How much power (mA) would I need in an AC-to-DC adapter for a TomTom?

[lanimilbus]

I've always just plugged my TomTom GO 920 directly into my car's cigarette lighter but now I need to power another item simultaneously which requires a DC-to-AC inverter (this one, to be exact) to be plugged into the cigarette lighter instead. The inverter has more than one outlet but none of them are DC so the TomTom can't be plugged into it, which is why I was thinking of purchasing an adapter like this which would plug in to the spare AC outlet on the inverter on one end and the TomTom would be plugged into the DC side on the other end.

I'm wondering though, would that be enough to power the TomTom? The adapter supposedly has an amp rating of 300 mA according to the reviews, but I haven't been able to find any mention of how many amps or milliamps the TomTom would require to keep its charge so I'm not sure if it would work or not.

 

[dhn]

Most dedicated car chargers that come with the TomTom units are at least in the 1A to 1.2A range.

I would think that 300ma wouldn't even be enough to run the application, let alone charge your unit.

It would probably drain the battery trying.

 

[lanimilbus]

Most dedicated car chargers that come with the TomTom units are at least in the 1A to 1.2A range.

I would think that 300ma wouldn't even be enough to run the application, let alone charge your unit.

It would probably drain the battery trying.

Well, if it makes any difference, the car charger I use for my TomTom isn't the original that came with it but rather a generic replacement I bought off eBay after I lost the original on a road trip. And just now after googling the code that's written on the side of the charger, I found this:.
MOT-V3 Car Charger

I'm pretty sure that's the same charger and the stats list it as being 550mA. So I guess the 300mA adapter is out. Also, if the original chargers are 1-1.2 amp then I guess that explains why my TomTom always dies from low battery within 30 seconds of being unplugged from the cigarette lighter...

So, this brings up two new questions. First, would a 5 amp/12V AC to DC power adapter like this one work instead? I can't see why it wouldn't but I'm not too experienced with this type of stuff so I just wanted to make sure.

Second, does anyone know the exact amperage for the original TomTom GO 920 car charger? I'll have to look into buying a new charger that will be able to adequately power it but I want to make sure I get the amperage right this time.

 

[Duckycards]

In reply to your first question: Yes, the 5 amp/12 Volt AC to DC converter should work fine with your TomTom. The actual current drawn from the converter is determined by your TomTom circuitry so as long as the converter has a high enough current rating and provides 5 volts DC to your TomTom it should work fine.

Sorry I can't help with the second question.

 

[mikealder]

Most of the car chargers supplied with the x20 models were 2 amp rated, although I have since used 1,2 amp chargers with my 720 with no issues at all.

To power all my different devices in the car I have wired in a four way switchable ciggy socket addaptor, in to this are mounted four different charger leads for the different devices, works a treat - Mike

 

[Michael Quinlan]

I tried a 1000ma-rated generic USB charger earlier this year with my Go 740, and if I recall correctly, the TomTom never really charged, although it worked okay when powered by this charger.

If you'll have to use an adapter either way, I think you're better off with multi-socket 12V adapter that will allow you to continue using the charger you're currently using (since it seems to work).

 

[nomenclator]

Converting dc to ac, then back again, doesn't make much sense to me. It is inefficient. Just get some kind of Y-adpater for the cigarette lighter socket. You can make one yourself - from 2 sockets, and a plug. Then plug the inverter into one socket, and the TT into the other. Or simply install a second cig lighter socket, into the car's accessory circuit (a circuit that stays on when the car is off). Or instead of a cig lighter socket and plug, install an alternative socket and plug, a smaller one that takes up less room on your dash. Install the socket into the cars accessory circuit, and put replace the TT's plug with the alternative you've chosen. Make sure you get the polarity correct.

 

[Duckycards]

I think he has another device that runs on AC so he needs the inverter. However, he might use a Y and plug the inverter in one branch and his TT in the other, provided his power circuit will handle the combined load.

 

[nomenclator]

"I think he has another device that runs on AC so he needs the inverter. However, he might use a Y and plug the inverter in one branch and his TT in the other, provided his power circuit will handle the combined load."

If the circuit will handle the load, with the dc device plugged into an ac to dc converter, plugged into the inverter, it will handle the load with the dc device plugged directly into dc. Plugging the dc device directly into dc will use less electricity. Every time you do a conversion, you use a bit of electricity to do the conversion, as well as use electricity to operate the load (its called inefficiency).

 

[lanimilbus]

I ended up ordering a replacement car charger from the TT website which actually charges it up now so it doesn't die immediately after being unplugged; it's 1.2 amperes, so the 550mA charger clearly wasn't enough. As for using both the inverter and the TT simultaneously, I don't know why I didn't just think of a simple DC splitter instead of plugging a converter into the inverter and the inverter into the cigarette lighter.

I think he has another device that runs on AC so he needs the inverter. However, he might use a Y and plug the inverter in one branch and his TT in the other, provided his power circuit will handle the combined load.

