A Very Simple Dim Bullb Tester

[FORTUNATA]

I'm sure someone here must have heard of a dim bulb tester or a "current limiter". I must admit I hadn't myself heard of it till a coup[le of weeks ago. I always assumed I'd wind up buying a Variac instead but find Maplins no longer sell them and kind of heavy to buy online (postal).

The aim of the device is that circuit resistance is very low but would dramatically increase if there is a fault in an appliance such as a short circuit.

Anyway, yesterday on my boat, I had a go at making a really simple dim bulb tester but admit I was a bit baffled by the result. So, I'll share exactly how I built it:

I decided to leave the switch out of the circuit and, given there are no switchless schematics online, I tried to simplify the device.

Material was simply a mains wall socket, A.C. wire, a plug, terminal blocks and a bayonet lamp holder.

Wiring was as follows (with the aim of passing the live hot wire through a bulb in series and back to the socket live).

First I wired a plug with live, neutral and earth. The other end of the plug was wired to the wall socket (except live) so that when you plug in, the socket is eventually equipped for A.C. supply. The wall socket is normally secured to a piece of wood.

Live is not fed to the socket at all but just to a terminal block. At the other side of the terminal block opposite, a wire is fed from it to "hot" on the bayonet lamp holder terminal.

A new wire is then fed from the other terminal on the bayonet lamp holder back to the wall socket live.

With some degree of trepidation I finally plugged it in, expecting a slight glow in the lamp, but nothing. Finally I plugged in a device to the wall socket and that worked O.K. and was getting an A.C. supply (presumably at reduced voltage with current passing through the bulb).

If this is correct, it may be my 100 watt dimmer bulb is only going to glow if there is some short circuit in the appliance under test in which case the heat would increase resistance.

Anyone ever made one of these and hopefully my switchless version is O.K.

[Murflynn]

easier to understand if you include a sketch.

 

 

 

or am I being very simple dim??

Edited April 27, 2015 by Murflynn

[FORTUNATA]

easier to understand if you include a sketch.

 

 

 

or am I being very simple dim??

The snag I had myself was very many of the schematics online were kind of vague. So, I just thought about how I could pass a live feed through a lamp in series then back to a supply point and that's how I winded up doing it. I left out the switch.

 

So when you plug in the live pin of a plug, it contacts a hot supply that's just been passed through a bayonet bulb holder (plus 100 watt bulb). Not the direct hot supply lead you get normally.

 

After all the messing about I later found out I probably don't need a dim bulb tester after all as some American engineers were saying the device is mainly useful when you're plugging in appliances with a transformer.

 

Anyway, I'm now also toying with the idea of just using a 500 milliamp inline fuse. The device I'm testing draws about 170 milliamps at 240 volts and I'm assuming 500 milliamps wouldn't be too critical an overload at point of circuit break.

 

 

 

[jonathanA]

Wtf ?

 

What are you trying to do ? So basically you have wired a 100w incandescent lamp in series with a 13a socket and with nothing plugged into the socket you expected something to happen ? Really ?

[MtB]

Wtf ?

 

What are you trying to do ? So basically you have wired a 100w incandescent lamp in series with a 13a socket and with nothing plugged into the socket you expected something to happen ? Really ?

 

http://www.radiomuseum.org/forum/current_limiting_with_a_dim_bulb_tester.html

 

And you thought computer programmers had the edge on us all for dull geekery!

[jake_crew]

I have used such a device as a voltage dropper.

 

I assume we're talking good old fashioned tungsten bulbs here - I'm sure LEDs won't behave the same.

 

But beware the non-linear resistance / temperature characteristsics of these bulbs - resisstance is low when cold, higher when hot.

[Murflynn]

After all the messing about I later found out I probably don't need a dim bulb tester after all

 

 

 

Classic !!

[Guest]

I have used such a device as a voltage dropper.

 

I assume we're talking good old fashioned tungsten bulbs here - I'm sure LEDs won't behave the same.

 

But beware the non-linear resistance / temperature characteristsics of these bulbs - resisstance is low when cold, higher when hot.

I can't be certain, but seem to recall my late brother in law doing something similar with an old army radio or similar. I know a "bulb" (incandescent lamp) came into the equation somewhere, and whilst it sounded rather Heath Robinson, he claimed a success with it. It was many years ago and I forget the full details now.

[Guest]

 

http://www.radiomuseum.org/forum/current_limiting_with_a_dim_bulb_tester.html

 

And you thought computer programmers had the edge on us all for dull geekery!

The logic behind it is easy enough when connected to a low current device. If all your electrickery flows to neutral through (say) a dodgy transformer, then a "full lamp" situation occurs. We used to do a similar test trick with an old fashioned test lamp across fuse holders. I could see this lamp thing being quite useful when pissing about with old radios and amplifiers etc.

