Powering the calculator from the USB port

Post here to share useful tips and tricks, to ask questions about using your DM42 or to report software-related problems
keithdalby
Posts: 545
Joined: Mon Apr 24, 2017 6:38 pm

Re: Powering the calculator from the USB port

Post by keithdalby » Thu Dec 21, 2017 7:47 am

Walter wrote:
Thu Dec 21, 2017 4:07 am
I'm a physicist.
Me too. I enjoy chatting with other physicists about the work they're doing, but as I am typing this I'm realizing that perhaps this isn't the appropriate place for that discussion.

mcc
Posts: 211
Joined: Fri Jun 23, 2017 3:10 am

Re: Powering the calculator from the USB port

Post by mcc » Thu Dec 21, 2017 8:24 am

Hi,

WARNING!
I ordered a DM42...but it has not yet arrived (I have asked Michael for a delayed delivery since I will not be at
home at Christmas). So everything will be a little "theorectical")...

Some thoughts about power supplies and their quality:
Powerbanks with USB 5V out:
These are driven from Lithium accumulators (3.7V) so the voltage needs to
be stepped up with a boost converter, which is included in the powerbank. These boost
converters work with a high frequency which may leak through into the 5V-path.
This may or may not interfer with the electronic of the DM42, which -- for example --
uses HF for its clock generator ("CPU clock" not the one telling the time :) ).

USB-Powersupply:
This is the same in the opposite direction:
220V/110V mains needs to be stepped down to 5V. Now you have a buck converter (or
even simpler circuits...IIIIIEEEEEKKKKKSSS!) to do that job. These also work with high
frequencies and the same problem may arrise.

USB connector of a PC:
Here 5V is needed for the whole PC and the transformation is done by a REAL BIG PSU.
If THAT 5v would be "poisened" with HF, the whole PC would have a problem.

El cheapo USB power supplies:
Especially when it comes to cheap powerbanks from questionable source another risk may be arise:
(This is only an example) Say the DM42 would take 5V 1A at full load. Furthermore the powerbank in
question is rated with 5V 1.2A...which "should be sufficient".
But: Boost converters especially those with less ripple, high effieciency and high current are not THAT
cheap. El cheapo sources may claim a higher current limit as the physics of the internal boost converter
may be able to deliver.
Consequence: At a certain point the current dropps, the voltage dropps while the internal temperature
of the boost converter jumps to the sky. AND: The ripple (that is the HF poisining of the DC) rises, which
is even worse.

The PC USB connector seems the most save aternative here.
On the other hand: An USB powerbank, which is save to use, would be quite handy.
But such a powerbank should be intensively checked with an oscilloscope (which I do not own) and
a experimental load to check its parameters.

HTH!
Cheers
Meino
DM 42 - SN: 00373, Firmware v.:3.14. / 3.14. as compiled by SwissMicros

grsbanks
Posts: 857
Joined: Tue Apr 25, 2017 9:23 am
Location: Preston, Lancs, UK
Contact:

Re: Powering the calculator from the USB port

Post by grsbanks » Thu Dec 21, 2017 9:29 am

mcc wrote:
Thu Dec 21, 2017 8:24 am
WARNING!
The DM42 has a boost generator of its own that generates the 5V needed for the LCD from the 3V coming out of the battery. I don't think it's that sensitive to HF ripple, plus all the sensitive components are decoupled.

The remark would be completely valid for el cheapo power supplies. They're usually not even regulated and you have no idea what junk is coming out of them unless you measure it.

USB power banks should be fine because the power being drawn by the DM42 is so low. In fact, half the time the power bank won't even realize that there's something there and will shut down. The same applies to phone chargers because they have to provide something fairly clean to the phone.
Not SwissMicros staff, just an enthusiast.

Michael
Site Admin
Posts: 236
Joined: Wed Apr 05, 2017 9:31 pm

Re: Powering the calculator from the USB port

Post by Michael » Thu Dec 21, 2017 10:14 am

Once the DM42 is powered from USB, the 5V are regulated to 3V by an LDO (LD3985).
The absolute maximum rating for the input pin of the LDO is 6V and is able to withstand non repetitive spike of 6.5 V for 200 ms.

Boub65
Posts: 89
Joined: Tue Sep 12, 2017 2:34 pm
Location: Rabat, Morocco

Re: Powering the calculator from the USB port

Post by Boub65 » Thu Dec 21, 2017 11:14 am

Just for your information...
I fried two (2) of my kids VTECH learning tablets (called INNOTAB in the US) by just using an original Samsung USB charger instead of the orginal VTECH USB charger.
I thought that 5V USB charger = 5V USB Charger...

My concern now is how to prevent my (4 and 6 years) old boys from using any charger to charge their tablets...
2xDM15L, DM41L, DM42 SN#00855
TI-89 titanium, CASIO fx-cg50 (to play with micropython)

Boub65
Posts: 89
Joined: Tue Sep 12, 2017 2:34 pm
Location: Rabat, Morocco

Re: Powering the calculator from the USB port

Post by Boub65 » Thu Dec 21, 2017 11:17 am

Michael wrote:
Thu Dec 21, 2017 10:14 am
Once the DM42 is powered from USB, the 5V are regulated to 3V by an LDO (LD3985).
The absolute maximum rating for the input pin of the LDO is 6V and is able to withstand non repetitive spike of 6.5 V for 200 ms.
I learned that Original Samsung fast USB Chargers are 5V/9V...
So it's a no no no!
2xDM15L, DM41L, DM42 SN#00855
TI-89 titanium, CASIO fx-cg50 (to play with micropython)

mcc
Posts: 211
Joined: Fri Jun 23, 2017 3:10 am

Re: Powering the calculator from the USB port

Post by mcc » Thu Dec 21, 2017 11:26 am

Hi,

will the calculator be harmed in any permant way, if underpowered?
Everything, which a reset could solve does not count here, even if I
will loose data...
DM 42 - SN: 00373, Firmware v.:3.14. / 3.14. as compiled by SwissMicros

grsbanks
Posts: 857
Joined: Tue Apr 25, 2017 9:23 am
Location: Preston, Lancs, UK
Contact:

Re: Powering the calculator from the USB port

Post by grsbanks » Thu Dec 21, 2017 11:53 am

The only possible thing I can think of is the power supply of the voltage booster or the booster itself. If you underpower it, it's going to be working overtime to step up to the required 5V.

Otherwise the only thing liable to suffer is your data as far as I can see.
Not SwissMicros staff, just an enthusiast.

mcc
Posts: 211
Joined: Fri Jun 23, 2017 3:10 am

Re: Powering the calculator from the USB port

Post by mcc » Thu Dec 21, 2017 12:15 pm

The background of my question is the existence of a brown-out detection circuit
on several ATMEL (now microchip) MCUs.
This circuit shutdown the MCU if voltage goes below a previously defined limit
to prevent damaging the EEPROM/FLASH.
DM 42 - SN: 00373, Firmware v.:3.14. / 3.14. as compiled by SwissMicros

tgray
Posts: 20
Joined: Fri Jul 28, 2017 1:36 am

Re: Powering the calculator from the USB port

Post by tgray » Thu Dec 21, 2017 1:12 pm

keithdalby wrote:
Thu Dec 21, 2017 7:47 am
Walter wrote:
Thu Dec 21, 2017 4:07 am
I'm a physicist.
Me too. I enjoy chatting with other physicists about the work they're doing, but as I am typing this I'm realizing that perhaps this isn't the appropriate place for that discussion.
That makes three of us.

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