Asked — Edited
Resolved Resolved by Dave Schulpius!

Robot Ower Supply Help Please?

Not sure if this is outside the scope of this forum or not. If so, my apologies in advance.

I'm modifying my life size Lost in Space B9 robot from static display to fully mobile and (apparently) autonomous. In doing so, I need lots of power. I'm far from an EE but had some electronics back in the dark ages in the USAF.

What I'm trying to do is power the tread section motors and controller by battery only (using two 24v wheelchair motors with a Sabertooth 2x25 controller with two 12v 35ah AGM batteries in series) and the rest of the robot by battery OR a plugged in power supply when he is not mobile, recharging and on display. I have the batteries connected directly to the sabertooth and motors and also feeding two, 200 watt 24 to 12 volt dc to dc converters to power the rest of the robot when on battery and a 400 watt power supply (120vac in, 12 vdc out) that powers everything except the motors and sabertooth when he's not mobile , charging and on display.

My question is: Where the output of the 400 watt ac to dc power supply connects to 12 volt system (which is also fed by the two 24 to 12 volt converters when on battery) do I need a diode there to prevent voltage bleed back into the converters? If so, any advise on type and/or size/rating or anything? I've really forgotten most of technical stuff I learned decades ago...

I don't want to smoke any of the components if I can help it... I have a fog machine for that come Halloween... :)

Any suggestions or comments appreciated as always :)


Upgrade to ARC Pro

Unlock the true power of automation and robotics by becoming a proud subscriber of Synthiam ARC Pro.


search the forums for b9


I would contact Dave Schulpius here on forum



@Gwen , Im happy to help with your power organization. I may not have had my coffee yet but I was confused about your actual needs for both current and voltage. I have noted you will use a sabertooth 2 x 25 which is more than enough.

You mentioned an inverter or power supply. i can repeat back to you the way I understand your desired configuration.

You want the robot to have batteries that power both the sabertooth and onboard systems and a inverter to power things made for 110v in the robot.

Then for stationary use you want the robot to run off a power supply that charges your batteries and powers your 110v items while plugged up to the wall... am i correct?

Assuming I have it right you may want to consider switching to DC power for everything for simplification , you could use step down converters for individual items that otherwise would convert 110v to dc for use anyways. A couple 20 amp hour 12 volt batteries should cover your needs with a descend time between recharch for demonstrations. Lead Acid batteries such as Power Sonic are very forgiving to start with. I recommend them first for any beginner as they will simplify your charging concerns. When charging you can have a switch that disconnects the battery from the robot only while charging. At this point you can plug up an external DC power supply for the robot to be powered on (but not moving) while batteries charge without the concerns of a short or ground feedback.


Hi @Gwen,

I noticed your arrival here on the EZ Forum last week. Welcome! Glad you made it over here. I've been following your B9 build over on the B9 Builders Club forum for the past few years and have been very impressed. You have great talent and a fine mechanical mind (wait, that didn't sound right but I hope you know what I meant).

I fully agree with the last post by He gives great advice and is spot on as usual. If I recall correctly you already have the tread section built and powered as you describe above. Do you already have the rest of the robot also wired as you mention above? If so and if all your asking is if you need a diode between your two systems I would have to say Yes. Adding a blocking diode is a good idea if placed in the proper spot of the circuit. It's really hard to advise without seeing your wiring schematics and the actual setup. So as a generic suggestion I'd advise to use a 40 Amp 600 Volt Stud Blocking Diode in the proper place. Here's a link to what I'm suggesting:

Auto Blocking Diode

User-inserted image

Again, can't really help with the placement or if it would be effective in your case without seeing what you have there. As mentioned you may need to have a switch to totally separate the two systems but unless you can find a affordable automatic cutout switch you would have to remember to open it manually each time you plug the B9 in to the wall.

I dont have my B9 running on batteries at present and don't really plan on it. He's only running on AC wall current that is converted to 12vdc by several Switching Power Supplies for most of the main power like Mike Joyce did in his B9's. I then lower the DC voltage used by some other sensors and motors with linear voltage regulators. You really need to watch your amp draws when you start branching out into some of the power hungry items like lights and big servos and DC motors. However I suspect you already know most of this stuff. :)

The fog machine with your B9 sounds over the top cool. I hope you take some video and pics and share with both EZ & B9 forums. :P

Have fun, Dave Schulpius


Hi Guys,

Thanks for the feedback.

Dave, I've see some of your stuff too and ditto back atcha! No offense taken...

I do already have everything running on DC except for an ac to dc inverter to power the 12 volt system and a 24 volt battery tender to charge and maintain the batteries when plugged in.