[Tony Brooks]

When I was an apprentice we used something similar as an insulation tester for starter and dynamo field coils.

[Guest]

When I was an apprentice we used something similar as an insulation tester for starter and dynamo field coils.

i believe I have also seen something similar used for armature testing, but that is not really my field.

[Tony Brooks]

i believe I have also seen something similar used for armature testing, but that is not really my field.

 

Yes, that as well.

[cereal tiller]

i believe I have also seen something similar used for armature testing, but that is not really my field.

Greenie for that Awful joke!

 

CT

[MtB]

I missed it first pass. (Virtual) greenie from me too!

 

(Run out of real ones.)

 

MtB

 

 

(Edit to add, strangely, actually I haven't. Real greenie awarded when I idly tried, to check!)

Edited April 28, 2015 by Mike the Boilerman

[cereal tiller]

i believe I have also seen something similar used for armature testing, but that is not really my field.

 

 

I missed it first pass. (Virtual) greenie from me too!

 

(Run out of real ones.)

 

MtB

 

 

(Edit to add, strangely, actually I haven't. Real greenie awarded when I idly tried, to check!)

Maybe Catweasel has become a devout ATHY-ist?

 

CT

[ditchcrawler]

Or limit inrush to a transformer. you just have to remember to switch it in before plugging the TX in

[FORTUNATA]

I have used such a device as a voltage dropper.

 

I assume we're talking good old fashioned tungsten bulbs here - I'm sure LEDs won't behave the same.

 

But beware the non-linear resistance / temperature characteristsics of these bulbs - resisstance is low when cold, higher when hot.

I understand the idea of it now but think I'd been a bit misled by obscure online explanations that seemed to suggest the lamp would glow dimly before you plug in the device under test. So, I was baffled as to how that could happen. I mean, normally you would wire live, neutral and earth and that would be that but what happens here is there is a halogen bulb in series on the live side.

 

Today I actually built my unit. It's maybe the simplest design on the web as I left out all by-pass switches. It consists of a block of plywood and a single wall socket is bolted down. A supply and plugged mains cable tucks under the socket and connects to it on neutral and earth. However, live doesn't go to the socket live pin at all. It goes to a terminal block. Opposite of the terminal block supply I ran another wire (bonded to the supply) and this runs to the actual bayonet lamp holder (live). Then, the second (neutral) wire of the lamp-holder is led right back to the vacant live terminal in the wall socket.

 

After this, I plugged in another lamp to the system and this lamp lit. The dimmer lamp didn't light. The dimmer light is 100 watts and the tester lamp I think is about 60 watts. Now ,assuming I plug in some equipment where there's a short circuit, my dimmer lamp ought to get hot and light up. At that point, it ought to become a viable resistor.

 

Late last night I looked at a few random figures in simple terms. The device I plan to plug in and test has a resistance of 1025 Ohms between live and neutral. The tester device I built is simply a bypass power supply with a heat resistor in series with live. This is 100 watts resistance. The mains supply as we know is 240 volts. So, converting the 100 watts to resistance, I was getting something like 580 odd Ohms.So, speaking very roughly there are 2 voltages which are the actual appliance and the series resistor of my dim lamp tester. So, I assume if the appliance is faulty and current increases, voltage will drop while it rises at the dim tester bulb - thus preventing excessive heat in the appliance and reducing danger to myself.

 

It's not an overly complicated circuit but food for thought such danger no longer exist with semiconductor appliances where large voltages aren't needed.

 

The only snag is I find American engineers are saying this is really an option when you test appliances that have a transformer. My appliance does not have a transformer (unless you count the output transformer). So, maybe all I accomplished was how to knock together a poor man's Variac.

 

 

 

 

 

 

Edited April 28, 2015 by FORTUNATA

[ditchcrawler]

I understand the idea of it now but think I'd been a bit misled by obscure online explanations that seemed to suggest the lamp would glow dimly before you plug in the device under test. So, I was baffled as to how that could happen. I mean, normally you would wire live, neutral and earth and that would be that but what happens here is there is a halogen bulb in series on the live side.

 

Today I actually built my unit. It's maybe the simplest design on the web as I left out all by-pass switches. It consists of a block of plywood and a single wall socket is bolted down. A supply and plugged mains cable tucks under the socket and connects to it on neutral and earth. However, live doesn't go to the socket live pin at all. It goes to a terminal block. Opposite of the terminal block supply I ran another wire (bonded to the supply) and this runs to the actual bayonet lamp holder (live). Then, the second (neutral) wire of the lamp-holder is led right back to the vacant live terminal in the wall socket.

 

After this, I plugged in another lamp to the system and this lamp lit. The dimmer lamp didn't light. The dimmer light is 100 watts and the tester lamp I think is about 60 watts. Now ,assuming I plug in some equipment where there's a short circuit, my dimmer lamp ought to get hot and light up. At that point, it ought to become a viable resistor.

