Using diodes to reduce voltage by 1-2 V under a very high load (e.g., 20A)

[TheElectricChicken]

Yes, you can fix the problem by adding low value series resistors. But it's more helpful to explain what the problem is and then how to fix it. If a beginner just understands "yes, no problem", he will have no idea about the reason if something fails because of bad wiring. The point of your suggestion is the wire's resistance and the way of wiring. It doesn't tell why that helps. So the beginner doesn't know, that he has to select the wire, i.e. the diameter, very carefully. IMHO, I'd prefer to add dedicated series resistors to make it more fool proof.

There are NO resistors to add. I've never said such a thing. In order to even out the current you simply follow the wiring diagram I drew. There is no need to understand or even know how much resistance the cable is providing, and no need to select. The method of wiring means that all resistances are reasonably equal and all currents are reasonably well distributed. SharpEars builds a powersupply, not an exploding space shuttle. I have built such a power supply, with diodes to drop the voltage.

If SharpEars follow me, then he has power supply same as I have a power supply. If he follow prophets of DOOM, he never start. Maybe you sell power supplies and not like someone build their own ? I dunno.

[TheElectricChicken]

Here is my power supply I built. I used some switches and a packet of 100 diodes which I already had. I also made a panel with square holes for the switches as well. No doubt people will say it's a mess and will cause the end of the world and burn down the city and dim the lights when switched on, and I hope so too, but so far it just works like a regular power supply. It just works like a regular 10-110v DC 2amp power supply. I flip switches on the front to short out the diodes. The diodes are in strings. Each string is half the size of the last string. The largest is I think 48 diodes from memory. It works well for it's purpose. Please feel free to say what YOU want a power supply for and claim that this wouldn't work. I'll be glad because I did not build it for naysayers, I build for myself. I think it is hilarious to use the old Marantz stereo I found, use the parts that are in it, and use the case of it as the case of the power supply and leave all the parts inside. Just add an alien face sucker to the front of the stereo (should show this to an audiophile who likes this one) and take control of the new Zombie stereo which now outputs voltage through the old speaker outputs. I think it's hilarious.

Here are some pictures how I amuse myself.

[TheElectricChicken]

IMHO, I'd prefer to add dedicated series resistors to make it more fool proof.

You appear to speak in contradiction here. resistors make it less foolproof because if the current is variable then using resistors will make the voltage drop variable. Diodes will clamp the voltage drop regardless of load.

[madires]

There are NO resistors to add. I've never said such a thing. In order to even out the current you simply follow the wiring diagram I drew. There is no need to understand or even know how much resistance the cable is providing, and no need to select. The method of wiring means that all resistances are reasonably equal and all currents are reasonably well distributed. SharpEars builds a powersupply, not an exploding space shuttle. I have built such a power supply, with diodes to drop the voltage.

And you wouldn't also add series resistors when paralleling BJTs?

Please see http://toshiba.semicon-storage.com/ap-en/design-support/faq/diode.html for "Is it OK to connect multiple diodes with the same part number in parallel?" for example.

[TheElectricChicken]

Please see http://toshiba.semicon-storage.com/ap-en/design-support/faq/diode.html for "Is it OK to connect multiple diodes with the same part number in parallel?" for example.

So not caring. this is like saying you require matched pairs of transistors or CPU's for a backyard job. You do not need oxygen free cable for this power supply either. Do you need oxygen free cable ? what do you think ? ridiculous. It's for people with lots of dollars and NO SENSE at all. same with quoting toshiba as if to suggest one diode will carry 4 percent more current than the next one. The amount of difference is proportional to the IQ of people who demand such precision for a DIY power supply. Hey, what do you think of my treating the stereo that way ? MwahHAhaHAhaHAHahAHAHhahaha

If you want to make ridiculous claims that  the diodes will have significant differences then quote the specification. It will state the tolerances of the diodes. A bag of diodes does not have enough variation to be measured at home.

