# How do I effectively use a DC power supply?

1. Dec 30, 2015

### gabede

I recently bought a DC power supply online to perform experiments that require electricity. I am not an expert with electricity so I have a few questions about it. It has a maximum of 10 amps and 30 volts, and both values are variable, however it does not show me the value of the current unless the electricity is flowing. It also won't allow any voltage to pass through unless the current passes a certain value. Can somebody please explain this to me and also give some tips of how to safely and efficiently use it?

I am currently trying to use it to build an electrolysis generator to use the gas for experiments including heavy water purification and another project in which I hope to build a hydrogen torch. I included this information to help you make better tips if you need it. Thanks!

2. Dec 30, 2015

### Staff: Mentor

Those statements make no sense. Please try again to explain better what you see and what you expect.

3. Dec 30, 2015

### phyzguy

Current is the flow of electricity. The current meter is reading how much electricity is flowing, so of course it will read zero unless electricity is flowing.

4. Dec 30, 2015

### gabede

Allow me to explain. When I turn on the power supply (plugged into a wall socket) it has two display screens. One is for current (measured in amps), and one is for voltage (measured in volts, obviously). There are two knobs under each display screen labeled coarse and fine for changing the values of each. When I turn the coarse current value up to about the middle, nothing displays unless the positive and negative terminals are applied to a conductive material (I used a sodium chloride solution). When the current knobs are turned to the left all the way (0 amps?), when I change the values on the voltage by turning the coarse knob under the voltage display, the voltage number remains at zero. I expect it to change to a higher number but it does not do this unless I turn up the current knob past a tipping point and then it shows me the value of the voltage. This makes no sense to me either and that is why I hope you can explain this to me. Thanks!

I also have a picture of the power supply and all the cords that came with it:

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5. Dec 30, 2015

### gabede

Thank you phyzguy, that is the explanation I expected, but I needed to be sure. Also, is there a way that I can measure the current without making it flow through a conductor? If not then that's fine, but I would like to know.

6. Dec 30, 2015

### phyzguy

Imagine current as water flowing through a pipe. Your question is like asking, "Is there a way to measure the amount of water flowing through the pipe without making the water flow through the pipe?" The current meter is measuring the amount of current flowing. If it is not connected to a conductive path, then there is no current flowing and the current meter will read zero.

7. Dec 30, 2015

### gabede

Thanks for the analogy, but I understood your explanation before it, and I realized how silly my question was right after that post. Thank you for taking your time to explain it though.

8. Dec 30, 2015

### phyzguy

What your power supply does is that it has a voltage limit, which you set with the voltage knob, and it has a current limit, which you set with the current knob. It increases the voltage until the first one of those limits is reached. So when you have the current knob set to zero, it will not increase the voltage because doing so would cause the current to increase above the limit which you have set (which is zero). As you increase the current limit knob, it is probably increasing the output voltage slightly, but it quickly reaches the current limit you have set. As you increase the current limit further, it reaches the point where the voltage limit is hit before it reaches the current limit, so you then see the output voltage increase. Does this make sense?

9. Dec 30, 2015

### gabede

Yes, I understand now what the current value means. What you are saying is that there can be no voltage if there is now current to carry that voltage correct? I did notice that if the voltage knob was turned up to it's maximum, that it would slowly increase as I turned up the current until it crossed a point where the the current was high enough to pop the voltage up to what I set the voltage knob to. This is certainly interesting and I have learned something from you so I thank you. It's good to know that I can get intelligent answers from people here.

10. Dec 30, 2015

### phyzguy

No, this is not correct. There can be a voltage even when there is no current flowing. It's just that the output will limit if it hits either the voltage or current limit. So if it hits the current limit, the voltage will be less than what the voltage limit is set for. As you increase the current limit above some point, then it hits the voltage limit first and the voltage limit then controls the output voltage.

11. Dec 30, 2015

### Staff: Mentor

If there is nothing connected to it (open circuit), you should be able to set the voltage regardless of the position of the current knob. But once you plug something into it, the current limit becomes active (if it is met).

