Justwondering321
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Practical uses of diodes

Sun Jul 29, 2012 1:10 pm

Again probably quite a noobish (?) question, but what is the purpose of a diode on a prototype breadboard? I know what they do its just what are they used for? Thanks for replies...

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redhawk
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Re: Practical uses of diodes

Sun Jul 29, 2012 1:42 pm

Where abouts are they located on the board??

Richard S.

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Montala
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Re: Practical uses of diodes

Sun Jul 29, 2012 1:44 pm

As this is a family forum, I shan't say what I read the last word of the title to this thread as, the first time I glanced at it! :oops:

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johnbeetem
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Re: Practical uses of diodes

Sun Jul 29, 2012 2:42 pm

Justwondering321 wrote:Again probably quite a noobish (?) question, but what is the purpose of a diode on a prototype breadboard? I know what they do its just what are they used for? Thanks for replies...
One of the chief uses of diodes in boards like RasPi is to protect IC inputs from voltages that are too low or too high. This is called using the diode as a "clamp". For example, RasPi clamps some HDMI data lines between 0V and +3.3V or +5V by using two diodes, one that clamps a negative voltage to 0V (actually approx -0.2V to -0.3V because of the voltage drop across a Schottky diode) and one that clamps a positive voltage to +3.5V to +3.6V. The two diodes are in the same package to save board space.

Another purpose of diodes is to OR together two power sources so that the higher voltage wins. Well-designed USB powered hubs should have a Schottky diode on the upstream +5V input so that if the hub is self-powered using an external power supply it does not try to power the upstream device.

This called an ORing diode because it performs the logical OR function -- actually it performs the MAX function, but that's the same as OR if you're using logic values. You can turn the diodes around and get the AND function, which is really MIN. Diode-Transistor Logic (DTL) was popular back in the days when you had to buy discrete transistors.

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reiuyi
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Re: Practical uses of diodes

Fri Aug 03, 2012 6:19 pm

This is a bit off-topic to the question, but I'm really glad I live in today's time. Diodes, transistors and all sort of logical gates such as AND, OR, XOR, NOR, etc are all super cheap these days! You can buy thousands of them for only 10 dollars! Experimentation with electronics has never been more fun and cheap.

I often use diodes as a poor man's way of dropping voltages. "This PSU gives 24v but I need 12v.. I'll just put some diodes in series and finish it with a 7812".

Diodes are used for all sorts of things. In the past they were often used for reverse-polarity protection (when stupid manufacturers would sometimes put positive on the outside of a 2.1mm plug!! WHY). They're used for rectifying AC of course (with smaller currents, otherwise people prefer bridge rectifiers; which are 4 diodes in a metal package). In DC-DC step-up there's always diodes for protection. In modern days, diodes can emit light (leds). I can't even begin to think of all possible uses of diodes. Make sure to stock up on the 1n4*** series, they're used in almost every single circuit and they're like a cent each.

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redhawk
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Re: Practical uses of diodes

Fri Aug 03, 2012 9:52 pm

Funny this topic cropped up again since I'm building a circuit which would require a diode for a very special purpose.
When the PI shuts down it doesn't kill the power to the USB ports so I'm creating a digital switch involving a relay that will self power down when the PI powers down.
Anyone who has built transistor circuits involving relays will notice there is a diode connected in parallel with the relay coil.
This is because when the coil is active and then stopped the collapsing magnetic field creates nasty voltages in the opposite direction high enough to give you an electric shock.
Transistors do not appreciate having current flowing in the wrong direction at such a high voltage (reverse bias) and in some cases may damage.
The diode connected at the relay coil makes sure the transistors are not subjected high voltage reverse bias damage.

Other uses for diodes are:
emitting light i.e. remote control
detecting light i.e. remote control
dropping voltage (poorman's regulator)
voltage reference in regulator circuits
poorman's voltage regulator zener diodes
connecting two power sources together with 2 diodes that may not at the same time have the same voltage output.
battery backup circuits i.e. clock radios that need to maintain the clock in the even of a power cut.
radio circuits detector stage (look up crystal radio)
variable capacitor (varactor) radio circuits tuning or at the transmitter modulation stage
rectifying AC to DC
transient suppressors - protect equipment from power supply spikes
voltage protection i.e inputs of amplifying equipment or digital meters
temperature detection circuits.
tunnel diode - microwave oscillators
lasers - perfect for tormenting next door neighbour's cat. :)

Richard S.

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johnbeetem
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Re: Practical uses of diodes

Sat Aug 04, 2012 10:25 pm

reiuyi wrote:This is a bit off-topic to the question, but I'm really glad I live in today's time. Diodes, transistors and all sort of logical gates such as AND, OR, XOR, NOR, etc are all super cheap these days! You can buy thousands of them for only 10 dollars! Experimentation with electronics has never been more fun and cheap.
I agree with you up to a point. But in the 1970s and 1980s you could get pretty much everything in 0.1" centers (DIPs, SIPs, or PGAs) which made wire-wrapping really easy. Plus wire-wrapping tools were very cheap because so many were sold. Nowadays most interesting components are SMT, so you need adapters to use them for prototyping.

Also, back in the 1970s and 1980s manufacturers were generally very open with their documentation, so you had the information needed to design fun things. Nowadays you need NDAs and if you're not one of their top 10 customers, fuggeddabouddit.

