# Mains plug help for circuit!

#### Chris15

Joined Apr 15, 2009
252
Hi
I have built a big circuit with 3 other circuits and they all require 9V batteries, (one is made for 12V but operates on 9V) i want to get rid of the batteries and connect a 9V Mains plug to stop having to change the batteries as this circuit will be on constantly. I know i cant just connect the adaptor. i have an adaptor that is 9V 350ma, what type of resistors do i need for the circuits to perform just like they would with the batteries. i dont know how much current the circuit will draw, Also i have a relay connected to an oscilator so the relay turns on, off, on, off and makes a noise is there a way to muffle it.

Any suggestions
Thanks, Chris

#### KMoffett

Joined Dec 19, 2007
2,766
Chris,
If you can post a schematic of the circuit(s), it would help a lot in answering your questions.

Ken

#### Chris15

Joined Apr 15, 2009
252
I dont have a computer schmatic i just made the 3 then connected but you wouldn't need it, i just need to know how to wire it up. in other words:
1. How much current (mA) is there in a fully charged standard 9V Duracell battery
2. How do i turn the 13V (from the 9V Mains plug, multimeter said) into a clean 9V with the same amount of current as a standard 9V

Another way of explaining, If i have a circuit that uses a 9V Battery how do i connect a Mains plug without blowing the circuit up.

#### KMoffett

Joined Dec 19, 2007
2,766
What the maximum capacity of a 9V battery is and what is need for your circuit with this wall wart is not too helpful.

With the minimal info you have provided I will venture a guess that this "might" work:
http://www.national.com/ds/LM/LM317L.pdf
Page 9: Adjustable Regulator with Improved Ripple Rejection.

On second thought, if you measured the output of the wall wart as 13V with no load, then it will probably not work, as it's voltage will likely drop under your load (???), and the regulator needs the input V to be at least 2V higher than the ouput V to work.

If you have a multimeter, why not just measure the current that the circuit(s) draws from a 9V battery.

Ken

#### Chris15

Joined Apr 15, 2009
252
Ok
How do i measure the current the circuit will use, and what is a voltage regulator? (is it the transistor thing with three legs), why are wall adaptors 13V when it says 9V 350mA, and knowlege for future use what steps do i need to take in order to get a circuit to work with a wall adaptor, for example i have built a circuit that needs 12VDC, i have a 14VDC 450mA wall adaptor, what are the steps i would take for it to work WITHOUT BLOWING UP

Thanks Chris, sorry about the lack of info

#### KMoffett

Joined Dec 19, 2007
2,766
Ok
How do i measure the current the circuit will use,
You place you multimeter, set on the highest current range and with the leads plugged into the "current" jacks (check you meter's manual), in series between one lead of the battery and the circuit. If the reading is very low, switch to the next lower range...and so on.

and what is a voltage regulator? (is it the transistor thing with three legs),
An electronic device that will take a varying voltage within a certain range and output a fixed voltage. Yes, they often look like a transistor.

why are wall adaptors 13V when it says 9V 350mA,
In an "unregulated" wall wart with no load, the output voltage will rise to the "peak" voltage of the secondary winding. This voltage (13V) is about 1.4 times the output voltage (9V) with the rated current (350mA) being drawn. If you attached a 27 Ω resistor to the transformer the voltage would drop from ~13V to ~9V.

and knowledge for future use what steps do i need to take in order to get a circuit to work with a wall adaptor, for example i have built a circuit that needs 12VDC, i have a 14VDC 450mA wall adaptor,
If the the wall wart is "unregulated", and your circuit doesn't draw more than 450mA, you could use a LM7812 regulator IC and two capacitors to bring the voltage down to 12V.
Figure 7/Page 22:http://www.fairchildsemi.com/ds/LM/LM7812.pdf

what are the steps i would take for it to work WITHOUT BLOWING UP
Additionally, some circuits that specify "12v" may work fine on 6v or 15v. Without more information, like schematics and what the circuits are supposed to do... I can't help you there.

