Thursday, July 4, 2013

12V DC UPS - Hobby Electronics

Welcome everyone. Hope you guys are doing great.

Tonight I'm going to write something about making a small DC UPS. You might not use this exact same diagram because it is not that great but you will get an idea of how it could be done. You will also get some basic knowledge about using a few components. 

Where it can be used? 

Now-a-days we all use routers, small DC fan & other electronic equipments which run on 12V DC. Most of these devices come with high voltage AC to low voltage DC adapter with them. Like a router might run on 12V only so it will come with an adapter that will lower the 110V/220V AC to 12V DC. So what if there is power outage or something? Well we can use an inverter like an UPS or IPS to run this device even after power outage. But inverters actually use 12V or 24V or even more DC battery and they invert it to high voltage AC like 110V/220V. And this procedure is not very efficient, a lot of heat will be generated and thus power will be lost. If you just want to run one router for internet it will be kind of an overkill. In that case we can use a circuit like this to power up the router or any similar device directly from a suitable battery, as these devices run on DC they will have no problem running directly from these battery.


For this project you will be needing

1. 12V Sealed Lead Acid Battery or Any other type of similar battery. (higher current Rating for higher load)
2. Transformers, resistor, diodes, regulator ICs, LED etc.
3. Electromagnetic Relay.
4. Multimeter for checking, wires for connecting and a PCB or any other kind of board to set the whole circuit.

Introduction To Relay:

I guess everyone of you already know about most parts. Let me tell you something about Relay. Relay is a Magnetic Switch. We all know about electromagnet. When electricity is applied to it, it becomes magnet. This principle is used in a Relay. When electricity is provided it turns on.

Let's talk some basic about relay,  Inside of the relay there is an electromagnet and a spring supported magnetic switch. When the electromagnet becomes magnet the switch changes its position and as soon as the magnet goes to normal state the the switch goes to its normal state too, yes it's because of the spring.

Externally Relays have 5 Pins, well there might be many different types of relays with different number of pins but let's just stick with the basic one for now. Two of these pins are used for powering up the electromagnet. as shown in the picture. We apply DC on it. AC can't be used to create electromagnet. And the rest three Pins are one is Common which can move and can touch the two different pins. One is Normally closed, when the relay is switched off, common stay touched with the normally close Pin, and the other Pin is normally open which normally stays floating because nothing is connected to it. When the electromagnet is powered up, common leaves the normally close pin and connects with the normally open pin. (As shown in figure)

So the switch acts like a SPDT(Single Pole Double Throw) switch.

Now understanding the label, On the relay two different ratings will be labeled. One is usually a DC part other is an AC part. The DC part is the rating of the electromagnet like 5V or 12V DC,if we want to activate the electromagnet we have to apply this certain voltage. 

The AC part is the switching portion, if it is rated at, suppose 5A 220V, that means we can use a load not more than this, if we need to go over that we have to find a relay with higher rating. Although it will be rated in AC it is absolutely fine to work with DC.

This is a typical 12V relay. As you can see it's rating on top of it. 

Determining the Pins

Usually it will be labeled on your relay, a coil between two pins represents the electromagnet.  And a switch between three of the pins represent the magnetic switch.

If there is nothing labeled on the relay just use your multimeter to find out the resistance and start labeling like this between 1 and 2 there should be around 500 to 600ohm resistance, between 3 and 4 pin there won't be any resistance meaning short & between 4 and 5 pin there will be infinite amount of resistance meaning open.

Diagram :

Let's take a look at the whole diagram.

Two different power supplies is being used to simulate a real life scenario like this, you can use your adapter to power up your device and you want to use a battery for powering it up during load shedding or any kind of power outage.

Now looking at the diagram we can see that when there will be no AC power available on any of the power supplies, the battery will keep the load running.

Then as soon as the AC power is available, it will be available to both the power supplies, and the relay will turn on thus connecting the Common Pin to the Normally open pin. Normally open pin is connected to the suppose the adapter given which provide 12V regulated power, so the load will keep running from the power supply 2. And at the same time via the regulator 7812 the battery will start charging. The diode there will prevent the relay from running on the battery when power supply is off or AC main power is off. 

Assume Power Supply 2 is the adapter provided with the device.


Like I said before this diagram is not that good, meaning this circuit shouldn't be used directly. Because,

1. 7812 will provide 12V, there is a slight drop in voltage on the diode, so the battery will actually see less than 12V. And less than 12V is not enough for charging SLA battery or any 12V battery. 

If the load is a 9V load and the battery is a 9V battery then it will be fine, all you have to do is to use multiple diodes in series so that the voltage can be dropped to around 9.5V or something.

2. Different techniques should be applied to charge battery, different battery require different circuits. Sealed lead Acid, Lithium ion, Nickel metal hydride these are the most common types and they need different charging circuits. 

Hope you will be able to improve this diagram. 


More about relay.

Unregulated power supply.

Regulated power supply.

Sealed Lead Acid Battery charger. 

Hope this will be helpful. Read my other posts here. Have a nice day!

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