Unconventional Power Supplies

[cr0sh]

All,

I'm in the middle of a robotics project where I need to, as part of my testing, power a couple of DC motors that I am pretty certain draw more than 10 amps; they power the rear drive-wheels of a Powerwheels H2 ride-on toy:

**broken link removed** **broken link removed**
​

I don't currently own a power supply capable of supplying the current they require; I am wanting to purchase such a supply in the near future.

I have contemplated simply running the motors using the large (12V 12AH) SLA battery that powers the toy, but I don't want to have to constantly monitor it and keep it charged if I can avoid it while testing, so I'd like to obtain a "bench" supply that can handle the current requirements. Furthermore, I don't know what make or model of the motors are (which is something I will attempt to find out, once I get to that portion of the system) in order to look up specs; if I am unable to find the specs, then I will need to have such a supply in order to "pony brake" the motors to determine approximate running and stall current requirements.

My plan of attack on this is to first look at a local electronics salvage yard (Apache Reclamation and Electronics) here in Phoenix, to see if maybe they might have an old switching or linear power supply that can handle the load, before investing money in some other solution.

One possibility I have thought of to use might be a PC power supply, but I wonder if there might not be other solutions - such as automobile battery charger, or maybe a DC welder? Could these be used for high-current supply? Any advice on current limiting as well?

Also - what would be the best way to measure the current? If the current draw is more than 10A, then my (admittedly cheap) multimeter(s) won't handle it. I was contemplating maybe a dash-mount amp-meter (for automobiles); does anyone have any other suggestions? I'm open to just about any possibility here; in the future I will likely wire up a hall-effect current sensor on the motors for monitoring via a microcontroller, but I am not at that point yet, so I am looking for something simpler.

Finally - I am open to any suggestions on how to properly check the current draw of these motors; as I noted above, my plan was to "pony brake" them in some manner, to simulate a loaded condition (but not fully stop them), and maybe add 10 percent or something for an estimated stall current. My goal is to gain the current draw information without damaging the motor and/or gearbox - or burning it out. So any information that could guide me in this process would be helpful.

Thank you.

 

[cr0sh]

I think I have a solution to my current measurement portion - Poor Man's Shunt:

YouTube - #43 Poor Man's DC shunt to measure high current

So I can measure that; but I would still like other ideas, as well as ideas on the power supply, perhaps something like this:

https://www.harborfreight.com/10-2-55-battery-charger-engine-starter-3418.html

Thanks again!

 

[Reloadron]

Hi Cr0sh

Would a basic ATX form factor computer power supply do the trick as to a power supply? They should give you a regulated 12 VDC @ 20 Amps no problem.

As to a shunt for the current if you want one I have a pile of 10 Amp shunts similar to these. I have the 10 Amp both 50 and 100 mV flavors, I suggest 100 mV flavor. If you want one, just ask as I have several and more than I will ever need or use. I may even have some old but like new 200 mV panel meters for use with them. Anyway, yours for the asking as I don't want anything for them. Sending through the post office is on me. Just let me know.

Matter of fact I used then with an old ATX power supply at one time.

Ron

P.S. Looking to get cold out your way!

<EDIT> If they run on a 12V 12AH battery I doubt they exceed 10 amps but I also may have some 25 and 50 amp shunts, no problem. I know I have a bunch of 10 amp versions. They would afford really good accuracy. </EDIT>

 

[cr0sh]

Hi Cr0sh

Would a basic ATX form factor computer power supply do the trick as to a power supply? They should give you a regulated 12 VDC @ 20 Amps no problem.

As I noted, that might be a possibility, provided I can find one cheap enough; maybe I can pull a working one from some case or something. So far, its likely the cheapest/best idea, at least up to a certain amperage level.

As to a shunt for the current if you want one I have a pile of 10 Amp shunts similar to these. I have the 10 Amp both 50 and 100 mV flavors, I suggest 100 mV flavor. If you want one, just ask as I have several and more than I will ever need or use. I may even have some old but like new 200 mV panel meters for use with them. Anyway, yours for the asking as I don't want anything for them. Sending through the post office is on me. Just let me know.

Thank you! I'll take you up on that offer. I'll PM you my mailing address.

P.S. Looking to get cold out your way!

Don't I know it - it already got down to "freezing" (sub-30 degrees) here last night; its supposed to be that way again tomorrow, and I think thru the weekend (not sure). Had I cover the plants last night (and will again tonight, of course). Really "crazy" weather we've been having; yesterday it snowed in the afternoon in the area our house is (I was at work in the middle of Phoenix - our house is on the Phoenix/Glendale border; my wife drove in it). "They" issued a no-burn day for this New Year's Eve, so we can't even have a fire for "ambiance"

 

[Reloadron]

OK, drop me a PM and a shunt will go out likely Monday USPO in a small mailer or whatever the wife finds at work. She sends plenty to Phoenix and plenty from phoenix finds its way here. she has a close cousin out there and they are always sending stuff back and forth. She also likes to get out there a few weeks every February as I freeze in butt deep snow here.

As to loading the motor, dynamic braking comes to mind using either pneumatic or magnetic brake. Trying to think of an inexpensive route. I wish I had an old torque cell lying around I could send you.

