Rapitest DM25 test meter

[stanbesida]

I have a Rapitest test meter which was given to me by my son, I want to check the output from an A/C adapter which is supposed to be 18.5V or 3.5A. Could anybody tell me where I should set the dial on the front, and where I put the red lead, i.e. A --- , A with a wavy line, V--
or V with a wavy line, also 10A, A, or V with what looks like a pair of earphones. I'm thick and don't know how to work it so Thanks in advance

[Buenchico]

Firstly, ignore the 3.5A figure. It tells you the maximum current that the adapter can safely supply to a device attached to it. Since you'll have nothing attached to the adapter (other than the meter), this figure is completely irrelevant.

What you need to check is the voltage supplied by the adapter. The phrase 'AC adapter' is loosely used to describe both something which simply drops the mains voltage down to (in your case) 18.5V but keeps it as an AC current OR something which drops the voltage to 18.5V but also converts it to DC. You need to know what it is that your adapter is designed to do.

If you're not sure whether the output is AC or DC, read the label to see if it tells you. If it doesn't actually say 'AC' or 'DC', it might use symbols. A wavy line like this ~ means AC. A solid line with a dotted line above or below it means DC.

If you decide that you're measuring an AC voltage, use the socket labelled V~. If it's DC, use V--.

Set the dial to the lowest voltage which is above 18.5V. So, for example, if your choice is 10V, 50V or 250V, you'd choose 50V (and then read the scale which runs from 0 to 50 to find out what the output of the adapter is).

Chris

[ianmunt]

you should be able to check the voltage easily enough.Check first if the AC adapter output is ac or dc.There should be an indication of this on the unit,ac will be an"s" sign on its side,dc will either be an equals sign or a dotted dash over a dash.To measure ac red to V with wavy line black to common.connect red lead to one part of the output socket and black to the other.For dc use v-- .If you get the polarity wrong you will get a negative reading.Measuring the current is less easy,and if you get the rated voltage on measuring it you can reasonably assume the current will be as specified

[stanbesida]

Thanks guys, it was DC and when I checked the reading was 19.81V, his this too high for my laptop? The A.C. adapter is for my laptop but when it is plugged into the laptop it isn't charging the battery and I am unable to run the laptop from it

[Buenchico]

Thanks for the reply.

Only you can answer the question "Is it too high for my laptop?". That's because different laptops may require different voltages. The required input voltage should be marked on the laptop. (It will probably be embossed into the casing alongside the socket or printed on a label somewhere on the laptop). If the nominal input voltage is stated as 18.5V, then 19.81V is probably within an acceptable range. (The actual 'potential difference' across the connectors from the adapter will probably vary slightly depending upon the 'load' across it. i.e. your meter might register the p.d as 19.81V but the battery might 'see' it as a p.d. of 18.5V).

However, if the required input voltage of your laptop is, say, 25V (or higher), the voltage from the adapter is too low and it won't charge properly. If the required input voltage of your laptop is, say, 15V (or lower), the adapter is providing too high a voltage and the battery may have been damaged because of it.

If the adapter output voltage matches (within reasonable tolerances) the input voltage of the laptop, then (since you've checked that the adapter is working) it seems likely that your battery has gone to the great electronic graveyard in the sky. (i.e. it's f*cked). Regrettably, that probably means that it needs an expensive replacement.

Chris

[Disco Fever]

You'll also find that due to the AC adaptor having no load then it will probably show 19.81 volts but when you plug your laptop in the voltage will drop to 18.5 volts also the 3.5 amps is the maximum current the AC adaptor can takew before you start to melt it. If you look on the back of your laptop it will tell you voltage and current taken by it just go to somewhere like Maplins and get a proper laptop adaptor.....