DC sub for AC voltage

Mooly

Administrator
Paid Member
2007-09-15 8:14 am
More information needed.

AC and DC are totally different. 8 volts AC will give around 11 to 12 volts DC when rectified internally in the phone, and any electronics ultimately uses DC to power it.

As to whether DC can be used from an adapter/PSU instead of AC depends on the internal circuitry but 7.5 volts probably won't be enough... and there are complex reasons why it could do long term damage if it did happen to work (such as an SMPS drawing more current as voltage falls).

I would advise you get the correct rated voltage charger and one of the correct type which you say is AC.
 

Rundmaus

Member
2005-08-21 10:46 pm
... always the same story in this forum, I am getting tired of it:


**********************************************************************

[Inexperienced Poster]: I want to do thing A. Is it ok or will I get problems?

[Electronics Guy 1]: No it is not ok, you will get problems.

[Electronics Guy 2]: You will get problems, better do it that way: thing B

[Inexperienced Poster] I did thing A once and didn't have problems. It will work!

**********************************************************************


Why asking in a forum in the first place, when you already decided for yourself?

Rundmaus
 

FoMoCo

Member
2012-12-04 10:04 pm
Someone told me most of these phones have a voltage regulator on the input .
That's probably correct. But as has already been mentioned, 7.5V probably won't power that regulator.

will the internal power supply bost the DC voltage if it is just made up of diode and Caps?
Absolutely not.

... always the same story in this forum, I am getting tired of it:
I agree. My recent experience was more like:
[Inexperienced Poster]: I want to do thing A. Is it ok or will I get problems?

[Guy who thinks he's an Electronics Guy 1]: Yeah. Go for it dude.

[Electronics Guy 2]: Nope and here's why...

[Guy who thinks he's an Electronics Guy 1]: Try OHMS LAW

[Guy who thinks he's an Electronics Guy 1]: Some people are idiots.
 

Rundmaus

Member
2005-08-21 10:46 pm
ian33,

what usually happens is:

1. The AC of the original power supply gets rectified and filtered. With the usual capacitor input filter, the raw DC voltage is approx. 1.4*V_AC - 2*V_diode. With V_diode being the usual voltage drop of a silicon diode, approx. 0.7 volts.

2. A voltage regulator sets the DC voltage to the value needed for the circuit. For the regulator to work properly, some 2-3 volts overhead are needed, otherwise regulation will not work.

Now, a short estimate of your circuit:

8VAC are needed in the original setup. This will give approx. 9.8VDC. A regulator probably regulates this to 5VDC or 6VDC, I don't know without knowing the schematic.

Now, if you feed in 7.5VDC, approx. 6.1VDC will remain after the diodes. The regulator will not work properly because it drops 2-3V in normal operation.

Rundmaus

PS. Also, reliability issues may occur. To save on costs, the rectifier diodes will usually not be over-dimensioned. With AC feed, only two diodes out of four in a full-wave bridge conduct at each instant. In the next half-cycle of the mains, the other two diodes are on.

So every diode has a duty cycle of 50%. If you feed DC, only two diodes will conduct permanently. If they are underrated, they will probably fail.