31, 2018, 07:55:55 pmIf you want lossless relay with minimal voltage drop and no noise then use... mechanical relay.

author: BrianHG
author_rank: Super Contributor
timestamp: March 31, 2018, 08:13:04 pm
content: How much voltage & current. Is this for speakers, or, is this for line level audio?

author: Zero999
author_rank: Super Contributor
timestamp: March 31, 2018, 08:17:12 pm
content: The only issue with an analogue switch is it needs an external power supply, other than just the switching signal.Yes, TRIACs are completely unsuitable. They're designed for 400Hz operation maximum, have a minimum hold current and a fairly fixed on voltage of about 0.7V.An alternative to an analogue switch is an opto-coupler with a photovoltaic cell output, driving some back-to-back MOSFETs.http://www.skyworksinc.com/uploads/documents/OLI920_203262B.pdfhttp://www.vishay.com/docs/83469/vom1271t.pdfhttps://www.digikey.com/en/product-highlight/v/vishay-semi-opto/vom1271-isolated-photovoltaic-mosfet-driverIf you only need to switch a small signal, the 2N7000 MOSFET will do.Quote from: ogden on March 31, 2018, 08:04:39 pmI agree, a mechanical relay will give the lowest possible on resistance. However, if this is only going to switch a line level signal (i.e. the signal from an MP3 or CD player), not a speaker or even headphones, a super low on resistance is not required. The 50R resistance of an analogue switch will be negligable, compared to the 10k input impedance of a typical audio amplifier. On the other hand, if it's for a speaker or headphones, then a mechanical relay really is the best thing.

author: FriedMule
author_rank: Frequent Contributor
timestamp: March 31, 2018, 08:52:32 pm
content: My goal is to build a input selector for a preamp that has only one input.Thanks a lot for all your answers, especially Hero999:-)Since i am totaly new to all this, i think that it would be easyer to solder a DIP chip, is ther anyone that is as usefull as those Hero999 linked to?EDIT: not preamp, integrated amp :-)

author: BrianHG
author_rank: Super Contributor
timestamp: March 31, 2018, 09:43:57 pm
content: Doing it electronically, you only need a CMOS analog switch. However, your control circuitry shares the same GND and power of your analog section.These devices will require +/-5v supply so they may cover your analog voltage range:Example of generic analog switches: 74HC4052 < This 1 IC will give you Stereo, 4 inputs, 1 output. < 100 ohm on resistance74HC4051 < This one IC will give you mono, 8 inputs, 1 output. (Just use 2 for stereo).Example of high quality analog switches: MAX4675 < The on resistance of this switch is only 3 ohm on resistance. (Getting close to a relay)For these, you don't need a negative supply with these, but, I would still protect my analog IOs and this will cost a whole lot more than 74HC405x analog switches since it is a single normally open switch. Your digital no longer is connected to your analog with these photo-mosfet switches.Optocoupled AC/DC mosfet switches: TLP222A < 2 ohm on resistance.TLP222A-2 < This has 2 switches of the TLP222 in 1 package.You may engineer a double break and mute in the middle design topology here if you think a 1 nano amp leakage current will be too much cross-talk on disabled audio channels.

author: Zero999
author_rank: Super Contributor
timestamp: March 31, 2018, 09:47:44 pm
content: Quote from: FriedMule on March 31, 2018, 08:52:32 pmHow many devices need to be connected to the input selector?Does it have to be stereo?Why can't you just use an ordinary manually operated switch?Is it being controlled from a microcontroller?As mentioned above, an analogue switch is another alternative. If it's stereo and four or fewer devices are required, then the 74HC4052 will do. It will simultaneously switch two channels between four different sources, using a binary control code. Unlike a relay, it's not isolated, so it will have to share a common 0V rail with the audio system. It will require a 5V power supply and some biasing to work.https://assets.nexperia.com/documents/data-sheet/74HC_HCT4052.pdf

author: DrGeoff
author_rank: Frequent Contributor
timestamp: March 31, 2018, 10:03:09 pm
content: Use a signal relay. Read about "wetting current". There are some small DIP-package types intended for signal use (eg NEC).

