Equipment Review: Want to Conquer the World? First You Need A Variable DC Power Supply!

So, Mr. (or Ms.) mad scientist, you’ve finished work on your Nuclear Confabulator Disinto Beam 3000 Mark IX at last! You’re all set to begin your conquest of the world, and you go to power up your NCDB3 to bring the world to its knees and…

Oh, $#%@&&@! The damned thing needs 17.3 V DC to run and you don’t have a 17.3 V power supply. Grrrr… Wait, you think. All is not lost yet! You sort of remember you had an old 17V power supply from an old fax machine that you kept because it might come in handy someday. (Us mad scientists never throw anything away. Never know when it might come in handy.)

I really do have a Giant Box ‘O Cables®. Need an MFM hard drive cable? I got ’em. Need a SCSI cable? Got them too. Old parallel printer cable? Yep. Yes. I am ashamed. Also afraid the family will start an “intervention” one of these days. I think there’s a US Robotics 1200 baud modem in there somewhere, too. I wouldn’t know, though, because I wouldn’t touch that thing with a ten foot pole. Afraid I’d lose my hand if I started digging around in there. Might be things living in there.

Now if you can just find the thing… Oh, wait, it must be in your Giant Box ‘O Cables®! You dig it out of the closet and eye it suspiciously. There’s no way you’re going to dig through that mess. And you can’t have one of your minions do it because your last minion left a week ago because you had to eliminate free coffee in the break room because, well, world domination costs money and you needed to cut back somewhere. Wait, did something just move in there? Eeek! Run and hide! It’s alive!

See? This is why you need a variable DC power supply on your workbench.

Long ago I got tired of fiddling around trying to find wallwarts, building battery packs or messing with transformers and rectifiers and all that nonsense to power some of the junk that comes through the doors here or that I tinker with myself. When you’re messing around building something, especially something that might not actually work anyway (i.e. most of my projects), you don’t want to go through all the trouble and expense of building or buying a power supply just for that one gadget. So I’ve had variable DC power supplies for a long time, and since my last one sort of … Well, it didn’t actually start on fire. I mean there weren’t actually any flames or anything. But it sure did smell funny.

So off I went scrounging around for a new one and eventually I ended up with this.

Should I confess that the first thing I did was immediately lose the damned manual for this thing? This is embarrassing, but yeah, I did. I remember that there was one. I remember taking it out of the box and setting it down somewhere. And when I turned around to pick it up and read it, it was gone. I think my house is haunted.

This puppy set me back about $60. It’s made in China… Well, just about everything is made in China these days, isn’t it? There are cheaper ones on the market, but with a lot of those the reviewers were, well, let’s just say the reviews were less than kind and leave it at that, shall we? The reviews for this one were pretty good, although you can’t believe that any more, either, because there are services out there which will, for a fee (of course) provide you with positive reviews for your products on just about any social media or sales site.

To be honest, I wasn’t sure what to expect from this thing. You can spend, heck, over $1,500 on a variable DC power supply depending on the quality, amperage and features you want or need, or as little as $30 or $40. So for a $60 power supply I would have been satisfied it was at least reasonably close to the voltage it indicated, and didn’t produce so much ripple that it looked like a roller coaster when I put it on the ‘scope. And not start on fire when I plugged it in and turned it on. Important, that. Not starting on fire I mean. One of the main questions you should ask is “How often does it start on fire?”

Let’s talk about power in general for a minute. If you already know all this stuff, just skip over this the following paragraph because you’ll find it boring. But there are people who don’t know, so let me give a very simplistic explanation of what’s going on.

We work with two entirely different forms of power. Both are electricity, yes, but the two systems, DC and AC, have entirely different qualities and uses. AC (alternating current) is fine for sending power long distances over power lines, running motors (some of them) and things like that, but for powering electronics, it sucks. Alternating current, well, alternates. DC doesn’t. And most electronics wants DC, not AC. That means almost all electronic devices that plug into your wall socket will have some kind of power supply which converts that 120V AC into a much lower voltage direct current which is fed to your device. Once upon a time this was done with bulky and heavy transformers, diodes and capacitors which took the AC, stepped it down to a lower voltage, and filtered out the sine waves, flattened out the curves, etc. and produced a constant DC voltage. Nowadays it’s mostly done with semiconductors and other lightweight components, but depending on the application you can still sometimes find equipment that uses the old transformer type systems.

So, back to the review of the Kuman.

As you can see from the photo it’s a nice looking little unit. It’s very light. I don’t think it weighs more than a pound or two. Fit and finish is nice. According to the specs it can put out up to 30V DC and handle amperages up to 5A. I plugged it in and hit the power button and…

Well, nothing happened. Uh ??? Oh, great. Was it DOA? I hit the power button a few more times and the display lighted up at last. I’m not sure what was going on there. That was the only time that happened, so I suspect there was some kind of coating or contamination on the contacts in the switch that wore off after I worked the button a few times. It’s worked every time since then.

