# DC Curent and Voltage regulation.

Discussion in 'General Electronics Discussion' started by Pexy, Feb 22, 2016.

1. ### Pexy

142
3
Feb 21, 2016
Hy guys. I am wondering if you could controll the curent from a DC PSU (12/15V 5/3A) just with a potenciometer (regulated resistor) or whould you need something else?
And what could you use to controll the voltage
I would prefere if the solution would not be a pre-build unit, but if it has to be one let mo know what should it be.

2. ### Gryd3

4,098
875
Jun 25, 2014
I'll give you a formula, and let you decide what you can and can't control.

Voltage = Current * Resistance

That DC PSU appears to be a constant voltage power source. No matter what you connect to it, it will always try to put out 12V or 5V from *it's output*.
If you connect a resistor or potentiometer in-line with it, then the resistor will drop some voltage across it based on the current passing through it... So then the voltage 'after' the resistor will be lower.
Special note here though... notice how I said the voltage depends on the current through the resistor? If you connect something that slightly changes how much current it draws, it will cause the resistor to change how much voltage is being dropped across it and your new output voltage will change! (Use the formula above to determine the relationship)
Here's the big kicker though...
If you have a 12V source, and a 50Ω light bulb that runs on 9V ... how do you provide 9V to it? Your method asks to use a resistor/potentiometer and it can be done... but lets look at the example with some math:
A 50Ω light at 9V draws (Current = Voltage / Resistance) 0.180 Amps ...
So we know the current through the circuit, and know that we need to drop 12V-9V = 3V across a resistor right? Resistance = Voltage / Current || 3V / 0.180A = 17Ω
There is just one more problem... this resistor might get hot... Power = Voltage * Current
Power = 3V * 0.180A = 0.54 Amps
It does not take much 'power' to make something as little as a resistor hot, and it takes FAR less to burn out a potentiometer... potentiometers are meant for signals, not for handling any kind of power. So although this is possible, you will find that you will need to buy specific high-power resistors instead of a potentiometer.

If you have a 100Ω Buzzer that runs at 6V, what value resistor do you need on your 12V PSU?

Bonus Question for you:
If you have a 50Ω Light bulb that needs 5V and 250mA, what value resistor do you need to use on your 12V PSU?
*This bonus question should help you understand the ability to control both current and voltage at the same time.
(Please no outside assistance on this one)

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3. ### Pexy

142
3
Feb 21, 2016
Question 1)
Amperage on a Buzzer would be 0.09A
Resistor should be 33.33ohms
And power over that resistor whould be 0.27w
Bonus question
Lightbulb whuld draw 0.12A
Resistor shuld be 50ohms
And a power across it would be 0.72w
Ohms law FTW!
Are my calculations correct?

Last edited: Feb 22, 2016
4. ### Gryd3

4,098
875
Jun 25, 2014
Buzzer Amperage of 6V / 100Ω = 0.06A
Resistor should be 6V / 0.06A = 100Ω
Power over the resistor would be 6V * 0.06A = 36W

Bonus Question required the 50Ω Lightbulb to draw 250mA at 5V.
Answers based on Lightbulb drawing only 120mA

5. ### Pexy

142
3
Feb 21, 2016
Ops the first time i didn't wrote it down so that is probably why I got that incorect.
P.S do you own a bench top PSU or some lab equipment like that?

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6. ### Gryd3

4,098
875
Jun 25, 2014
Unfortunately, I no longer own a benchtop PSU so I have been using an old computer desktop PSU.
It's not perfect, but is more than good enough for the applications I use it for.

If you are asking to inquire about their Current & Voltage settings, I can sum it up quite easily:
- When using a PSU, there are two modes you can operate in, Constant Voltage, and Constant Current.
- When in 'Constant Voltage' mode which is how the majority of PSUs you will encounter operate, they will work their hardest to always put out the set voltage. Amperage will vary, but once it hits the upper limit or setting, the voltage will usually dip, or the PSU may literally shut-down. The Amperage of course it dependant on the voltage setting and the load you connect to it.
- When in 'Constant Current' mode, the PSU will actually vary it's output voltage to make sure the current stays the same... again, if you try to draw too much from the PSU, the voltage will stop increasing when it hits it's limit causing the current to dip or PSU to shut-off.

