Hi,
I’m not exactly sure how to phrase this; but I occasionally have need of a source that delivers a specific current and voltage. Usually it’s for determining if a camera or other small dc fed device is/will work. The range would be somewhere between 100ma to a few amps and a couple of volts to 25 volts. Does such a thing exist?
Thanks,
… john
Yep, they definitely exist. I have a couple. Typically they max out at about 5 to 10 amps and around 30 to 50 volts. Anything more than that and they tend to get very expensive.
They are usually a voltage source (voltage regulated), meaning that you adjust the voltage, and the current will be determined by the load that you connect them to. You usually can’t set them to a specific current (voltage and current are related to the impedance of the load, so you either set the voltage or set the current, but not both, and very few power supplies are current sources). From your description, a variable DC voltage source sounds like what you want, so that should be easy enough to find.
You are probably looking at somewhere between $100 and $300 for a decent one. Make sure you get one with short circuit protection since you are testing things that might not work and could potentially short the output.
Some things to google that will help you find one: “variable dc power supply” “bench power supply”
The ones that I have a fairly old so I’m not going to bother giving you the make and model.
You can get a constant current out of those power supplies by setting the current limit to 100 mA (or whatever) and cranking the voltage all the way up.
This may be a nitpick, but a power supply cannot deliver a specific current and specific voltage. The relationship between voltage and current is determined by the load (the thing receiving power).
But most benchtop power supplies have variable output voltage and variable current limit. Which means the power supply will try to deliver a specific voltage, but if the load pulls more current than the current limit, the power supply will go into current-limited mode - i.e. decrease the voltage until the current is just under the current limit.
(Or, another way to think about it is, both the voltage and current settings are maximum limits. The power supply ramps up the output until one of the limits is reached.)
Just search for “adjustable regulated power supply” and you’ll see they range from <$100 to thousands of dollars. I recently bought this $60 supply and it does the job, but the adjustment knobs are rather coarse and difficult to set a very precise value. Also it’s a switching power supply, which has some ripple in its output, while a “linear” power supply would not. More expensive supplies may have digital control of output, multiple output channels, more stable output, computer interface, etc.
Just to add on to engineer_comp_geek’s post: What you are looking for is a DC power supply with an adjustable current limit. Such a supply will provide a specified output voltage until it’s output current reaches a specified limit. Once it hits the current limit it operates in “constant current” mode, regulating the output voltage to a level that keeps the output current at the limit. Mine has a couple of LEDs which indicate if it is in constant voltage (CV) or constant current (CC) mode.
ETA: Not all bench power supplies have programmable current limits. You have to look at the specs.
I have something like this
They are available with different voltage and current ranges (I have the 50 Volt 20 Amp version). They are available from many sellers but I think that particular model is made by one company.
They take a DC input so you have to add a separate supply for that. There are also “lab power supplies” with AC inputs that do the same.
IME they are fantastic. Great controls, great display. Some can be controlled from a PC via Bluetooth and/or USB, with nice graphing, ramp and other elaborate features.
You do set the specific voltage and the specific current.
As long as the load draws less than the set current, they are voltage regulated. Once the current exceeds the set value it seamlessly turns into Constant Current mode, reducing the voltage such that the current is exactly maintained.
Not affiliated, I am just a happy user.
And if you are old like me, you have one like this which I have been using since 1972.
It was made sometime between 1947 and 1957.
That unit still needs a power supply. What are you using for that?
If you don’t have to go more than 30 VDC at 5 amps, you can find perfectly good units on Amazon, eBay, and other sites for $60-$90. I’ve got a Tekpower I’ve used for several years with no problem and it cost me $75. It doesn’t sound to me like you are going to need more than a couple amps at 24 VDC (at most).
It is always a good idea to use leads with in-line fuses…just in case. I have a little bowl of fuses sitting on the bench. Unless I know and trust the equipment I’m powering up, I install an appropriate fuse. It’s suspenders and belt, but I’ve screwed up many times before.
I use this type of power supply. (Amazon.de link, couldn’t find it on Amazon.com, surely something equivalent can be found.)
