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Newbie electriconics question..

Posted: Sat May 25, 2013 8:52 pm
by Lennie
Hi

I have a question about electronics that is very down-to-earth. Here goes: The GPIO feature of a RPi provides a +5V pin and a 0V (GND) pin. This is just similar to a 5V battery, nothing abstract here. Now I insert a resistor of R Ohm between these two pins, so from Ohm's law I get that the current though the resistor is 5V/R. I can measure the voltage across the resistor with a multimeter placed parallel to R.

My question is, will my multimeter always show +5V, regardless of what value R is?

Re: Newbie electriconics question..

Posted: Sat May 25, 2013 9:01 pm
by mline
Lennie wrote:My question is, will my multimeter always show +5V, regardless of what value R is?
Yes, because you measure always between +5V and GND.
If you want to measure the battery voltage, use a voltage divider and ADC with (i.e.) 1.1V reference voltage.

http://en.wikipedia.org/wiki/Voltage_divider
http://en.wikipedia.org/wiki/Voltage_reference
http://en.wikipedia.org/wiki/Analog-to- ... _converter

Re: Newbie electriconics question..

Posted: Sat May 25, 2013 9:03 pm
by Lennie
mline wrote:
Lennie wrote:My question is, will my multimeter always show +5V, regardless of what value R is?
Yes, because you measure always between +5V and GND.
Thanks, I will check up in the links you provided. In that case, is it even necessary for me to even have a resistor placed there, or can I simply leave it out and measure the voltage directly with my voltmeter?

Re: Newbie electriconics question..

Posted: Sat May 25, 2013 9:08 pm
by mline
You can directly use +5V and GND pins (or TP1 and TP2 pads) to measure the voltage of your power supply.

Re: Newbie electriconics question..

Posted: Sat May 25, 2013 9:17 pm
by Lennie
mline wrote:You can directly use +5V and GND pins (or TP1 and TP2 pads) to measure the voltage of your power supply.
Thanks, I will definately try that. What are the TP1/TP2 pads?

Re: Newbie electriconics question..

Posted: Sat May 25, 2013 9:23 pm
by mline
TestPads for easy voltage measure. Check this:
http://elinux.org/RPi_Hardware#How_Can_ ... dequate.3F

Re: Newbie electriconics question..

Posted: Sun May 26, 2013 2:07 am
by johnbeetem
Lennie wrote:
mline wrote:
Lennie wrote:My question is, will my multimeter always show +5V, regardless of what value R is?
Yes, because you measure always between +5V and GND.
Thanks, I will check up in the links you provided. In that case, is it even necessary for me to even have a resistor placed there, or can I simply leave it out and measure the voltage directly with my voltmeter?
Well, actually you can't use any value of R. If you use a very small value of R, say 1 Ohm, you'd need 5A of current to get 5V. That would blow RasPi's polyfuse F3, which is rated at 1A. You probably don't want to draw more than 300 mA or so from the GPIO 5V pins -- see the RasPi Hardware Wiki for details: http://elinux.org/RPi_Low-level_peripherals#Power_pins

As mentioned above, you don't need a resistor. Even if you did, the entire RasPi board acts like a resistor which draws 500 mA or so, depending on how busy the SoC is.

I recommend using TP1 and TP2 for checking 5V. Be very careful with the 5V GPIO pins, because it's easy to short one accidentally to a neighboring pin, possibly killing your RasPi. I recommend covering the 5V pins with short bits of wire insulation to prevent this if you're not using those pins.

Re: Newbie electriconics question..

Posted: Mon May 27, 2013 3:21 pm
by Jock in a Frock
A voltmeter presents a high impedance (i.e. resistance in a d.c. circuit) to the supply under test. The better your voltmeter, the higher the impedance (and the less current flowing through your voltmeter).

Conversely, an ammeter to measure current has a very low impedance. Ammeters are designed to be placed in series with the circuit under test, and the low impedance means that it does not inttorduce more load on the circuit.

That's why you should never put an ammeter across a circuit in parallel, as you're effectively shorting out the circuit. If there was no impedance between the ammeter and the circuit under test, you would blow the supply rail.

Re: Newbie electriconics question..

Posted: Tue May 28, 2013 8:52 am
by jwlawler
Lennie wrote:Hi

I have a question about electronics that is very down-to-earth. Here goes: The GPIO feature of a RPi provides a +5V pin and a 0V (GND) pin. This is just similar to a 5V battery, nothing abstract here. Now I insert a resistor of R Ohm between these two pins, so from Ohm's law I get that the current though the resistor is 5V/R. I can measure the voltage across the resistor with a multimeter placed parallel to R.

