VTECH cordless phone standby power

Discussion in 'Homework Help' started by DonJuane, Dec 17, 2013.

  1. DonJuane

    Thread Starter New Member

    Dec 17, 2013
    [[Who created the conventional wisdom that posts must be closed after a specific amount of time? People use these as a reference book and when all are shut out and can't return to correct or answer an old question, the batch of miss-or-inaccurate info must remain indefinitely for all the world to struggle through. So, is an answer to a mostly timeless question left unanswered after a specific amount of time, no longer of value? Since the answer here is an implied "yes", now I beg to differ.]]

    Ref: https://forum.allaboutcircuits.com/showthread.php?p=327650#post327650

    Old post but apparently a question that can't be answered ....

    Simple enough, provide a DC source for a cordless phone power supply but after spending several hours on this project this past week, I don't think it can be done.

    I started with ordering a VTech cordless phone but my research somehow missed the fact that the unit ran on a 6VAC power supply (instead of the DC I would provide from the cheap regulator I also ordered off eBay). The 12.5V from my 12VDC lead acid battery could easily be stepped down to power the phone via a step down regulator to supply the needed voltage to the cordless phone base. WRONG!

    First of all, finding the voltage specs online for the VTech phone was impossible. I finally found the specs to a "replacement" power supply on eBay claiming to power it, rated at 6VDC so I thought I was set. WRONG AGAIN!. When it arrived I plainly saw 6VAC - not DC. But before I gave up, I decided to try supplying the 6VDC using a test of both polarities into the phone base station. The phone appeared to be charging and all the lights and LCD gave the proper indications, but the dial tone would never appear. After multiple resets and polarity reversals, the results were the same.

    I therefore abandoned that project and went looking through my workshop to find an old GE Portable phone that claimed 7.5DC input on its base station. Alas, I thought my problem was solved and ordered 2 replacement batteries for the set of base and remote handset. With my meter I first checked the transformer output and it was reading 12VDC ! Switching to AC on my ancient analogue meter, I saw 12VAC as well. Well "way out of spec" I thought but I also new the circuit must be super flexible in handling a higher voltage. But not so!

    I inched up the voltage out of my variable regulator to 7.5 VDC and observing polarity, applied it to the GE cordless phone. Same symptoms as with the VTech, the phone charged, lit up but never gave me a dial tone. (Note that with all phones, I am switching back and forth with the AC charger/adapter just to prove that all my other connections are working and that something else is not the problem).

    So I decided since the AC adapter for the GE was putting out 12VDC (pulsing DC which would show that there was 12AC or DC on the output) that maybe my regulator has spurious RFI or something and since the phone had no problem with the 12DC from the factory wall adapter, I'd simply apply the 12V directly from the battery. Same scenario, everything appeared to work, just no dial tone.

    So knowing there was likely a very complicated circuit on a small board inside the small base of these phones, I skipped tearing them all down to test and try to determine what odd issue it is that these phones require a voltage source that is pulsing, and in the VTech case that would be the 6VAC and in the GE case, the 7.5VDC which in reality was 12VDC pulsing (non filtered).

    So at this point I did what all wise men do and gave up :). I'll use the corded phone when the power goes out.
  2. MikeML

    AAC Fanatic!

    Oct 2, 2009
    So why not use DC chopped to a 60Hz or 120Hz square wave?... It means using a 555 oscillator, and maybe a PFET between the external DC source and the device...