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| stirlinggaz |
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stirlinggaz
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Joined: 22 Jul 2007
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 Posted: 10:58 - 24 Feb 2015  Post subject: CHEAP MULTIMETERS |
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hi,
a wee warning about buying cheap mulitmeters off ebay, in particular the DT-830B which comes in black or yellow.
i have a half decent multimeter (a gunson) but mislaid the leads so figured i would be as well buying a cheapo multimeter complete with leads, which costs around the same as buying the leads alone.
some people will say, "what do you expect from a tool that costs less than a fiver?"
thats not the point, i think these meters are unsafe. heres why.
i bought a new battery for my bike, a little 5ah one for my wee nsr & wanted to know if i needed to charge it up.
so using the 10a MAX socket (red) & black (com) & within 5 seconds the wires got incredibly hot & went tacky, as they were starting to melt!
like most other meters this function is unfused & the instructions clearly state 15 secs "max" but i only touched the 5a battery for a couple of seconds!
furthermore, when i opened up the meter, i discovered BOTH red sockets are unfused.
tyl/dr?
avoid cheapo multimeters, i reckon they are a fire hazard,
cheers,
GAZ |
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| Copycat73 |
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Copycat73
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Posted: 11:02 - 24 Feb 2015  Post subject: |
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so you set your multi meter to amps. and connected it across the battery terminals ... is that correct ? 
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| Taught2BCauti... |
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Taught2BCauti...
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| Posted: 11:04 - 24 Feb 2015 Post subject: |
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Did you put the black lead to the battery negative terminal, and the red lead to the battery positive terminal?
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| Skudd |
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Skudd
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| Islander |
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Islander
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Posted: 11:11 - 24 Feb 2015  Post subject: |
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Step1: Learn how to use a multimeter correctly.  
Reading current (amps) requires the meter to be in series with the circuit you are reading. You connected it across (i.e. in parallel with) a battery capable of dumping around 100 amps into the load the meter presents then wonder why it overheats?
In summary: Voltage checks (AC or DC) go across (in parallel with) the battery or circuit you're testing. Current checks go in line with (in series with) the circuit you're testing.
Oh and resistance checks are made with no power applied to the circuits. |
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Skudd
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| The Artist |
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The Artist
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| Skudd |
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Skudd
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| Posted: 11:19 - 24 Feb 2015 Post subject: |
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Brilliant...................... now how do you use it and what are all the buttons for?
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Petty Anarchists look at "1984".............. The Visionary looks at "Animal Farm". |
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| Islander |
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Islander
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| Posted: 11:23 - 24 Feb 2015 Post subject: |
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| Skudd wrote: | I just need to know about a very basic multi meter, Halfords one would be fine for ease of finding and buying. How to use it and when to use it. What what the buttons and readings all mean and how it will help me sort out a certain 125cc bike.
Just a basic idiots guide please. |
Voltage checks on the circuit side of the regulator rectifier are made using the DC ranges. To test to see whether a voltage is present, select the range nearest and above 12v, connect the black lead to a good earth point or the battery negative terminal and the red lead to the point that you're testing.
To check battery charging, test the battery voltage with everything off - a healthy battery should be around 12.5v. Then check with the engine idling and it should remain around that or slightly higher. Finally rev to around 3-4k and you should see it rise to around 13.5v to 14v.
To check the alternator output you need the AC ranges. Select something around 100 or 200v, and check the alternator leads in pairs on the alternator side of the regulator rectifier. You should see something around 50v AC.
To check whether a wire or fuse is intact, select a low resistance range (1 ohm or 10 ohms or similar), make sure the ignition is off (we don't want power applied to the wire or fuse) and place a probe at each end of the wire or fuse you're testing. It should give as near to zero as possible.
Similarly, checking for shorts use a low resistance range, ignition off, and place a probe between the two thing you suspect are shorting out. This time you want as high a reading as possible - a low reading means a low resistance which means a short.
That's as basic as it gets. 
I'm sure there are a zillion youtube videos out there as well. |
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| Skudd |
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Skudd
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| Posted: 11:27 - 24 Feb 2015 Post subject: |
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Oh it needs to be more basic........... VERY basic.
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Petty Anarchists look at "1984".............. The Visionary looks at "Animal Farm". |
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| Islander |
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Islander
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| Posted: 11:40 - 24 Feb 2015 Post subject: |
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Oh dear...