Here is the circuit for above power supply.

[madires]

IMHO, I'd prefer to add dedicated series resistors to make it more fool proof.

You appear to speak in contradiction here. resistors make it less foolproof because if the current is variable then using resistors will make the voltage drop variable. Diodes will clamp the voltage drop regardless of load.

Sorry, Vf depends on the load current. Most diode datasheets provide a nice diagram showing the relation between Vf and If.

[TheElectricChicken]

http://rvroadtrip.us/graphics/12v_4batteries.jpg
http://ww3.wholesalesolar.com/battery-folder/parallel-batts.gif

here are two diagrams for wiring batteries in series parallel combination in such a way as the losses in the cables are evenly balanced same way that the diodes should be wired.

Sorry, Vf depends on the load current. Most diode datasheets provide a nice diagram showing the relation between Vf and If.

"Most diode datasheets" ...you can't provide one, or don't have one.

[madires]

Sorry, Vf depends on the load current. Most diode datasheets provide a nice diagram showing the relation between Vf and If.

"Most diode datasheets" ...you can't provide one, or don't have one.

Ok, let's take a classic diode: https://www.fairchildsemi.com/datasheets/1N/1N4001.pdf Figure 2. Based on your lack of basic knowledge I have to assume you're a beginner.

[SharpEars]

Please see http://toshiba.semicon-storage.com/ap-en/design-support/faq/diode.html for "Is it OK to connect multiple diodes with the same part number in parallel?" for example.

So not caring. this is like saying you require matched pairs of transistors or CPU's for a backyard job. You do not need oxygen free cable for this power supply either. Do you need oxygen free cable ? what do you think ? ridiculous. It's for people with lots of dollars and NO SENSE at all. same with quoting toshiba as if to suggest one diode will carry 4 percent more current than the next one. The amount of difference is proportional to the IQ of people who demand such precision for a DIY power supply. Hey, what do you think of my treating the stereo that way ? MwahHAhaHAhaHAHahAHAHhahaha

If you want to make ridiculous claims that  the diodes will have significant differences then quote the specification. It will state the tolerances of the diodes. A bag of diodes does not have enough variation to be measured at home.

Here is the circuit for above power supply.

Wait, how many amps do you expect this thing to put out continuously, before you smell some diode smoke? Those diodes are rated for 1 A of forward current.

I guess if your intent was to build a 1 A power supply, then yes, possibly mission accomplished...

[PSR B1257]

batch has nothing to do with it.

Well, it has. Did you think, semiconductor manufacturer producing A, B, C-grade transistors intentionally?
They don't. They producing just one type, but due to imperfection during manufacturing the acutal parameters of each exemplar spread more or less.
The same is true for diodes of course.

hot leads have higher resistance, in the real world it works fine.

They have. But who wires dozens of diodes in parallel using separate wires? Long and thin wires providing a voltage drop which compensates for the decreasing forward voltage - true.

BETTER is to give newbies good advice they can always use.

25 diodes in parallel rather than one diode bridge capable of carrying the whole current - best advice i've ever read...

It also very popular, to put LEDs in parallel on a circuit board. There you have almost no trace resistance - hence the LEDs die quite often (self-experienced due to lack of knowledge).

[SharpEars]

It also very popular, to put LEDs in parallel on a circuit board. There you have almost no trace resistance - hence the LEDs die quite often (self-experienced due to lack of knowledge).

But wait, if you put them in parallel at the current entry point of their 220-330 ohm current limiting resistor, things will be OK, because the current limiting resistor will also act as a ballast resistor I think.

Maybe I'm wrong.

What I am saying is if two or more LEDs are connected in the following parallel configuration, things should be OK:

       /-\/\/\/--|<--\
___/                       \____
      \                       /
       \-/\/\/\--|<--/

..., things are much worse if instead you use the following configuration with a single current limiting resistor:

                  /--|<--\
__/\/\/\__/            \____
                 \            /
                  \--|<--/

[TheElectricChicken]

Wait, how many amps do you expect this thing to put out continuously, before you smell some diode smoke? Those diodes are rated for 1 A of forward current.