My question is, will my multimeter always show +5V, regardless of what value R is?
If you had an ideal 5V power source then this would be true. With a 1kOhm resistor 5mA would flow, with 1Ohm it would 5A, with .1Ohm it would be 50A, etc. Sadly, ideal power sources don't actually exist and most deviate from ideal in several ways. Many batteries which are slightly but not greatly overloaded behave as if there was a resistance within the battery called the "internal resistance". This is not a resistor deliberately placed within the battery just the appearance of one. If you battery had an internal resistance of 1Ohm then with high value resistors, a few hundred Ohms or more, you could fairly much ignore it. With a lower resistance, the effect will be more noticeable, with a 1Ohm resistor attached only 2.5A will flow since the total resistance will be 2Ohm rather than 1Ohm. Also, both the resistor and the battery will get warm. Even of the battery is shorted, only 1A will flow. Car batteries have a particularly low internal resistance which makes them good for their job, they can supply the hundreds of amps needed by the starter motor. I would not recommend shorting a car battery.

With other power sources and more serious overloads, different behaviours may occur. It could be dangerous to both the equipment and you to experiment with overloading them. Fuses are commonly used as protection but I would not rely on them. Modern circuit breakers usually do a better job but again I would not recommend experimenting and assuming that the circuit breaker will keep you and the equipment safe.

As a kid, I once miswired a circuit and I had a toggle switch directly across the 240V house mains. There was a 13A fuse in the plug and a fuse in the house switch board. When I closed the switch, it exploded but the fuses were fine. I was lucky not to get hurt and I did not make that mistake again.

Re: Newbie electriconics question..

Posted: Tue May 28, 2013 11:15 am
by pluggy
jwlawler wrote: As a kid, I once miswired a circuit and I had a toggle switch directly across the 240V house mains. There was a 13A fuse in the plug and a fuse in the house switch board. When I closed the switch, it exploded but the fuses were fine. I was lucky not to get hurt and I did not make that mistake again.
Plain old fuses are often very slow to blow. Depending on the switch the contacts (maybe rated at 1 or 2 amps) would have vapourised long before the 20 or 30 amps required blew the 13 Amp fuse. I once had to have the electric board out to replace the household main 100A fuse, because a fault in the 40A fused electric shower blew it before it blew its own fuse. The big cartridge fuses they use are quick blow and the fuse in the shower wasn't. Its on a circuit breaker these days, so hopefully it won't occur again.

Re: Newbie electriconics question..

Posted: Tue May 28, 2013 11:27 am
by jwlawler
pluggy wrote:
jwlawler wrote: As a kid, I once miswired a circuit and I had a toggle switch directly across the 240V house mains. There was a 13A fuse in the plug and a fuse in the house switch board. When I closed the switch, it exploded but the fuses were fine. I was lucky not to get hurt and I did not make that mistake again.
Plain old fuses are often very slow to blow. Depending on the switch the contacts (maybe rated at 1 or 2 amps) would have vapourised long before the 20 or 30 amps required blew the 13 Amp fuse. I once had to have the electric board out to replace the household main 100A fuse, because a fault in the 40A fused electric shower blew it before it blew its own fuse. The big cartridge fuses they use are quick blow and the fuse in the shower wasn't. Its on a circuit breaker these days, so hopefully it won't occur again.
Yes, I don't think that I ever remember one of those 13A fuses actually blowing. The other day, I noticed loads of old electrical junk in a friend's garage. I asked why she had not thrown it away. She said that she planned to salvage the fuses as spares. I said that it was probably a waste of time.

I used to like to substitute lower value fuses in the many items that don't need anywhere near 13A. I don't bother now. It seems very unlikely that replacing a 13A fuse with a 3A one will save me from a house fire.

Re: Newbie electriconics question..

Posted: Tue May 28, 2013 11:50 am
by pluggy
The 3 amp one would blow long before a 13 amp. They'll take their rating on a continuous basis, they'll blow inside a second or two at three times their rating. Very little stuff that isn't permanently wired needs 13 amps (~ 3kW at nominal 230V) . Its not a good idea to use 13A fuses for everything. The wiring on most stuff won't stand 13A and would get hot in seconds before a 13A fuse let go. If a 3A parted at 7 or 8 amps, its unlikely thin cable would get hot in the couple of seconds it took to break. The power socket circuits probably have a 30 amp circuit breaker in your house. A 3 amp will blow before that trips, a 13 Amp probably won't. A thin cable has a fairly high resistance and would probably limit the current to less than the trip current of the circuit breaker if it short circuited. But the current would be high enough for the cable to get red hot and set fire to the insulation.

Re: Newbie electriconics question..

