# increasing voltage or current

#### kourosh daryaee

May 12, 2016
38
i choose 2N2222 transistor since it can handle the load i need (up to about 500 mA).
but i don't know if its suitable for board or not since my signal voltage is 1.8 and load is 12v 120ma .
so according to equation on the https://www.electronicspoint.com/resources/using-a-bipolar-transistor-to-turn-a-load-on-and-off.30/ article
when
V(signal voltage)=1.8 (red diode voltage on the board)
VBE=0.8
hfe 50 when ic =150
n = 3(i dont know exactlly)
so R will be 111ohm and i choosed 100ohm resistor for it.

if i'm wrong correct me please.

#### (*steve*)

##### ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
Moderator
Jan 21, 2010
25,510
You say you want up to 500mA. Let's say the gain of the transistor is 50 at that current.

Your base resistor is about ((1.8 - 1)*50) / (0.5*3) = 50/1.5 = 33 ohms. Use a 33 ohm resistor

May 12, 2016
38
Thanks

#### kourosh daryaee

May 12, 2016
38
i have some questions if you do not mind please
in this circuit

where should i connect the signal voltage ,power supply, and what is that earth sign there, i mean where should i connect it?
and by the way supply has 2 wire , how can i connect it to 1 like that vcc?
i know this questions is stupid enough to not to be answered but i have more of them

Last edited:

#### davenn

Moderator
Sep 5, 2009
14,297
where should i connect the signal voltage ,power supply, and what is that earth sign there, i mean where should i connect it?
and by the way supply has 2 wire , how can i connect it to 1 like that vcc?

I will give you one answer .....

and what is that earth sign there

it goes to the -V of the battery or other power supply

now that leaves 2 other terminals for your other 2 choices .... what do you think is the most likely solution ?

Dave

#### kourosh daryaee

May 12, 2016
38
well i think you said the hard part.
but this is my guess

but i do not understand if supply going to be connected on VCC how it's going to work with that diode direction?

Last edited:

#### BobK

Jan 5, 2010
7,682
That diode is in this circuit because it is running a motor. You would take out the diode and motor, and replace it with your string of LEDs and resistor, all in series. The LEDs should point in the opposite direction of the diode in your schematic.

FYI the diode in the schematic you posted is called a freewheeling diode. When you stop current to motor it becomes a generator, generating a voltage opposite to the supply voltage that runs the motor. This diode shorts out that opposite voltage, which does 2 things. It keeps the voltage from becoming very high and blowing out the transistor, and it acts like a brake on the motor, using up the excess energy by making it a generator with a short circuit on the output.

#### kourosh daryaee

May 12, 2016
38
Thank you Bob that was very interesting ,what a single diode can do !
so it is my final design i think :

it's still confusing me , specially how the transistor is not between supply and diode?

#### Alec_t

Jul 7, 2015
3,647
Q1 is acting as what is called a 'low-side' switch. If it were between the supply and the diode it would be a 'high-side switch', but would be more dificult to control with the 1.8V signal.

#### Gryd3

Jun 25, 2014
4,098
Thank you Bob that was very interesting ,what a single diode can do !
so it is my final design i think :

it's still confusing me , specially how the transistor is not between supply and diode?
If you are on the highway and an accident occurs... police stop traffic...
Traffic no longer flows before or *after* the police-man
Funny way of looking at it, but it's true.
In electronics, you need a circuit... and you can break that circuit anywhere along the path to stop current flow. Just so happens in this example, you are breaking the circuit between the LEDs and negative supply terminal.

#### kourosh daryaee

May 12, 2016
38
and i hope i be lucky with the circuit , i am going to build it tomorrow .i liked to do it tonight but stores are closed and i came 20 minute ago from work.

#### BobK

Jan 5, 2010
7,682
Getting there, but you still have the LEDs wrong. You cannot parallel them with one resistor. The reason for going to 12V was to allow you to put several LEDs in series. You should be able to two string of 3 LEDs + a resistor in each string. Look at the LED resource again for how this is done.

Bob

#### Gryd3

Jun 25, 2014
4,098
Getting there, but you still have the LEDs wrong. You cannot parallel them with one resistor. The reason for going to 12V was to allow you to put several LEDs in series. You should be able to two string of 3 LEDs + a resistor in each string. Look at the LED resource again for how this is done.

Bob
The op may not be able to do so... It's been discussed in earlier posts, but they have already been connected in parallel and glued in place :s
Hopefully enough is still exposed, or can be exposed to rewire.

#### kourosh daryaee

May 12, 2016
38
The op may not be able to do so... It's been discussed in earlier posts, but they have already been connected in parallel and glued in place :s
Hopefully enough is still exposed, or can be exposed to rewire.

Bob is Right that problem still exist...
and i made circuit it works well but the 33 ohm resistor act as 52 ohm and i do not know why,maybe it be the problem too.

