# determining voltage drop on a componant

If you don't know the voltage drop across a component - with component in the case being a single electronic piece or a "black box" device. All you have are the two wires. I get putting voltage onto the circuit then measure the voltage across the leads to get the voltage drop. But what if you don't know what the safe voltage threshold is on the device? Do you start out with using a 1.5v source, and slowly increase the voltage until you see a drop across the component?

I have a tipper rain gauge circuit that will close the circuit based on the magnet/hall effect transistor.

What I don't know, is whether it's a 3.3v - 5v or 9v circuit , so before I try to figure out it's current draw - I need to run current through it, and don't want to fry it trying to determine the voltage drop.

This is more of a general question, if I come across this for something else where I don't know anything about a device.

What is the procedure to determine voltage drop - and current used - both require power up the device/component?

I currently do not have a variable power supply (which is why I'm looking at stepping up in 1.5v increments aka "AA"'s) - but it seems I might need one in the future.

Thanks

If there are only two wires, it probably functions as a switch. That means you should NOT put a "voltage" (like a battery or power supply) directly across it without a resistor (or something) to limit the current. Take a look at the [u]Button Example[/u] which uses a pushbutton switch and a pull-up resistor.

When the switch is off, all of the voltage is dropped across the switch. When the switch turns-on, all of the voltage is dropped across the pull-up resistor and there is no voltage across the switch.

There's no way of knowing the "breaking point" (or the rated voltage & current) without the specs.

12V with a 10K resistor is probably a good guess. But since I assume you are using an Arduino, 5V is probably OK too.

You are about a voltage drop without specifying what voltage you are suoplying to the device. If you are asking how to know what voltage to supply , the answer is that's what datasheets are for. Without some reference documentation there is no wsy to know. If you want to start with a very low voltage and gradually increase it that's your call. We don't know the inputs outputs or any information so we can't answer your question .

That's my question - when you don't have specs on the device or component, and need to determine what limiting resistor value to use. In my case, this particular device will be used with a pi, but it could have been an arduino (yes, I know these would most likely use two different resistors).

I'd hate to have the notion that just because you don't have specs on something, you can't use it...

So how does one go about determining how much voltage is required, without frying it. Which was why I thought about starting low, and increase until I get some form of output when it's supposed to output. (in this case, when the magnet activates the circuit).

This was more of a general question - not so much specific to one device or component.

Thanks

In general terms there is no reliable way other than what is either written on the device or determined from manufacturer's literature. You try a "nominal" voltage and if the device works you win. Alternatively, you could connect a variable power supply and monitor the current. As you increase the voltage you will reach a point where the current increases dramatically and the magic smoke leaks out. At that point you know you have exceeded the maximum working voltage - you lose.

Hopefully, it will work prior to the magic smoke.

So, lets say it doesn't start to work until 2V is applied (from 1V). Would I consider that to be it's current drop, then base the value of the limiting resistor on the actual voltage I'm going to use?

would it's current draw remain the same regardless of the voltage? So if it works at 2V and has a draw of say 20ma, when I base the value of the limiting resistor, would I still assume it's draw will be 20ma?

If you don'tknow how the two wires are connected you should not do anything unless you are prepared to destroy it.

Before you start putting voltage on it, try to figure out how it works. Try checking resistance across the two wires, and see what you get, and whether it changes when you manually trigger the tipper.

I’ll wager that that’s not a hall effect sensor at all, but just a reed relay. Hall effect sensor would require 3 wires (power, ground, and signal), unless there’s some funky means of powering it (some sort of parasite power, like they do in OneWire). Whereas a reed-relay would just require the magnet and 2 wires.

Measure continuity with a magnet close to it.

Measure continuity with a magnet close to it.

true. I didn't actually look at the circuit board that close - but great way to tell.

Thanks

Hey, look, if you want to use the collective knowledge of the people here, you post a picture of the thing!

gginnj: true. I didn't actually look at the circuit board that close - but great way to tell.

:D (classic photo. I'm going to save it)

raschemmel:
(classic photo. I’m going to save it)

Oh, we’re going to see that photo a lot more!

Love the pics :) - but in my defense - the board is not easy to get to without breaking the thing - otherwise I would have taken a picture of it - so I can zoom in on things to read them. But the question wasn't specific to the sensor - it was a general question of what to do when you don't any information, which many of the responses were helpful, and some not so helpful - but on the giggle factor, I'd give them a 4 (out of 5).

it was a general question of what to do when you don't any information,

Then you can't. You need information to narrow things down, even if that is only the function of a device, you need to know something about what you are measuring.

Your description of what a voltage drop is leaves a lot to be desired.

OK. Maybe my terminology is incorrect for what is a voltage drop - I'm using it based on the following you tube video: Lighting an LED with a Raspberry pi

The video does a nice job (in my opinion) in how to determine the value of a limiting resistor based on the "voltage drops" of the other components in the circuit (in this video, an LED). As stated in the video, he knows the voltage drop of the LED (I'm assuming based on a spec sheet).

However, if you have something that does not have a spec sheet, how would you go about trying to figure out the voltage drop it would use?

To say - just throw in a 10k resistor doesn't help (it may work, but you really didn't learn much). To say - you can't doesn't help either - since you have it and it might be useful - why not try

in my limited electronics knowledge that I have been learning so far - in order to use ohms law to figure voltage, you would need to know the resistance and the amperage. resistance - not a problem - meter it. amperage - you would need to a meter in series of it functioning - however, in order to get it to function safely, you would need to know the safe voltage to run it at - but that is what you are trying to determine.

hmmm. so what would be the best way to attempt to find it? start low and increase? If that's the case, how do you know when to stop?

My original question was not based on anything specific - so I couldn't really post pictures, or give specs. It was solely to gain more information, on how to go about trying to figure out electrically more about something - what steps would give what kind of information.

If my usage of "voltage drop" was incorrect - please let me know a better term that would fit.

Thanks GG

gginnj: This is more of a general question, if I come across this for something else where I don't know anything about a device.

What is the procedure to determine voltage drop - and current used - both require power up the device/component?

Back to the original question. There is no procedure to determine voltage drop if you don't know what the device is. If the device is to be used in series with other devices, "voltage drop" may be an appropriate term. If the device is simply to be powered by some source, the term "input voltage" might be more appropriate.

Not all components produce a voltage drop like the LED. This is more correctly called the forward voltage drop or simply forward voltage. This is in the data sheet, without a data sheet you can have a guess as to the resistor value and then measure the voltage across the LED with a meter. Then refine the resistor value as a result of the measurement.

The LED is one of only a very few components that work like this. Things like a resistor does not have a voltage drop as such. There may be a voltage drop if it is in series with another component like another resistor. Then the voltage drop is determined by "the other resistor" because the original resistor has current flowing through it.

But suppose you take that resistor and place it across 5V and ground, there is no voltage drop, well it is always 5V, what changes is the current and that is determined by the size of the resistor. Other things too have no voltage drop. Say the processor inside the Arduino board, you apply 5V to that and there is no voltage drop you just get 5V. Same with a relay coil or most other things.

So a voltage drop only occurs as a result of current flowing through a component and then through an other component. So the concept of a voltage drop is of little use in determining what voltage to feed most devices.

hmmm. so what would be the best way to attempt to find it? start low and increase? If that's the case, how do you know when to stop?

You don't, there is no way of knowing when it will go pop without information about what you are testing.

gginnj:
start low and increase? If that’s the case, how do you know when to stop?

Buy two, burn one and then run the other just below that voltage