Ken

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#### Chris15

Joined Apr 15, 2009
252
Yes thank you alot ken you are very helpful, these questions have bothered me for ages, so when u say connect a 27ohm reseistor to the transformer u dont mean open the wall adaptor and put the resisitor u mean connect it to the wiring where the jack goes in, (sorry but i want to be sure before i do things). also thanks for the regulator link, im not a beginner to electronics i just have only usead a voltage regulator once for a AC/DC voltage controller LM350T, so anyway how would i wire up the regulator to the capacitor?, and yes i tried a 6V wall adaptor on the 12V timer circuit and it worked thanks for that part. How do i know if a wall adaptor is unregulated, the regulator would need a heatsink if i were to run it all night i know that bit, and for the reading the current that a circuit one probe goes on the battery or power supply
(+ or - does it matter) and the other goes on the circuits output?

i am just reading the instructions that come with the circuit and there is a small paragraph that says

"The circuit is powered from 12V with diode D1 included to prevent reverse polarity. if operated with a 12V plugpack, S1 could be replaced by a wire link as the untriggered current drain is only about 10-12mA. The uF capacitor decouples or smooths the supply rails to provide reliable operation of IC1"

Hopefully that will help,

Sorry about all the questions but thanks

Chris

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#### KMoffett

Joined Dec 19, 2007
2,766
Chris,

The 27Ω resistor, placed across the output connector (not inside the case), was just a way to demonstrate the change in output voltage of an "unregulated" power supply when it is unloaded and loaded. If it were a "regulated" supply, you would see very little change in voltage between loaded and unloaded conditions.

The LM350T is an adjustable version of the LM78xx series if fixed output regulators. You could use the LM350 regulator, with the correct resistor values for your circuit.

Since it sounds like you are using a commercial circuit, do you have a web link to the ones you're trying to power?

Sorry about all the questions but thanks
Questions are how you get answers!

Ken

#### Chris15

Joined Apr 15, 2009
252
http://jaycar.com.au/ShowLargephoto.asp?id=5810&PRODNAME=Short%20Circuits%20Three%20Project%20-%20Universal%20Timer&IMAGE= The closest thing they allow me to get is a picture but i could tell you about it. So with the 14v 450mA plug the only thing i have to do is put a 27ohm resistor on the output before it gets to the circuit and it will work without getting blown up?. and where was it that i put the probes for the multimeter one on the battery (12V 1.2A) and the other where on the circuit?, also how would i wire up the Regulator and the two capacitors and what are their values, e.g 2X 1uf, connect to input and output gnd goes to gnd.

Thanks chris

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#### CDRIVE

Joined Jul 1, 2008
2,219
Judging by the photo and the URL, that's a 555 Timer. If so, it's doubtful the the supply voltage is critical or requires regulating.

#### KMoffett

Joined Dec 19, 2007
2,766
Chris,
I think you missed the point of the 27Ω resistor. It was just to test whether the wall wart was the regulated or unregulated type. Without the circuit attached, measure the voltage from the wall war wires. Then place the resistor across the two wires and measure the voltage again. If there is a significant reduction in the second voltage reading the wall wart is unregulated.

For current metering see the attached. The regulator circuit is also there. The capacitors' values are not critical. C1 can be anything from 0.1uF to 100uF. If you use a polarized cap for C1, make sure the + on the cap goes the the + from the wall wart. C2 is anything from 0.1uF to 1uF.

I agree with CDrive...a garden variety 555 monostable circuit. These can easily run from 5VDC to 18VDC. If you're running one timer circuit off of one wall wart, unregulated is fine. If you intend to run all three timers (you did say you had three timers?) off one supply, and they may be timing at the same time, you should use a regulated supply. When they switch on and off it will change the load on the supply, changing the voltage, and can affect the other circuits timing.

ken

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#### Chris15

Joined Apr 15, 2009
252
Ken
The circuit works with the 14Vdc plug pack, there is only one 555 timer i said there was 3 Light activated switches, and the 50W light wont work when i connect it to the 14V wall adaptor i think because it needs at least an Amp but i have a 12V 1.2A Battery for that.

Thanks chris

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#### CDRIVE

Joined Jul 1, 2008
2,219
So ken
How would i connect the regulator (the one in your link) and the two capacitors? i want to use the wall adaptor 14V 450mA, the light on the circuit is 50W 12V so that would suck alot of current so explain in detail how i measure the current, one on battery/supply and the other where?, then once i define the current i decide whether to make a regulator for it or not

Thanks chris
Why power a 50W 12V lamp from your circuit? Use your circuit to drive a relay who's contacts are connected to the 120VAC line. Then you can use any common 120V lamp. One of those little night lights will throw enough light to prevent a toe stub.