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reiuyi
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Re: Practical uses of diodes

Sun Aug 05, 2012 6:10 pm

johnbeetem wrote: I agree with you up to a point. But in the 1970s and 1980s you could get pretty much everything in 0.1" centers (DIPs, SIPs, or PGAs) which made wire-wrapping really easy. Plus wire-wrapping tools were very cheap because so many were sold. Nowadays most interesting components are SMT, so you need adapters to use them for prototyping.

Also, back in the 1970s and 1980s manufacturers were generally very open with their documentation, so you had the information needed to design fun things. Nowadays you need NDAs and if you're not one of their top 10 customers, fuggeddabouddit.
Wire-wrapping is a thing of the past; we have breadboards nowadays. There is no advantage over any of the two besides practicality.

Many chips are SMD and BGA nowadays, that is true. At the same time, prototyping boards are really cheap nowadays. I remember Intel System Development Kits being sold for upwards of $1200 (1980s dollars!!). A while back I bought an atmega2560 demo board (arduino mega) for like 20 dollars, and an ARM Cortex M3 development board for 15 dollars. If you can prove your background (read: if you work for a tech company), you can get lots of these things as commercial samples for free nowadays. Lots of interesting chips have been put on DIY-friendly adapter boards, which I find more user-friendly than having to solder them myself.

Unfortunately I must agree that the current patent environment has gone very grim, with many companies choosing not to release documentation that was once standard issue to release. I fully understand (for example) Broadcom's decision not to release the full documentation and datasheet of their latest Videocore4 GPU. They're merely protecting their intellectual property, though. All companies are more than allowed to do that. The technology industry is one of the few that has a long-standing tradition of publishing as much technical data as possible, without any law making such publications mandatory!

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redhawk
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Re: Practical uses of diodes

Tue Aug 07, 2012 10:51 pm

Image

This photo isn't fake I really am powering an LED with only 1 wire and 2 germanium diodes. :lol:

Richard S.

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morphy_richards
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Re: Practical uses of diodes

Fri Sep 07, 2012 6:43 pm

Diodes could also be used to replace the ones that ache all down the left hand side of Marvin the Manically Depressed Android. :D

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morphy_richards
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Re: Practical uses of diodes

Fri Sep 07, 2012 6:47 pm

redhawk wrote:
This photo isn't fake I really am powering an LED with only 1 wire and 2 germanium diodes. :lol:

Richard S.
With no other power source? How does that work?

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redhawk
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Re: Practical uses of diodes

Fri Sep 07, 2012 8:07 pm

It's basically a circuit that converts radio wave energy into electrical current which is used to power the LED via my FM transmitter.
This works because a wire subjected to electromagnetic radiation (radio) generates a small electrical current at both ends.
To increase the current flow you make the wire resonate at the same frequency as the transmitter this is the purpose of the coil and capacitor.
Now you have AC flowing in the wire (comparable to a mains transform) you convert this into DC using 2 germanium diodes which feeds the LED.

It's a neat trick but it does have a practical use if you need to optimising your FM transmitter power output. :)

Richard S.

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Re: Practical uses of diodes

Fri Sep 07, 2012 10:45 pm

redhawk wrote:It's a neat trick but it does have a practical use if you need to optimising your FM transmitter power output. :)
Or want to reduce your electrickery bill by stealing electricity from the airwaves to light up LEDs... :mrgreen:

The secret life of Machines shows how the Diode came about, and what it is used for in most situations up to the 1990s, specifically shown in The Radio:

http://www.youtube.com/watch?v=2roG4jIjvEk

One of my favourite shows to watch that... :)

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morphy_richards
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Re: Practical uses of diodes

Fri Sep 07, 2012 11:07 pm

I once made an AM radio that collected enough from the 'air waves' to power a little earpiece thing and pick up Atlantic Radio (or something like that). I never realised you could power an LED.

Have you ever tried powering multiple LEDs in a relatively small area - for example a 3X3 or more grid using not too much more length of ariel? (or not too much more physical space taken up by ariel while increasing its length)

ambi dextrous
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Re: Practical uses of diodes

Fri Sep 07, 2012 11:13 pm

Ah! Practical uses of diodes OK misread was going to report inappropriate subject but it just an imagined L and misplaced O. :oops:

twocvbloke
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Re: Practical uses of diodes

Sat Sep 08, 2012 9:08 am

morphy_richards wrote:I once made an AM radio that collected enough from the 'air waves' to power a little earpiece thing and pick up Atlantic Radio (or something like that).
Like this one in the bottom half of this Tim Hunkin doodling?

Image

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morphy_richards
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Re: Practical uses of diodes

Sat Sep 08, 2012 9:43 am

That's the one! :D

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reiuyi
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Re: Practical uses of diodes

Sat Sep 08, 2012 9:58 am

I'd almost say it's ironic that with modern-day transisors, it's easier and cheaper to build a DIY FM-transmitter than an AM-transmitter, despite the fact that in theory AM should be way easier to construct! Those damn copper prices...!

The device that always comes to mind is Talking Electronics' "FM bug", see here: http://talkingelectronics.com/projects/ ... Intro.html. It's a very fun little project and I bet lots of raspi users who want to get introduced into RF could appreciate such kits

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