Transformer or Power Adaptor which one is best ?

Guest

I'm picking up a Roomba Discovery this week for use in the UK.

After reading some posts on power for the UK it's my understanding that you need 22V DC/1.25A output to the cradle charger.

With regards to power which is the best option.

A Laptop Style power charger
http://www.cybermarket.co.uk/ishop/923/shopscr3897.html

The same company also do the following transformer which may or not be a better option based on some other posts.

http://www.cybermarket.co.uk/ishop/923/shopscr1576.html

Any comments appreciated.

Thanks

Gary
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Slash · Guest

Go the laptop switchmode transformer, in my opinion a much safer option.

or mod the old unit if your good with electronics

-B

vassilis_eco · Posted: February 22, 2005, 3:17 am Post subject:

Definitely go for the laptop switchmode power supply.
Vassilis

G · Guest

24v @ 1.25a
http://cpc.farnell.com/jsp/endecaSearch/partDetail.jsp?SKU=PW01490&N=401

or

24v @ 2.5a
http://cpc.farnell.com/jsp/endecaSearch/partDetail.jsp?SKU=PW01496&N=401

Any comments on one of these babies ? The first seems to hit the specs exactly.

Thanks

Guest · Guest

As my 1st post above I think the Roomba Discovery needs 22V.

The 24V you list may work but for £10 more I think the one I posted above is a better option as you can set the Voltage in 1V increments so 22V can be setup exactly.

wildit · Guest Joined: 07 Sep 2006 Posts: 19

hello
the same pb for france, ...can you teach me please:
i don't understand very well the power supply for me

vasalli was talking in one post of 220v to 22 "vcd" 2amp (we don't have "vcd" in france???, or another word??)

someone can help me to know which one to buy (and put e link for example will be lovely+++ to understand better for me!!)

thanks+++

JohnAllison · Guest Joined: 13 Aug 2006 Posts: 10

Watts = Volts * Amps

Volts and amps are most important when choosing a power supply. The Roomba Red Charger draws 0.3 amps at 120V. This is equivalent to 36W of power. This is the maximum threshold of the unit.

Also listen on the power supply is the 16W of input power. The roomba itself
uses 16W to charge the battery out of the 36 available from the charger.

The charger also changes the voltage of the input from 120VAC to 22VDC. In this conversion we lose heat and gain available amperage because of the lower voltage. As specified on the charging unit the output is .75A at 22VDC, or 16W. :) Isn't math fun?

So at 16W we are not talking alot, and if you wish, you could have a power supply capable of supplying 1 MegaWatt but the charge would only draw 16 W. So don't be afraid of too much wattage specified on the conversion unit.

I would recommend buying what is known as a "Step Down Voltage Converter." These are sold to americans so thier laptops and cellphones won't explode when overseas. They are very common and very cheap. $10US. go to froogle.google.com search of a step down volatege converter. Rank the listings from low to high price. Half way down you should see pictures of plugs that are similar to what you have in Europe.

Let me know if you have any other questions.

wildit · Guest Joined: 07 Sep 2006 Posts: 19

[url]http://www.castorama.fr/boutique/sku/sku.jhtml?elementId=Casto890607&productId=CastoECO0502

thoose i find

tell me if its ok pleas
thanks[/img][/url]

Gordon · Posted: September 9, 2006, 8:32 am Post subject:

wildit wrote: {this I find}tell me if its ok pleas
thanks
That 'transformer-converter-adapter' is OK.
BTW, "vcd" means nothing; it was probably mis-typed, and should have been "Vdc" = "Volts, direct-current".

JohnAllison · Guest Joined: 13 Aug 2006 Posts: 10

This is correct. You can purchase that device and it will charge your roomba.

Gordon · Posted: September 9, 2006, 11:27 am Post subject: Re: Watts Volts and Amps

JohnAllison wrote: Watts = Volts * Amps Agreed, for direct-current cases and for purely resistive alternating-current cases.
Quote: Volts and amps are most important when choosing a power supply. The Roomba Red Charger draws 0.3 amps at 120V. This is equivalent to 36W of power. This is the maximum threshold of the unit. John, its not clear to me just what power-supply, PS, you have there, for the Red-Disco(?). I recall the Reds were first delivered with a transformer-wall-wart PS (similar to the Originals), but after a few months into production of the Discovery series, iRbt decided to instead ship APS Fast Chargers with the Red Discos. I assume yours must also be a Fast Charger, simply because a wall-wart type would not draw 0.3Aac primary current; and, for the fact you later mention its 22Vdc output.

Here are some data from my APS Fast Charger ("FC", hereafter): With my mains-voltage between 116 & 117 VAC, the Model #10556 FC draws 0.57 Aac input current while delivering nominally one ampere of dc charging current to the robot. The FC's label indicates: "Input: 120VAC 60Hz 0.6A".