Very happy with that, too, even though I blew one up running at close to max. (Seemed fine, then blew up later.) Ran others a bit less close to max for extended times with no problem. Very good power factor (95%+). Very loud sound, I modded one with quieter fans.
I also use it with this kind of battery, very convenient and works great too, works as promised.
Again, no affiliation, just happy customer.
Better link, closer to what I bought, can’t tell if it’s the exact same product. (There are multiple sellers selling the same product.)
And this is my battery, I have the 20Ah which gives a full 1kWh of energy.
Thank you all for your help. It appears that there is something out there that will do exactly what I want. I don’t like buying on line, so I will probably go to our local Sayal electronics store. I’m sure they will have it.
Many of the devices I’m trying to fire up will be cameras, laptops, tablets, etc., with low voltage/current demands, that usually have the voltage requirement imprinted on them. My concern is mostly with getting the current right. If I understand many of you, you’re saying that most devices will self regulate the current they receive? Is there any danger of the circuitry being fried by too high a current?
… john
Power supply current figures are a maximum for the specified voltage. The supply current must exceed the current drawn by the device. You won’t fry the electonics by having a supply with higher current capabilities than required. Imagine a battery in a three mode (high, med, low) flashlight. The battery can supply the current needed for high mode but nothing is damaged when using the light in low mode with its lower current draw.
For the small, battery powered devices you mention, a few amps of supply current is probably just fine. Practically, I recommend that you may as well purchase a supply with at least double the rated current capabilities, especially since some supply ratings fall a bit short on the current draw high end where the voltage may dip a bit. The cost difference shouldn’t be too significant.
I’ll put in greatly simplified form:
It takes one volt of electrical potential (“pressure”) to push one amp of electrical current through one ohm of resistance (“load”). This is Ohm’s Law. For a given load, you can connect your power supply and adjust the voltage as needed. You can then calculate the current that is flowing. If you lower the resistance of the load, the current will increase if you keep the supply voltage constant. The end result is that you can either adjust the voltage to the desired level OR you can try to keep the current constant by adjusting the voltage, but you can’t do both (i.e., you can’t arbitrarily choose both the voltage and current for a fixed level of resistance).
Yea, an inline out fuse is a good idea.
Some power supplies have an adjustable current limit, which is a nice feature. If the current exceeds the current limit set-point, the power supply will either crowbar (cheaper units) or go into constant-current mode (more expensive units). The cool thing about the latter is that you’re also able to configure the power supply as a constant-current source, which can come in handy when powering things like LEDs.
Edit: I should have read the above replies more carefully, as others have already addressed the topic of an adjustable current limit.
At a given voltage the current is determined by the device you are powering.
So a power supply that is rated 12v, 10A will only supply 12V, 1A if that’s all the device required. You can’t have too "much current.”
That said, there is one “gotcha” that I learned about the hard way - old-style transformer/rectifier/capacitor “wall-wart” power supples are rated for X volts at Y amps. That means that if you are drawing much less than Y amps, the voltage can rise substantially. I fried a device by using one of these cheap power supples that was rated for 12V @ 2A, when the device only pulled a few mA. The “unloaded” voltage was almost 20V, which killed the device.
Modern power supplies which use “switching” regulators don’t have this issue.
Yes.
Not if the device is in working order and you are supplying the specified voltage.
The “crow-bar” is called foldback. you don’t want foldback if you want a constant-current source.
There is actually a third option. Some current-limited power supplies are actually just very sensitive /unstable around the current limit, and some voltage-limited power supplies are just very sensitive / unstable around the voltage limit. So you can set a voltage or a current, but it won’t stay there unless the load is very stable.
To be honest, that is likely to be more of a problem with cheap high-current units – your 1200 W 40-100 Amp units – but it’s a reminder that you need to be sure you are getting what you want.
Alright. Thanks again
I won’t pretend to understand (most) of what’s being said here;
but you seem to all be singing from the same songbook.
If I’m hearing you correctly, it’s more likely too high a voltage,
than too high a current,
that presents the most likely senario for frying my devices?
… john