Semi-OT: mix and match plug packs - what is allowable in terms ofVolts and Amps?

[spodosaurus]

How does one determine when a left over powerpack from one dead device
can be used on another device whose plug pack has returned to the wild
to join the feral plug packs? What are the rules with volts, amps, and
watts to prevent ‘bad things’ happening? For instance, I have to ADSL
modems. One is dead, but with a 10V,1A power pack. The other is alive,
but needs a 7.5V,1A plug pack. A friend of mine wants to relieve me of
my modem and plug pack, and I want to know if ‘bad things’ are going to
happen with this compination? (such as fire, release of magic smoke from
the working modem, etc)

I ask here because I know we have a handful of resident experts in this
area

Ari

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[JAD]

that would be ok as long as the POLARITY is the same....he 'may' find that it wont
function as expected...it should be fine

 

[Bill]

How does one determine when a left over powerpack from one dead device
can be used on another device whose plug pack has returned to the wild
to join the feral plug packs? What are the rules with volts, amps, and
watts to prevent ?bad things? happening? For instance, I have to ADSL
modems. One is dead, but with a 10V,1A power pack. The other is alive,
but needs a 7.5V,1A plug pack. A friend of mine wants to relieve me of
my modem and plug pack, and I want to know if ?bad things? are going to
happen with this compination? (such as fire, release of magic smoke from
the working modem, etc)

I ask here because I know we have a handful of resident experts in this
area

Ari

Sounds like a good way to make a smoke offering to tron gods to me.
What happens to your processor if you overvolt it by 33%?

Your power pack puts out a fixed voltage < 10V > with a maximum amp
output < up to 1 amp > example for your one power pack. That means
what ever you attatch should handle 10 volts and not draw more than 1
amp.

Putting your two good components together is overvolting the modem by
2.5 volts. Not good.

Bill

 

[spodosaurus]

In simple terms you wanna connect a 10V 1A power pack to a modem that
requires 7.5V at 1 amp.

It's 'risky' to say the least although it may well work* but certainly
not advisable*, what happens if the thing catches fire... you'll be for
the 'high jump' simply because the power pack wasn't made for that
particular modem.

It would be quite ok to use a 7.5V 3 amp power pack on the modem...
because it's the modem what determines what power is drawn and would run
cooler than the 1 amp version at full load.

The power in Watts is derived from the Voltage x the Current consumed,
so the modem consumes a maximum of 7.5 Watts, with the 10V power unit
this would be 10 Watts... flee power of a difference I know but what if
a fault develops ?? Keep on the safe side I say.

davy

Thanks guys, that's what I suspected. :/

Ari

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Many people around the world are waiting for a marrow transplant. Please
volunteer to be a marrow donor and literally save someone's life:
http://www.abmdr.org.au/
http://www.marrow.org/

 

[Paul]

Thanks guys, that's what I suspected. :/

Ari

I can give a practical example that happened to me.

I bought a label maker (LCD display, keyboard, motor etc), got
it home and discovered it runs on batteries. Well, I hate that
kind of shit. I checked and there was a barrel connector for
an adapter.

I went back to the store, and they wanted $39.95 for an
adapter. As I'm a cheapstake, I decided I could do better
than that.

One of the requirements for barrel connectors, is that they
at least be labeled for "center pin positive" or "center pin
negative". So I had that to go on. The adapter itself was
7VDC at 1.2A at the store. That is all it said.

I bought a 7VDC adapter with more than 1.2A rating. The
adapter I bought was *regulated*, which means the output
will never change from 7V. It will always be exactly that
voltage, from zero amps, to at least 2 amps load.

I plugged in the adapter. The label maker LCD display lit up,
but as soon as the motor engaged, the thing shut down.

I ended up buying that $39.95 adapter. It turned out, the
proper adapter was *unregulated* DC. With my multimeter,
I got 9.4V when zero amps are being drawn. The adapter
output voltage would drop, as more current is drawn. I
suspect if I were to draw exactly 1.2 amps from the
adapter, I'd have seen 7V as the result. (The label on
the adapter says 7V 1.2A, and yet under no load, I got
9.4V.)

I don't think they are required to list whether the
adapter output is regulated or unregulated. They may
distinguish AC versus DC (as some adapters may choose
to output AC, such as a transformer based one). If a
bridge rectifier and filter cap are added to the
transformer, that makes it an unregulated DC adapter
(with voltage falling as more current is drawn). The
third kind (the kind I tried to run the label maker
with) was a switching adapter. A switching adapter
converts AC to regulated DC, just as an ATX supply
would. But since the voltage didn't "pop up" to over
9 volts, that seemed to cause an issue with the label
maker.

So I was as careful as I could be, to match what I
thought the requirements were, but it still didn't work.
Since the polarity and type (DC versus AC) was right,
there was no damage.

If you had a device with a 5V input spec, it is possible
the device has no internal regulation at all. In that case,
the input is likely expected to be tightly regulated.
Not too much deviation would be tolerated in that case.
Even the friendly "unregulated" adapter, could blow
such a design into the weeds, due to the "voltage popup"
under no load. (Different logic families have different
tolerances to excess voltage. For example, there used to be
a CMOS logic that could take 5V to 15V, and for that,
virtually any DC adapter in that range, would work.
But that is decades old technology. Newer stuff is less
tolerant.)

With some internal dissection and investigation, maybe you'd
have better luck matching them up. I got screwed out of
$40 on my attempt, just to be taught a lesson.

Paul

 

[spodosaurus]

Further to Paul's comment... you only need to get the +/- polarity wrong
and thats enough.

I checked that first It's printed on both the modem and on the 10V/1A
power plug I tested it with (and posted here with).

I've found a 1A 7.5V (multi volt power plug) at Altronics for $25AUD
that'll do what I need (and has polarity set by how you connect the plug
to the plug pack's cord).

Ari


--
spammage trappage: remove the underscores to reply
Many people around the world are waiting for a marrow transplant. Please
volunteer to be a marrow donor and literally save someone's life:
http://www.abmdr.org.au/
http://www.marrow.org/