Ron

 

[tcmtech]

For a cheap 12 volt power supply with a built in amp meter you cant beat a good used battery charger.

 

[cr0sh]

OK, drop me a PM and a shunt will go out likely Monday USPO in a small mailer or whatever the wife finds at work. She sends plenty to Phoenix and plenty from phoenix finds its way here. she has a close cousin out there and they are always sending stuff back and forth. She also likes to get out there a few weeks every February as I freeze in butt deep snow here.

Alright - I sent it. Thanks again!

As to loading the motor, dynamic braking comes to mind using either pneumatic or magnetic brake. Trying to think of an inexpensive route. I wish I had an old torque cell lying around I could send you.

I was kinda thinking to just grab the spinning wheel with my hands while wearing some leather gloves!

 

[cr0sh]

For a cheap 12 volt power supply with a built in amp meter you cant beat a good used battery charger.

Yeah - I'm likely to go either route; I might play with what I got this weekend (rummage around my junk pile for a PC power supply), but next weekend I'll probably see if there's anything at Apache Reclamation that might be in good enough shape to work - I might be able to find an old battery charger there or at Goodwill that would work (worst case, I'll spend the money at HF - I have a 20 percent off coupon to use anyhow).

 

[Reloadron]

Got the PM and it will leave Monday. Just drop a PM when it gets there.

Yeah, real good leather gloves.

Ron

 

[cr0sh]

Got the PM and it will leave Monday. Just drop a PM when it gets there.

No problem - will do!

Yeah, real good leather gloves.

I have a good pair of welder's gloves, which should do the trick. Another possibility would be to find a scrap of carpet, attach it to the body of the vehicle with some rope or such, and place it under the wheels; that might give enough friction...

 

[kinarfi]

from what I have found out about power wheels, the motor is or very similar to a **broken link removed**. My power wheels had a 27 amp circuit breaker on it. I used one of the motors on a power steering unit I built for my off hiway vehicle for a while some time back. I have since changed over to a **broken link removed**. I may have a few more answers for you if I knew you questions.
Kinarfi

 

[cr0sh]

from what I have found out about power wheels, the motor is or very similar to a **broken link removed**. My power wheels had a 27 amp circuit breaker on it. I used one of the motors on a power steering unit I built for my off hiway vehicle for a while some time back. I have since changed over to a **broken link removed**. I may have a few more answers for you if I knew you questions.
Kinarfi

Well my main questions are "what is the running and stall current of the motors" in my Powerwheels H2; if the motors are anything like that BaneBots motor (and I do know that it is a 550 of some sort, based on what I have read on the Modified Powerwheels forum), then that is a quite high stall current - though I think the gearbox would likely strip long before then (since they are plastic gears). I think I am going to need both a large set of fuses (20-30A) for these motors, plus a power supply of at least 20A to test them with, based on your post. My initial controller will likely be relay based, but I will probably purchase a PWM h-bridge in the future. Thank you for the post of those specs.

 

[kinarfi]

Running current depends on load, If a big kid is going up hill, you may stall it, if going down a steep hill, it will turn into a generator. That's how the powerwheels stop. Dynamic braking, They disconnect the power and turn into a generator into a low resistance load. Do you have the schematic for you powerwheels.
Another method of measuring amperage is to use a **broken link removed** hall effect device,
You should disconnect one motor and stall the other out, the gears can handle it and so can the motor if you only stall it long enough to make the current reading, I have one not in a car and will stall it later today and post for you, you could hook it to you car battery which will have all the power you need with out the internal power supply losses to give you the truest max amperage.

 

[cr0sh]

Another method of measuring amperage is to use a **broken link removed** hall effect device,

Yes - I noted that a hall effect device might be a way to go for on-board measurement and control (if the h-bridge I ultimately purchase doesn't have a current measurement feature); but in the meantime I would want to use some simpler method like a shunt and a standard meter. That hall effect device isn't too bad, and I will keep it in mind for later, though.

You should disconnect one motor and stall the other out, the gears can handle it and so can the motor if you only stall it long enough to make the current reading, I have one not in a car and will stall it later today and post for you, you could hook it to you car battery which will have all the power you need with out the internal power supply losses to give you the truest max amperage.

Alright - I'll keep that in mind, too; now that I think about it, I have a spare car battery I could probably use, once I get it charged up. Thanks for the info...

 

[KMoffett]

I modified one of those pink jeeps for our students' Computer Science robotics lab. I knew the run and stall currents for those motor, but don't recall the numbers right now. I probably have them in my notes somewhere. I chose a Sabertooth 2X25 regenerative dual motor driver to run between the motors and an Arduino microcontroller. So, I think the stall current was between 12A and 25A, because of my choice.

Ken

 

[cr0sh]

I modified one of those pink jeeps for our students' Computer Science robotics lab. I knew the run and stall currents for those motor, but don't recall the numbers right now. I probably have them in my notes somewhere. I chose a Sabertooth 2X25 regenerative dual motor driver to run between the motors and an Arduino microcontroller. So, I think the stall current was between 12A and 25A, because of my choice.

Ken

I've looked at that h-bridge before, and I've kept it in mind; until I know the actual numbers, I won't make a full decision, but thanks for pointing it out...