#### kourosh daryaee

May 12, 2016
38
The op may not be able to do so... It's been discussed in earlier posts, but they have already been connected in parallel and glued in place :s
Hopefully enough is still exposed, or can be exposed to rewire.

So i have to rewire them,but the new problem i found is that i cant mix any color with each other like last time because of difference in working Voltage(even for two series of three paralleled LED).
these are colors that i think can be mixed together parallel:

___|___
___|_____|___
(?) (?)
|blue |red |
|white |orange|
|green|yellow |

the two question marks are resistors that i must use to reduce Voltage Gryd3 Told once that they should be between 470 to 510.
if found this calculator that might be useful http://ledcalc.com/. it suggested using 120 ohm for each of strings.

Last edited:

#### BobK

Jan 5, 2010
7,682
You can place LEDs with different forward voltages in series. As long as you set the current correctly, they will split the voltage as each requires. For example;

2 red LEDs Vf 2.5V
1 blue LED Vf 3.3V

Total voltage of LEDs 9.1. So you need a resistor that drops (12 - 9.1) = 2.9V at 20mA, which is 145Ω.

Bob

#### Gryd3

Jun 25, 2014
4,098
So i have to rewire them,but the new problem i found is that i cant mix any color with each other like last time because of difference in working Voltage(even for two series of three paralleled LED).
these are colors that i think can be mixed together parallel:

___|___
___|_____|___
(?) (?)
|blue |red |
|white |orange|
|green|yellow |

the two question marks are resistors that i must use to reduce Voltage Gryd3 Told once that they should be between 470 to 510.
if found this calculator that might be useful http://ledcalc.com/. it suggested using 120 ohm for each of strings.
Well... if you have the ability to re-wire more, it would be in your best interest *not* to put any LEDs directly in parallel with each other.
The resistors I quoted, are based on using one resistor for each LED in case you can't manage to re-wire any of them in series. If you run *any* in series, or *any* directly in parallel you will need different value resistors.
The tricky thing about this, is that LEDs are *current* driven devices, which is why (at a bare minimum) we use a resistor to limit the current flow... now, if you have 2 LEDs in parallel, your resistor must be sized such that it allows enough current for two LEDs to light. If the 2 LEDs in parallel are not *exactly* the same (There is no *close enough* here) then the current through them will not be split evenly. This means that one LED will *always* be fed more current than intended. This will shorted the life of the LED and if the worst case happens (it burns out) then the other LED(s) in parallel will end up being force fed the current that was meant for the now dead LED resulting in a chain reaction of your LEDs in parallel dying.
Don't get me wrong, it's possible to use LEDs directly in parallel without an LED on each one, but this is not one such instance.
If you can't rewire any in series, I strongly urge you to place a single LED in-line with each LED on your device. You will need 60 resistors, and it will draw more current, but it's a better option than direct parallel LEDs. You just need to cut the lead, or wire on either the anode or cathode of each led, then insert a resistor.

#### kourosh daryaee

May 12, 2016
38
i am planning to rewire it,even if i have to rewire the whole thing from scratch i will do.
but i have limit of voltage since we the circuit to work in with 12 voltage dc i do not think we can put more than 4 in a string.
if there's any solution to put 6 string in one string please tell me.

#### Gryd3

Jun 25, 2014
4,098
i am planning to rewire it,even if i have to rewire the whole thing from scratch i will do.
but i have limit of voltage since we the circuit to work in with 12 voltage dc i do not think we can put more than 4 in a string.
if there's any solution to put 6 string in one string please tell me.
Simply add the Vf of the LEDs you want to add in series and make sure it's less than the source voltage you want to use. If the source voltage will vary.. (ie, in a car that often runs between 12, and 14.4V.... use the *higher* voltage for your calculations and omit one LED from the string... ie, if you can *fit* 4, only use 3)
It's important that the LEDs you choose to run in series with each other all have the same mA rating, because the mA through every item in a series string will be the same. A resistor is then chosen to provide the desired current (mA) when you provide the *left-over* voltage from the calculation you have done above. (Source V, - Vf of each LED in series)
You can make as many 'strings' as you want, and you can then connect almost any number of strings together in parallel...
So ... no, you cannot have 6 LEDs in series with a 12V source, but you can easily make two strings or 3 LEDs + 1 resistor each. Then simply attach both strings in parallel.

#### kourosh daryaee

May 12, 2016
38
what happens if the LED's doesn't have same Current consume for example if first is 15 second is 20 and third is 25? will it be like the voltage in parallel i mean they wont work ?
so i have to control the current,did we do that in parallel?why we have to do that now?
and can i give every string 12 v since they are serry?( i mean i need not to care about the voltage anymore?)
and since the 2 strings are going to be parallel does they have to have same Voltage consume?

Last edited:

Replies
4
Views
2K
Replies
30
Views
5K
Replies
9
Views
3K
Replies
4
Views
2K
Replies
22
Views
11K