It's not possible to control both the voltage AND current into a device unless you can also control the resistance of the device. So devices that offer control for both simply offer an adjustable output for one, and an adjustable 'limit' for the other.

142
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Feb 21, 2016
8. ### Gryd3

4,098
875
Jun 25, 2014
What do you plan to work on?
Like I said, I use an old computer PSU... so my needs and experience allow me to work with inexact voltages.
All 3 looks fine to me to be honest, but others on here may have a preference based on the name-brand.

9. ### Pexy

142
3
Feb 21, 2016
I plan on doing little hobbies and projects. For now as a power supply i have a car battery charger
(http://products.einhell.com/com_en/cleaning-car-accessories/battery-chargers/bt-bc-4-1-p.html) like this one, but the problem is controlling the output values. So i planed on buying one of these and be set for a long time on that regard.
The only thing that draws me to the UNI-T one is that there are a lot of copies of the same supplies like the other two and i haven't seen any copy of UNI-T one. ( not the brand)

10. ### Gryd3

4,098
875
Jun 25, 2014
Well... you would be far better off using an old computer PSU than a car battery charger.. they put out roughly 12V and are capable of many amps.
I can't really provide much feedback on the PSUs you linked though..

11. ### Pexy

142
3
Feb 21, 2016
That is why I am planing on buying one of the linked ones.
How do you controll the output current and voltage on your PSU.
Do you have for the voltage part something like 3v 5v and 12v output holes that you just plug your conections to or did you make a more "sofisticated" way of doing that?
Could you also tell me more about your PSU.
how you built it and did you built it by some video and stuff like that?

12. ### Gryd3

4,098
875
Jun 25, 2014
It's not as fancy... the PSU has +12V, +5V, +3.3V Ground -5V and -12V.
My work is almost always with 12V or 5V, so I have modified a 4-pin molex connector to plug into my board to provide 12V and 5V.. I simply use a jumper on my breadboard to get to either one...
It's cheap... and I cannot adjust the current or voltage with this model...
It can put out over 10A, but when shorted, the PSU shuts down. ( I simply reset it when this happens )
This let me work on automotive accessories like instrument clusters and audio amplifiers.

13. ### Pexy

142
3
Feb 21, 2016
Cool sounds very simple and i have seen a few examples of PC cpu so i might build one just for fun

14. ### Gryd3

4,098
875
Jun 25, 2014
There are some extended build modifications that actually alter part of the circuit inside allowing you to slightly adjust the output voltage of each rail... great care should be taken as the capacitors inside can pack a punch!
Otherwise, if you have 11.8V , your 12V circuit should still operate fine.

15. ### Pexy

142
3
Feb 21, 2016
Y
Yes some caoacitors can zap you like hell.
What do you think about using a AC motor speed controller as a transformer?
And how many wats does your PSU have and what kind of transformers can be found in them?

16. ### Gryd3

4,098
875
Jun 25, 2014
I wouldn't use an AC speed controller as a transformer.. too much going on. Is it a variable frequency drive?

The PSU I'm using puts out 16Amps on the +12V rail... So, just shy of 200W, but you can also use other rails at the same time.
*note that some PSUs require you to put a load on the 5V rail to trick them into thinking they are being used properly A high power resistor usually does this well.

17. ### sureshot

234
13
Jul 7, 2012
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18. ### Pexy

142
3
Feb 21, 2016
Not sure if variable frequency drive.
It's this one
http://m.banggood.com/4000W-220V-AC...lectric-Motor-Speed-Controller-p-1004793.html

142
3
Feb 21, 2016
Ok thank you

20. ### Gryd3

4,098
875
Jun 25, 2014
Please don't use this as a power supply... it looks like it's simply a beefier version of a light dimmer... which means it operates on a sort of 'PWM'.
It chops up the AC waveform as it goes out... the more that is chopped off, the dimmer/slower the light/motor goes. It does not actually regulate voltage in any way.

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