Typical cheap digital multimeter looks like this:
https://cdn.shopclues.net/images/detailed/459/Multimeter_1367903256.jpg
The ranges are boxed to keep the similar ones together.
Voltage ranges first:
On the left of the dial you'll see a V with a solid line above a dotted line. That's the DC or direct current ranges. You use those with things powered by batteries. On the bike you'll be using the 20 range.
On the right you'll see a V with a tilde next to it (V~). Thats the AC or alternating current ranges. You use those to test alternator output. You'll be using the 200 range.
Stay away from the ranges on the right marked with the A with a solid line above a dotted line. Those are the DC current ranges and you need to know what you're doing before you try to use those (see the OP for why...  )
The ranges with the omega symbol are the resistance ranges. On the meter in the picture they range from very high (2000k or 2 megaohms) down to low (200 ohms). You'd be using the 200 ohm range most of the time.
Ignore the other range settings apart from OFF.
Black lead goes in the common or black terminal, red lead goes in the + or V terminal. Don't use the one marked 10A.
Use that in conjunction with the advice above and/or youtube videos.
Any better? |
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| Skudd |
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Skudd
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| Posted: 11:47 - 24 Feb 2015 Post subject: |
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Getting there
Just a bit of kit that I've never needed to use. Didn't really do it at school either. (many years ago) I now I will probably need to use one soon as I'm sure I will find a few niggles on the 125. Hopefully all will be fine, but you just know something will be better if you had an idea what was wrong in the first place.
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Islander
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| Posted: 11:53 - 24 Feb 2015 Post subject: |
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Be methodical and think it through and you'll be fine. |
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Skudd
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| stirlinggaz |
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stirlinggaz
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Islander
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| stirlinggaz |
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stirlinggaz
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stirlinggaz
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stirlinggaz
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| Taught2BCauti... |
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Taught2BCauti...
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| Posted: 12:22 - 24 Feb 2015 Post subject: |
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| Skudd wrote: | I just need to know about a very basic multi meter, Halfords one would be fine for ease of finding and buying. How to use it and when to use it. What what the buttons and readings all mean and how it will help me sort out a certain 125cc bike.
Just a basic idiots guide please. |
For most basic fault-finding work, you will probably start with the meter set to 'DC Volts'. If the meter is not an 'auto-range' type, you will need to select a range higher than the maximum voltage you would expect to find - e.g. 20V for a 12V system.
Make sure the test leads are plugged into the correct sockets on the meter, and use them to test the voltage.
Most common tests - directly across the battery terminals, when:
1. Battery is disconnected from the bike.
2. Battery is connected normally, but everything switched off.
3. Battery is connected normally, ignition on, engine stopped.
4. Battery is connected normally, engine running at tick-over.
5. Battery is connected normally, engine running at 4,000 rpm.
Between battery positive, and various earthing points around the bike - e.g. frame, engine casing, bulb holders, etc. with the red lead on the battery positive terminal, and using the black lead as a probe.
Between battery earth, and various points on the electrical system - e.g. fuses, connector blocks, bulb holder center contacts, etc. with the black lead on the battery negative terminal, and using the red lead as a probe.
Take care not to short anything with the metal end of the test lead when doing this - wrap some insulating tape around it so only the tip is exposed if necessary.
Testing AC (Alternating Current) Volts.
The only part on the bike that works in AC Volts, is the alternator, and when the engine is running, it can put out anything between 40 and 100 volts when not under load. - so take care!
Best way to test it, is to disconnect it from the Reg/Rec (usually 3 yellow wires) and label them A, B, and C. With the meter set to 200V AC range, connect the probes to A and B (doesn't matter which way round) and start the engine and note the reading at tick-over and 2,000 rpm. Stop the engine and do the same for B and C, then stop the engine and do the same for B and C. The 3 sets of readings should be the same, or there is a problem with one of the coils.
Take care not to touch any part of the wiring when the bike is running - AC voltages are dangerous!
Resistance Testing.
This will tell you how good the connection is between one point and another on the electrical system. A good connection will give zero, or a very low reading - a bad connection will give a high reading, or no reading at all.
Some components, like coils and bulbs, will give a reading above zero if they are working, and no reading at all if they are faulty. Simple switches will give a zero reading when on, and no reading at all when off. A Fuse will also give a zero reading if good, and nothing when blown.
LED's and Diodes will only give a reading in one direction - swap the leads around, and there will be nothing.
When testing resistance, start with the highest range, and gradually decrease the range to get a meaningful reading.
Current Testing.
