Anyone converted SLOW charger (0.75A) to 230V?

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Anyone converted SLOW charger (0.75A) to 230V?

Postby Mibars » December 30th, 2010, 1:58 pm

Hi!

I see that tweaking fast charger into 100-240V model is as easy as capacitor and varistor replacement, but what about "slow charger"? The one that's rated 22V 0,75A (20W)?

As for now we are using 45W 230->115V converter (yeah, i know...), I thought about buying laptop charger (rated 3A or so...), but what if this mod is as simple as with fast charger?

Anybody knows what's inside and if it's worth dissasembling?
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Re: Anyone converted SLOW charger (0.75A) to 230V?

Postby Gordon » January 1st, 2011, 11:38 pm

Using the top-of-page/RHS Advance search function, do an Author search for red_sword_fish, then study his work. There may be a Standard PSU conversion in his posts, I just can't recall.

Even though you find none, I think the Standard PSU's circuit might be very much the same as in the Fast Charger (1.25A). Remove the box cover from yours and compare it to pictures you find in the RSF thread(s).

Do not use a third party PSU having dc-current delivery capability in excess of 1.25A. Roomba depends on the PSU rolling current off if charging-current-demand exceeds ~1.3A.
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Re: Anyone converted SLOW charger (0.75A) to 230V?

Postby Pyrofer » January 5th, 2011, 9:48 am

I converted my original charger (22v .75a) using the same method, One capacitor and remove the blue round thing.
Just upgraded to the new charger and did the same mod again, almost exactly the same!
To get the parts I just cracked open another UK charger from something and it had the right 400v 47uf Cap in it.
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Re: Anyone converted SLOW charger (0.75A) to 230V?

Postby Mibars » January 5th, 2011, 3:58 pm

Pyrofer wrote:(...)and remove the blue round thing.(...)


You mean "remove surge protection"? :D This blue round varistor is there for a reason. I live outside of town and I don't have power lines dug in ground, let's say, that as for now I've one blown surge protector in my home - and I've alredy fried 3 modems during thunderstorms :)

It's good to hear that design is similar to 1,25A model.

One more question: Gordon wrote that Roomba depends on current supplied by charger - Does it mean that it decreases charging current when it notices voltage drop? What's the charging voltage then?
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Re: Anyone converted SLOW charger (0.75A) to 230V?

Postby Gordon » January 5th, 2011, 10:15 pm

Mibars wrote:...It's good to hear that design is similar to 1,25A model.
Use a little caution with that 'jump'. While the HV sections tend to be very similar (depending on which switching controller happens to be incorporated) the low-voltage sections (which we don't expect to become involved during a 120VAC to 240VAC re-work) undergo more design changes than one might expect! E.g.: Member red_sword_fish discovered a small diode in the LV section (of certain PSU models) died after the conversion, and normal use began. If you see a diode labeled "D11" on your board, you might want to review RSF's postings about PSU re-work.
One more question: Gordon wrote that Roomba depends on current supplied by charger - Does it mean that it decreases charging current when it notices voltage drop?
Yes. The PSU falls out of its assigned 22Vdc regulation point and droops in accordance with load current. I can show you a rough plot of that function, if you want to see it.
What's the charging voltage then?
Of course it depends on the battery's relative capacity (i.e., how empty is it, relative to its rated capacity?). Lets say you had left a battery in the robot but failed to keep its powered charging PSU connected, and that you eventually found the battery showed only about ten volts across its terminals (w/o any load on the battery).

If you were to then direct connect a 22-volt source to that depleted battery a very heavy (amps) current would begin flowing into the battery. An over-pressure of 22V - 10V ~ 12V would exist, when it would supplying a charging voltage of about only 12V or 13V would be preferred, i.e., just a couple volts over the present battery voltage.

Well, under that over-pressure duress, the 22V iRobot PSU will develop its own protective voltage drop by lowering its output voltage down into the 11 to 13V region (I don't know these numbers, exactly, I'm just tossing out likely WAGs). By operating in that manner, the PSU's output current will be on the order of 1.3A to 1.4A. As I recall, if one tried to pull more than 1.45A from a Fast Charger, its output voltage will have fallen to zero.

Now, if instead of directly connecting to the battery, you plugged the PSU's cable into the robot's side jack, the scenario would be just a little different. There would be some voltage dropped inside the robot before it gets to the battery terminals. So, where we had a 12V "over-pressure" via direct connect, we might have only 10V (which is still about eight more than we would like), hence the PSU is still over loaded.

However, charging is going on, and as charge is stored in the battery it terminal voltage rises and eventually lets the PSU return to about 1.3A output, which means PSU voltage is around 20V, and terminal voltage is approx. 18V.
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Re: Anyone converted SLOW charger (0.75A) to 230V?

Postby Mibars » January 30th, 2016, 11:38 am

I thought that it would be worth mentioning that I did the conversion last year after Roomba was used just occasionally (like 5-10 times/year) and now I'm charging with no issues at 230V few times a week.

Hard to believe that my Roombais now 5 years old ;)
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