9v adaptor - does the mA figure matter?

[deeks]

Hi
I have a baby monitor where the 9v adaptor has just given up the ghost. On the adaptor it also says 300mA, but I don't know what that means. I have another 9v adaptor, but it says 600mA on it. Can I use this adaptor on the monitor, or would this be dangerous?

 

[Al-Glasgow]

Im thinking, the new adaptor will be using 50% power.

Just check the polarity of the final connector is same.

A

ps. Im NOT an electrician.l

 

[breezer]

mA is the amount of current the device can supply

your monitor requires 300mA at most, so a supply of upto 600mA will be fine.
PROVIDED:

1)the new power supply is stableised (it will say so)

2) the plug is the same polarity*

some will allow you to "turn the plug round" so what was +ve is now -ve

(make sure the drawing on each is the same for polarity)

I can't easliy do drawings but it looks like an almost circle with a line coming from the gap, make sure they are both the same, usualy the almost circle is -ve

 

[deeks]

Excuse my ignorance Al, how do I test the polarity? I've tried plugging it in for a few seconds, and it appears to work with the new adaptor
++++++++++++++++++++++++++++++++

thanks Breezer. Looks like the polarity is the same, but the new adaptor doesn't seem to state stabilised anywhere
_________________________________
Lynda, moderator

please use the edit button

 

[Taylortwocities]

Excuse my ignorance Al, how do I test the polarity? I've tried plugging it in for a few seconds, and it appears to work with the new adaptor

You're lucky it was the correct polarity. Even a fraction of a second could have kaynackered the monitor if it was the wrong way round. If it works, cool!

TTC

 

[davelx]

It's pretty unlikely these days that connecting with the wrong polarity will cause any damage, as any half competent designer is going to put a blocking diode in line. Some even put a bridge in, so the polarity is irrelevant.

 

[Softus]

It's pretty unlikely these days that connecting with the wrong polarity will cause any damage, as any half competent designer is going to put a blocking diode in line.

What does a non-blocking diode look like, davelx?

 

[sm1thson]

It's pretty unlikely these days that connecting with the wrong polarity will cause any damage, as any half competent designer is going to put a blocking diode in line. Some even put a bridge in, so the polarity is irrelevant.

my friends laptop battery cable broke so he had soldered on a universal fitting. One day his girlfriend had been using his laptop then came over to sit with him he looked over at his laptop and say smoke poring out, -she had pulled the cable and the universal end came off and she plugged it on the wrong way round the laptop continued to run on the battery but its PSU circuitry (on the motherboard) was fried. I fixed it for him, but it could have been an expensive lesson on getting the polarity right!

 

[davelx]

It's pretty unlikely these days that connecting with the wrong polarity will cause any damage, as any half competent designer is going to put a blocking diode in line.

What does a non-blocking diode look like, davelx?

ha-ha - it looks the same but is used in a different part of the circuit for a different purpose. Blocking diode is a valid term, just like coupling capacitor, bias resistor, etc etc. are valid terms.

 

[davelx]

It's pretty unlikely these days that connecting with the wrong polarity will cause any damage, as any half competent designer is going to put a blocking diode in line. Some even put a bridge in, so the polarity is irrelevant.

my friends laptop battery cable broke so he had soldered on a universal fitting. One day his girlfriend had been using his laptop then came over to sit with him he looked over at his laptop and say smoke poring out, -she had pulled the cable and the universal end came off and she plugged it on the wrong way round the laptop continued to run on the battery but its PSU circuitry (on the motherboard) was fried. I fixed it for him, but it could have been an expensive lesson on getting the polarity right!

What make was it - just so I know not to buy one, as they are clearly skimping.

 

[MattChan]

Blocking diode is a valid term,

Normally call them Idiot Diodes though

 

[sm1thson]

What make was it - just so I know not to buy one, as they are clearly skimping.

it was an Aldi special el cheapo, I cant remember the exact make. I don’t agree that they were skimping, a diode has a voltage drop across it so that has to be considered in the design, (so no good when you have a 12V PSU and 12V parts etc) and the current going through the diode combined with its voltage drop turns it into a heater, on a laptop the designer doesn’t expect the user to supply it with any old PSU.

I have put a diode backwards across the supply rails before to ensure the fuse would blow if connected the wrong way round without having the sacrifice of voltage drop under normal operating conditions.