How much power should i supply?

[stuboy78]

Hi all

I'm new to electronics.

i have built a board with 24 relays on it of 12V each (ohm resistance of coil is 750 ohms) i have connected the pins of power of all relays in parallel mode. now i want all of them to work simultaneously when i supply power. how much power will it require? will the volts be same 12V and i have to increase the ampere?

Stuart

 

[dknguyen]

Yes, they will take the same voltage since they are in parallel, but each will require a certain amount of current. Since you have 12, they will require x12 as much current. Whatever the datasheet current the datasheet says the relay needs x12. If it is saying the coil needs 12V and the coil resistance is 750 ohms, then each relay will draw 12V/750ohms = 16mA. Current is the number of electrons so it will split down each path to each relay (in this case they split equally since each relay is the same resistance). Voltage is a property of each individual electron has, and so voltage stays the same when they are in parallel.

You are talking about the coil that controls the relay, not the main terminals of the relay right?

 

[stuboy78]

Got the data sheet

Hey thanks for the reply!

I just checked the manual. it says 16.70 mA at 12V which means

16.7 x 24 = 400.8 mA

So if i have an AC adaptor that says 12V 500mA, it will work? or should i look for 400mA only?

What if i use a 1000 mA? will it burn the coils?

Thanks again

 

[dknguyen]

Hey thanks for the reply!

I just checked the manual. it says 16.70 mA at 12V which means

16.7 x 24 = 400.8 mA

Where did the 24 come from? Don't you only have 12 relays?

So if i have an AC adaptor that says 12V 500mA, it will work? or should i look for 400mA only?

What if i use a 1000 mA? will it burn the coils?

Thanks again

Now...when we talk about anything that supplies power it is usually a voltage source. What this means is the voltage is kept constant and the load draws whatever current it needs. The specified current rating on such a voltage source power supply means that it can maintain the specified voltage *UP TO* that amperage. So no, it will not damage it. THe voltage source will "force" that voltage into the load, but it won't force the current into the load.

A current source power supply on the other hand (very rare) will force the specified current into the power supply, and it will be able to do this *UP TO* a specified voltage. The voltage will be adjusted to be whatever is necessary to get the specified current into the load *UP TO* the specified voltage. If the voltage required to get the specified current into the power supply is higher than the specified voltage, the current source won't do it's job properly.

So one type of specification *IS* what the device will do. THe other is a rating (or a number indicating capacity) which is what a device is able to tolerate (ie. the maximum output it can produce properly or conversely, the maximum/minimum input it is able to handle like).

 

[carmusic]

the 500ma will be perfect for that

 

[stuboy78]

i gave 9V 500 mA power and the relays r perfectly running fine.

I have one adaptor that says 9 VAC 1000 mA but when i use it the relays give a trembling noise.... any idea why???? what is VAC?

 

[dknguyen]

Trembling noise? As in the noise of the relays clicking as they repeatedly open and close?

VAC means AC voltage- to be specific it means the AC voltage coming out of your wall.

 

[stuboy78]

yes... the noise is like the relay is contnuously going on and off.... is it because of the 1000 mA?

i tried it with 1 relay and 24 relays.. the result is the same...

 

[dknguyen]

Is the coil supposed to be an AC or DC coil? Sounds to me like the AC is causing it to switch on and off at 120Hz.

It is not the 1000mA. We just went over this. That just means the adapter can output the proper voltage up to 1000mA. The coil itself will draw only as much as it needs.

 

[Menticol]

VAC - VDC ... What is Polarity?... Imagine an hypothetical AA cell that keeps rotating its poles, the one that used to be the + then turns into -, and the - turns into +, X times per second, over and over again without stop.

On DC current, instead, the + stays as + and the - as -, always. Polarity doesn't change. Like real AA cells, or any other battery.

Got it?

Alternaing current, like the one that's coming out from your wall, changes its polarity 60 or 50 times per second, depending on the country.

Noise is caused by the relays continuously turning on and off, 60 or 50 times per second, because they are feed with AC. I guess it may fatigate the relays mechanical system, so don't do it.

You need a DC source, because Direct Current keeps its polarity constant, so your relays stay on or off as desired.

As I said, batteries are DC sources. For your application, you can also get a wall mart labeled as Output: x volts VDC, x mA

Your second Question

Using the 1000 mA source to feed (for example) a 500 mA bulb is not a problem, because the bulb will take the 500 mA it needs.