author: BrianHG
author_rank: Super Contributor
timestamp: March 31, 2018, 10:03:24 pm
content: If you go for mechanical relays, I would just use telecom 'latched' DPDT relays. You power them once for half a second, and then don't power anything. They then use no power at all and hold your settings. I once designed a complete solar/battery powered pre-amp with these, full relay switched inputs plus relay switched volume attenuation. The MCU had an IR receiver I designed which ran on 2ua while waiting for the remote control to wake up the PIC mcu. Then, with charge which was left in a small cap, it had enough to step the relays to a new selection, then went back to sleep. No connections to any AC or outside power. Lasted on 3x D cell batteries indefinitely. 3x AA batteries would probably lasted 2 years.

author: FriedMule
author_rank: Frequent Contributor
timestamp: March 31, 2018, 10:23:08 pm
content: It is a joy to ask on this forum!!To answer all your question:-)I am thinking about max 2-3 stereo input that should not do any crosstalk if posible.The reason that it have to use solid state are that i am trying to make a input switch to a place where my mental handicapped sister leves.It is a part of a sensory room and will be used to switch among different audio stimulus.When finished it will be incased in epoxy or somethin like that, so it cant get wet or don't get damaged by hard handeling.EDIT: forgot to say that it is important that the switch can be powered by i.e. batteries and the input signal do not get to cross path with the battery power.

author: BrianHG
author_rank: Super Contributor
timestamp: March 31, 2018, 11:32:11 pm
content: You might be looking for something like this:This battery switcher portion of my pre-amp handled 4 inputs, 1 output, but, it needed the relay switched volume board which connected at the bottom left. The 4 relays were Telecom Latched relays which the MCU just applied a positive or negative voltage for 1/2 second to latch them one way or the other. After that, the MCU went to sleep. This unit was RS-232 control input with 3 solder-dot option switch on the PIC MCU so I could gang multiple units together.Though the connector footprints were for XLR-Balanced audio needing 2 PCBs for stereo, you could easily change the mono-XLRs for Stereo RCAs.

author: iampoor
author_rank: Frequent Contributor
timestamp: April 01, 2018, 06:30:32 am
content: Douglas Selfs small signal audio design has an entire chapter on this. If you are seriously interested in audio design, pick up a copy!

author: Zero999
author_rank: Super Contributor
timestamp: April 01, 2018, 10:42:28 pm
content: Quote from: FriedMule on March 31, 2018, 10:23:08 pmThree stereo channels should be no problem.How long does the control cable need to be? Presumably it's going to be wired, rather than wireless. What are you actually going to be using to control it?QuoteDoes that mean you need the batteries to be isolated from the audio signal? If so why? Do the batteries power anything else?Having the control circuit isolated excludes using analogue switched.

author: FriedMule
author_rank: Frequent Contributor
timestamp: April 02, 2018, 04:51:54 pm
content: Quote from: Hero999 on April 01, 2018, 10:42:28 pmSorry for my late answer!My thought is that the batteri do not have to be isolated, worst that can happen is that someone touch the 1.5v batteries.I also think about using a tilt-censor so you only have to tilt the box on one of the sides to change input signal.It all shall inside a soft toy dice ore something.EDIT: forgot, the cable should be about 2 feet and the cables are screwed to the wall, so the users, just have to turn the dice to change signal source.

author: Richard Crowley
author_rank: Super Contributor
timestamp: April 02, 2018, 05:10:14 pm
content: Why not simply use a 4052 CMOS switch? 2 x 4 perfectly matches your requirements.Widely available, dirt-cheap, easy to use DIP package. Available in a variety of chip families.It is used in pro audio and broadcast gear. It should be good enough for your application.Keep it simple and get on with your project.

author: Zero999
author_rank: Super Contributor
timestamp: April 02, 2018, 10:31:25 pm
content: Quote from: FriedMule on April 02, 2018, 04:51:54 pmHow many batteries? What sort of battery life do you require?A bipolar power supply is ideal. The CD4052 and 74HC4052 use so little power the self-discharge of alkaline AAA batteries, will exceed the power consumption of the IC. The 74HC4052 alone, will happily run of four alkaline, or two lithium, 25mA button cells for over 3000 hours.How much power does the tilt sensor use and what sort of output does it have? The '4052 needs a binary coded input to tell it which channel is selected.