Yeah, this is what I was thinking of. Any way to find out beforehand whether or not it would handle the load? I would be using a DC splitter like this one with the TomTom plugged into one of the sockets and the inverter plugged into another with one or two AC devices plugged into the inverter (most likely a phone charger and a camera battery charger). Would that be likely to overload it or blow the fuse or anything?

 

[ccarson]

Most automotive 12v sources are rated at 10 amps or less. You shouldn't have any problem with a selection of phone or camera battery chargers plugged in with the TomTom. However, the inverter you're using is rated at 200w continuous which would exceed the 10 amp limit by quite a bit. I would simply add up the loads you anticipate connecting and make sure they total less than 10 amps at 12 volts. You can also check the fusing for your accessory circuit to see what fuse is used. By the way, Radio Shack has a Y cable that I've been using for my TomTom and a USB charger for an iPhone and it has worked fine. The 3-way adapter you show wouldl be a tidier solution if it will physically fit. Some auto connectors have limited room around them, though.

 

[dash8]

I measured mine (XL 340) using a Kill-A-Watt meter at 6 watts. The meter will only work on the AC household power mains end. Since the AC current is in RMS value (the one normally used), there's no problem comparing it with DC. Also, 1000 mA = 1 amp

240V AC @ 25 mA = 6W (Europe & Australia)
120V AC @ 50 mA = 6W (North America)
12V DC @ 500 mA = 6W (Battery voltage in most vehicles)
5V DC @ 1200 mA = 6W (All USB devices including TomTom)

Since power converters aren't 100% efficient, the 1200 mA should be considered a maximum value. In other words, some of the 6 watts were lost in the conversion, and you could get by with a little less than 1200 mA.

 

[nomenclator]

dash8, where did the "6 watts" come from? Is that a published spec?

 

[canderson]

dash8, where did the "6 watts" come from? Is that a published spec?

He directly measured the input power of his AC adapter. The 6W he saw combined with about 85% power conversion efficiency would indicate that the TomTom was drawing about 5W @ 5V = 1 amp... just about what folks say they believe the car chargers supply on a good day.

These things are designed to be charged (albeit a bit more slowly) from a computer's USB port. The voltage off the port is 5V. The maximum power draw from a computer's USB port is 500mA. 5V x 500mA is 2.5W if you go that route.

So 500mA would be the minimum a TomTom would expect to see. It's obviously adequate to run the unit (they run in the PC dock), and it's enough to charge it (they charge in the PC dock). However, if you tried to do both at once (charge in the PC dock while operating the unit), I doubt it would charge in much less than a full overnight attempt.

 

[nomenclator]

I see, I think. He measured (on the AC side of the converter) the total power in watts being used by the TT, plus the 15% being wasted in the process of AC to DC conversion. I knew there were current measuring tools, but I didn't know there were watt-measuring tools available. I wonder how many amps you can supply to the TT, before you are charging the battery too fast and damaging it. ??

What is the "pc dock" ??

 

[canderson]

I see, I think. He measured (on the AC side of the converter) the total power in watts being used by the TT, plus the 15% being wasted in the process of AC to DC conversion. I knew there were current measuring tools, but I didn't know there were watt-measuring tools available. I wonder how many amps you can supply to the TT, before you are charging the battery too fast and damaging it. ??

What is the "pc dock" ??

You can't deliver too much current to the device. The internal charger will regulate that. Lithium ion chemistry doesn't use constant current charging in the way NiMH would.

There are all kinds of tools for measuring what's being piped into an AC connection. Some do some statistical evaluation for power factor correction as well. You can even buy them as panel mounts for building directly into equipment / racks (http://www.murata-ps.com/data/meters/acm20.pdf?ref=e-frontline-ACM20)

He just measured the raw 6W. I made the 85% assumption myself as that's not atypical for a wall wart.

As for the PC dock, that's the device that TT sends out with their units that you plug into your PC's USB port and set your unit into. You can do the same thing on some models by just using a USB mini-B cable. Other models require the "active dock" that you also plug into your computer's USB port. However you do it, it's the same thing. What you can receive from a USB port is 5V @ maximum 500mA.

 

[dash8]

You can't have too many amps in your power adaptor (other than being too expensive or bulky). These amps aren't necessarily being used, but are "stand-by" amps for when needed, with the max. available as the given spec. A power hungry device will have a low resistance, and use a lot of current (measured in amps). The only way to burn out an electronic device is to raise the voltage beyond what it was designed for.

"Forced" amps are used in arc welding, where you set the amps depending on the type of weld needed, and the machine raises the voltage accordingly. But just about anything else, the input voltage is a fixed value and it's the current that varies. As long as the current is less than what the power adapter is capable of supplying, there's no problem. If it's more, the power adapter -- not the device -- is in danger of burning out or blowing a fuse.

 

[nomenclator]

Thanks dash8. Great information.