Posted: Tue May 28, 2013 12:02 pm
by jwlawler
pluggy wrote:The 3 amp one would blow long before a 13 amp. They'll take their rating on a continuous basis, they'll blow inside a second or two at three times their rating. Very little stuff that isn't permanently wired needs 13 amps (~ 3kW at nominal 230V) . Its not a good idea to use 13A fuses for everything. The wiring on most stuff won't stand 13A and would get hot in seconds before a 13A fuse let go. If a 3A parted at 7 or 8 amps, its unlikely thin cable would get hot in the couple of seconds it took to break.
My fuse switching habit was back in the days when appliances frequently didn't come with plugs. You had to buy the plugs separately and attach them yourself. The plugs typically came with 13A fuses since you might be using them for things that required that much power. If you wanted a lower rating, you had to buy additional fuses separately. As a result, I still have a tin in my garage with loads of unused 13A fuses.

Now, that most appliances come with plugs attached, I would hope that the fuses are more appropriately rated but I have not checked recently. I may do a little survey of my house next weekend.

Re: Newbie electriconics question..

Posted: Tue May 28, 2013 12:31 pm
by pluggy
In general they do, but it doesn't stop some one one replacing them with a 13 A which rarely blow as you've noted and so have a long life. In the UK, fuses have a colour coding which is almost universal for the cartridge fuses in plugs, so you can tell their rating without seeing the figures. Red for 3A, Black for 5A and brown for 13A. You don't see so many black ones. Old CRT monitors and televisions were devils for fuses, they drew little current (less than an amp usually) and so needed small fuses, but they had a habit of drawing a very large current when they were first plugged or switched on in which would sometimes blow a small fuse.

I've done a PAT course got the ticket so I supposedly know what I'm talking about. ;)

Re: Newbie electriconics question..

Posted: Tue May 28, 2013 12:45 pm
by jwlawler
pluggy wrote:In general they do, but it doesn't stop some one one replacing them with a 13 A which rarely blow as you've noted and so have a long life. In the UK, fuses have a colour coding which is almost universal for the cartridge fuses in plugs, so you can tell their rating without seeing the figures. Red for 3A, Black for 5A and brown for 13A. You don't see so many black ones. Old CRT monitors and televisions were devils for fuses, they drew little current (less than an amp usually) and so needed small fuses, but they had a habit of drawing a very large current when they were first plugged or switched on in which would sometimes blow a small fuse.

I've done a PAT course got the ticket so I supposedly know what I'm talking about. ;)
Yes, I remember the colours. A bit simpler than the colour codes for resistors. Some plugs allow you see the use rating without opening them so my survey may not be hard. I guess that 5A fuses are rare because common appliances either tend to be very high power (kettles, irons, hair driers) or very low power (too many things to list). Not many common things happen to be in the middle range. I just checked my lap top power supply; it was possible without a screw driver and it has one the 5A fuses that we just agreed were rare. I wonder if it needs it, could it reasonably draw over 700W?

I remember that it was advisable to turn off the TV before trying to reset a circuit breaker. Even if the TV was not the item which caused the fault, the surge that you mentioned, together with lots of other stuff starting up, may trip the circuit breaker.

What's a PAT course?

Re: Newbie electriconics question..

Posted: Tue May 28, 2013 1:51 pm
by pluggy
Laptops usually pull around 40-50 watts which is about 200mA at 230V, they can run to around 150 watts flat out charging the battery which less than an amp but they use switch mode power supplies which can have a large surge current when you plug them in. I just checked the wife's, that has a black fuse in it as well. I'd have thought 5 amp was on the generous side.

PAT = Portable Appliance Testing.

http://en.wikipedia.org/wiki/Portable_appliance_testing

I'd suspect Wikipedia is a bit vague because, the legislation surrounding PAT is vague. It generally speaks of 'competent person' without defining what is meant by competent for example. Courses are usually run by electricians with professional body membership. And the instructors are usually the examiners as well. Pay the requisite fee (typically £100 for a single day course) and away you go......

Re: Newbie electriconics question..

Posted: Tue May 28, 2013 2:55 pm
by btidey
The main purpose of these fuses is to protect the cable. The actual power drawn by the appliance doesn't really come into it. It's when the appliance goes faulty that the fuse fulfils its purpose in life. So a 100W appliance with a 6A rated cable is fine with a 5 amp fuse.

Re: Newbie electriconics question..

Posted: Tue May 28, 2013 3:20 pm
by pluggy
You usually find the cable for an appliance is appropriate for the device its fitted to (which is part of the Pi's power issues, the power supplies/leads sold/used on the Pi probably weren't made with the Pi in mind), so the power consumption of the device isn't entirely irrelevant (and the cable doesn't usually have a power rating on it). I would agree that the main purpose of the fuse is for the cable, but the fuse must be able to allow enough current to feed the device.

