Cheap low-power 12V DC-DC isolator?

[Scott Willing]

Hiya folks,

We just got a cordless phone with the ubiquitous wall-wart AC adaptor.
(9V) Obviously I'm not keen to keep my main inverter running 24/7 for
the sake of this thing. I have a 12V outlet in the perfect location
but due to the telephone line positive ground issue, I can't just use
a cheap linear step-down. (Creates a permanent off-hook situation.)

I need something like an isolated DC-DC 500mA converter. 12V in, with
an adjustable output,would be great, but 12V -> 12V would do.

There are wonderful little modular PC-mount converters available but
I'm not sending Digikey $100 to fix this little problem. I didn't
spend that much on the phone.

Would be a fun project to design/build, if I had time for fun. I need
a built unit or a kit.

Meanwhile I'm going 12V outlet->cheesy 125W inverter->AC adaptor.
Works and is even pretty efficient, but... yuck!

-=s

 

[George Ghio]

Last time I bought a power supply as a kit to run my phone it cost
$8.50. Looking at the catalouge right now, the price is $6.95 Aud. The
cat No. is KA1797. www.jaycarelectronics.com for the US and CA

George

 

[[email protected]]

Hiya folks,

We just got a cordless phone with the ubiquitous wall-wart AC adaptor.
(9V) Obviously I'm not keen to keep my main inverter running 24/7 for
the sake of this thing. I have a 12V outlet in the perfect location
but due to the telephone line positive ground issue, I can't just use
a cheap linear step-down. (Creates a permanent off-hook situation.)

I need something like an isolated DC-DC 500mA converter. 12V in, with
an adjustable output,would be great, but 12V -> 12V would do.

There are wonderful little modular PC-mount converters available but
I'm not sending Digikey $100 to fix this little problem. I didn't
spend that much on the phone.

Would be a fun project to design/build, if I had time for fun. I need
a built unit or a kit.

Meanwhile I'm going 12V outlet->cheesy 125W inverter->AC adaptor.
Works and is even pretty efficient, but... yuck!

-=s

Just make a cheap and dirty multivibrator circuit feeding an isolation
transformer (can be a simple "filament transformer" type unit with 2
low voltage secondaries) and rectify and regulate the output. Set to
600hz or higher filtering is relatively trivial.

 

[Vaughn]

Hiya folks,

We just got a cordless phone with the ubiquitous wall-wart AC adaptor.
(9V) Obviously I'm not keen to keep my main inverter running 24/7 for
the sake of this thing. I have a 12V outlet in the perfect location
but due to the telephone line positive ground issue, I can't just use
a cheap linear step-down. (Creates a permanent off-hook situation.)

Three thoughts:
1) Before you do anything else, try swapping the wires on your phone line to see
if the off-hook situation goes away. I think that there is a 10 to 20% chance
that this will cure your problem.

2) Assuming that you run your inverter for a few hours a day for other reasons;
use a 12 V. Gel cell and a charger as a combined DC/DC converter and UPS for
your cordless phone. I actually saw a commercial version of this in an office
supply store recently.

3) If you still want a DC/DC Converter, check out Marlin P. Jones
http://www.mpja.com/ . I don't see exactly what you want on their web site, but
it is worth a phone call. Some of their DC/DC converters are less than $10.00.

Vaughn

 

[Scott Willing]

Just make a cheap and dirty multivibrator circuit feeding an isolation
transformer (can be a simple "filament transformer" type unit with 2
low voltage secondaries) and rectify and regulate the output. Set to
600hz or higher filtering is relatively trivial.

Appreciate the thought, and I hear you bro, but that's why I said
"Would be a fun project to design/build, if I had time for fun." And
because I know how easy it should be, I'm not keen to spend a lot.

Truth is I probably even have all the parts I need on hand. But I also
had the parts on hand for a little motor speed control project for
over two years -- schematic ready to go -- and that didn't get built
either.

I ordered a kit to solve the speed control issue for $10. I was buying
something else on a website, and this wasn't going to add any shipping
cost so I went for it. Felt like a beginner hobbyist.

But when a had a brief opportunity, I put it together in 10 minutes. I
didn't have to think, plan, lay out. It was a little holiday for my
weary brain, instead of the mental torture I usually subject myself to
when I built from scratch.

****
BTW I see Backwoods Solar offers a solution to this very problem for
US$40. I love those guys, and I bought my first gear from them after
moving into the electrical nightmare of my home. But the shipping cost
sometimes kills me when importing small items.

***

-=s

 

[Scott Willing]

Three thoughts:
1) Before you do anything else, try swapping the wires on your phone line to see
if the off-hook situation goes away. I think that there is a 10 to 20% chance
that this will cure your problem.

