DC Adapter Voltage question

[markopolo]

Hello everyone, new to the forum. Rather busy today, so will just say

I'm not an Electrician, but a musician. So Hi.

What I thought would be a quick question is turning out a bit more

problematic. I have spent hours today on the phone trying to track down

an adapter - 9-volt DC 200mA Min.

It is for an overdrive pedal for guitar - pretty simple thing 'powered' by

a 12AX7 tube.

The company that makes them have gone bust. Here is a link to the PDF

that I found though:

http://www.genzbenz.com/img/manuals/tw/910_user_manual.pdf

I have a 9V adapter that fits the socket in the back, but it is rated at 0.5A.

It also say '4.5VA' on it, whatever that means. Apart from that it just

says '230V ~ 50Hz'.

An electrical guy suggested an adapter for me to buy that was 0.55A.

But then contradicted himself by saying that the jump from 200mA to

500mA was a 'big' one in electrical terms.

I have heard of people 'overpowering' these slightly to get a better sound.

But I have no idea if the jump from 200mA to 500mA would be a bit

much. Or even if that is what they are talking about.

I'm confused. So would this adapter I have be Ok to try? Or could someone

advise me the best place I might be able to buy one of these.

Thanks a lot for your time.

cheers.

 

[Doc Hudson]

Welcome to the forum markopolo. You electrical appliance will only take as much power (current x voltage) that it needs. If your transformer can give more power, (supply a larger current), it will not be a problem. The important thing is that the voltage's match. Your description says you need a 9vDC 200ma (Min), that means your new transformer must be at least 200ma. If it is less than 200ma output it will not provide enough power for the device to work correctly. A 500ma transformer output will comfortably supply 200ma to your device.

Doc H.

 

[Doc Hudson]

I have a 9V adapter that fits the socket in the back, but it is rated at 0.5A.

It also say '4.5VA' on it, whatever that means. Apart from that it just

says '230V ~ 50Hz'.

4.5VA is another way of saying how much power the transformer can supply.

this is Volt x Current, 9v x 0.5A = 4.5VA or approx 4.5watts.

Doc H.

 

[Apache]

Any electrical shop will have them, B&Q, Curries, even bigger supermarkets.

http://www.maplin.co.uk/Module.aspx?ModuleNo=35927

Loads on ebay etc etc

 

[kme]

As doc hudson succinctly says above: if you`ve got a 12V 0.5A supply (500mA) ; it will happily run any 9v appliance, that uses 500mA or less.

Think of a standard 13A socket in your house. You can connect a little nightlight to it, that only uses a few milliamps; even though the socket can deliver 13A.......

Does that help, mate?

(n.b. I`m trying to work out how they can drive a vacuum tube (valve) with 9V ?)

They need an anode voltage of a few KV to operate - but that isn`t important to your question; that`s just me thinking.............

KME

 

[Apache]

(n.b. I`m trying to work out how they can drive a vacuum tube (valve) with 9V ?)

They need an anode voltage of a few KV to operate - but that isn`t important to your question; that`s just me thinking.............

KME

A very big transformer?

 

[kme]

Transformers don`t work on DC mate; they need the changing AC waveform to generate the magnetic field for inductance. You can get DC-DC converters, as solid state modules; but they don`t generally produce sustained KV outputs.

 

[ADS]

Hi,

I might be totally on the wrong track here, lads, but isn't it a case of power out etc etc.

If I remember right, the output power is going to be constant so if:

9V @ 0.5A gives 4.5 VA (this being the power output of the transformer)

If you only draw 0.2A, would the voltage across the load not increase -

i.e. 4.5 VA divided by the current drawn....0.2A which equals 22.5 V - which probably wouldn't be good for his pedal.

A bit like the opposite of volt drop.

I'm sure someone will point out where I'm going wrong in my thinking.

 

[kme]

No mate; the voltage will only rise a small amount; and only if the power supply is "unregulated", which isn`t many of them, any more.

The transformer & rectifier bridge are designed for a max current draw, but the regulated units use a zener diode to hold the output constant.

If they actually worked in the manner you`re thinking, and you had a 105VA transformer on 2 x 50W dicroic lamps, and 1 lamp blew, there`d only be 50W of load, so the voltage would effectively double; thereby killing the other bulb!

Its a "maximum deliverable"; not a continuous output of that magnitude.......

KME

 

[ADS]

I knew it would be the electronics side of things that I wasn't getting

 

[SPECIAL LOCATION]

Hi,I might be totally on the wrong track here, lads, but isn't it a case of power out etc etc.

If I remember right, the output power is going to be constant so if:

9V @ 0.5A gives 4.5 VA (this being the power output of the transformer)

If you only draw 0.2A, would the voltage across the load not increase -

i.e. 4.5 VA divided by the current drawn....0.2A which equals 22.5 V - which probably wouldn't be good for his pedal.

A bit like the opposite of volt drop.

I'm sure someone will point out where I'm going wrong in my thinking.

Arhhhhhh!!!

Totally wrong! :_| :_|

I do hope the laws of Physics haven't changed!

Power is the amount of work done to achieve a task or the amount of electrickery consumed in a load...

