Using a Multimeter
Switching gears a little bit, I was going to start talking about some different tools that are used to kind of look at analyze amplifiers and amplifier parts, et cetera and I just thought I'd go through and kind of talk a little bit about the multi meter so multi meter does a bunch of different things um the handiest things that do are full and you're dealing with guitar amplifiers is measuring voltage measuring resistance and measuring current so starting with voltage if you have a multi meter it will have some selections that you need to know about, um, which function you're going to use this again a major voltage or resistance are current and some multi meters have different selections for different ranges of voltage or current so let's see here I kind of have organized yeah, so you need to know if you're going to measure voltage that you are in the voltage measuring set up. You also need to know that you have the probes that you would use connected to the right ports because a mult...
i meter will often have different ports for measuring uh, the current or a different one for measuring voltage or impedance. So you just have to get familiar with the actual meter that you're working with and know how that works, so once you know you're on the right setting and you're in the right ports, then you need to connect you're probes to a device that you want to measure the voltage across that so so you have a resistor in a circuit you have the circuit energized it's all going and you take your two component every two probes you find your way across the component and then you measure the voltage across it sometimes you need to adjust the range of the meter so if a meter is set for measuring voltage is on the order of milan's our nobles or bolts or tens of bolts or hundreds of olds sometimes you have to take between the different settings to get through the right setting if you have it in the wrong setting it might just tell you zero you know it's not zero silly anyway so just be aware of that um a good rule of thumb is to use alligator clamps um like this kind of thing whenever you can because that will save you the threat of shock a bit keep is keep two hands out of the amt whenever possible and if you do have to use two hands uh well don't use two hands but if you have to get one hand in there you might clamp the ground inside of something or a clamp across one end of the the component you want to measure the voltage take the other one, put your hand in your pocket where you can't and I accidentally reach out and touch something for yourself and you like that just trust me it hurts so that's ah good rule of thumb oh important in measuring voltage so if you're doing ah voltage measurement of a component that you know there's a dc voltage across that you want to measure you'll need to be set for that and multi meters will often have a button that selects between a c or d c or sometimes there's a notch setting specific for a c versus d c so dc measuring voltage pretty basic you just go across the component and I'll tell you a voltage and then you know the voltage it's a dc voltage it's that simple a c voltage is a bit trickier because there's something to know about what the media is telling you and meters air different from each other some meters will tell you what's called a peak voltage the's air a bit more rare but it'll actually tell you so you're measuring this voltage here in the picture three hundred forty forty bullets on this graph is what you're saying is that this is unsighted just explain this graph here this is ah time is this what we call the x axis moving from left to right through time and the why we call the y axis is the bolt indigenous is telling you how much voltage as you move through time so looking at this voltage wave form here uh there's a peak voltage, which is the highest point on this, a c way form, and that's that's useful to know. But what a multimedia typically tells you is not that what it'll typically tell you is some average about the bulletins that you're measuring and what's called the r m s voltage is usually the way they're set up. So if you had something you were measuring and a c voltage across and it told you, like in the picture here, two hundred forty volt that's what you measure to forty both they see it's important to know in some cases that that's not the peak voltage, but rather they are a mess. Voltage it's an average and there's a formula you can use to get from the r m s voltage to the peak. If you need to know that assuming that you're measuring a sign, you saw it which he will, in some cases, in an amplifier again, the fine destroyed is this perfect kind of curved, symmetrical way form you just multiply the roms, fall to you, get by route too square ready to go so that peak voltage is getting very twinges default it is going to equal the voltage are a mess that you measure take that and multiply it by the square root of two so say you needed to know what the pete baltar's was for any purpose like a rectifier we'll get into this would be the way to find out exactly what you can expect to get but that needs some more explanation, but we'll get there so yeah, anyway, just keep that in mind when you're measuring a c voltages resistance works the same way as faras I'm sorry, can I go back? I realized something I didn't say that is important when you're measuring voltages, you need to apply the probes in parallel to the component that you're measuring, so this picture here is handy for that shows you the meter is connected in parallel to the light bulb on dh that's important because we get to current, you'll see why so in measure resistance the same way you would measure in parallel to the resister, meaning one lead across the top of the resister one lead across the bottom of the resister or left to right and again make sure that you're set to the right mode. Make sure you're in the right probes position, and another important thing is if you've got a resistor in an amplifier and you want to check and make sure that it is what it's supposed to be it's not damage say, for example, you wouldn't need to make sure you power down the circuit for one that way the voltages going through the circuit wallets energized aren't going to throw off your reading, and another one is you would need to disconnect it from the circuit completely. In most cases, you would actually have to remove the component and then measure across the resister in parallel to get a resistance measurement. Um, and another handy thing you can do with your meter when you're testing through an amplifiers something's not working and you suspect that there's a short meaning that something that's supposed to be presenting a resistance say is connected together for whatever mistaken reason a component fails. Oh, weir and support should be whatever reason you can set up the meter with this there's a little beat functionality pushed the button, and it tells you it's going to be when things short together, and that could be handy thing to go through. So, um, not associated with the resistance, measuring setting current is a tricky one. You can use the multi meter typically to measure current, but now you need to make sure that again you're in the right setting for measuring current. But you also need to make sure that you have your probes in the correct port and because current can cause things to blow up, they have separate ports for different ranges of current sometimes will have different selections as well. But you need to know exactly how much can you expect to see to know which port to be in otherwise we'll just pop the fuse so it requires a little bit of knowing ahead of time what you're getting into before you made your current and the other really important thing about measuring current is rather than being in parallel with the device that you're trying to measure, you need your probes in siri's with the thing, so in this picture here you can see you've got a battery and you're trying to measure the current going through this resistor on the right hand side well, instead of applying the probes in parallel to the resister, you have it in siri's so it's in line with the circuit on its way through the resistor and back and that'll tell you how much current you're getting now. I think that whenever you can avoid measure and current with the meter, you should because it's much easier there's a much easier and safer way to get to that information. If you want to know how much current is going through this resistor in this circuit, for example, you can go back to your volt meter make sure that you're back on the right setting for measuring voltage and you just measure the voltage across that resistor and now you have this handy equation that I knew a dude with before voltage equals current times resistance, and you can just use that equation to solve for the current. You know the voltage across that from your measurement, you know the resistance value. Maybe you need to take it out. Measure that, too, and you can just do the division, divide the voltage by the resistance, and it will tell you the current and it's, totally accurate and it's safer. And you're not going to go through a bunch of meters tryingto measure current and blowing things up.
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