I'm attaching my primitive schematic as best I can draw it OMG it took me a half hour and siz attempts to figure out and draw what I did lol...) I've forgotten most of what I learned about schematics (in the dark ages) so I hope its clear enough.

Drawing it out I think I have the fuse on the wrong side of the switch... I'll have to change that. Right now the switch is only rated at 15 amps. When I had a 15 amp fuse in the fuse holder, it popped when I accelerate the robot to full speed instantly but does not with a 20 amp so I know I'm drawing somewhere between 15 and 20 at surge (albeit without another 50 or more pounds on board). I'll eventually replace that switch with a 30 amp relay circuit to eliminate that week spot.

I have TWO diodes in place now as shown in the schematic but based on your input they're woefully inadequate (50 PIV 3A rectifier diodes). I have run the robot on both systems but with only one half of 12 volt system in place. Not having an EE I was going to slap him together and run him into the ground and see if the diodes or the switch are getting too hot... I did try to over rate everything wherever I could so I don't think I'll over load and of the wiring or circuits anywhere else.

I also figure I'll add an ac switch to the power cord when I plug the robot in when he's on display. The duplex receptacle is inside the G-Bot and is accessible by just lifting his rubber leg a bit. I can just unplug the battery tender when its not needed.

I've attached a link to the battery tender I'm using. What are the ramifications of leaving it plugged in all the time using this charger/maintainer? The product literature says its designed to be used that way... In theory he will always be ready to go mobile and I can control his 12 systems with toggle switches in his programming bay (hmmm, maybe I'll put an extra smoke detector near him... err on second thought I'll just plug him in when He needs charging or when I have people over... :)

Do you agree the switch and the diodes are the weaknesses as I've drawn it? I'm thinking those two puny little diodes will be smoking soon once he's all built up and under load... Should I just go ahead and order two of the ones you recommended and place them or do I need to rework something else?

I am interested in feedback on all these issues.

Thanks again in advance!

Deltran Battery Tender at Amazon


Here's the schematic again. I don't think it got uploaded...

User-inserted image


Oh yes, and of course the are two EZ Robot boards. One in each 12 volt box in the above diagram... (board 0 in the lower half and board 1 in the torso). I'm using the EZB v3 since I purchased them almost two years ago then took contract work in Hawaii for a while... They seem to have stored well.

I will eventually upgrade to v4 or whatever is current I'm sure... Wifi must be far superior... OMG, that being said, after i do all this work and get the EZB v3s set up and configured, can I plug and play with the EZB v4 or above? Or will I have to re do whatever I setup?


Going from v3 to v4 will be close to plug and play. Main difference from a wiring perspective is that the v3 has a 5volt regulater on the +vcc pin to all digital ports. The v4 passes the input voltage directly to the +vcc pin, and the v4 runs best at 7.4 volts or higher, so if you have ping sensors or other 5v devices, you will need to either put regulators inline (available in the store soon, or very inexpensive on eBay) or power them on separately from the EZ-B.



Hi Gwen,

I'm at work at lunch so I dont have a lot of time to considder this. At first glance though:

The two diodes you show below the regulators are proper and needed. If you leave it like this I'd add a third one on the positive output of the 12v 400w power supply with the band away from it to keep the batteries from backfeeding it through the regulators. However I would not tie the battires into your circuit while the power converter is runnng ad you show it. I think the phaseing differance will show up at the regulaitors and they may burn up. I'm not totally sure of this however. You may have to disconnect your 24v battires before you start up your 12v power supply.

You may want to considder changing your power supply to 24v and paralleling your batttiries to it. Put the power supply first and put a blocking diode between the two to keep the batteries from backfeeding the PS. You can then tie in your step down regulators between the diode and battries. You can then run our battery tender and Sabertooth after the batteries. Again I dont have a lot of time right to think this through all the way but I think this will work. You may want to run this idea by the guys at B9 builders club. We have a few guys over there that know more about this stuff then me and are happy to help. I did something like this in my leg section and have a thread here on this forum with deawings and pics. I dont have the link right now but do a search useing my name and Sabertooth. That may get you there. I'll post a link later tonight if you cant find it.

Good luck and keep me posted. ;)



Here's the link of my thread I mentioned and the schematic. This drawing is for a high side switch to turn off the sabertooth but you can see how I configured the battery and the power supply.

User-inserted image

Turning off a Sabertooth

By the way, the diode I pointed to may be overkill. Looking at where you need them installed in your circuit you need to find out what your Amp pull is on your two 12v circuits. Depending on what you have in your B9's bubble I doubt your pulling more then 3 amps up there. You need to add up the amps on each circuit and match the rating of the diodes to it (and add some overhead). My suggestion is to get a Amp probe and measure what you're pulling after each regulator. That way you can size both your diodes and the fuse properly.

Good luck, Dave