5v source pin

Wael Chatila · Post by **Wael Chatila** » June 14th, 2007, 1:14 am

Hi

I need a 5v pin to power an "alien" circuit which draws somewhere between 0.5 - 0.8A. Im currently using a linear regulator connected to the 16v battery...its getting very got even with a heat sink. Does any one know of a 5v pin I can draw my power from?

vic7767 · Post by **vic7767** » June 14th, 2007, 11:07 am

Most of us use the 7805 regulators. There are units available that will handle up to 1 amp (T05) case. Easiest way is to search ebay.

roadknightlabs · Post by **roadknightlabs** » June 14th, 2007, 1:21 pm

Vic is correct. You will have to step down the 16V coming off the SCI pin to 5V using a voltage regulator like a 7805 or 78L05 because there is no 5V pin on the SCI port. Some cables and adapters out there have 5V pins, but that's only because they're doing the stepdown internally.

I've been able to power my Zaurus PDA off the Roomba battery by using a 7805 on a breadboard circuit to drop the voltage down to +5V.

Wael Chatila · Post by **Wael Chatila** » June 14th, 2007, 2:00 pm

Yep, im using one of those. However, i've modded my roomba far enough already to not be afraid of getting to a 5v pin some where inside the roomba's gut which im looking for.

Gordon · Post by **Gordon** » June 14th, 2007, 8:51 pm

Wael Chatila wrote:...i've modded my roomba far enough already to not be afraid of getting to a 5v pin some where inside the roomba's gut which im looking for.

I can tell you exactly where to attach the pick-off leads. But no one, except the EE-designer can tell you how much (current) margin remains for you to play with.

I can also suggest that drawing as much current from the on-board 7805, as you indicated in your first post, is not a wise thing to do!

Is it not the case that the 7805 will be dissipating about eight watts?

Is it not the case that the main_PWB has a hard time getting rid of heat developed by its hot components already?

Why make it any harder for the 'bot to stay alive?

If you go ahead with this destructive plot, I think we should start up an "anchor pool" -- you know, where we all place a 'bet' and guess how many minutes your robot will operate before the regulator gets hot enough to de-solder itself and disconnect from the PCB!
Trouble is, we'd have to hire a qualified time-keeper; whose fee would likely drain the pool.

roadknightlabs · Post by **roadknightlabs** » June 14th, 2007, 9:18 pm

Gordon wrote:
Wael Chatila wrote:...i've modded my roomba far enough already to not be afraid of getting to a 5v pin some where inside the roomba's gut which im looking for.
I can tell you exactly where to attach the pick-off leads. But no one, except the EE-designer can tell you how much (current) margin remains for you to play with.

[...]

Gordon wrote: If you go ahead with this destructive plot, I think we should start up an "anchor pool" -- you know, where we all place a 'bet' and guess how many minutes your robot will operate before the regulator gets hot enough to de-solder itself and disconnect from the PCB!
Trouble is, we'd have to hire a qualified time-keeper; whose fee would likely drain the pool.

I'm in a for a spinny brush

Wael Chatila · Post by **Wael Chatila** » June 14th, 2007, 9:45 pm

I'm not quite sure you grasped the whole purpose of my original post Robot Master.

It seems like the best option to go from here is using a DC-to-DC converter which costs >$10.

vic7767 · Post by **vic7767** » June 14th, 2007, 11:51 pm

I understood that you asked for a pin internal to the Roomba that would provide you a 5v dc output at .8 amps.

If you look at the PCB traces there don't appear to be many that are beefy enough to support the amps and the traces may not be at the 5 volt potential.

The reason I suggested a hardy 7805 regulator is to help you avoid the meltdown that Gordon mentioned. Most of us hackers don't mind experiments on the Roomba robots but we like to avoid workbench mini fires.

The dc to dc converter you mention I would like to know more about.

Wael Chatila · Post by **Wael Chatila** » June 15th, 2007, 12:16 am

see http://www.dimensionengineering.com/DE-SW033.htm

it's pin compatible with the 7805.

Also, for the record i meant to address "Gordon" in my previous post.

Gordon · Post by **Gordon** » June 15th, 2007, 12:32 am

Wael Chatila wrote:I'm not quite sure you grasped the whole purpose of my original post...

The only thing there for me to grasp was you want to steal 0.5->0.8A from Roomba's +5VREG buss to support your external elex.

Tapping raw battery power at SCI-connector, as suggested by Vic & RKL, is good to a certain load level, but it won't support your requirement. There is a 72V, 0.25A resettable fuse in series, so you are liited to something less than 1/4 amp.

You could buy a TO-220 cased 7805, give it a good heat-sink, where cool air can flow over it (which means mounting it outside of Roomba's shroud), then power it directly from the battery terminals.

While I was typing this, new posts arrived. I did take the link to this product: 10W Step down adjustable switching regulator; Product ID: DE-SWADJ, which looks like it should work for you, and is nicely priced! Its efficiency ought to help keep the heat down. IMO, it must still be connected directly to the battery harness, given next.

To tap the battery power, you will have to splice into the RED & BLK harness that goes from the battery-connector to P7/J7 PCB connector. See zones ZN-D2 and ZN-E2 in this schematic.

Added via edit: It may be useful to know that the raw battery voltage will vary from a high (freshly charged battery) of 17 to 18 volts, down to Roomba's automatic, operations-cut-off at 12Vdc.