Electrical question

SimbaDog

SimbaDog

Well-Known Member
Joined
12 May 2004
Messages
8,008
Location
Devon
Visit site
On a 240v to 12v transformer, if I use an electrical screwdriver on the 2 12v output terminals, should I expect the light in screwdriver to show?
It's a std 220-240v clear plastic electrical screwdriver.
:confused:
 
No, the tester is expecting 220v AC. It won't illuminate on 12v DC. Multimeter ?

If the light does come on, then you have a leak across the coils. Achtung !
 
The mains tester lights up with a current to ground (through you). Consequently the resistor inside is quite important.

If you are getting a bright "signal" from the 12v output I suspect the secondary winding is short to the primary. I recommend caution (or bin it and buy a new one).

edit - Sarabande said that, didn't he?
 
Last edited:
On a 240v to 12v transformer, if I use an electrical screwdriver on the 2 12v output terminals, should I expect the light in screwdriver to show?
It's a std 220-240v clear plastic electrical screwdriver.
Click to expand...

Those screwdriver test things would not tbe given house room by an electrician.

It should not light but I am not at all surprised that it does.

Check that your transformer is an isolating transformer, not an autotransformer. A multimeter on ohms range will give a low ohms reading between input and out put if it is an auto transformer but should give infinity on an isolation transformer

If thats Ok check the insulation resistance between the input and output with a "Megger" that's like an ohms meter but puts 500volts between the terminals to test the insulation.
 
Last edited:
It's actually a brand new LED bathroom mirror, didn't work so I checked there was power into the switch (OK), then that there was power into the LED transformer that say's 220/240V AC in & 12V DC out, there was power in but was then surprised to get a light on the 12v outlet to LED's :confused:
 
provenjohn said:
It's actually a brand new LED bathroom mirror, didn't work so I checked there was power into the switch (OK), then that there was power into the LED transformer that say's 220/240V AC in & 12V DC out, there was power in but was then surprised to get a light on the 12v outlet to LED's :confused:
Click to expand...

It'll be an auto transformer, cheap and nasty. Those screwdrivers actually use a very small neon bulb as the indicator that will begin to illuminate from about 80v onward and as a previous poster mentioned they use the stray current to ground that you provide. The 12v for the LED's could be being delivered at 110v or more away from ground so quite feasible that the transformer is OK. Your best bet is a proper multimeter set to volts and connected across the output. If testing the actual output of the transformer then set to AC if through a rectifier or similar then DC.
 
Hummm after working and teaching electrical/electronics maintenance for total 30 years, I would see if that "electric screwdriver" will float :D and pray that it doesn't. Trash throw it away..... You can purchase a small electrical "multimeter" at your automotive supply (or boating supply) house.
FIRST THING if you do not know how to use it, have someone (electrician electronics) person show you and it will save you many problems in the future. They are pretty simple/easy to use but used improperly, they can explode :eek:Especially using one around 120/240 volts. Problem is the meter MUST be on the correct scale to prevent damage.
I have several sets of mechanical and electrical/electronic tools, as I am retired now. I do have one of the test lights that has a light bulb in the probe end and an aligator clip on the other. These are great for tracing 12 volts or 24 if needed. They are great for checking ground connections also.
I do not own one of the "electrical screwdrivers", as they can prove to be very dangerous to the user.
Also, when checking "mains" you need to be able to accurately read 120 and or 240 volts. Read across both hot leads and from each to neutral. Across should read 240 and either to neutral should read close to 120. A neutral open or bad connection can cause all kinds of problems and possible damage to your expensive equipment. There is no way to detect these kinds of problems using a "electrical screwdriver". You can, with the meter, see fluctuations in your input power that may be caused by a loose or corroded connection.
BE SAFE AND PROPERLY INFORMED, buy you a multimeter, there are pretty cheap ones for the home/boat owner. You can always spend several hundred bucks on a quality meter if that is what you really need but the cheap ones will give you the ability to make SAVE AND ACCURATE readings.
Just be sure, as I said to read the instructions or better yet and easier to understand, have someone with electrical knowlege show you how to safely use it and to properly read it.

Good luck.;)
 
sea_goin_dude said:
Read across both hot leads and from each to neutral. Across should read 240 and either to neutral should read close to 120.
Click to expand...

