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# Current vs. Voltage

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#### nyeboy2000

##### New Member
Assuming I want to run a device at 54 watts, and I could run it at 9 volts, 6 amps or 6 volts, 9 amps, is there any possible advantage to running it at 6 volts 9 amps? In other words, is it always the case that I should run with lower amps and higher volts if I have the choice? If so, why would anyone want to run a device with such high current and low voltage?

well, if you have long calbles, it is better to use a low current beacuse you will loose less power on the lines.
also, a power suply that will provide a highter current will have a lower efficency that a suply that will give a highter voltage, both of them giving the same output power.

So is there ever a case where higher current, lower voltage is better?

In electric arc welding machines, low voltage but very high current is used to avoid shock hazard.

Is this true in general for products that are going to be handled by humans? Meaning, is higher voltage more dangerous that higher current per se?

well of you have a high voltage is easyer to gete a shock.
consider that you only need a few 10' of mA to get a strong enough shock to die.......
you can get a nasty shock from a 9V batt in some cases.

Maybe my bad english is the problem, not understand correctly.
If You have a load with 54W and designed for 9V (e,g. a bulb) it suck 6A. When connected it to 6V, cant suck 9A,only about 5A.
If You thinking about switching mode supply with 9V output and 54W, it can deliver 9V 6A to output, and the input current depend from input voltage (if 6V,about 9A) ,because the power (45W) is constant.(no thinking about losses.)

Since I said that current was the problem, I would like to see more opinions on this. For my logic I am thinking about a 12 volt battery (pure DC) Duracell #MN21 (it's about the diameter of an AAA cell and about 1" long. It is used in a garage door opener remote control. Now, it delivers 12 volts but the current is a few miliamperes. For sure, you will not feel anything if you touch the terminals with your fingers.

At the other end of the logic is a 12 volt Die Hard battery with 500 Cold Cranking Amps. If you drop a screwdriver on the battery terminals (don't do this on purpose) you will get sparks and a mangled screwdriver. The current flow could litterally melt solid steel - you sure would not want to touch the terminals.

Both batteries are 12 volts but there is a great difference in current available. The battery with little current will not hurt you - I don't even want to think about the current in the car battery.

12V is 12V also no problem when touching the 120Ah car battery.
The standard safety voltage is 65V, in this case cant flow dangerous current via human body. (but over 24...30V can sensing with wet hands,and this depend also from frequency.)

I agree with the example given with the car battery and garage opener battery. That is why I was a little bit confused about the earlier post about arc welder's being low voltage high current to avoid shock. This still leaves the question: With safety a priority, would it be wiser to go with higher current or higher voltrage?

In your example, I don't think it makes much difference. The oddity about this thread is that ususally a device has a voltage limitation - rather than a choice. Normally, we design devices around a specific supply voltage and do the math for the circuit to compensate for the difference between supply voltage and each part. The way you have described your item, there is not even a lamp that is designed to operate at a specific voltage. I guess the manufacturer dosen't even have a recommendation. If it were me, I would opt for the lower voltage, understanding that the current difference between the two options makes little difference, and just keep a keen level of respect for the circuit as I developed it.

about the voltage you can get shocks:
you can get really electrocuted at 12V!!!!!
i'll tell you how...the skin is quite a bad conductor comepred to what's below it.
once i cut myself and blood ws coming out. i wasnt careful and i was tuching with one hand the ground of a circuit and(this seems quite odd) with the finger from the other hand wich i have cut i touched the +V wich was around 12-13V and really i got a nasty shock!!!
thats because the blood come in direct contact with the electric current and i had a lower resistance so more current was flowing.

And remember 10uA current through human heart is enough to kill him.

yes that's right. and if i remember a 50mA trough any part of the body is deadly

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