Re: Newbie electriconics question..

Posted: Tue May 28, 2013 4:08 pm
by plugwash
pluggy wrote: PAT = Portable Appliance Testing.

http://en.wikipedia.org/wiki/Portable_appliance_testing

I'd suspect Wikipedia is a bit vague because, the legislation surrounding PAT is vague. It generally speaks of 'competent person' without defining what is meant by competent for example. Courses are usually run by electricians with professional body membership. And the instructors are usually the examiners as well. Pay the requisite fee (typically £100 for a single day course) and away you go......
The real problem with PAT testing is that the theory and reality are very different.

AIUI In theory in service testing of electrical equipment (and installations for that matter) should be under the purview of someone who understands the organisation's use of electrical equipment and based on that and on official guidence can make risk assesments as to how often things need to be tested and who is responsible for doing it. Being competant to organise the testing is not the same as being competant to actually do the testing. Users should be educated so that obvious faults are spotted sooner rather than later.

In reality a company either pays someone to go on a short PAT course or pays an outside firm of PAT contractors. Either way there is often noone arround who sees the bigger picture. Fixed equipment in particular can easilly drop through the cracks between installation testing and portable appliance testing. Like with many H&S things people treat it as a tick in a box rather than really thinking things through.

Re: Newbie electriconics question..

Posted: Tue May 28, 2013 4:10 pm
by mline
btidey wrote:The main purpose of these fuses is...
...to protect the PCB.

Re: Newbie electriconics question..

Posted: Tue May 28, 2013 4:23 pm
by Burngate
pluggy wrote:I've done a PAT course got the ticket so I supposedly know what I'm talking about. ;)
So have I, but it didn't stop techpaul shouting at me :(

Re: Newbie electriconics question..

Posted: Tue May 28, 2013 4:39 pm
by jwlawler
In the days when appliances typically came without plugs, I would use the smallest standard fuse that was sufficient for the load. Obviously, this should be less than the rating of the cable but that was normally supplied with the appliance and hence you would hope that the manufacturer fitted an appropriate one. Most people didn't take this trouble and hence most appliances had 13A fuses even though the appliance did not need this and the wiring was not suitable (e.g. a desk lamp). Some of you may not be old enough to remember these days. I can't remember when prefitted plugs became normal but I remember fitting plugs to my HiFi which means that plugless appliances were still common in the mid 80s.

Of course, appliances do typically come with plugs and I more lazily assume or trust that an appropriate fuse has been supplied. There is no obvious good reason for the manufacturer to do otherwise.

As I mentioned earlier, I was a little surprised that my lap top has a 5A fuse. Not the max 13A but not the apparently sufficient 3A one either. A possible explanation for that is that it is capable of working on 110V and hence is liable to draw twice the current that it would here in the UK. 3A still seems sufficient but it would allow less of a margin for the initial surge.

Re: Newbie electriconics question..

Posted: Tue May 28, 2013 6:29 pm
by btidey
mline wrote:
btidey wrote:The main purpose of these fuses is...
...to protect the PCB.
The reference here was to the fuses in the plugs on the AC mains (UK style plugs) and their purpose is to safely protect the cable from overheat and resulting fire risks. They would happlily supply enough energy to reduce the Pi to a small molten doorstop.

The polyfuse on the Pi PCB does have the role of protecting the PCB.

Re: Newbie electriconics question..

Posted: Tue May 28, 2013 6:42 pm
by btidey
jwlawler wrote: Of course, appliances do typically come with plugs and I more lazily assume or trust that an appropriate fuse has been supplied. There is no obvious good reason for the manufacturer to do otherwise.
An appropriate fuse BS1362 will almost certainly be fitted in the UK but a manufacturer might choose to standardise on plugs, fuses, cables over a range of appliances. Also if the cable is detachable like in many laptop bricks then the manufacturer can't predict if the lead will be used elsewhere. So the plug protects the cable and the appliance should look after itself.

Re: Newbie electriconics question..

Posted: Tue May 28, 2013 9:35 pm
by jwlawler
btidey wrote:
jwlawler wrote: Of course, appliances do typically come with plugs and I more lazily assume or trust that an appropriate fuse has been supplied. There is no obvious good reason for the manufacturer to do otherwise.
An appropriate fuse BS1362 will almost certainly be fitted in the UK but a manufacturer might choose to standardise on plugs, fuses, cables over a range of appliances. Also if the cable is detachable like in many laptop bricks then the manufacturer can't predict if the lead will be used elsewhere. So the plug protects the cable and the appliance should look after itself.
Good point.