quiet 12v relay

Hi,

Does anyone have any recommendations for a very quite, even solid state, 12V relay for automotive applications similar in operation to the standard Bosch relay? The SS relays I always see for sale are more industrial DC to AC type.

I have an application where silence is golden.

Thanks,

D (in orbit)

By similar in operation are you looking for a SPDT configuration or a switch that can handle 30A or both?

If you application is only 1A  but needs both NC and NO contacts you'll have some options.

If you applicaiton is higher current but you can use a SPDT relay you'll have some options.

If your application requires a 30A SPDT relay then you have 1 option - and it's posted above!

Why such the big base resistors?

What would the maximum current output be of that circuit be?

The resistors cut the voltage to less than 1 volt for proper base triggering, Otherwise it would fry the Transistor from too much voltage input.

Kind of like over driving an RCA input on a Stereo with high wattage..

A single resistor won't cut the voltage, it will only limit the current.  The circuit shown above wouldn't be a good amplifier because the base resistors are WAY too big.

The current going in to the base of the 2n2222 would be limited to ~0.12mA at 12vdc by the 100K resistor.  With a beta of 50 for the 2n2222 it would output a maximum of 6mA. 

The MJ2955 has a beta of ~20 which means if you put 6mA in to the base you will get a maximum of 120mA out the collector.

Here is a circuit I designed to run an electrolosis cell.  Same basic principal (Darlington transistor array) but drastically different resistor values.  The pot in this circuit controls how much current is sent to the cell and the calculations shown above have been proven in "real life" (at least the lower limit, my cell will only pull 20A at max power).  This circuit uses a regulated 5vdc input so that small fluctuations in battery voltage won't effect the current coming out of the final output stage.  It has a static 30 ohm resistor to set the min current that the circuit will see.  Since the current is limited at the first transistor there is no base resistor needed at the 2nd transistor.

Hi,

Thanks for the suggestions and for the questions and sorry for the delay in getting back to you. The relays are just switching contacts - no AC or DC current is being switched. It triggers the "next song" on an MP3 player. The relay is being driven by a standard 12V automotive battery.

I would like to use off the shelf components rather than wire up a board.

Perhaps silencing them in an insulated box is best - I just thought there might be a small off the SS relay available.

Thanks again,

David

p.s. I have posted another question and I'll try to give plenty of details this time.

Radio Shack sells a 12 volt reed relay.  It pulls way less current than an automotive type relay.  The only problem is that it is a single pole single throw device.  It will only work if the sensors you are using are able to close the circuit when they sense motion.  According to your other thread, you need it to output when the circuit is opened.  If there are a common, NO and a NC connection on the sensors, you will be able to use this reed relay.  It is silent and pulls way less current.

http://www.radioshack.com/product/index.jsp?productid=2062479&cp=&sr=1&origkw=reed+relay&kw=reed+relay&parentPage=search