That's an intriguing prospect. It's been a few decades since I took
analog telephony stuff in school (as related to modem circuitry) and
the memory is not good beyond "telco = positive-grounded system." I
dunno wether that will fly or not... but with a powered phone... hmm.

I'll certainly look into it. It would be a riot if that worked. I'd
owe you a case of brewskis fer shur.

2) Assuming that you run your inverter for a few hours a day for other reasons;
use a 12 V. Gel cell and a charger as a combined DC/DC converter and UPS for
your cordless phone. I actually saw a commercial version of this in an office
supply store recently.

I'm ahead of you there. For some reason I love little indendent
systems. I do have some gell cells and a few surplus 3W amorphous
panels... you see where I'm headed...

3) If you still want a DC/DC Converter, check out Marlin P. Jones
http://www.mpja.com/ . I don't see exactly what you want on their web site, but
it is worth a phone call. Some of their DC/DC converters are less than $10.00.

OK I will.

Sheesh, three clear concise possibilities in one post. You're a credit
to usenet sir. Thanks.

-=s

 

[Scott Willing]

Last time I bought a power supply as a kit to run my phone it cost
$8.50. Looking at the catalouge right now, the price is $6.95 Aud. The
cat No. is KA1797. www.jaycarelectronics.com for the US and CA

George

Thanks George. That's a non-isolated unit, and I'd bet the price of it
that it's another linear LM317-based circuit, with a pot to set the
output voltage.

But this is in the right direction. I've never run across these folks
before, so I'll check them out regardless.

Cheers,
-=s

 

[Hank McCall]

That's an intriguing prospect. It's been a few decades since I took
analog telephony stuff in school (as related to modem circuitry) and
the memory is not good beyond "telco = positive-grounded system." I
dunno wether that will fly or not... but with a powered phone... hmm.

I'll certainly look into it. It would be a riot if that worked. I'd
owe you a case of brewskis fer shur.




I'm ahead of you there. For some reason I love little indendent
systems. I do have some gell cells and a few surplus 3W amorphous
panels... you see where I'm headed...




OK I will.

Sheesh, three clear concise possibilities in one post. You're a credit
to usenet sir. Thanks.

-=s

Actually the fellow who said to use an astable multivibrator and a
filament transformer had a good cheap isolating device. The other way is
to make a simple flyback mode switched mode power supply. I think
National Semiconductor or T.I> have complete schematics on their web sites.

 

[wmbjk]

Hiya folks,

We just got a cordless phone with the ubiquitous wall-wart AC adaptor.
(9V) Obviously I'm not keen to keep my main inverter running 24/7 for
the sake of this thing. I have a 12V outlet in the perfect location
but due to the telephone line positive ground issue, I can't just use
a cheap linear step-down. (Creates a permanent off-hook situation.)

I need something like an isolated DC-DC 500mA converter. 12V in, with
an adjustable output,would be great, but 12V -> 12V would do.

There are wonderful little modular PC-mount converters available but
I'm not sending Digikey $100 to fix this little problem. I didn't
spend that much on the phone.

Would be a fun project to design/build, if I had time for fun. I need
a built unit or a kit.

Meanwhile I'm going 12V outlet->cheesy 125W inverter->AC adaptor.
Works and is even pretty efficient, but... yuck!

At the risk of being burned at the stake... ;-) I can see the
attraction of not having the extra drain on system capacity, but is it
really worth much trouble to save running the main inverter 24-7? At
say, 10 Watts idle and 100 hours extra run time, the total is 1 kWhr
per week, perhaps 50Watts PV. Which is a still sort of a lot if the
phone is the only appliance wanting full time power. But I expect
there are others, or will be. Perhaps it's time to bite the bullet and
run the inverter 24-7?

Wayne (the heretic)

 

[[email protected]]

Newsgroups: alt.energy.homepower
Hiya folks,
We just got a cordless phone with the ubiquitous wall-wart AC
adaptor. (9V) Obviously I'm not keen to keep my main inverter
running 24/7 for the sake of this thing. I have a 12V outlet in the
perfect location but due to the telephone line positive ground
issue, I can't just use a cheap linear step-down. (Creates a
permanent off-hook situation.)
I need something like an isolated DC-DC 500mA converter. 12V in,
with an adjustable output,would be great, but 12V -> 12V would do.
There are wonderful little modular PC-mount converters available but
I'm not sending Digikey $100 to fix this little problem. I didn't
spend that much on the phone.
Would be a fun project to design/build, if I had time for fun. I
need a built unit or a kit.
Meanwhile I'm going 12V outlet->cheesy 125W inverter->AC adaptor.
Works and is even pretty efficient, but... yuck!
-=s

Some phones have no problem with telco's positive ground. I use a
cheap GE (the one without caller ID and display). Last cheap Uniden
worked fine for a couple years, died. I take a guess at Walmart, try
it, exchange till I find one that works. Don't much want noise from
converter being radiated around my place.


