laptop power cord as power source

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laptop power cord as power source

Postby fireflyva » 13 Oct 2014, 19:52

Hey guys - I'm new at this and don't want to electrocute myself so I thought I'd better ask.

I have an old laptop power cord that is rated at 19.5 v DC @ 3.16 A that I have used to provide power to my breadboard. Since most of the projects require 9 volts, I placed a guitar pot in between the power cord and the breadboard, and using my multimeter I dialed the pot down to get 9 volts at my breadboard. I've read on this forum somewhere that too much power is generally not a problem because the circuit will only "pull" what it needs, and that power won't be "forced through" the circuit causing anything to burn up.

Question: I'm not likely to cause any harm providing power this way, am I? I get tired of buying 9 volt batteries.

Thanks for your help.
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Re: laptop power cord as power source

Postby tabbycat » 13 Oct 2014, 20:21

fireflyva wrote:Hey guys - I'm new at this and don't want to electrocute myself so I thought I'd better ask.

I have an old laptop power cord that is rated at 19.5 v DC @ 3.16 A that I have used to provide power to my breadboard. Since most of the projects require 9 volts, I placed a guitar pot in between the power cord and the breadboard, and using my multimeter I dialed the pot down to get 9 volts at my breadboard. I've read on this forum somewhere that too much power is generally not a problem because the circuit will only "pull" what it needs, and that power won't be "forced through" the circuit causing anything to burn up.

Question: I'm not likely to cause any harm providing power this way, am I? I get tired of buying 9 volt batteries.

Thanks for your help.

i'm not an electronics graduate but i'd say for the sake of £5 don't risk frying yourself or your circuits and buy something like this:
http://www.ebay.co.uk/itm/AC-DC-600mA-Universal-Mains-Plug-Power-Supply-Charger-Adapter-Adaptor-3v-12v-/351180450991?pt=UK_ConsumerElectronics_PowerAdaptors_SM&hash=item51c3fc6caf
you will get all the voltages you'll ever need 3V, 4.5V, 5V, 6V, 7.5V, 9V, 12V (these things are pretty accurate) with the option of reversible polarity (not sure if this one has but mine is similar and does, worth checking), you can also use it with a daisy chain for your effects, and it will work for anything else around the house that ever needs any kind of mains power supply.

don't take risks with your health for a few pounds.

tabbycat.
"be a good animal, true to your instincts" (d.h. lawrence).
"there is no excellent beauty that hath not some strangeness in the proportion" (francis bacon).
"ni dieu ni maître" (anarchist slogan).

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Re: laptop power cord as power source

Postby Nocentelli » 13 Oct 2014, 20:44

The main risk with using a "voltage sag" pot and a higher supply voltage is that you'll fry the pot if you dial too close to the end of the pot's travel (providing you're not opening the chassis of the adaptor - that is asking for death). 18-19v may only be double the voltage, but 9v is too low to be conducted by dry skin, 19v and you may feel a tingle.

Plenty of people use a standard 18v power supply and have options for 12v or 9v: You could use a series resistor of 1k (a higher power rating of 1/2W or more) leading to the pot would prevent a large voltage being presented with a very low resistance at the end of the pot's travel towards zero ohms which would allow a large current to flow if the circuit draws a lot of current and causes a lot of power to be transferred to heat in the pot's carbon track - You do the maths (V=IR, P=IV, I think). A 9v regutator and large cap would make it less likely to fry components by mistake)

A decent 9v adaptor is maybe a better idea, but beware super cheap unregulated 9v jobs as they may deliver significantly more voltage than the switch says, and are also more likely to inject noise into the breadboard than a decent regulated switching adfaptor designed for pedals, e.g. Onespot and the clones on the market.

There are two really good reasons to use a 9v PP3 battery DC supply instead of a mains AC->DC power supply (especially a cheap one) - If you have a short on the breadboard, a battery will get hot quick and eventually fully drain if you don't realise in time: A mains adaptor may well be killed by such a mistake. The second is that batteries cannot inject noise from the mains supply into your circuit.

I have two very basic breadboard/breakout looper arrangements to play with - one has a battery and one has a onespot style 9v adaptor: The onespot and similar designs have some sort of cut-out protection - If there is a short or near short between +9v and ground, the power shuts off momentarily and tries again a few seconds later: If the short is still present, it shuts off again, and it cycles around doing this until the fault is resolved or the plug disconnected. I've had a couple of circuits that have worked for a few seconds, then shut off until I disconnected and re-connect something, then they would fire up again and shut off.
mmolteratx wrote:absolutely zero commercial use allowed. If I find anyone selling these, I'll fly to your house and kick you in the nads. And you may or may not find yourself in trouble.

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