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I am new to electronics and am still just learning. I saw this project on Kickstarter for a small USB based DC power supply and was wondering what some of the more experienced people thought of it.

I am considering buying two for experimentation/hacking at home. Is this a good idea or should I be considering an alternative product?

Link to the Kickstarter project: http://www.kickstarter.com/projects/210251816/usb-based-variable-power-supply-for-small-projects-0. UPDATE: It is closed and didn't meet funding but was relaunced here http://www.kickstarter.com/projects/210251816/usb-variable-power-supply-for-small-projects-relau.

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Was just reading another question and it said shopping advice was off-topic here. Sorry about that, please feel free to close if this is indeed not a good question to have asked. – spoon16 Jun 9 '12 at 21:46
The "500A" in the project/part number stands for 500 amps, right? – Nick T Jun 9 '12 at 23:57
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@NickT, no it is 500 mA. USB 2.0 is limited to 500 mA. – Brian Carlton Jul 5 '12 at 18:04
@BrianCarlton, ever the stooge to my comedy. – Nick T Jul 5 '12 at 19:08

3 Answers

up vote 11 down vote accepted

Here's my thoughts, and hopefully I'll be able to swing this back around to be design centric rather than shopping centric.

The whole USB thing really isn't interesting. It uses USB for power only, so basically USB is an over-glorified wall-wart. Having the ability to power it off of a computer isn't a huge benefit compared to the voltage and current limitations it gives.

The voltage regulator is a linear regulator with a pot for adjusting the Vout. Any halfway competent hobbyist could hack something together in 30 minutes. 10 minutes for someone that is experienced. They would also likely use a better heat-sink to the max power is higher. And using some leftover wall-warts from "the box of orphaned wall-warts" makes this super cheap. If you hacked something together using spare parts lying around then you might spend US$5 and it would work just as good.

The 7-segment display is a waste. It is useful, sure, but there is so much more that could have been achieved with that micro-controller.

Here's how I would have designed that device to be much more useful:

Use a micro-controller with an actual USB interface. Using a small app on the PC you could control the output voltage and monitor the output voltage and output current. It could implement some sort of programmable overvoltage/current protection as well. Controlling multiple vout's with sequencing or other simple "waveform generation" could be very useful.

The power supply itself would be a switching regulator based on a SEPIC topology. This way the output could be either a higher or lower voltage than the input. The output voltage/current is controlled from the micro-controller. It is actually fairly easy to control the output voltage. Output current is a little more difficult, but not impossible.

If a low-noise output is required then I would follow the SEPIC regulator with a programmable LDO regulator. Again, the output voltage would be controlled by the micro-controller. Usually the LCD output would be about 0.5v lower than the output of the SEPIC. In this way efficiency is still mostly high, but the output noise would be very low.

Then I would design a similar device, but instead of a programmable supply it would be a programmable load. USB controlled.

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This product uses a MP1410 2A buck regulator. However, it has a maximum duty cycle of 90%. Instead of SEPIC, I would use a buck regulator IC which can go up to 100% duty cycle. That helps the output go to the point that is almost equal to the input. They are demanding $14K! I am shocked how they could have pledged over $6K! – abdullah kahraman Jun 10 '12 at 8:01
@abdullahkahraman It looks like the picture on the kickstarter page is an old version since it is obviously using a linear reg and not a switcher. Also the text describes an MP1401, while the schematics have the mp1410 that you mention. I still don't understand why you would use a simple buck regulator instead of a sepic, since the sepic would give a larger Vout range than the sepic. – David Kessner Jun 10 '12 at 14:53
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Make an edit to improve the original post @DavidKessner and then ask the OP if it fits their needs afterwards, I dont want to close a post with 2 nice answers. – Kortuk♦ Jun 10 '12 at 15:07
I don't think they even have thought on the topology. Probably this design is for hobbyist use for microcontrollers, LCDs, LEDs etc. So, one will use mostly 1.8 to 5.5V. However, if, say, 9V is needed, it probably will be for motor driving and such. Well, 250mA may be enough for lots of hobbyist motors such as small DC motors and servos. Now, a SEPIC sounds very logical. – abdullah kahraman Jun 10 '12 at 17:48
Sir, I'd like to buy your product. – Joel B Jul 5 '12 at 18:22
up vote 6 down vote

This is a bad and dangerous design.

Without USB negotiation you can only draw 100mA from Vbus. 500mA is only allowed after the device asks the host if its OK. This design has no USB end point at all and no current limiting (except the regs internal limiter) so it breaks specifications if it tries to draw > 100mA whenever it wants.

It also doesn't have detection for USB Charging Ports so it can draw full power whenever it wants from VBus rather than when it is safe to do so.

Additionally there is no inrush current limiter but a pretty substantial input capacitance. Again the host better have ample protection.

In general this is a pretty dangerous device to use unless your really sure your host port has good solid current limiting. I wouldn't plug this into an USB port I cared about continuing to work afterwards.

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up vote 5 down vote

A good power supply will have its outputs isolated from mains ground. This power supply does not, as far as I can tell.

A good power supply will go to +5V, because that was once the most commonly-used supply for digital circuits and is still popular. This supply does not.

A good power supply will have an adjustable current limit, for powering things like LEDs, and for safely powering up new circuits for the first time. This circuit does not have an adjustable current limit.

Also, $14000 for this design? Any competent hobbyist could do better than this.

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I am guessing that they could have designed an isolated power USB power supply with real USB communication using $14K! Hobbyist market is really big.. – abdullah kahraman Jun 10 '12 at 8:03
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I agree the design sucks but presumably the $14k isn't for the design but to do a largish production run to keep costs low, $14k sounds about right for a 1000 unit run of these with cases/knobs/etc. – Mark Jun 10 '12 at 20:08
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But this is the kind of thing that is easily bootstrapped. There's no need for $14k to make a thousand mediocre power supplies. Hand-assemble fifty and see how that goes. – markrages♦ Jun 10 '12 at 22:29
@Mark Ah, I thought $14k was just for developing and they are going to sell each unit.. I'm very unfamiliar with kickstarter. – abdullah kahraman Jun 11 '12 at 14:36

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