negative powered relay

I suppose I could experiment with this, but I'll see what kind of response I get here first.

Question : If I were to wire the neg line of an amp, head unit, or some other 12 volt power device thru the coil on a 12 volt relay (85 & 86) . Would it trigger the relay.  In other words connect 86 to the ground wire of the device and 85 to ground. I my limited understanding of electronics it seem to me ground is not ground until it's grounded. The ground wire still carries current, and thus should still energize the relays coil.  ?????  Yes,,,,,,No

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Only the strong survive.
Eat the weak.

It would depend on what 12V device you're hooking up along with it.

Generally won't work even if the relay switches because the relay will drop the voltage so your device will no longer get the full 12V.

Or if your 12V device draws very little, it wouldn't pass enough current to switch your relay.

No way . The relay needs a positive and a negitive. You could control the relay with a switch or the radio to break the Hot or Ground of the amp.

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John DeRosa (Hotwaterwizard)
Stockton California
When in doubt, try it out !

HOTWATER

In theory it'nt the device side or the coil, for example sake a head unit. The device side of the coil considered hot? I know if I put a multimeter on a unconnected negitive lead and the other to ground I'll get a reading.

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Only the strong survive.
Eat the weak.

You get a reading because your DMM is usually 1M ohms or more, which doesn't draw any current worth mentioning. It's still a voltage divider, and you just read all the voltage because it's all dropping across the DMM leads.
Are you trying to do something with this or is it just the theory you're interested in?

They do make Special Current relays that do operate as you discribed but usually at higher AC voltages like 300 vac

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John DeRosa (Hotwaterwizard)
Stockton California
When in doubt, try it out !

Are you just being curious or do you have some kind of plan that you are contemplating?  Years ago Linear Power built a current sensing turn on circuit.  We could possibly help you with one, if that would work for you.

dualsport

Both. I want to understand the theory so I can figure out any other hair brain ideas I might come up with. This one is fueled by a desire to turn on the amps in my daughter's car I'm tweaking, with the head unit. She bought it acouple months ago from from the 'mp3playerstore" https://www.mp3playerstore.com/stuff_you_need/dvd/index.html#2 to be exact. Nice unit but there is not a remote turn on lead. I've installed an isolator and an extra kenitek battery in the trunk. I have a fatal flaw in that I like to tweak the hell out of stuff and I really don't want to hook the amps up to the accessory hot lead. I'd rather they did not come on until the head unit is powered. I'd like to avoid the constant draw on the power system.

i an an idiot

If you know of such a circuit I'd love some help, thanks. I'll do a search to see what I can find. thanks

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Only the strong survive.
Eat the weak.

Does the unit have a Power Antenna wire?  If it has power antenna and no amp turn on wire, chances are the power antenna wire will be hot anytime the unit is turned on.  If no power antenna wire, it is not uncommon to connect the amp remote wire to the switched power source for the radio.  (The red wire of the radio).  The amp will be on anytime the key is on, this is generally not a problem with the younger crowd, if the key is on the radio is on.    I know that there are several manufacturers out there that make audio sensing switches for this exact purpose.  Maybe someone else can let us know who that is.   The problem with the current sensing device made by Linear Power is, when it was around, the switched wire of a radio actually pulled a little current, today the red wire on a radio is just a sense wire itself, they do not pull enough current to work.

It would be much simpler if you just switched it like iam suggested, from a power antenna or ignition accessory line.

If the amp really draws a lot of power for some reason even when there's no input, you could do it by using a comparator circuit to drive your amp relay.
The comparator would detect the presence of your low level audio signal to turn on, and when you have the head unit off, the zero level signal would turn your amp back off.
If you don't want to have to tap into your actual signal, you might put a low resistance current sense resistor in line with your head unit power, and use the comparator to detect when it's turned on. You'd want to use a very low resistor value so it doesn't drop too much of voltage going to your head unit, and that would be where the comparator comes in; being able to detect the very small changes in voltage when it's on or off. It shouldn't be too difficult, but still,
I don't know if it'd be worth the trouble unless the amp power draw is unusually high when it's on without any signal input, and you regularly sit around for a long time with the ignition on accessory and the radio off.

Thank you for input.

i an an idiot

Yes there is an antenna lead, but from what I understand it's only hot when the radio is on. Not the cd. This is what i was going to use to switch a relay for the main power for the amps. I have 2 Infinity's 1 ref611a for the sub & ref7541a for the front and rears. Closing in on a 1000 watts with the 2 combined. This rig is for my daughter so I have no doubt she'll be sitting around with just the stereo on. Everything is wired through the remote battery in the trunk just so she can, and not kill the starter. If the antenna wire does not work, going straight to the hot lead (red wire) on the head unit is my backup. I'm just an anal retentive tweaker that wants it my way. Amps on only when the head unit is on.

dualsport

I like your idea of a voltage comparator. Any ideas of where I can go to due some research on them and figure it out?

Seeker1                                                                                                                                                                                                                                                                                                       The anal retentive tweaker

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Only the strong survive.
Eat the weak.

In most cases the power antenna wire is only hot when the radio is on. Since this unit has no power amp remote wire, I really think that the power antenna wire will be hot anytime the deck is on, cd, radio, usb, whatever it has. Connect it and try it. Years ago before there was a separate amp turn on wire, the power antenna wire was the amp wire and they were hot anytime the radio was on.

You can't tweak it until it gets in the car.

Here's a datasheet for a comparator you could look at for ideas-

comparator

If you use the example in Figure 2 for inverting comparator with hysteresis, you could use the output to control a transistor driver to turn on a relay when the Vin input goes above a set threshold. The comparator can only sink a small amount of current, so it won't be able to drive a relay directly.

If you use a current sense resistor for the input signal, you should be able to tweak the resistor values so that it'll turn on when the head unit is powered on. Just use a small enough resistance so it doesn't affect your input power too much, but large enough to get a signal for the comparator to work with.

Here's a link for a nice description of its uses:
comparator info