Charging voltage? and crowbar protection

JohnTT · Occasional Visitor Joined: Nov 27, 2006 Posts: 55

I've tried searching but can't find it. Does anyone know what the correct charging voltage for a TomTom ONE V2 should be?

My original TomTom car charger outputs 5.2V on the mini USB but another charger I bought outputs 5.0V and is it my imagination? - it seems to take ages to charge. I might be wrong. I'm not sure if the voltage difference is just tollerance variation though, as the spec might be 5V+/-5% which would allow 4.75V to 5.25V.

I bought the other charger because it is neater and allows dissassembly to incorporate a crowbar protector http://www.mitedu.freeserve.co.uk/Design/overvoltage.htm . There have been a few TomTom chargers go faulty and output 12V which blows the TomTom up Rolling Eyes

.

Andy_P · Posted: Fri Apr 20, 2007 11:00 am Post subject:

I wouldn't have thought that amount of variation in voltage would make a difference, but the maximum current capacity of the charger might.

As the TomTom One uses the USB socket to charge, it will conform to the USB power specifications.
Wikipaedia has a good layman's guide HERE
("power" is about half way down).
The main jist is:

JohnTT · Occasional Visitor Joined: Nov 27, 2006 Posts: 55

Thanks for that. Yes it should conform to the USB standard but sometimes manufacturers tweak things a bit. The lead for the PC is of course reliant on the PC for voltage.

Good point about the current. I'll measure the current draw with both chargers to compare.

It still might be worth fitting a crowbar in the unit as with all these things they are made cheaply so are as basic as possible. They use buck regulator switchers and the failure mode of these devices is often a shorted switch which will connect input to output. There is a fuse but that will only blow after the damage is done.

Andy_P · Posted: Fri Apr 20, 2007 11:41 am Post subject:

JohnTT · Occasional Visitor Joined: Nov 27, 2006 Posts: 55

I measured the following charger current draws from 12V:

TomTom turned OFF:
Standard TomTom charger = 250mA
Ebay TomTom charger...... = 250mA

TomTom turned ON:
Standard TomTom charger = 350mA
Ebay TomTom charger...... = 270mA

Clearly there is little spare power left to charge the battery with the TomTom turned ON when using the ebay charger as the current only rises by 20mA.

Presuming 85% efficiency that would make the battery charging current at 5.2V on the Standard TomTom charger ~490mA with the current rising to ~686mA to both charge and run the TomTom.

Further testing showed that the 5.0V output of the ebay charger drops to 4.55V under load with the TomTom on or off. Clearly it needs some re-design!

If anyone's interested here's the Mini USB pinout of th eebay charger:

Pin 1 = V BUS = 5V
Pin 2 = D- = joined to Pin 3
Pin 3 = D+ = joined to Pin 2
Pin 4 = ID = 20k resistor to GND
Pin 5 = GND = GND

Although the TomTom charger has the voltage output on pins 1 + 5, I could not find any evidence of the resistor and joined pins.

JohnTT · Occasional Visitor Joined: Nov 27, 2006 Posts: 55

Right, I changed the inductor in the switch mode for a slightly larger one, so it won't saturate, reduced a resistor in the current limit and changed the output voltage to match the Tom Tom charger. The switcher employed is a MC3406A.

It now draws 360mA @ 12V and maintains 5.2V out so the output current is around 750mA.

I also added a 5W 5.6V zener across the output, so there is no way the output can go over voltage and destroy the Tom Tom. You don't need to include the thyristor shown in the crowbar circuit elsewhere on this site as the zener is more than capable of blowing the fuse by itself.

I wish I could give the ebay seller some negative feedback after all that effort. I might as well have made one myself! Rolling Eyes

Oldboy · Posted: Thu May 10, 2007 9:04 pm Post subject:

JohnTT · Occasional Visitor Joined: Nov 27, 2006 Posts: 55

Oldboy · Posted: Fri May 11, 2007 7:31 am Post subject:

JohnTT · Occasional Visitor Joined: Nov 27, 2006 Posts: 55

That document does not make it clear and indeed the fact that a step down or "Buck" regulator type switch mode can produce more current out than it takes in is often overlooked.

Here's a basic Buck regulator. Energy is only lost when current and voltage appear together through and across the same component. As you've pointed out, the switch is either conducting with no volts across it or is off with volts across but no current (so power loss is zero). The inductor and capacitor only store energy and no power is lost. The diode only looses a little and nearly all the power is delivered to the load:

You need to look at it in terms of energy and power. There is a basic principal that you can't get more energy out than you put in and you can only approach near to 100% efficiency without actualy quite getting there - there are always some losses - hense 80-95% is typical from a switch mode supply.

If a power supply draws 6 Watts from the 12V battery and is 100% efficient (let's be perfect for a moment) - then it must be both drawing 0.5 Amps (12V x 0.5A = 6W) and delivering 0.5 Amps at 12V (12V x 0.5A = 6W).

But let's say you halve the output voltage to 6V. If it's still drawing 0.5A (12V x 0.5A = 6W) and is still 100% efficient then it must still be outputting 6W (i.e. no energy losses), so the current must be greater (6V x 1A = 6W)

i.e. Power in = Power out
12V x 0.5A = 6V x 1A

That's how payphones you see on the street manage to run microprocessors and electric motors and modems from a telephone line current of 20mA. They step up the current by lowering the voltage.

See here for more information:
http://www.hills2.u-net.com/electron/smps.htm
http://www.onsemi.com/pub/Collateral/SMPSRM-D.PDF

Skippy · Posted: Fri May 11, 2007 8:54 am Post subject:

Oldboy · Posted: Fri May 11, 2007 12:19 pm Post subject:

JohnTT · Occasional Visitor Joined: Nov 27, 2006 Posts: 55

Skippy · Posted: Sun May 13, 2007 8:25 pm Post subject:

JohnTT · Occasional Visitor Joined: Nov 27, 2006 Posts: 55

	PocketGPS
	Web