Tom Willmon
near Mountainair, (mid) New Mexico, USA

Net-Tamer V 1.12.0 - Registered

 

[Scott Willing]

Three thoughts:
1) Before you do anything else, try swapping the wires on your phone line to see
if the off-hook situation goes away. I think that there is a 10 to 20% chance
that this will cure your problem.

Follow-up:

Sonava gun, it turns out the wires *were* reversed and have clearly
been since before we moved in. However correcting the reversal didn't
fix the line-holding issue; I still need isolation. Too bad, you'd
have been twice the hero if that little trick worked.

2) Assuming that you run your inverter for a few hours a day for other reasons;
use a 12 V. Gel cell and a charger as a combined DC/DC converter and UPS for
your cordless phone. I actually saw a commercial version of this in an office
supply store recently.

While out of town I picked up another 12V gel cell. I already have
some surplus 3W panels, so I'm going to put together a little
independent system to run the phone and a 12V radio that is often our
only other load when the office isn't running. The latter doesn't draw
enough juice to keep the inverter out of search mode even at its
highest sensitivity, so it normally runs on rechargeable NiCds.

I also picked up a very simple phone line tester when I was at it.
Just a bicolour LED molded into an RJ-11 plug with a little handle on
it. At the price, not even worth the trouble to make myself.

I just thought I'd toss it into the kit for troubleshooting elsewhere,
but (though I'm embarrassed to admit it) the dollar tester is what put
me onto the line reversal at our place. Could've found out with a
meter - and confirmed it with one - but since the wiring colours match
up correctly at the distribution point inside the house, I've always
assumed it was right.

I should know better.

-=s

 

[Vaughn]

I really appreciate "follow up" posts like this.

...correcting the reversal didn't
fix the line-holding issue; I still need isolation.

Oh well, I still think it was worth the try.

While out of town I picked up another 12V gel cell. I already have
some surplus 3W panels, so I'm going to put together a little
independent system

I think that is a good choice. Don't forget to cobble up some type of
regulator. I have ruined more than one battery with an unregulated trickle
charge.

Vaughn

 

[Scott Willing]

I think that is a good choice. Don't forget to cobble up some type of
regulator. I have ruined more than one battery with an unregulated trickle
charge.

No worries. I bought a little multifunction device that I ran across
after buying my 13W suitcase solar panels. It's such a nice
combination of features in a compact (cigarette package-sized) metal
chassis that I couldn't resist. Indeed I think I'll get a couple more.

You can see it here:

http://www.designnotes.com/Merchant...=PROD&Product_Code=SOL4UCN2&Category_Code=sop

I managed to find it for CDN$38, and then from another source for
CDN$28, so at nearly US$40 it's pretty seriously overpriced at this
site.

It combines a shunt PV regulator with a LVD, and has a couple of
outputs on 3.5mm phone jacks that aren't even mentioned in the
descriptions - 6V (7806 regulated) and a 3V output that looks to be
zener-regulated off the 7806 output. (I didn't bother to desolder the
component inside the spagetti to be sure.)

The main control circuit is clever, if cheap. The quality of
construction was a little iffy too - I had to resolder part of the
terminal block and reposition one of the LED's. Still, to wire such a
thing point to point and work up a nice little chassis for it would
take a lot more than $28 worth of my time.

I'll probably fudge the 7806 up to 9V with a couple of resistors and a
cap to provide my phone power supply, and use the 12V output to run
the radio. Badda bing.

Cheers,
-=s

 

[Sylvan Butler]

No worries. I bought a little multifunction device that I ran across
after buying my 13W suitcase solar panels. It's such a nice
combination of features in a compact (cigarette package-sized) metal
chassis that I couldn't resist. Indeed I think I'll get a couple more.

You can see it here:

http://www.designnotes.com/Merchant...=PROD&Product_Code=SOL4UCN2&Category_Code=sop

I managed to find it for CDN$38, and then from another source for
CDN$28, so at nearly US$40 it's pretty seriously overpriced at this
site.

Sure is.

Does look/sound like a clever gadget. Google 'sol4ucn2' turned up:

http://www.allelectronics.com/cgi-bin/category.cgi?item=SCN-2 (back
order until about 7/15), US$28

http://www.calcentron.com/Pages/velleman2/velleman_solar_cells.htm , $30

sdb