I guess if your intent was to build a 1 A power supply, then yes, possibly mission accomplished...

They don't put out smoke !, they work perfectly because I know what I'm doing. Yes, it's just a few amps because it goes from zero to well over 100Volts DC. If you have two 300 Watt supplies and one is 10 volts and the other is 100 volts then one will have 30 amps and the other just three.

I believe they were 3A diodes but I should have to fine the top of the pack wherever it went.  You just double them up until you have enough to handle the total current. Even if one diode is out by 2 % tolerance it's completely irrelevant considering you will get a few milliamps difference between diodes at 1amp each for example. If it worries you, just add one more diode for paranoia's safety's sake.

somone posed a datasheet which proves my points and that there won't be any smoke.
https://www.fairchildsemi.com/datasheets/1N/1N4001.pdf

For the 4001 you can overload it without the smoke, up to 30a instead of 1amp, for short periods. the smaller the overload, the longer it can take it. It also shows that the current will distribute evenly by itself, there is a graph in there showing that the voltage drop increses with increasing current, comparable to a resistor. So as one gets 'overloaded' it's voltage drop increases and the current shunts to other diodes.

Take into account that if all the elaborate studies and calculations and advice is all wrong, then one of the string of diodes is going to blow up, oh the humanity, and like a fuse, protect the rest, so you lose one 5c diode which you can salvage from any circuit board. Eventually you should say 'this is my fourth year of theorizing what these diodes could do, I've given up family, social life and my health, I know some people say I should just go ahead and risk a diode, but they are all crazy ~!!!!!'

I really think you should think four times before you try any of this insanity because what would a person who has built one know anyhow.

[TheElectricChicken]

25 diodes in parallel rather than one diode bridge capable of carrying the whole current - best advice i've ever read...

I'm sorry you must be looking for the thread titled " Using bridges to reduce voltage by 1-2 V under a very high load (e.g., 20A)

http://imgur.com/gallery/PZRWvjd

[TheElectricChicken]

But wait, if you put them in parallel at the current entry point of their 220-330 ohm current limiting resistor, things will be OK, because the current limiting resistor will also act as a ballast resistor I think.

Maybe I'm wrong.

What I am saying is if two or more LEDs are connected in the following parallel configuration, things should be OK:

       /-\/\/\/--|<--\
___/                       \____
      \                       /
       \-/\/\/\--|<--/

..., things are much worse if instead you use the following configuration with a single current limiting resistor:

                  /--|<--\
__/\/\/\__/            \____
                 \            /
                  \--|<--/

you are correct. The thing to remember with LEDs is, if you want them to last for ages, then run them at less than their rated maximum. If it works well at half or a quarter it's rated current, then give it just that.

Leds have great differences in voltage drop according to colour and type, much much more than say power diodes.

[SharpEars]

25 diodes in parallel rather than one diode bridge capable of carrying the whole current - best advice i've ever read...

I'm sorry you must be looking for the thread titled " Using bridges to reduce voltage by 1-2 V under a very high load (e.g., 20A)

http://imgur.com/gallery/PZRWvjd

Since I created this thread, I will chime in here.

Bridge rectifiers are made from diodes, so bridge rectifiers are an acceptable response as was the suggestion of using power resistors with the caveats given.

I am open to suggestions outside of the strict boundaries implied by the thread title.

[SharpEars]

For the 4001 you can overload it without the smoke, up to 30a instead of 1amp, for short periods. the smaller the overload, the longer it can take it. It also shows that the current will distribute evenly by itself, there is a graph in there showing that the voltage drop increses with increasing current, comparable to a resistor. So as one gets 'overloaded' it's voltage drop increases and the current shunts to other diodes.

So, if the voltage drop increases with the current flowing through the diode, why is it a bad idea to connect them in parallel?

