Microscope Lamp Power Supply and Inc. Bulbs question, AC/DC?

[Gully]

Hi DIYnot, I've got a new problem I thought you may be able to help me with. I recently bought an old (~'70s?) microscope from the USA and after eagerly awaiting it's arrival it finally arrived today. By chance I had a universal adaptor upstairs and waisted no time plugging it in. Upon flicking the switch the microscope's power supply (only for the bulb beneath) emitted a loud buzzing noise. It was on for 5-10 seconds before I rushed to turn it off but I fear the damage is already done, when I flicked the switch the buzz stopped but I was struck by a strong smell reminiscent of burning plastic

Before plugging it in I had already considered a difference in voltage, but assured myself that I had looked it up previously and found that these days the UK & USA both used ~230V... of course my memory served me wrong, the power supply is rated at ~115V, as are all USA consumer electricals.

I read somewhere that the power supply 115V to 6.5V transformer, with a variable rheostat (I don't know if that's accurate). The outputs are 4.5, 5, 5.5, 6.5 or 7.5V, controllable by a knob on the front. I couldn't find out if that was AC or DC. If my memory serves me correctly, the power light on the power supply stayed on until I disconnected the power. I think the microscope's incandescent bulb didn't illuminate and appears to have survived (element is intact).

So my questions are: Can anyone theorize what damaged I may have caused to the power supply by running double the voltage through it? What component could have caused the burning plastic smell? Is there any chance it has survived or that I could just replace a component and reuse, this time with a step-down transformer?

If I am to replace the whole thing, I don't think I will buy an exact replacement. I believe it is a simple device, essentially just a transformer and a variable resistor. So what exactly would I be looking for? Do you think it's output is likely to be AC or DC? I've read both work with incandescent bulbs, but it can effect the brightness/longevity of the bulb...

Also I have something like the plug below, could that work? It's DC and supplies some voltages with in the correct range:

I'm sorry for the barrage of questions, if you can answer any of them I will really appreciate it

 

[Gully]

Think I may have released the magic smoke

 

[jj4091]

You should be able to get it repaired somewhere like this http://www.rjweng.com/Electronics-repairs?gclid=CKmywqefiLYCFaLHtAodG18APw or maybe even a local TV or computer repair shop try google or yellow pages. Not sure if it will be economical or not. You need more info about the rating of the lamp to be able to source an alternative ie operating voltage range & wattage.

 

[viewer]

Yup; smoke all gone!
To check for damage to the controller, you'll have to look inside and check for melted material, continuity and resistance. You may be lucky, but still need a 110V supply (shaver socket?).
Assuming it's died;
First check the lamp / bulb. It's probably 6V if 1970. It won't care whether you give it AC or DC, but probably old supply was DC
A simple volt drop by resistance is all you need so a multi-volt output wallwart should work, but the differences may be too crude for the user. There are a number of toy/model 6V units for trains etc which should work.
An alternative work around would be to change the whole to 12V. There are lots of cheap 12V toy controllers and lots of lamps, but you'd need to check the lamp holder.
BTW I was once asked to put a 13A plug on a lead to a microscope. I did without asking which lead it was - yes it was the transformer to lamp lead. The resulting pop reminded me that I should have asked.

 

[bernardgreen]

It is good practice to run microscope lamps from smoothed DC. The light from many small lamps run on AC has a ripple of brightness which can increase eye strain in the user of the microscope.

 

[Scoby_Beasley]

Yup; smoke all gone!

Agree with viewer. You need some new smoke.

Harness Smoke

• Scoby_Beasley
• 19 Feb 2013
• 2

 

[Gully]

Agree with viewer. You need some new smoke.]

Haha, thanks I came across this fantastic product shortly after learning about magic smoke.

To check for damage to the controller, you'll have to look inside and check for melted material, continuity and resistance. You may be lucky, but still need a 110V supply (shaver socket?).

Unfortunately my multimeter is broken so I am currently unable to test. I did consider a shaving socket, but unfortunately I don't have a 110V one here. I will try and borrow a multimeter. Inside it comprises primarily of one`lump with a rotatable switch attached by various wires coming off each point. Superficially the lump resembles this, without the writing:

The surface of the bit in the centre (coils?) looks like melted plastic and is a dirty brown colour reminiscent of ancient sellotape®. On one side a small amount looks to have seeped out. If I understand the cause of magic smoke correctly this should be the coil insulation? It doesn't look like a lot so perhaps there's a chance there's still enough magic smoke inside? Haha...

Yup; smoke all gone!
A simple volt drop by resistance is all you need so a multi-volt output wallwart should work, but the differences may be too crude for the user. There are a number of toy/model 6V units for trains etc which should work.

I tried my plug (wallwart - haha), but got no light from my bulb. I'm guessing not enough current? Or perhaps my wire-round-pins connection was too crude/ the plug could be broken. I'm now going to have a root through the attic for my old Hornby controller, thanks for that idea

You need more info about the rating of the lamp to be able to source an alternative ie operating voltage range & wattage.

I forgot to mention, the bulb is rated at 2.75A and 6.5V = 17.88W. The power supply output range is 4.5 to 7.5, I guess that's in volts so the bulb must be able to take a little more than 18W.

