# Basic Electrical Question

Discussion in 'General Electronics Chat' started by Pika, Aug 18, 2013.

1. ### Pika Thread Starter New Member

Jul 27, 2010
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I'm a newbie in electronics. Suppose I have a circuit board that requires 28V, 3A. Apparently if I set the power supply to 28V and 4A, the circuit board will only draw the required amps it needs (3A). Will the same apply if I set the voltage to say 30V, 4A, would the circuit board only draw 28V and 3A or would that cause an over voltage to the board? Additionally, sometimes I get an OVP on the power supply. What causes it?

Jul 18, 2013
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It all depends alot on the circuit, but in the simplest sense, you increase the voltage, the current will increase accordingly.
Also you may start to exceed the voltage rating of certain components.
Max.

3. ### crutschow Expert

Mar 14, 2008
13,511
3,385
Typically circuits are voltage operated. They operate at their rated voltage and take whatever current they need. Thus the power supply needs to be set at the rated voltage and able to deliver as much current as the circuit requires. If the supply can deliver more current that's ok since the circuit only takes what it needs. Circuits do have some tolerance on their operating voltage. ±5% is a common tolerance.

4. ### Metalmann Active Member

Dec 8, 2012
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"If the supply can deliver more current that's ok since the circuit only takes what it needs."

I'm still trying to wrap this brain around that fact, concept.

5. ### WBahn Moderator

Mar 31, 2012
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4,918
Take a 12V car battery and connect it to the lightbulb used in the dome light. It draws a certain, relatively small, amount of current; probably well under 1A. Now connect that same dome light to two 6V lantern batteries wired in series and it will probably light about as well drawing about that same current of something under 1A.

Now connect that 12V car battery to the starter (by turning the ignition switch) and you may well be drawing 300A to 1000A or more. Try doing that with your two 6V lantern batteries and it will probably seem as though nothing is happening. In reality, which is happening is that the lantern batteries are putting out as much current as they can, probably a few amps (I don't have a feel for what lantern batteries are capable of) and the voltage is dropping to near zero in the process.

You shouldn't find any of this unexpected. You wouldn't expect a car battery to be supplying 1000A to a dome light just because that same battery is capable of supplying 1000A to a starter. You wouldn't expect two 6V lantern batteries to be able to start a car just because they can power a dome light from the car.

Metalmann likes this.
6. ### Metalmann Active Member

Dec 8, 2012
700
223
Yeah, I learned all that about vehicles at an early age; it's just a problem for me when it comes down to those pesky, tiny, voltages/currents. It's going to be a long haul for me to figure all this tiny, digital stuff out.

Must be a mental block, but thanks again to you all, for giving your info to help everybody out.
I'm reading like a crazy old goat!

7. ### WBahn Moderator

Mar 31, 2012
18,093
4,918
We all have those blocks about this or that. What I find helps is to try to think of something that I'm struggling with in terms of an analogy, even if it's a pretty bad one, that I am comfortable with. I usually think of most voltage sources in circuits as just a battery. It works just fine for the overwhelming majority of cases.

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8. ### crutschow Expert

Mar 14, 2008
13,511
3,385
How about a water analogy? (I know, that's frowned on in some circles.) If you have a water pump putting out a fixed pressure (voltage) and it's going through a small orifice (resistance) at 1 gal/min for that pressure, then it makes no difference whether the pump can pump 1 gal/min or 1000 gal/min. As long as the pressure is the same only 1 gal/min will flow through the orifice.

9. ### studiot AAC Fanatic!

Nov 9, 2007
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Don't.

What do you think would happen if you took some american 120 volt equipment and plugged it into the european 230 volt mains?

10. ### Rbeckett Member

Sep 3, 2010
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Just take your time and re-read everything at least once and keep working on it and it will all click one day and start making sense. When I first started studying the Ebook it went completely over my head till I reread it about the third or fouth time and kept doing the experiments. I still get hung up calculating resistor values, but it is just one of those mental blocks we all get from time to time. Just don't get discouraged and quit trying before your miracle happens.

Wheelchair Bob

11. ### wayneh Expert

Sep 9, 2010
12,403
3,249
Another analogy is the wall outlet. Just because it can supply an "infinite" amount of power (until the breaker blows) doesn't mean everything you plug into it will draw the full amount. It's about the load, not the supply.

If your supply is the rate limiting device, that's either a bad design or a very clever one, where the load and supply were carefully matched.

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12. ### BobTPH Active Member

Jun 5, 2013
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Yes, many people seem to have a problem with that. But no one ever thinks twice about plugging a 60W lamp into the same socket that can run a 1500W heater.

Bob

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13. ### Veracohr Well-Known Member

Jan 3, 2011
559
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I like to explain it like this:

Voltage is pushed.
Current is pulled.

Therefore, connecting a supply with excess voltage is pushing the circuit beyond what it was designed for. Connecting a supply with excess current rating doesn't matter because the current isn't being pushed by the supply, it's being pulled by the circuit.

This assumes, as mentioned, that the circuit is voltage operated, and thus uses a voltage source. Not a current source.

14. ### studiot AAC Fanatic!

Nov 9, 2007
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515
This view has some attraction but a word of caution.

The damage cause by overvoltage may be due to excess current flowing through the circuit or it may be due to some threshold (such as a breakdown voltage) being exceeded without excess current being drawn.