It seems like the rising forward voltage drop (with current) will balance them all out (i.e., no thermal runaway scenario).

[TheElectricChicken]

Bridge rectifiers are made from diodes, so bridge rectifiers are an acceptable response as was the suggestion of using power resistors with the caveats given.

I am open to suggestions outside of the strict boundaries implied by the thread title.

Yes it would work, I am confident that the respondent may have possibly been having a bit of a troll with some of their responses. There are times when you just have to pick up the soldering iron, make it, and then watch the entire world NOT come to an end as the device simply works as advertised.,

[AF6LJ]

There isn't really enough information to make intelligent suggestions...

What is the nature of the load?
Why are you stuck with a 9V supply?
What enviroment is this gear used in?
What is it for that matter?
How many 5V loads are there and why not use distributed regulators?
What about air flow, or is this water cooled?
EMI considerations?

[TheElectricChicken]

So, if the voltage drop increases with the current flowing through the diode, why is it a bad idea to connect them in parallel?

It seems like the rising forward voltage drop (with current) will balance them all out (i.e., no thermal runaway scenario).

Exactly correct. But I liked tricking them in to getting a datasheet which they thought would prove their point but actually proves mine.

For power diodes soldered together in parallel, it would be enormously difficult to measure any discrepancies in the differing currents. Now different colour LEDS, yes, simple and easy and some won't light. But power diodes ? forget it.

[PSR B1257]

It seems like the rising forward voltage drop (with current) will balance them all out (i.e., no thermal runaway scenario).

The thermal runaway is exactly the problem which TheElectroicChicken supposedly simply ignores.

The bridge rectifier gets rid of this issue, since all diodes are on the same substrate and therefore at the same temperature. And they are manufactured literally at the same time, so they is basically to parameter drift.

That's all I say.

[TheElectricChicken]

It seems like the rising forward voltage drop (with current) will balance them all out (i.e., no thermal runaway scenario).

The thermal runaway is exactly the problem which TheElectroicChicken supposedly simply ignores.

The bridge rectifier gets rid of this issue, since all diodes are on the same substrate and therefore at the same temperature. And they are manufactured literally at the same time, so they is basically to parameter drift.

That's all I say.

I'm not ignoring it, I'm making fun of it. I drew a picture didn't you see ?

Locked to the same temperature makes it worse because it doesn't allow the temperature coefficient of resistance even anything out.

[madires]

So, if the voltage drop increases with the current flowing through the diode, why is it a bad idea to connect them in parallel?

It seems like the rising forward voltage drop (with current) will balance them all out (i.e., no thermal runaway scenario).

Exactly correct. But I liked tricking them in to getting a datasheet which they thought would prove their point but actually proves mine.

As I said before, Vf decreases when the junction temperature increases. And of course you're smarter than a company producing semiconductors   

[TheElectricChicken]

As I said before, Vf decreases when the junction temperature increases. And of course you're smarter than a company producing semiconductors   

Oh, I'm smarter than several for sure.

[c4757p]

I see we're having fun with the "screw learning and understanding, let's all just derp our way to a working circuit" brigade.

[TheElectricChicken]

I see we're having fun with the "screw learning and understanding, let's all just derp our way to a working circuit" brigade.

All I'm saying is you won't find videos of diodes having runaway nuclear meltdowns and you won't have to dynamically load share between them with coils because in the real world you are going to have a hard time finding this ever happen. ever. connect a Laboratory power supply to a half dozen identical diodes from a packet, twist and solder the leads together and send the total rated current through. See what (doesn't) happen. You can make caps, leds, batteries, and lots else explode, but diodes running in parallel at or below rating ? Nope. not easy. Just build it. I'd bet all my money on it, so long as you don't go over the total for half a dozen.

Anyhow, it's whether you can use diodes, not diodes in parallel that's the topic, and the answer either way is yes, and I have uploaded the photos to prove diodes are fine to drop voltage.