| Electronics > Beginners |
| Some noob questions |
| << < (10/28) > >> |
| rstofer:
--- Quote from: Mr D on July 17, 2018, 06:07:57 pm ---Many thanks to all for being exceedingly patient with me, i'm gonna take my time and read through this thread a few times. I want to ask one more favour. Could someone tell me the values of a few resistors i should buy that i can use to connect to my Fluke and power source so i can experiment with calculating ohms law? I guess the resistance should be in a range that it'll be clear on my multimeter, but not so low that i risk damaging my power supply or Fluke. --- End quote --- I bought an assortment kit from Jameco many years ago. Maybe something similar is available to you? https://www.jameco.com/z/00081832-540-Piece-1-4-Watt-5-Carbon-Film-Resistor-Component-Kit_81832.html 100, 220, 330, 470, 1k, 2.2k, 4.7k, 10k, 22k, 47k, 100k all 5% ought to cover it. Notice how the values are just 10x multiples of the original sequence? That's a property of common resistors https://ecee.colorado.edu/~mcclurel/resistorsandcaps.pdf --- Quote ---And does anyone have some links to a good lab power supply that will be useful for testing Ohm's law? Doesn't have to be very cheap, but i also don't want to spend more than necessary. There's so many available, i have no idea what to get. --- End quote --- I didn't have a lab type supply for decades. I used batteries or wall warts and occasionally a dedicated open frame power supply. I finally broke down and bought the Rigol DP832. It is not inexpensive and there are many cheaper alternatives but it works really well and I'm glad I bought it. A 3 output supply is handy when working with dual rail op amps where you need +15V, -15V and maybe 5V for some kind of supporting logic. Having a supply with adjustable current limiting saves on parts. It does no good to have an analog knob with no display of the limit value, you can have it set too high and not even know it. A true lab supply has an indication of the current limit value. Very important feature. You can search the forum for 'power supply' and find MANY threads. A lot of people want to build their own and there are a number of users recommending it. Maybe so... I buy power supplies, the only one I built was an old Heathkit back around '70. There are some power supplies on eBay if that's a help. Some are older HP supplies and these are probably quite good. There are also some Chinese versions of dubious quality. So, prices are from cheap to expensive with features to match. Of course, I have built the project level supplies with LM7805s and things like that. They were specific to the project, not general purpose. |
| rstofer:
--- Quote from: Rick Law on July 17, 2018, 07:02:16 pm --- Why your initial test was unsafe: If you understand what is described thus far, I can explain why your initial connection (with DMM measuring current directly connected to the output) is unsafe. Without the FAN in between, nothing is there to use the power. So, the DMM is taking the full load. Good that your DMM limits that to just 2A - it is still not good for long, but it survives for the short duration of heating. --- End quote --- Well, we know the 2A number but we really don't know the voltage at the DMM. The power supply has some internal resistance (known later in your studies as the Thevenin Equivalent Resistance) so some of the voltage was dropped internally and some of it was dropped in the meter. We don't know the resistance of the current measuring circuit in the DMM but it's going to be LOW. Say it's 0.1 Ohm... If so, the meter was only dissipating 0.4W (I squared times R). The rest of the voltage was dropped internally within the power supply and there are a number of ways that can occur and not all of them result in excess heating. Why do we care about this? Well, Thevenin is going to play an important part in circuit analysis but more important, it's going to lead us to the fact that we need 2 DMMs (minimum). It does no good to measure the current through a device unless we can simultaneously measure the voltage across it. The power supply may be the limiting factor, not the device. This is all a lot of fun. Ohm's Law, Kirchhoff's Laws, Thevenin's Theorem - these are the tools of circuit analysis. You go nowhere without understanding them. |
| Rick Law:
--- Quote from: rstofer on July 17, 2018, 07:40:24 pm --- --- Quote from: Rick Law on July 17, 2018, 07:02:16 pm --- Why your initial test was unsafe: If you understand what is described thus far, I can explain why your initial connection (with DMM measuring current directly connected to the output) is unsafe. Without the FAN in between, nothing is there to use the power. So, the DMM is taking the full load. Good that your DMM limits that to just 2A - it is still not good for long, but it survives for the short duration of heating. --- End quote --- Well, we know the 2A number but we really don't know the voltage at the DMM. The power supply has some internal resistance... ... ... This is all a lot of fun. Ohm's Law, Kirchhoff's Laws, Thevenin's Theorem - these are the tools of circuit analysis. You go nowhere without understanding them. --- End quote --- Don't scare the guy... Let him in gently. I think he has to figure out a lot more before he gets there. I merely want to let him know he can't stick the probes into just anywhere when it is set to current... Like sticking it into the AC outlet. Now we can relabel the DMM settings as Crispy and Extra Crispy. |
| rstofer:
--- Quote from: Rick Law on July 18, 2018, 01:39:38 am --- Don't scare the guy... Let him in gently. I think he has to figure out a lot more before he gets there. I merely want to let him know he can't stick the probes into just anywhere when it is set to current... Like sticking it into the AC outlet. Now we can relabel the DMM settings as Crispy and Extra Crispy. --- End quote --- One thing we all know: If you try to measure volts with the probes in the current position, the fuses may blow. Or the meter may just do an imitation of a hand grenade. The user just has to pay careful attention to where they put probes and what they expect to have happen. LOOK at the meter and verify the test lead connections BEFORE sticking a probe somewhere. Some meters (like my Fluke 189) will flash a warning if the probe position isn't correct for the range selected. That's a nice feature There was a time before meters had fuses and CAT ratings are kind of a new thing as well. In spite of that, many of us are still here. NOTE: The EEVblog Brymen BM235 warns when the test leads are in the wrong position. So does the new EEVblog 121GW. These are professional meters. The Aneng 8008 does NOT warn of incorrect test lead connections. Apparently, it is expendable. As is the user... I like the meter and use if in preference to my other meters but I do need to be aware of the limitations. Since I work on low energy projects, it's not much of a risk. |
| Brumby:
Firstly, let me address the water analogy. It is a very good analogy when taken as a guide to help your thinking, but - like all analogies - it will break down if you try and take it too far. This is what you are doing when you start talking about the speed of the water flow. You will get caught up with details that simply are not part of the analogy and you will get totally confused. It would be better (not perfect) to think of the quantity of water flowing - not the speed. Secondly, you absolutely have the right idea of wanting to understand Ohm's Law!! Once you do, the water analogy will make a lot more sense and you will probably see where a few of its limitations arise - but you will also be less likely to need it. Third - but perhaps the most important at this juncture - DO NOT put your meter into current mode and connect it directly across a power source unless you really have to and already know what to expect. It can be very dangerous and it is not a habit that you want to develop. To understand why, you really need to know Ohm's Law and apply it in the real world. |
| Navigation |
| Message Index |
| Next page |
| Previous page |