If we compute input power via volts times amps, E * I, and based on the label-data, we get 72W. But, the very next line on the FC's label says: "Input Power: 33W"! 33 vs. 72! That is telling us we must take into the account the AC effects of the FC's input. To do that, we use this extended expression: AC_power = Eac * Iac * cos(phi), where "phi' is the phase-angle between the voltage and current sinusoidal waves.

While the value of that angle is of no interest to us, the Power-Factor, PF, sometimes is. "Power Factor" is a name applied to the "cos(phi)" term by the power generating utilities. We can solve for it (taking iRobot's "33 watts as fact), to obtain cos(phi) = PF = 0.46 (approx.). It simply says the FC is using 46% less power when AC-powered, than it would if DC-powered (which, BTW, is a perfectly feasible option for running this switcher!).

Quote: Also listen {listed} on the power supply is the 16W of input power. The roomba itself uses 16W to charge the battery out of the 36 available from the charger. That "16W of input power" is one of the mystery numbers for that Red's PSU, John. The FC I am looking at claims: "Output: 22VDC 1.25A". Now, we *can* say: Edc * Idc = 22 Vdc * 1.25 Adc = 27.5 watts, available to the robot for charging its battery. We can then compute the Efficiency, "eff", of the FC as: 100 * 27.5 / 33 = 83%; a nice looking efficiency.

Quote: The charger also changes the voltage of the input from 120VAC to 22VDC. In this conversion we lose heat and gain available amperage because of the lower voltage. As specified on the charging unit the output is .75A at 22VDC, or 16W. Isn't math fun? I would couch the thermal characteristics of the conversion by saying '...we *generate* heat within the FC, and gain...". Then, to the numbers again... I don't fathom that ".75A", since output current of the FC is quite close to *1.25Adc* while actively charging a battery (it lowers to about 0.05A upon switching into trickle-charging mode; and the FC's input current lowers to approx. 0.11Aac).

Quote: So at 16W {or even 33W INPUT} we are not talking alot, and if you wish, you could have a power supply capable of supplying 1 MegaWatt but the charge{r} would only draw 16{33} W. So don't be afraid of too much wattage specified on the conversion unit. ... That would be reasonable advice IF all those transformer-type converters provided *voltage-regulation* of their outputs! I am certain they are not well regulated, else their prices would not, could not be so low. I would be cautious about buying more power capability than needed, without knowing that the over-size transformer's unloaded secondary (output)voltage level remains within the specified upper limit of the equipment to be powered with it.

*My* problem with supplying firm advice in this regard is I have no such converter on hand which I can measure loaded and un-loaded output voltages. I *do* have access to two extreme cases, however. One is from a EE text ("Industrial Electricity". Dawes), and the other is empirical data from a lab power-supply transformer / rectifier circuit.

My textbook example is for a 2300VAC to 230VAC step-down transformer that is capable of handling 100,000 watts. Obviously, an *industrial* item! The nice thing about its design is voltage regulation is better than one percent from no-load to full load! Very impressive; but it was designed for electric power-distribution to consumers.

Countering that wonderful design is a transformer in a Heathkit, 50Vdc, 1.5A power supply I have on hand. I have made dc-voltage measurements at zero-load and 1.5A load right at the bridge-rectifier's output (so, while this is not *exactly* the same as measuring secondary AC-voltage, there is no meaningful exacerbation of voltage level; and, the data already exists!). The test data indicate a no-load voltage of 71Vdc, and a 1.5-ampere-loaded voltage of 60Vdc. Note that this xfmr must continuously handle a power-dissipation on the order of 60V * 1.5A = 90W; which suggests it was designed and built with some positive margin in it. It would be a good reverse-engineering project to work backwards from the amounts of iron and copper it contains (which could be estimated) to estimate the nominal power dissipation capability for it! I'll leave that for the reader -- just ask for dimensions.

I'm assuming it to be a nominal 200W transformer. When it feeds 1.5-amps to a load, the voltage across the load is 11-volts less than when no load is connected. This xfmr is nominally a 2:1 step-down device. If this same design were to be scaled up for 230VAC input, and 125VAC output, a user might measure 125V + 2 * 11V = 145V under light, to no load. That could be worrisome when a FC drops into trickle-charging.

DO NOT construe what I say here to be factual as applied to commercial transformer-type converters, adapters. Those manufacturers have been making these things for a long time and have likely put just enough iron & copper in them to generally make them less threatening (as an excess voltage threat). All I am saying is the above effect almost certainly must be present -- it is just a matter of *magnitude*, so be careful about buying excess power-handling capability. Know what you are getting (impossible from ordinary suppliers!). If you buy excess power-capability, you will pay more at the front, you will pay a little more while it is powered (even with nothing connected to it), and you will have a larger and heavier unit to deal with.

wildit · Guest Joined: 07 Sep 2006 Posts: 19

thanks for the answer!!!

you are all 2 friendly!

but i 'm going let you discuus from technological discus to hight for me, poor poor woman with her vacuum!!!