Whoa, don't forget this is a UK forum. We don't have "hot" leads (ok, that's just terminology), we only have one "live", and from it to neutral or earth should be 240ish volts. Nothing should be at 120v to anything else.

I agree with you on the worthlessness of neon screwdrivers though :). I have one because it's the correct size for the screws in electrical terminals, but I wouldn't dream of using it as a voltage tester.

Pete
 
ooops sorry about that one. I am on the other side of the big pond here. ours is two sides of 120 and 240 across both. I hope I got my suggestions across though. I have seen some of the old fuse panels of years ago and they were dangerous as the neutral could open leaving the device hot for anyone that might contact it and ground (earth I guess) that was before the ground leads were added and many many updates to our electrical code were added.

I will try to be "technically correct" if I can..:D

Ps do I understand you only have 240 for "mains" power. I will try to find your "code" information and update myself on your system. ;)


Some of our appliances are dual voltage devices like electric clothes dryers use 120 to drive the drum and blower while using 240 for the heat circuit.
 
Last edited:
sea_goin_dude said:
Also, when checking "mains" you need to be able to accurately read 120 and or 240 volts. Read across both hot leads and from each to neutral. Across should read 240 and either to neutral should read close to 120.
Click to expand...

Sorry but dangerous info, wrong country. UK is 230/240 phase to neutral in the domestic environment.

Never have been impressed with the US method of domestic wiring. It's that plug above the bathroom sink that gets me every time.
 
Last edited:
sea_goin_dude said:
Ps do I understand you only have 240 for "mains" power.
Click to expand...

That's right. Only one phase goes into each house, and from that phase to neutral is 240 volts[1]. Neutral is tied to earth either at the meter or back at the substation (transformer), depending on the system. The latter is normal in towns, the former in the countryside to save cable.

Running at a higher voltage means we pull less current, so have less of a problem with heating the cables.

Pete

[1] On paper, to comply with European standards, it's 230volt nominal with a wide upwards tolerance; in practice I believe it's still the old value of 240.
 
It's that plug above the bathroom sink that gets me every time.[/QUOTE]

The bathroom outlet is prob.the safest outlet in the house due to our GFCI, ground fault interruptor. It will trip the circuit if it detects any imbalance in the two wire circuit such as would be detected if ANY current should try to travel through any kind of short/leak or if someone comes in contact with a live/hot conductor. These are required any where in the house with outlets anywhere near water lines or outside outlets. They (GFCIs) will turn the circuit off but can be reset by a simple button on the device. Kitchens, bathrooms, washing rooms etc are protected by these devices. They will not permit enough current to even be felt by a person who contacts a hot conductor. Also where the floor might be concrete or other conductive material.
I am trying to find and download your electrical standards manual. ;)

Jerry :D

ps do you not have outlets in your bathrooms?
 
sea_goin_dude said:
The bathroom outlet is prob.the safest outlet in the house due to our GFCI, ground fault interruptor.
Click to expand...

We have the same device, although it's called an RCD ("residual current device") here.

Current building regs effectively require every circuit to be protected with one, with only a few exceptions. Although of course older houses won't be to current regs.

A while ago it was common to fit a single RCD for the whole house (when they were more expensive) but that is now discouraged and I think may actually be prohibited under current regs as it makes nuisance trips both more likely and more serious. You can either have several (but not all) circuits sharing an RCD (but with their own MCBs for overcurrent) or fit RCBOs to each circuit which combine the function of MCB and RCD and are the preferred solution (but slightly more expensive).

I'm starting to forget some of the details of this stuff :)

No, we don't have sockets in bathrooms, and conventional switches are also not allowed. They have to be waterproofed to an appropriate standard, which varies depending on how far from sink/bath/etc they are. The usual UK solution is a switch on the ceiling, operated by pulling a string, although personally I think that's a bit naff and have fitted some nifty capacitive sealed switches instead.

Changes in the most recent version of the regs do now allow sockets, but the required distance from bath/shower/sink is much bigger than any normal bathroom so for most people there's no change. I think the change was made to allow for modern open-plan designs where there might not be a clear distinction between a bedroom and an en-suite bathroom.

Pete
 
Last edited:
You must log in or register to reply here.

Other threads that may be of interest

P
Adding fuses to my batteries
2 3 4
Replies
61
Views
2K
AngusMcDoon
AngusMcDoon

Share this page

Top