5V Trigger for 12V Relay

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jaspershotrods

Member - Posts: 8

Joined: February 09, 2010

Location: Arizona, United States

Posted: February 09, 2010 at 7:43 PM / IP Logged

i need to know if there is a way to trigger a 12v spst relay using 5 v the switch im using cuts the volts more than half. thanks

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i am an idiot

Platinum - Posts: 13,677

This member consistently provides reliable information

Joined: September 21, 2006

Location: Louisiana, United States

Posted: February 09, 2010 at 8:03 PM / IP Logged

Is it a lighted switch. There is no other reason for the voltage being cut down by the switch other than you having it wired through the lamp in the switch.

KPierson

Platinum - Posts: 3,527

Joined: April 14, 2005

Location: Ohio, United States

Posted: February 09, 2010 at 9:36 PM / IP Logged

Yeah, the switch shouldn't do that.

But, to answer your question there are two routes you could take -

a 5vdc relay with the contacts wired to the coil of the 12vdc relay

or use a npn transistor (base - 5vdc switched input, emitter to ground, collector to (-) side of coil of 12vdc relay)

Kevin Pierson

oldspark

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Joined: November 03, 2008

Location: Australia

Posted: February 09, 2010 at 9:54 PM / IP Logged

I agree with the above.

Usually it's a 5V circuit to switch a 12V relay. That's easily done by ground switching (assuming their 0V aka grounds are common).

[ FYI: That's the common technique used for many modules and circuits - ie an input out output low or ground is the "active" state. EG - Kettering and electronic ignitions, computers (internally between chips).

Also called or akin to "Open Collector Outputs".

But not common in "powering" scenarios as on vehicles - ie - (except for horns and some lighting systems) most switch power to the load - ie, switch the +12V, not ground the load (but the controlling signal may well be a ground - eg ECU, alarm, etc). ]

howie ll

Pot Metal - Posts: 16,466

Joined: January 09, 2007

Location: United Kingdom

Posted: February 10, 2010 at 5:29 AM / IP Logged

Or using a "normal" 12v+ switch where the illumination is in parallel, I've never seen one that didn't output 12volts!

oldspark

Gold - Posts: 4,913

Joined: November 03, 2008

Location: Australia

Posted: February 10, 2010 at 6:57 AM / IP Logged

Oh Howie!

You silly-billy - it's obviously a 7V bulb! (Must be French). 5V Trigger for 12V Relay -- posted image.

I assumed that our clever Idiot is correct - the 5V being measured with a load connected to the lamp's grounding terminal or similar.

Jasper may have to clarify his description.

[ FYI - If it's a 12V bulb-illuminated switch off 12V, then it will be 12V if on, else off and hence "floating" else 0V (gnd) if connected to a load.

The bulb is connected between output (internally) and a terminal that you connect to GND.

Usually GND is the middle terminal with the +12V in and switch output at the ends. ]

If it's a 12V or other illuminated switch connected to a 5V supply, then yes, it will be switching 5V (with a dull bulb).

Otherwise it is connected wrong, or a faulty switch, or....???

jaspershotrods

Member - Posts: 8

Joined: February 09, 2010

Location: Arizona, United States

Posted: February 10, 2010 at 11:49 AM / IP Logged

it is a flat pannel switch and with 12.5 in and the switch closed it has 4.8 coming out so it will not acctivate a 12 v spst relay just wondering if there is a relay i could use that would trigger off of less than 5 volt but still let 12 v through to the load (window motors)

jaspershotrods

Member - Posts: 8

Joined: February 09, 2010

Location: Arizona, United States

Posted: February 10, 2010 at 1:08 PM / IP Logged

i can only get about 66ma through the switch and need 130 ish to trigger a relay

thanks again

KPierson

Platinum - Posts: 3,527

Joined: April 14, 2005

Location: Ohio, United States

Posted: February 10, 2010 at 1:46 PM / IP Logged

How many pins does the switch have? It still sounds like you are running the load through the lightbulb instead of around it.

Try this - with the load completely disconnected, but the power in and ground still connected, does the light still light up in the switch?

Kevin Pierson

jaspershotrods

Member - Posts: 8

Joined: February 09, 2010

Location: Arizona, United States

Posted: February 10, 2010 at 3:35 PM / IP Logged

there is no light it is a flat pannel switch 1 common 4 out puts i cant get any more than 66ma to come thru can i use a transistor to up the output and if so what am i looking for

thanks

i also tried to use the common on switch as - trigger wont trip relay that way either

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