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

 

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?

 

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

 

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).

 

the 500ma will be perfect for that

 

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?

 

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.

 

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...

 

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.

 

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

__________________
Felipe

 

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.

 

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.

__________________
Uncle $crooge

 

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.

__________________
Felipe

 

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

__________________
Uncle $crooge

 

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