This measures how much power is being used by a circuit under test. You almost always have to move the red lead to a different socket on the meter for this, especially for currents above 1/5th of an amp.
Never connect the test leads across a battery when testing for amps, as this will short-circuit the battery!
To test for current, the meter must only ever be placed 'in series' with the circuit under test - e.g.
Earth----Battery----Meter----Bulb----Earth
Most common use for this test, is to see if anything is draining the battery when everything is supposed to be switched off.
To do this, disconnect the earth wire from the battery, and hold it away from the terminal. With the meter set to the highest amp range, connect the red lead to the earth wire, and connect the black lead to the battery negative terminal.
+Battery- ----Meter----Earth Wire
Start on the highest range, and gradually decrease the range to get a meaningful reading - if something is draining the battery when it shouldn't be, this will help to trace it by disconnecting things (like the ECU or the Reg/Rec) or pulling fuses, until the reading drops to zero.
Make sure you put the red test lead back into the correct socket after testing amps!!!
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G
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| stirlinggaz |
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stirlinggaz
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| Posted: 12:31 - 24 Feb 2015 Post subject: |
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thanks guys, i've learned a lot today.
(like not to f*ck about with my "good" meter)
cheers,
GAZ |
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| Islander |
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Islander
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| Posted: 12:33 - 24 Feb 2015 Post subject: |
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| stirlinggaz wrote: |
sorry, i dont know the difference between "in parallel with" or "across"
what is wrong with putting black lead (in COM socket) to NEG post on battery.(Im genuinely interested & want to learn btw)
& red lead (in middle v ohms symbol mA socket) to positive post on the battery, with meter set to 20v DC? |
If you'd have stopped there then you would have been fine.
| stirlinggaz wrote: |
& when testing current (DC) leads in same sockets but meter set to DCA? |
And that's where you went wrong. You only ever read current with the meter in series with the circuit you're testing as it presents a very low resistance in that mode.
| stirlinggaz wrote: | i presumed (wrongly it seems) that if i overloaded anything on the middle socket, the glass fuse would blow?
which was what happened when i 1st used my gunson meter & was trying to figure out this witchcraft called elicktrickery. (hence the reason i have a pack of thse glass fuses  ) |
Fuses can tolerate a large overload for a surprisingly long time.
| stirlinggaz wrote: | ps : how is the battery capable of dumping 100 amps into the meter? this i really dont understand, sorry! |
It may only be rated at, say 12Ah or so but that's constant output. For instantaneous output it can supply a lot more if the load has a low enough resistance. The CCA rating (cold cranking amps) gives you an idea of the instantaneous current available.
Last edited by Islander on 12:39 - 24 Feb 2015; edited 1 time in total |
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| Taught2BCauti... |
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Taught2BCauti...
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| Posted: 12:35 - 24 Feb 2015 Post subject: |
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| stirlinggaz wrote: |
see, your the 1st person to tell me not to use the 10a socket.
why not?
& how do i test the amps of a battery? (as an example)
telling me NOT to do something just makes me curious enough to try it
cheers,
GAZ |
An ammeter (or a multi-meter set to read amps) works in a different way to a volt meter and has very little internal resistance. Between the test leads, there is nothing more than a small coil of thick wire, which creates a magnetic field that is measured by another coil inside the meter - so it's just like putting a length of wire straight across the terminals.
There is a special type of meter that is used to do what you want to do - called a 'Drop Tester' which works by measuring the voltage across the battery terminals whilst the battery is connected to a heavy load resistance.
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Islander
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| Posted: 12:38 - 24 Feb 2015 Post subject: |
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| stirlinggaz wrote: |
see, your the 1st person to tell me not to use the 10a socket.
why not?
& how do i test the amps of a battery? (as an example)
telling me NOT to do something just makes me curious enough to try it
cheers,
GAZ |
You can use the 10A socket if you know what you're doing but using the current ranges requires that the meter be connected in series with the tested circuit and not in parallel. Getting it wrong can damage your meter and the heat generated can lead to some nasty burns. The reason I advised Skudd to forget it is that he doesn't know how to use it at this stage.
Testing the current output of a battery to test its functionality is a specialised test that requires a meter capable of dissipating a large amount of energy - not a multimeter!
You can test the current draw through specific circuits though as long as you don't exceed the meter's rating (by trying to measure starting amps, for example...) |
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Old Thread Alert!
The last post was made 11 years, 136 days ago. Instead of replying here, would creating a new thread be more useful? |
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