Essentially, you need to decide, when you use it, what you want the limits to be. Depending on the experiments, you may not care about the current limit, so you can turn it all the way up and just let the voltage be set at what you want.

FYI, I bought a very similar PSU last year -- it may be the same one. I found (after much head scratching) that the provided leads had a high resistance and were messing with my experiments. You should look into that and consider making yourself a new set of leads.

I used this PSU for a couple of peltier cooling experiments - devices I was making/modifying. I was trying to find the optimal voltage and determine the current requirement for these devices. I'd start low with the voltage and gradually ramp-up the voltage, while keeping the current limiter out of the picture.

12. Dec 30, 2015

### gabede

Ok, so there can be voltage when there is no current. Why does my power supply say there is no voltage when the current is 0 even though the voltage limit is set to maximum (while the terminals are connected through a piece of metal). Also why does it say there is no current when I set the current limit to maximum and the voltage to 0? If there is a video explaining this in detail I would greatly appreciate knowing about it, because I cannot find anything about what I want to know on the internet video wise, and I probably shouldn't be asking for an entire electricity lecture on a forum. heh.

13. Dec 30, 2015

### Staff: Mentor

You didn't read what I wrote: I said "If there is nothing connected to it (open circuit)". If you have a piece of metal connected to the terminals, then you have a short circuit, not an open circuit.
Because there is no current. Voltage is the force that pushes current. The voltage isn't a "limit", it is just the voltage. In a normal situation, the voltage just is what it is - whatever you set it to - and the current will be whatever the load demands. This power supply (pretty much all benchtop type power supplies) just has an additional feature that enables it to avoid the problem of too much current by providing a current limit, which pulls down the voltage if the current limit is reached.

I'm not sure it is safe for you to be playing with this power supply given your level of knowledge of electricity.

14. Dec 30, 2015

### phyzguy

I agree with russ_waters. You should study about electricity and play with something safer, like a AA battery, some wires and some resistors, until you understand what you are doing.

15. Dec 30, 2015

### gabede

You're right, but I am really curious to learn about how it works and I wan't to use it effectively and safely. I don't want to waste your time and ask you to explain everything to me, however I would like to be pointed towards some sort of resource, because I am having trouble finding one.

16. Dec 30, 2015

### gabede

This is my extent of knowledge now: voltage is the measure of potential energy to move electrons from one point to another and it determines the kinetic energy that the current will have. This explains why my power supply can have a voltage without being a closed circuit, because it is showing me the potential energy of the current before I close the circuit with something like metal. The current is the actual flow of the electrons through materials so it can only be measured when there is a closed circuit, which is why my power supply says 0 amps when the circuit is open. When I set the value of the amps on my power supply, I am limiting the amount of current that the voltage can push through a conductor. If I set the limit to zero, the voltage won't push any current through and it will tell me that there is not voltage even though I set it to a high number, because if there was potential energy that would mean that when I closed the circuit it would allow current to flow through, but the machine won't allow that because I set the current limit to 0. Resistance is the friction or opposition to current, so if I have a resistor, it will lower the current because it is resisting their flow. If this is all correct I know everything I need to know to complete my projects and that should conclude this post. If it is not correct I will continue to research and find as much information as I can. Thank you for your help!

17. Dec 30, 2015

### Staff: Mentor

Not to be rude, but have you read the manual? And what kind of experiments are you hoping to run? The one you're on right now - playing with a short circuit - is a really, really bad idea.
Basically correct.

18. Dec 30, 2015

### gabede

19. Dec 30, 2015

### CWatters

The knobs under the current Meyer set the current limit. This is a safety feature to protect your circuit and the power supply in the event of a fault like a short circuit. When the set current is exceeded the power supply limits the current by turning down the voltage. So if your circuit is designed to draw say 1A set the current limit a bit higher than that, say 2A. If you set the current limit too low eg zero then it will immediately turn the voltage down or off. This is what you are seeing.

20. Dec 30, 2015

### CWatters

Messing around with hydrogen and electricity is dangerous. More than a few people have got hurt. Don't do it.