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### Topic: Diode voltage drop (Read 6175 times)previous topic - next topic

#### Domino60

##### Jul 25, 2015, 02:03 pmLast Edit: Jul 25, 2015, 02:04 pm by Domino60
Hi,

I've been looking a while on internet how to drop some voltage from a power supply and I found that
people say that you can drop some voltage using a single or more diodes in series. I have a 20~22V
4.7A power supply do you know what kind of diodes is better to use to drop down to/in between
12~15v ?

I used as a test 1N4007 3x of them in series and the voltage dropped to 19v is there any other kind
of diode that i can drop it more but with a single diode not more, does the diode gonna get hot? and does that gonna affect the Amps?
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#### septillion

#1
##### Jul 25, 2015, 02:28 pm
The voltage drop between different diodes is always around 0,7V-0.9V (depending on the load). For Schottky diodes it's a bit lower. So diodes with a bigger voltage drop are rare (because most of the time you don't want a voltage drop...).

And yes, diodes can get hot. If you'll draw the full 4,7A you dissipate around 0,8V x 4,7A = 3,76W and that can make a diode hot.

Diode as a voltage drop is kind of fine if the drop is small and doesn't need to be accurate. But to go from 22V to 12-15V it's not the way to go. Depending on your load (what do you want to power?) you can use a simple 7812 of buy a cheap step down converter (around 2 dollar in China).
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#### Paul__B

#2
##### Jul 25, 2015, 02:38 pm
Oh dear.

You refer to a 4.7 A power supply but you fail to explain how much current you actually need to draw,

And you fail to explain why you need 12~15 V.

Yet again, the favourite "XY problem".

If you were to explain what you really need to do, we would know what the specifications are and could suggest an optimum solution.  It clearly is not for an Arduino anyway.

#### Domino60

#3
##### Jul 25, 2015, 02:49 pmLast Edit: Jul 25, 2015, 02:49 pm by Domino60
I need to power up a imax B6 80W (11v~18v), I got a 20~22V 4.7A power supply.

Asking something about power supply and drop down the voltage doesn't mean that i "fail" really/necessary need to say what I need to power up but nvm some people are curious.
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#### Wawa

#4
##### Jul 25, 2015, 03:12 pm
How long ago.
How much did it cost.
Do you really think we are interested in that.

If you want to use that supply, the only way is a buck converter. Try ebay.
Anything lineair (diodes, regulators) would get too hot (voltage difference x current = watt).
Leo..

#### Domino60

#5
##### Jul 25, 2015, 03:39 pm
Then i will buy other power supply, can't wait months till the converter comes.

Quote
How long ago.
How much did it cost.
Do you really think we are interested in that.
really?

Internet
20days
19\$
no

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#### septillion

#6
##### Jul 25, 2015, 03:42 pm
really/necessary need to say what I need to power up but nvm some people are curious.
Yes you do. Or at least tell us what it draws.

But in this case I think the easiest way (and good and cheap) is to buy a new (laptop) PSU (eBay.
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#### russellz

#7
##### Jul 25, 2015, 04:32 pm
The voltage drop of any diode is a function of the current, it is not fixed but can be calculated from the equation:

0.7 V is a good rule of thumb for most applications for a silicon junction diode working in it's normal range.

Russell.
Retired after 40 years as a chartered engineer working mostly with RF and analogue electronics.

#### Domino60

#8
##### Jul 25, 2015, 08:51 pm
Quote
Yes you do. Or at least tell us what it draws.
Why people on this forum are so curious? no offence but his topic name is called "Diode voltage drop"
and what i described in the 1st commend is an example how i found people dropping down the voltage
i said ofc the range of the voltage i need to get so people wouldn't ask me after that how far you wanna drop, after that I don't think it's really necessary to tell what i need to power up, I can power up anything i like in the range of 12~15v example the toilet paper auto roll motor so I really don't see a point.

Quote
But in this case I think the easiest way (and good and cheap) is to buy a new (laptop) PSU (eBay.
What you linked it's a really good idea and can be used in any kind of other projects, I already got a laptop power supply so it's 19v 4.7A (90W) but when i plug my voltmeter it says 22v that's why i wanted to drop the voltage.
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#### septillion

#9
##### Jul 25, 2015, 09:33 pm
Why people on this forum are so curious stubborn?
As Wawa pointed out we don't give a rats ass about what you want to do. But you ask us a question and we say it really depends on the type of load / the current it will draw. And, no offense, because you ask the question we assume your electrical knowledge isn't super great (otherwise you're not asking questions about it) so it's wayyyyyyyyyyyyyyy easier to just freaking tell us what you want to power then to rely on your know how about electronics.

It's like only telling us you want to make you're car go slower but only tell us it's red....

The voltage of the PSU you have will probably drop to 19V under load. But yeay, even that's to high if it can only handle up to 18V.
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#### Domino60

#10
##### Jul 25, 2015, 09:45 pm
Quote
The voltage of the PSU you have will probably drop to 19V under load. But yeay, even that's to high if it can only handle up to 18V.
Even if the PSU gonna drop I can't work with the iMax B6 because has a protection, when i plug the input PSU the imax start bipping and write "input voltage error" I can't do anything, pressing the buttons have only few sec effect then again start bipping. I need lower voltage
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#### runaway_pancake

#11
##### Jul 25, 2015, 11:49 pm
10 silicon rectifier diodes in series, each rated at least 5A, will do this. [NB - At Least.]
(If you aren't going to need that "4.8A", diodes rated for less current will suffice.)

Are you going to do this or is this just so much MM?
"Who is like unto the beast? who is able to make war with him?"
When all else fails, check your wiring!

#### MarkT

#12
##### Jul 26, 2015, 12:10 amLast Edit: Jul 26, 2015, 12:10 am by MarkT
There is a standard circuit for dropping some non-trivial number of volts using a zener and a power transistor:

The darlington already drops about 1V, so for a 5V drop choose a 3.9V zener, etc  The 1k
resistor should be increased for light loads (< 10mA).

For high power you bolt the darlington to a suitable heatsink.
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#### Paul__B

#13
##### Jul 26, 2015, 12:31 am
I need to power up a imax B6 80W (11v~18v), I got a 20~22V 4.7A power supply.

#### Domino60

#14
##### Jul 26, 2015, 10:03 am
Quote
10 silicon rectifier diodes in series, each rated at least 5A, will do this. [NB - At Least.]
(If you aren't going to need that "4.8A", diodes rated for less current will suffice.)

Are you going to do this or is this just so much MM?
I think it's kinda long.

Quote
There is a standard circuit for dropping some non-trivial number of volts using a zener and a power transistor:
Are you sure this is working? I can try it.

Paul__B

I know about 12v 5A power supply but i already got a lot of them but no one 12v I got under 12v and above 19v about 8~10 power supplies.
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