It’s Alive!

I suppose the first thing I should have done was hook it to the VOM to make sure it was putting out power reasonable close to what the display claimed it was, but me being me, I just set it for 5V and plugged my clock kit into it and it worked just fine and dandy.

Then I thought oh, wait a minute, maybe I should check this thing out before I actually use it to try powering something expensive, so I got out the meter and hooked that up to it.

Voltages were within 0.01V or better across the entire range. Oh, I should point out that the Current knob doesn’t change the current it puts out, it limits it to a specific value. So you can set it to limit itself to only, oh, 1 Amp, let’s say if you want to do that for some reason. The voltage knob does indeed change the voltage. The knobs are a bit on the stiff side, but not enough to be a real issue.

The voltage tests were very good. According to my Fluke the voltage of the PS was generally within 0.01V or better across the entire range that the PS puts out.

Next it was time to hook the oscilloscope to it and see how bad the ripple was.

Ah, what is ripple, you ask? Well, okay, you didn’t ask but I’m going to tell you anyway. To explain that we have to go back to the difference between AC and DC again. If you want a detailed explanation you can scoot over to Wikipedia and read this, or if you can’t be bothered, well, I don’t blame you because it’s boring unless you’re into electronics.

AC current, the stuff that comes out of the plug in your wall, is a sine wave. Which is shown in this little graphic over there on the right that I just stole borrowed from Wikipedia. The red line shows how the AC current alternates over time, going from positive to negative, and doing so at about 60 cycles per second, or 60 Hertz. But as I said before, most electronics doesn’t like AC current and wants to be fed DC. Your electronics don’t want that current cycling from plus to minus like that, it wants a nice, flat line that doesn’t alternate at all.

Ripple.

So, to see how good a DC power supply is, you need to look at the voltages it is producing and how it changes over a period of time, like plotting it on a graph. And that is exactly what an oscilloscope does, measure voltages over time and plotting it on the display (graph). What you want to see on the scope is a flat line, not something like, well, this over there on the left.

So I hooked it up to my scope and this is what I got

And, well, that’s not bad at all, really. In fact, that’s a hell of a lot better than I expected it would be. The PS was set to 4V. You’ll see in the lower left corner than I have the scope set so each division is 2V, the line is two divisions up from center, so that’s spot on 4V. And the line itself is pretty much smooth.

Well, you see a tiny bit of hash. The line isn’t perfectly smooth. But I zoomed in farther and fiddled with the timing and other technical stuff that would bore you to tears (it did me) and found out that while there is a tiny, tiny bit of ripple, that hash you see on that line works out to a deviation of about 0.0014 volts. And while some purists might be irritated by that, out here in the real world where I live that’s pretty darned good, especially for a sixty buck power supply, and isn’t going to be an issue for anything I’d use this PS with.

So, let me wrap this up because I’ve been babbling along here long enough.

For $66 this is a nice little power supply. Well made, attractive, small, light weight, handles 0 – 30V, and up to a little over 5 Amps. Calibration of the display is spot on according to my meter. And as for ripple, that’s down to around 0.001V which is pretty good for a PS in this price range.

And – you can’t get it any more. At least not off Amazon where I got this one. Sigh… It’s listed as unavailable now. I have seen it available from other vendors, though. And I’ve seen units that look exactly the same except for the brand name on Amazon. But watch out because this same PS, although with a different brand name on it, is going for nearly $100 and you can get it for at least $30 cheaper than that if you shop around.

Author: grouchyfarmer

Yes, I'm a former farmer. Sort of. I'm also an amateur radio operator, amateur astronomer, gardener, maker of furniture, photographer.

2 thoughts on “Equipment Review: Want to Conquer the World? First You Need A Variable DC Power Supply!”

  1. I still have my Micronta (Radio Shack) bench power supply. I think I bought it in the late 1980s or early 1990s when even Radio Shack had excellent build quality. Several years ago I thought it was time to upgrade and found a power supply on Amazon for I think $80 on sale or something like that. It was damaged in shipping so I returned it. By time the return was processed (at that time returns didn’t go through as fast as they do now) the sale was over and they would not send me a replacement at the sale price, so I just dropped the matter and kept my Micronta. It’s been chugging along just fine ever since.

    Like

    1. Radio Shack used to carry some pretty nice equipment. I used RS brand multimeters, logic probes, soldering equipment for years. Think I still have a VOM from the mid 1980s that still works fine. RS also contracted with a lot of name brand manufacturers to source their RS brand equipment. If I remember right a lot of their audio gear back in the day was actually made by RCA. I really miss that place. I still have a local one. Sort of. It still has the Radio Shack sign outside and in the store and carries some RS branded stuff, probably left over in the warehouse, but I don’t think it’s actually a Radio Shack store any more.

      Liked by 1 person

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