Operates as a switch! Relay or transistor?

[ericmar]

hi,

I want to do some switching on my circuit so I need either a DPDT relay or 2 transistors to act as switch. I wonder which one is better and can last longer. Relay takes more power and they r usually bigger in size, right? But if I m going to use transistor as a switch, how can I make it behaves like a double throw relay (i.e. either in operation mode 1 or 2)?

Thanks.

Regards,
Eric

 

[flatulent]

There are many things to consider when selecting.

Relays have low on resistance, no distortion, slow switching times, frequency responses into the microwave region, high power switching capacity, about a million switching cycle limit.

In critical applications in systems where cost is not as important as performance, I frequently design in relays.

 

[golfbumb]

Flatulent's comments are quite correct. I would add, you can use transistors to source and sink current within your circuit but the beauty of relays is that they provide isolated contact closure--no references to anything else in your circuit. Two transistors will never exactly behave like a relay.

The primary drawback to the relay is those slow switching times, as stated, often exceeding 50-250mS, depending on the frame size. Secondarily there is significant power consumption in operating the relay (hold currents are fairly low). But it costs no energy when the relay is "relaxed", i.e., design to use the relaxed mode (coil off) as much as possible.

If a 1/4 second switching time is not cutting it close then a relay should work. But always stay within the current ratings for the operating voltages suggested by the relay manufacturer. Even on the low side of the current spec. Otherwise, someday, you will likely find your contacts stuck together.

Your need for two poles implies that you might need electrical isolation. If you need higher switching speed and also need to decouple your circuit's logic from the "relay" section, consider a pair of optical isolators. An optical insolator is a photodiode optically coupled to a photo transistor. Flow enough current through the diode, and the phototransitor biases on. That phototransistor is completely electrically un-referenced to your circuit.

And when using a relay, remember that as the relay is turned off and the coil field collapses Faraday's law says you will get a voltage spike (which can be supressed with a reverse biased diode across the coil).

 

[Johnson]

Optocoupler sometime is good choice!

 

[djalli]

How about diodes? I designed a 9GHz switch just using diodes. Diodes are fast and are not a big challenge in circuitry.

 

[ericmar]

How about diodes? I designed a 9GHz switch just using diodes. Diodes are fast and are not a big challenge in circuitry.

How did u do that? Do u mind giving me some examples?

Thanks!

 

[onemilimeter]

SSR = Solid State Relay

Opto + SSR could be an alternative to mechanical relay.

 

[ericmar]

I was planning to use 3 relays on my circuit but my power supply couldnt provide enough power if I use 3 relays!

 

[flatulent]

There are such things as magnetic latching relays for small signal use. they cost more and require more complex drive circuitry.

 

[platonas]

There is another one option, use CMOS IC switches like the 4016 and 4066. There are more ic simillara to the above. Chech the RS or Farnell catalogue for analogue switch ICs.

 

[djalli]

How about diodes? I designed a 9GHz switch just using diodes. Diodes are fast and are not a big challenge in circuitry.

How did u do that? Do u mind giving me some examples?

Thanks!

You can use diodes that at fast pin diodes, schottkys... transistors, anything that is fast. Depends how you look at the problem.

See figure!

 

[cent24.3]

There is lots of solutions depending of the application.

As previously said:

- you can use "flip-flop" relays OR... some models specialy design to draw less curent (have a look on online catalogs). I remenber changing one relay and fall from 90mA to 30mA of current.

- You can use 4066 (I think 4016 is out of date by know) OR... some Maxim IC like the MAX4760, datasheet : "The MAX4760/MAX4761 (DPDT) analog switches operate from a single +1.8V to +5.5V supply.", you've got four of them inside the IC.

bye