This is the second post in a series of posts on making your own mini UPS for DSL modem. Be sure to read the first part before you read below
The design of our mini UPS depends upon our DSL modem. Specifically, the current and voltage ratings of our DSL modem. If you have read the previous post then you should already know that a DSL modem works on DC power. These dc ratings are typically printed at the back of the modem.
I’ve this modem (see Figure 1) provided by my internet service provider PTCL (Pakistan Telecommunication Company Limited).
|Figure 1: Front side of FiberHome AN1020-25U DSL modem distributed by PTCL to its customers|
A look at the back of this modem (see Figure 2),
|Figure 2: Back side of FiberHome AN1020-25U DSL modem|
As seen the ratings are 12 V, 1 A DC. The symbol having solid and dotted line indicates that the DSL modem works on DC power. The 1 ampere shown is the maximum current draw. The typical current draw will be lower than this maximum value.
The power adapter supplied with this modem has similar ratings as seen in the figure 3.
|Figure 3: Power adapter supplied with FiberHome AN1020-25U|
Since the 1 A is the maximum current draw we need to measure actual current drawn by the modem. The process of measuring current is a bit tricky than measuring voltage as it involves breaking the path and connecting a multimeter inline. The multimeter is an electronic test instrument used to measure voltage, current, resistance and other quantities. A multimeter can be analogue or digital. Obviously a digital multimeter will be better than an analogue one. Fluke USA manufactures the best digital multimeters. Other reliable brands are Uni-T China, Kyoritsu Japan, GW Instek Taiwan.
Here is a graphical illustration that will help you in measuring the current drawn by your modem. If you don’t have a digital multimeter then I suggest to buy it now if you want to make your own mini UPS (see Figure 4).
|Figure 4: An illustration of the process of measurement of current flow through a DSL modem|
In the illustration above the red line coming off the adapter is positive lead and black line is the negative lead. As you can see we need to cut the negative wire of the power adapter. You can also cut the positive wire and leave the negative wire intact. For this tutorial I chose to cut the negative wire.
It is also possible to measure current without cutting a wire but that would require a clamp meter which costs more than a digital multimeter. Since these posts are meant to be equally useful and understandable to beginners so I thought about describing the process using a digital multimeter. The first electronic test instrument that a beginning electronics student encounters is a multimeter.
If you don’t want to ruin your existing adapter you can use any spare adapter with the ratings similar to the adapter supplied with your modem. Double check the voltage ratings of both the DSL modem and the spare adapter and make sure they are same. If your DSL modem works at 9 V dc and you connect a power adapter of 12 V dc, your modem is very likely to get damaged.
While in the illustration above the wires coming off the adapter have two colours, actual power adapter wires are of black colour. So your adapter has two black coloured wires, which of these would be a negative wire? This varies from manufacturer to manufacturer and all manufacturers mark a wire in one way or another. Some adapters might have positive wire marked while others would have negative wire marked. Thus it is a good practice to actually check first. My adapter had negative wire marked. In Figure 5 you can see the dashed white lines on one wire while the other wire has something printed on it. The one with dashed white lines is a negative wire and the other is positive. I found this using continuity test option on my multimeter.
|Figure 5: Polarity markings on the wire of power adapter|
After cutting the wire remove about 10mm of insulation from both ends. Connect the end of the wire near to the power adapter with the black test lead of the digital multimeter and connect the red test lead of the digital multimeter with the end nearer to the modem. This arrangement will give us positive reading. I really like to have positive reading. See the Figure 6 which shows how I measured the current draw of my modem.
|Figure 6: Measurement of current flowing through a DSL modem|
Although the picture shows the modem FiberHome AN1020-25U drawing 0.194 A (same as 194 mA) this is not the value that we are looking for.
Because the DSL line is not connected. Neither any Ethernet cable is connected to the modem (there are four Ethernet ports on this modem) nor is any other device connected to this modem via Wi-Fi. The absence of all these connections has given lower reading. To make this tutorial I could not connect the DSL line. I had already found the current draw of my modem connecting all the needed devices. The reading which I obtained was 230mA.
Now that we know how much current our DSL modem draws we now have to select a battery. It is the battery that will power our modem during a power outage so whatever battery we select should have enough capacity to run our modem for whatever is the duration of the power outage.
We will discuss the types, selection and other aspects of a battery in the next post.