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Asked — Edited

Usb Or Ac-Dc Conversion As Lipo Battery Replacement?

So, here’s the deal. My project will place, say, 25 robo heads in a (college, university, or high school) computer lab with 25 PCs, one head at each PC work station. In that environment, it will not be feasible to schedule battery recharge on a daily basis. So I am looking for a direct connect scheme from the EZ-B to the PC or to a 110 AC outlet, that will replace the LiPo battery pack.

The first idea would be to consider a USB connection between the PC and the EZ-B, with a DC-DC boost converter, boosting the USB supplied 5V to 7.5V. However, while the EZ-B required voltage of 7.5 volts can be met, the real question is whether the USB/converter combination can supply required amperage. In another thread, Steve G described the variation in USB ports available on typical PCs, USB 1.0, 2.0 and 3.0, and how each supplies different current ranging from 5V at 500mah to 5V at 900mah. In another thread (now I can’t find it) one of our community members reminds us that we are working with a moving target: as we design products based on currently available technology, we need to understand that such technology will likely change between now and the completion of the development project. So it looks like I would need to investigate the amperage generated by typical DC-DC boost converters, as a function of supplied amperage by the USB port, to see if required amperage (500mA?) is available. Do I have this right?

The second idea is to require an additional 110 AC outlet at each workstation for the robo head and its EZ-B. Then a straightforward AC/DC conversion (with the required amperage) would solve the problem. However, it’s not clear that, with all that’s going on at a typical PC workstation in a lab, that another AC outlet would be available there. But if such an outlet is available, isn’t this a straightforward solution?

Thanks for any light you can shed on this design issue.

United Kingdom
#1  
Hey Ron.

So what sort of gizmos will a typical "Head" you would be using have (servos, sensors, camera ect), just so we can get an idea of what power requirements they will need?
#2  
It always comes down to the amps, not the volts. The EZ-B will actually run on 5v, you just need to disable the low battery warning. But 1/2 amp is barely enough to keep the EZ-B itself running. Certainly won't be able to use the camera or LED array. Can probably use the speaker.

Alan
#3  
Good questions Steve and Alan,

For prototype head, I see just the gizmos that come with the JD head. At this time, I don't have an inkling what head or heads will be used in production, and what amp needs they will have. So for now, I guess I should be able to answer the question for the vanilla JD.

Ron
#4  
So does the 2 LiPo battery pack provide 7.5 V at 500mA? Should that be the target of a direct connect?

Ron
United Kingdom
#5  
Actually the 7.4v LiPo battery pack sold in the store could potentially provide about 30 amps, but that would only be for a few minutes. As Alan has mentioned, is really not a lot of power at all. Most sensors need very little amperage, a camera will need a little more as with LED arrays, and sevos will need the most amperage to deal with the initial in-rush current to make them move.

To run an EZ-B with a JD head with the included LED array and camera (not forgetting the head comes with two micro servos), you should get a good couple of hous run time with a 7.4v 1300mah battery. Or you could use a power adapter which can knock out a few amps, and should run the v4, camera and LED array okay (if you intend to use servos as well, you will need to look at double figures for amps, around 15 to 20 depending on how many servos you would use).
#6  
Yeah, Steve, the neck servos will be important.

Wouldn't the best bet be to try to reproduce the "power" provided by the typical LiPo battery pack? That profile would be able to handle most anything a prototype would need.

Ron
United Kingdom
#7  
Yeah, if you can reproduce the power (voltage and amp output) that a LiPo or Nimh battery can offer, than that will be fine.
#8  
To handle the servo inrush current you'll need at leasr a 10 amp supply at 7.5 volts. 20 would be better but with just the 2 head servos 10 should do. Most of the time the draw will be much less, but whenever the servos start to move they pull a lot of amps for a very short time (inrush). Search the forum for inrush current or in-rush current and there are dozens of posts about EZ-B 's resetting and every one of them was using an insufficient plug in power supply or alkaline batteries.

Alan
#9  
@Alan

Ok, so a 20 amp supply is a good target. Just looking around a little, I found this:

P/N: RSP-150-7.5
7.5vdc Power Supply
Approx Power: 150 Watts
Max Current: 20 Amps
Enclosed 1U low profile 30mm, built-in active PFC function, built in constant current limiting, LED indicator for power on, adjustable output voltage, and universal AC input range: 85~264vac.
Dimensions: 199x99x30mm
More Info, Quotes, Purchase $36.90 ea.

This quick result gives me a pricing basis for my business plan.

Thanks to both Steve and Alan.
PRO
Canada
#11  
A 5VDC, 20Amp supply can be used. The camera and LED eyes will still operate just fine as they are powered by the regulated to 3.3V. But as mentioned the low battery warning setting will need to be adjusted to use 5V.

4.5V is the minimum voltage input before the regulated 3.3V is affected.

Edit: The ez-b plus camera only draws ~180mA peak but servos can draw crazy amounts of peak current.
#12  
Just remember. Any voltage can be stepped down, so a power supply with higher voltage than needed can be used. Just get converters that handle high amperage.

I have had a hard time trying to find wall adapters that supply 2amps! So your quest might lead you to a bench test power supply, or a PC power supply. With a PC power supply, check what each voltage line can output. The 5v line could do one amperage while the 12v does another.
#13  
OldBotBuilder, Jeremie, and Technopro,

Thanks for the info and suggestions. So, Jeremie, looks like 5V/20A might be the best route for my experimenting. However, if as Technopro says it's not easy to find such a wall adapter, maybe the most straightforward approach is to look at a PC power supply - I can see how the two supply voltages could handle multiple amp requirements. I'd need to compare cost of a PC power supply with the approximate $37 cost of the converter I found and posted in #10 above.

Thanks again for your help!

Ron
#14  
I think the $37 one you found is probably a good bet. If you were to buy a battery and charger for each location, you would be talking about $45 per location, with less convenience. 20amp at 7.5 volt is sufficient to drive a whole JD, not just the head (and probably enough for something like an Alan or InMoov head too).

Alan
#15  
Cool, Alan. Saving $8 per unit is good news.

Ron
#16  
Do you have a link for that power supply? I might want to test one.

Alan
#19  
Please specify the power consumed by your one head means the total power consumed by the all components consisted in one head. According to that calculation you can select that which power supply will be suitable for you. But it is looking that the power through USB will not be enough you must think about the direct power from the wall outlet.