 

Late last night I looked at a few random figures in simple terms. The device I plan to plug in and test has a resistance of 1025 Ohms between live and neutral. The tester device I built is simply a bypass power supply with a heat resistor in series with live. This is 100 watts resistance. The mains supply as we know is 240 volts. So, converting the 100 watts to resistance, I was getting something like 580 odd Ohms.So, speaking very roughly there are 2 voltages which are the actual appliance and the series resistor of my dim lamp tester. So, I assume if the appliance is faulty and current increases, voltage will drop while it rises at the dim tester bulb - thus preventing excessive heat in the appliance and reducing danger to myself.

 

It's not an overly complicated circuit but food for thought such danger no longer exist with semiconductor appliances where large voltages aren't needed.

 

The only snag is I find American engineers are saying this is really an option when you test appliances that have a transformer. My appliance does not have a transformer (unless you count the output transformer). So, maybe all I accomplished was how to knock together a poor man's Variac.

 

 

 

 

 

 

If you dont have a switch how are you going to put full mains voltage and unlimited current on to the appliance you have plugged in?

[FORTUNATA]

If you dont have a switch how are you going to put full mains voltage and unlimited current on to the appliance you have plugged in?

Just unplug tester and then plug device in normally through my RCD. If on the first trial the tester bulb lights up because it's drawing lot of current, then that would caution against plugging directly to mains.

[FORTUNATA]

Yeah, I just found one diagram where it's wired the way I did it - mega simple. Others do have a switch in circuit or even switches to bypass the dimmer.

So far as I'm aware the only thing I need to watch out for is if the 100 watt lamp was dead you wouldn't know without plugging the lamp in directly to mains which is where a bypass switch is more convenient. However, the bulb does work.

Edited April 28, 2015 by FORTUNATA

[Keeping Up]

The resistance of the bulb when not glowing may be as low as one tenth of the resistance that it has when it is hot. This makes the calculations complicated but is incredibly useful if you want something that will drop only a small voltage under normal conditions but will drop a larger voltage during switch-on conditions or faults.

Edited April 28, 2015 by Keeping Up

[16csvt]

If I ever have to look at a switched mode power supply that is blowing fuses, putting a bulb in series is a useful way of being able to find what is short, while keeping the current 'within limits'.

One can also use different sized bulbs to give a different 'current limit'.

It is still worrying when you think you've sorted it, and switch the bulb out of cct.

Expect a bang, and if there isn't, bonus.

 

I've also used this technique on the van, when one cable went solid to ground over a sharp edge along its length.

Putting a 12V bulb instead of a fuse enabled alot of cable wriggling to show where the short was (when the bulb went out)

Regards all

Kevin

 

[FORTUNATA]

The resistance of the bulb when not glowing may be as low as one tenth of the resistance that it has when it is hot. This makes the calculations complicated but is incredibly useful if you want something that will drop only a small voltage under normal conditions but will drop a larger voltage during switch-on conditions or faults.

It was a happy ending. On my boat today I plugged in the 1947 radio I'd been working on through the dim bulb tester device. I admit I was still scared to death as the last time I had plugged the wireless in, the mains filter capacitor started to smoke and even bubble wax. I just pulled the plug pronto at the time. I eventually replaced all the dodgy capacitors.

 

Anyway, I plugged in today on the boat via the current limiter and wasn't sure what would happen as resistance between live and neutral on the set itself seemed healthy. Well, the test bulb didn't light and in time the wireless produced some crackling. Finally I removed the dim bulb tester from the circuit and, maybe due to a bit of luck, the wireless finally came to life (a tap with my meter on a Mica cap triggered reception somehow).

 

So, I guess I've learned something new but I ask myself often would I really have the nerves required to do the kind of work the old T.V. service engineers used to do. High voltage stuff kind of scares me still; after I once got zapped on my boat.

 

 

 

 

[Keeping Up]

... the kind of work the old T.V. service engineers used to do. High voltage stuff kind of scares me still; after I once got zapped on my boat.

 

No it was great. When a customer came into the shop complaining at how long it was taking to mend his TV we used to let him look inside at the back, then we'd just draw a huge arc off the EHT line with a screwdriver and say "No sorry it's really not safe to let you have it back yet" and they'd retreat shaken and white faced. If they'd been rude you'd add an extra item to the bill for making it safe!

[Guest]

No it was great. When a customer came into the shop complaining at how long it was taking to mend his TV we used to let him look inside at the back, then we'd just draw a huge arc off the EHT line with a screwdriver and say "No sorry it's really not safe to let you have it back yet" and they'd retreat shaken and white faced. If they'd been rude you'd add an extra item to the bill for making it safe!