Leave a safe margin, i.e. don't use a 1000mA source to drive a 1000 mA load. That varies with each application. peak current etc... but on layman terms, more is better: a supply with excess of amps is better than one that lacks them

 

[stuboy78]

Hey that was very easy to understand both of u! thanks a lot!

Yes i think the adaptor i have is AC... it was of some modem or router....

the normal one i use is DC which works properly.

 

[audioguru]

I have never used an AC-DC adapter at its max allowed rated current so I don't know how hot it gets.
A minute ago I unplugged a 1000mA adapter that has a load of only 425mA and it was pretty warm.

 

[Menticol]

I have never used an AC-DC adapter at its max allowed rated current so I don't know how hot it gets.
A minute ago I unplugged a 1000mA adapter that has a load of only 425mA and it was pretty warm.

That's true Audioguru, these wall marts are true vampires. Even without load the heat and consumption... you can fry eggs on them!

At full load the wall marts overheat badly, strange odors are felt, then open circuit. Not great thing to see.

 

[audioguru]

My 12VDC/1000mA wall wart has an output of 16.7VDC without a load. It does not get warm without a load.

 

[carmusic]

anyway all adaptor sold by wallmart are UL listed so don't panic with overheating thay have been fully tested and all have inside fuse if overheating, it is normal under full load that adaptor become at around 50C

 

[stuboy78]

hey menticol

u said AC changes polarity. but the wall outlet always gives LIVE at one wire and ground at the other... where does the polarity change happen? and how does it affect us?

 

[dknguyen]

If AC is floating you can imagine that line A and line B repeatedly have A and B go above and below each other (sometimes A>B, other times B>A).

If B, for example, is tied to a ground, then what results is that you can still describe them using the above description (which is to describe their polarities relative to each other). But now you have an additional description you can use: you can describe their polarities relative to ground. If B, is tied to ground for example, you can say A goes above and below ground (or B since they are the same in this case). The first description still holds true because you can actually say that from B's polarity goes above and below A's polarity (if you observe B's polarity using A as a reference).

It's just like relative motion and reference frames!

 

[Menticol]

Household AC polarity changes from +120 to -120, taking the neutral wire s a reference. The AA battery was a "For dummies" example, didn't wanted to confuss you!

For dknguyen, the spanish joke: El que sabe, sabe! (Who know... knows!) Good explanation

 

[dknguyen]

If you ran AC into a rectifier to turn the AC into DC, you could actually call either line AC line DC ground. It would be okay as long as your circuit is "isolated" and does not plug into something else that actually has the true AC neutral as it's neutral. As soon as that happens...

BTW. AC has 3 plugs. THe two "eyes" are hot and NEUTRAL (very often mistakenly called ground). ANd the "mouth" is actually ground. Neutral carries the return current, in a perfect world ground is supposed to be at the same voltage as neutral (don't stick your finger in to test it because this isn't a perfect world), and ground only carries fault currents like short-circuit currents. So really, most of the time when we are saying ground in any circuit (AC or DC) we should be saying neutral.

THe difference? Something like the metal outside of your toaster might be connected to ground while the current actually powering the toaster flows between hot and neutral. THe intent is that if a neutral or hot wire goes loose inside and touches the outside of the toaster and you touch it, the current (mainly from the hot line) won't flow through the metal case, through you, and into the ground. It will flow through the ground line which is a lower resistance to the ground than you are and save your life (and also helps for detecting faults in your house since it should normally be zero current in the ground line). You can't just connect the metal case to neutral to do the same thing (well you can in a perfect world) because current is actually flowing through here during normal operation and even though it's "supposed" to be at the same voltage as ground, this often makes it not so and the neutral could actually be at a dangerously high voltage.

You could almost imagine it like neutral is the voltage of the physical ground of the power plant (where is staked into as a reference)) far far far away from you and is the voltage your live line is referenced to. And that the ground in your plug is the voltage of the ground right at your house. They are not necessarily the same. Atmosphereic charge build-up on the long transmissions lines, the fact that it carries current over a long distance, and the large distance in the soil between the power plant has the potential to make the neutral be dangerously high voltage relative to the ground you are standing on at your house.

For dknguyen, the spanish joke: El que sabe, sabe! (Who know... knows!) Good explanation

Yeah, sometimes I wish I could quickly doodle out quick little animations with a pencil and post the animation on here.

 

[stuboy78]

oops i should have said One wire is LIVE and the other NEUTRAL... sorry there....

Third we say Earthing or Ground