24volt DC Tester

You cannot use a neon as it it below the strike voltage of a neon. Another way is two LEDs and a resistor for each one, red/green facing backwards with a bypass 5 v zener on each in case the PIV is too high. only use if under 120 V or so, but you can make it for 240V with a larger resistor = dimmer at 24 V

That could be used to tell you what is + or =

Aurizon, you wrote: "with a bypass 5 v zener on each in case the PIV" but really it would be better to wire the LED's together in parallel with opposite polarity and just use one resistor. This way, the most PIV either LED will see is the forward voltage of the other LED and you only need one resistor and no other components.

yes, That is correct. I had them separate in my minds eye

Another technique is to use a hall effect sensor to detect the magnetic field around a conductor. The advantage of this technique is that you don't need to make an electrical contact with the conductor to detect if there is power on it. The disadvantage is that you cant actually read the voltage. I have seen these units given away as samples by electrical equipment suppliers so they cost next to nothing.

I would however recommend investing in a pencil or pen style digital multi meter. Here are a couple of examples.

http://www.instrumentation2000.com/catalog/electrical/0805.html

they are not overly expensive and give you the actual voltage rather than just an indication.

If I understand correctly, the poster is really interested in logic levels on control circuits. If this is correct he or she only needs to know whether the voltage is above or below one or two threshold level(s). It is quite possible for a point in a control circuit to have voltage present with zero current (as in a circuit to detect switch closure to ground). In that case the Hall-effect probe would not work, because it must have a current flowing to create the magnetic field.

There are battery powered probes from Fluke and others which use a very sensitive FET to detect charge or voltage. They are non-contact, and only need to be close to the source (closer for lower voltages). The one I have (Fluke VoltAlert, model # rubbed off) is several years old, and only detects voltages above 90V, but I know they now have more sensitive ones that work below 24V. Unfortunately I do not know if they reliably detect DC. By the way they are GREAT for finding flaws in series Christmas or other decorative light strings - best if the longest wire segment is on the grounded side of the circuit.

The more I think about it the more I believe that dkwarner is correct and the devices I have seen use a FET rather than a hall effect device to detect the presence of and electric field rather than a magnetic field.

If you consider that there are a wide rang of voltages that you can come across in control systems and you can have neutral switching and induced voltages that can give a false reading there is only one guaranteed solution. For a quick test to see if a voltage was within limits I would stick with using a pencil or pen type digital multi meter.

I agree with your DMM solution (pencil or regular). Since my last post, I did a bit of research, and the FET devices are only reliable with AC. They will make one flash as they approach a DC voltage source, but that can just as well be from static on the operator's body as from the object being tested. I use 5V as well as 24V control circuits, and at 5V it is even more important to know the exact voltage, measured by a high impedance device.

From what I have understood, grdpande wants a single touch point tester same as a "Neon tester". The regular neon tester points out the existence of AC voltage only without any quantitative measurements.

With the Hall-effect transducer you are getting just this. (masu)

If you want to know the exact voltage, that is a whole different story.

Some time ago I was working on the development of a single point, wireless DMM probe, with direct interfacing into a PC. By touching a point in the circuit, you are getting the information on voltages, currents, resistance and polarity. With auto ranging and data logging etc. It is a rather complicated matter, and still under development.

Wangito.

wangito: Please contact me by email either english or spanish. Gracias! Dick Warner <dkwarner@mindspring.com>

Ok before you get your self or someone else killed:

single phase AC power power is only reliably tested/checked using a multimeter period end of story!!!!!!!!!!!

All the tester is good for is confirming if voltage is present and thats all!!!!

earth is also the completing medium fo re3 phase power as well.

Wow, how things become convoluted. If the goal is a "hold it close to the wire and watch it glow" it isn't going to happen with 24VDC. Yes Hall effect could detect the presence of CURRENT FLOW in a wire, but not simply the presence of voltage without current flow. And the Earth does not constitute a current carrying path for any kind of power transmission. With three phase all the current flows between the phases, on the power grid. Once transformed to a lower voltage for end use, one point of the transformer secondary might be connected to ground depending on the type of service. Current may be provided from one phase of the circuit to a common that is served by a wire and may be at ground potential but the Earth is not used to carry current.

What's wrong with using one of the simplest and oldest devices ever used for this purpose?

A solenoid is connected across the terminals using probes and the slight current produces a magnetic field which pulls the iron core into the solenoid against a spring...

The depth the core is pulled in is read of the side against a voltage scale, for dc it works fine for ac you will feel the vibration or the core which tells you its ac and to read off the ac scale...

They can be really sensitive as well as covering a wide range... You would still need two connection points though.

John.

As I drew an arc from the flyback transformer to my index finger, I turned to my fellow high school senior electronics class student and I said "Yeah that feels like about 14,000 volts to me." Subsequently, my instructor asked me to not to encourage other students to engage in suicidal behavior. Don't get me wrong, if you don't know what you are doing or your luck simply runs out you can end up dead before you can say what happened. But that wasn't the only time I used the "direct experience" method of voltage detection. Fortunately my luck hasn't (yet) run out. Don't try this at home kids, I am a professional at engaging in suicidal behavior and surviving, so far.

- Dangerous misconception there.

In a correctly wired single phase circuit
earth is NOT the circuit completing medium,
it is a reference point.

Low voltage D.C. test lamps are available, or purchase a
24V. lamp & socket, add pigtails, & there you have it.

Since the original poster is talking about control circuits, the current required by an incandescent lamp (or by the solenoid suggested by someone earlier) will in some cases drop the voltage sufficiently to give a false low indication, when the open circuit voltage (the logic level) should be high.

I believe the poster wants an indicator that does not require a connection to ground (using operator's body as capacitive ground). If a ground connection is acceptable the the LED-resistor (or two LED's opposed parallel if polarity is unknown) as suggested earlier is pretty hard to beat. By the way , a second resistor can be added in parallel with the LED(s) to form a voltage divider and establish a desired threshold voltage.

Engineers are Engineers are Engineers?

You can purchase a simple Automotive tester that incorporates ground wire and a the probe located on the tip of a screwdriver type handle with a 12v lamp inside. You can either replace the bulb with 24v or place a resistor that is the same value as the 12 volt lamp in series.

Everybody always want to re-invient the wheel.

My two cents, P(Power)ircus(circus), Pircus