It is Not how much stuff you are providing to put into a load..

It is how much power the load is consuming...

a-la a shower may consume 8.5KW @ 240v but only 8.1KW @ 230v

A certain voltage applied across a load will draw a current equal to V/R...

It is this Current x Voltage that gives us the power

or the Voltage squared / Resistance.

e.g. 4ohms load, 12v across it, draws 3amps

Power = 12 x 3 = 36watt

or

(12x12) / 4 = 36watt.

The voltage and the current either side of the transformer will always be proportional to each other..

Think of a low voltage light..

50watt 12v lamp fed via a tranny.. (this will be drawing approx 4.16amps)

on the mains side of the tranny V will be 230v I will be approx 0.217 amps

0.217i x 230v = 50w

4.16i x 12v = 50w

AND

4.16/0.217 = a ratio of 1:19.17

203/12 = a ratio of 1:19.17

Voltage AND Current are inversely proportional to each other either side of the transformer..

the Power is the same....

and the power is governed by how much juice the load is taking.

You can stick a 20w or 35w 12v bulb off your 50w transformer all ok...

But a 20watt transformer will not be able to pass enough current for a 50w lamp to work!

Out friend at the top will be ok with his 500ma transformer...

all that will happen is the transformer won't need to work so hard and probably wont get quite as warm as a 200ma transformer IMHO!

 

[SPECIAL LOCATION]

Ole slow paws been pushy shoveyed by KME again!

:coat

 

[ADS]

Okay, I'll take on board what you are saying, but answer this, then -

Why are there about 15000 different 'power adapters' (for want of a better word) with ratings of - voltages ranging from 3V, 5V, 6V, 9V, 12V, along with current ratings of 50mA, 100mA, 200mA....etc etc, and about 50 different sized, round shiny-chrome plugs to plug into your device.

If what you are saying is true, you would only need maybe 3 of each voltage, with current ranges of say 0-500mA, 0.5A to 1A and 1A-2A

I have about 25 of these things at home - from numerous phones, game consoles etc over the years, and I've found that only the right one will make a particular device work - it's like they have to be pretty accurately matched???

 

[SPECIAL LOCATION]

Okay, I'll take on board what you are saying, but answer this, then -Why are there about 15000 different 'power adapters' (for want of a better word) with ratings of - voltages ranging from 3V, 5V, 6V, 9V, 12V, along with current ratings of 50mA, 100mA, 200mA....etc etc, and about 50 different sized, round shiny-chrome plugs to plug into your device.

If what you are saying is true, you would only need maybe 3 of each voltage, with current ranges of say 0-500mA, 0.5A to 1A and 1A-2A

I have about 25 of these things at home - from numerous phones, game consoles etc over the years, and I've found that only the right one will make a particular device work - it's like they have to be pretty accurately matched???

Cost of manufacture..

Size of adapter..

Heat dissipation..

etc..

etc..

One biggun with a voltage selector range would do for many applications..

but that just ramps manufacturing costs up!

as with all things it comes down to money...

Just consider the size of conductors needed for carrying 200ma -vs- 2A

much finer smaller conductors etc.. etc..

Don't forget also some are for DC applications others for AC applications as well!

 

[steptoe]

Cost of manufacture..Size of adapter..

Heat dissipation..

etc..

etc..

One biggun with a voltage selector range would do for many applications..

but that just ramps manufacturing costs up!

as with all things it comes down to money...

Just consider the size of conductors needed for carrying 200ma -vs- 2A

much finer smaller conductors etc.. etc..

Don't forget also some are for DC applications others for AC applications as well!

+ polarity on different sides of the pin/jack/whatever you wanna call it

 

[SPECIAL LOCATION]

+ polarity on different sides of the pin/jack/whatever you wanna call it

:Applaud

I was just thinking how long till someone spots the other bit I missed off!!!!

:innocent :BlushingO)

:coat

 

[Apache]

Transformers don`t work on DC mate; they need the changing AC waveform to generate the magnetic field for inductance. You can get DC-DC converters, as solid state modules; but they don`t generally produce sustained KV outputs.

yes of course :Blushing

 

[kme]

Technically, you could have ONE multi-voltage supply; rated at something daft like 60VA; with a switchable bridge rectifier, and polarity switch (we used to have oen that went from 3V to 15V in 1.5V steps, DC only, stabilised, 10A output - it was called a "Brazenose" IIRC)

Or.....get a bench power supply; you can dial in a constant voltage; or constant current output, select DC or AC, and bingo. All you need are the different plugs.........

There`s where the manufacturers make their extra dosh - their power pack doesn`t fit owt else, and nobody else`s fits their stuff...

That said, more & more usb-style plugs are being used, with the added bonus that they`re all the same pinouts, polarity and voltage - the way forward, we can hope, to a "standardized connector"?

KME

 

[steptoe]

unless you are Mr Dell

 

[Doc Hudson]

That said, more & more usb-style plugs are being used, with the added bonus that they`re all the same pinouts, polarity and voltage - the way forward, we can hope, to a "standardized connector"?KME

Very true KME. Don't all new mobile phones now have to have a standard power adapter, I am sure I heard something about this several months ago?

Doc H.