Volt-control relay

Thread Starter

schmoofer

Joined Aug 26, 2013
4
Hi all,

I'm a novice with electronics so please bear with me. I'd like to build a voltage-controlled relay, specifically a sort of "gate" that will allow current to pass (turn on) above 12.8 V or so and close at voltages below that. I'm looking to use it in my motorbike to connect it with my phone charger so it will only charge when the bike is running and being charged by the alternator (at which point the terminals will read out a voltage above 12.8 V or so).

From perusing the forum and the web, it seems a comparator (an LM311 or so) coupled with zener diodes will do the trick, but I'm dumbfounded when it comes to building it and choosing what resistors and capacitors to use. Can anyone offer advice or point me to a circuit diagram? I've looked at the diagram here. How valid is it? Thanks!

David

P.S. Can a moderator change the title to "Help with Voltage-Controlled Relay"? I'm getting the bug where I can't submit a thread unless the title is 20 characters or less. And I can't PM since I just joined =( Thanks!
 

MaxHeadRoom

Joined Jul 18, 2013
28,617
The LM311 should do the trick, I used one on a 240vdc mobile generator circuit to control the field above a certain voltage.
You just need a scaled down version.
Max.
 

strantor

Joined Oct 3, 2010
6,782
Why do you need to do this? Are you worried about power flowing the wrong direction; worried about your phone trying to charge the bike?
 

Thread Starter

schmoofer

Joined Aug 26, 2013
4
I'm more so worried that the phone or, rather the charger will drain the bike, which has happened. I also want to power a gps tracking device on my bike, so I want it to only charge when the bike's running and use it's own battery when it's not.
 

piranha3380

Joined May 28, 2013
30
just hook it up to your 12v headlight, ime powering my handle warmers from 1 headlight and my gps from the other, when ya start it it goos on and when ya stop the bike it goos off, no fancy stuff that could break or freek up your bike/Phone due to fibration, just be sure to hook it up to your city light and not your high beam,

keeping it simple is usualy the better option

(btw it wil not pull your lighting down as the provided amp are more than your gps and headlights need, done this on my own FJR 1300 never had any problems)
 

wayneh

Joined Sep 9, 2010
17,496
Maybe I'm missing something but you could likely replace the relay with your load and power it directly through the transistor. If you need more current, just use a beefier transistor.

(assuming you don't just use the headlight idea above)
 

Thread Starter

schmoofer

Joined Aug 26, 2013
4
Yeah, I'm trying to avoid tapping any existing wires for the sake of cleanliness and as an exercise. I'll see if I can simulate the circuit in LTspice before I buy the materials.
 

wayneh

Joined Sep 9, 2010
17,496
Me, I'd reverse the comparator logic (switch inputs) and use an N-channel MOSFET (such AS IRF540N) positioned below the load, dropping the relay.
 
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