low voltage relays?

Printed From: the12volt.com
Forum Name: Relays
Forum Discription: Relay Diagrams, SPDT Relays, SPST Relays, DPDT Relays, Latching Relays, etc.
URL: https://www.the12volt.com/installbay/forum_posts.asp?tid=6906
Printed Date: June 24, 2025 at 5:29 AM

Topic: low voltage relays?

Posted By: SiDogg
Subject: low voltage relays?
Date Posted: December 19, 2002 at 11:02 PM

I'm looking for a low voltage relay? What I need is something that will trigger at approx. 1v or even less if possible (just to be safe), but can output as much as 12v or more...

so it would be something like this:

trigger: 1v
constant: 12v
switched: 12v
ground

Is there anything that does this? some kind of low voltage relay or a step-up relay?

Thanks!!

Replies:

Posted By: go2pac
Date Posted: December 20, 2002 at 5:08 PM

Use some transistors to trigger the relay

-------------
Karl

Pacific Accessory Corporation
Mobile Audio Interfacing Equipment
Connecting You to Your Music Since 1976

Posted By: SiDogg
Date Posted: December 20, 2002 at 8:20 PM

huh? what would transistors do? please elaborate.. this site is terrible at the traffic it gets - such little traffic so sporadically...

if you guys could elaborate that would be pimp.. :)

thanks!

Posted By: capple
Date Posted: December 21, 2002 at 6:28 AM

a transistor is an elctrical switch. where a small voltage on the base will turn on a higher voltage device. here I used the output of a eprom 5vdc to turn on a 12vdc relay. the first diod is to protect my eprom. the resitor is to limit the base curent the diod around the relay coil to to protect the transistor from brake down voltage spikes. Use a 2n3904 NPN type I can help or others may too just what is your load IE voltage and power draw (amps) and what is your turn on voltage? posted_image

Posted By: capple
Date Posted: December 22, 2002 at 6:01 AM

here is how to find the values you need.

1 triggger voltage (1VDC)

2 take trigger voltage -.7 (.3)

3 input to trasistor will have a resitor on it = to .3 / 3mA (100 ohms) find a even R value that will keep you current low.

4 relay draw or other device must be less then 700mA and less then 30VDC (a bosh type is less then 500mA)

5 place a 1n914 diode with the cathode to the + voltage. This is to protect the trasistor

example 2

trig voltage (5VDC)

5VDC-.7=4.3VDC

4.3VDC / 4mA = 1075 ohms round to 1K ohms (current will be 4.3mA)

4.3VDC / 3mA = 1433.3 ohms rouns to 1.4k ohms (current will be 3mA)

trigger voltage =12VDC

12VDC-.7=11.3

11.3VDC / 4mA = 2800 ohms round to 2.2k get you 5.1mA

nothing changes as for the relay side.

the values for the resistor I just pick because they are common. you can chose other too.

it is not a bad idea to put the diode on the input side of the trasitor to protect the device driving the circuit. here is where the current above realy comes into play and should stay well below 75mA and will need more then 1VDC to run the circuit. and the (-.7) will now be (-1.7)