It is good practice to run microscope lamps from smoothed DC. The light from many small lamps run on AC has a ripple of brightness which can increase eye strain in the user of the microscope.

That makes sense, thanks a lot

 

[viewer]

Sounds like the insulation on the windings has melted and let the smoke out. Very difficult to get back in so probably not worth any further work as you still need to get 110V to it. Multimeter is a must.
The wallwart you're using probably has a rating of only 500mA hence the failure to light the bulb (assuming the bulb is OK - multimeter again ). To get the 3A you need it will be hard to find a cheap multi-voltage unit and the 4.5 to 7.5 range is quite small so the rummage in the attic seems the most promising option. Best of luck; let us know how you get on.

 

[maltaron]

That transformer looks very much like a 100volt line loudspeaker transformer. I suppose someone may have used it to replace an American mains transformer.

 

[ericmark]

That transformer looks very much like a 100volt line loudspeaker transformer. I suppose someone may have used it to replace an American mains transformer.

That explains the watts I did look at that and try to work out why is was rated in watts.

As to replacement likely a motor cycle battery charger just over 6 volt and DC would work.

The problem I have with my microscope is there is not aperture control so I need to alter the amount of light. Mine a cheap modern one has a LED lamp above and below the slide with a control which will alter the amount of light and since LED the colour balance is unchanged. But with a simple bulb changing the voltage will also change the colour.

I find LED lighting works well where I want to photograph the image but I suppose through eye piece not that important.

 

[Gully]

That transformer looks very much like a 100volt line loudspeaker transformer. I suppose someone may have used it to replace an American mains transformer.

That's because it is a line transformer

It's just something I came across to give an idea of what kind of lump I was talking about. I said without the writing because I wanted you to ignore it and not be lead into thinking I have a line transformer. Or maybe I do, I don't know what a line transformer is inside. I should stop being lazy and just take a picture of it, here you go:

Untitled

• Gully
• 20 Mar 2013
• 1

Seems unlikely, but any chance this component is replaceable with a Line transformer?...

Anyway, this is what happened today: I rooted through my attic and found a bunch of train controllers, unfortunately they're all rated at very small currents too so it's the same problem. I do have one more modern controller however which has a dual output for two train tracks. Each one is rated at 0-12V and a maximum of 12VA. So 1A each.

So I decided to try connecting them in parallel under the theory that I could add up the current, yielding 0-12V and 2A; maybe with a bit of leeway stretching to my required 2.75A. It doesn't really matter if I release the magic smoke from this power supply. I set it all up and flicked it on. The power supplies started up fine but the bulb didn't light

The good news is they didn't trip either. They have an LED indicator light which flashes if they trip. I then checked the bulb again and upon inspection the filament dropped off it's connections inside the bulb!

So now I don't know if the bulb was the problem or the power supply. Was the filament just hanging on by a thread from the start and never had a chance of illuminating... or was it fine at first failing to light because of a problem with my improvised power supply and coincidently fell off with a jolt after my experiment? I guess the former - I've been experimenting with a dead bulb.

Well luckily I ordered a new multimeter this morning before all this happened, same day dispatch first class delivery. When it arrives I'm going to test the hell out of everything!

But maybe I should just pack it all in and find a suitable LED to go in its place. I was going to try my LED torch but conveniently I can't find it! I do have an aperture on mine. What do you use your scope for eric?

 

[Monkeh]

The 'molten drip' appears to just be a bit of rust.

I'd say you've either blown the contacts of that switch to hell and back or blown a fuse in the transformer. Simply increasing the voltage wouldn't have killed it, but the current might have.

 

[Gully]

Molten drip isn't rust, if you can see it up close. I wish it was. And remember there was a strong smell of burning plastic... The supply might have survived but I don't hold out much hope. I was curious about the dangers of excess voltage vs. excess current though. I always thought current was the circuit killer.

Well anyway, I think I've got a good alternative solution. Now my bulbs gone as well as my power supply, I'm thinking LED conversion. It should be cheap quick & easy + in some ways it's an upgrade from my out-dated incandescent set up, so it won't have all been for nothing. That helps puts my mind at rest and ease the frustration.

I can even recycle my old switch and box, hooking it up to resistors to gain a range of brightnesses and keep the elegant vintage appeal of my miscrosope and matching power-supply

Here's the microscope and power supply in case anyone was interested:

 

[viewer]

Sounds like ericmark's system is a sensible answer. No problem with colour and cooler running - those small lamps get rather hot and a problem with live material. You could hide the control gear in the old transformer housing and keep the look.

 

[bernardgreen]

I forgot to mention, the bulb is rated at 2.75A and 6.5V = 17.88W. The power supply output range is 4.5 to 7.5, I guess that's in volts so the bulb must be able to take a little more than 18W.

In which case a variable DC power supply with its current limit set to 2.75 amps will supply the lamp. If you accidently turn the voltage too high then the current limit will take over and prevent the voltage going higher than the lamp can tolerate.

American Optical used to have a daughter company in the UK. If they are still active and you can track them down they may be able to help.

An LED would need to be selected for the spectrum of the light it provides. Although they appear to the human eye as "white" some of the spectrum may be missing ( compared to incandescent white ) and this will affect how the eye sees some items in the specimens.