24V rectifier for lighting.

I'm looking to replace the 24V AC 50W halogen capsule lamp on my lathe:

It's a horrible lamp. Hard to position, not very bright and the mount attached to the saddle can get in the way for some operations.

Lots of possibilities I know, but there might be an advantage in using the existing 24V AC supply. Something I'm thinking about is using a 27W Sealey LED array . This works from a 12-36V DC supply, so I'd need a rectifier. A couple of questions:

Q1. I have diodes suitable to make a full wave bridge rectifier myself but don't know how to calculate the appropriate value for smoothing capacitance. Any advice?

Q2. Is it nuts to contemplate making something myself? Perhaps there is some off-the-shelf unit which would do what I want? I've looked but so far failed to find, perhaps because I don't know the right search terms.

Comments along  'if I were going there I wouldn't start from here' lines also welcome of course!

Robin.

Edited By Robin Graham on 18/08/2023 22:07:00

I needed to replace the 48V light on my lathe - not so easy to find bulbs for it these days, and in any case as I was going to run the machine off a 240-415V inverter anyway the light would only have come on with the motor. I bought a couple of small 12V LED spotlights intended for car bumper mounting. I was able to modify the mountings to fit one of them in the existing shade and reuse the wiring and the support arms. I then removed the 415V-48V transformer from the LoVoLight switchbox and replaced it with a surplus 12V power supply that I think had run an old broadband modem/router. Adequate power for the LED. Gives a decent amount of light. I plan to mount the other one (I think they were about £15 for the pair) to do a similar job on my mill. Cheap but effective.

I also mounted the light on the splash guard - could you do something similar with yours? Looks like you already have a small clip-on light on the splash guard?

Edited By Nealeb on 18/08/2023 22:31:20

Ebay is awash with machine led lights. I bought 2 for my machines some time ago & they are pretty good & very bright. Have a look at this item No on ebay. There are lots to choose from. Your only drawback is these are 230Volt.

Steve.

Led light

To answer your initial query, I don't think you need a smoothing capacitor, just run it off the rectifier

Run the lathe light from 240V and use a 12V LED transformer for house down lights. Available from your local hardware

In answer to your original post.

If it is "a horrible lamp" dump it and buy something new, but, if you are like me..... recycle, re-use!

First question is: does the lamp have a transformer, or is there a switched mode power supply?

If it does have a transformer, I will check my 1970's DC power supply textooks and report back.

Some time ago Lidl had Led lights for sale for less than 10Euros, I bought two of them for my lathe and mill.

They came in two types, one with a base for table use, the other with a clamp for clipping to a shelf. The latter type is simple to fit to your lathe and the light is both bright and easy to focus. Like you, I had/have two of those horrible lights, now consigned to the dark corner of my cave. The Lidl lamps operate from a plug-in transformer, not a problem as I found, so, might be worth while looking out for them.

Just a thought !

You may experience strobe effects with unsmoothed DC, fluorescent lighting is also a strobe risk but most of us use it in the workshop without having a problem. Being aware of the possibility is probably enough to avoid grabbing hold of a rotating job that appears stationary.

Mike

I used an LED floodlight on a 24v transformer and rectifier - it flickered so I used the first cap that was to hand 1000uf at 50v that stopped it ! Noel.

Without smoothing, your LED light will switch off every half cycle, so it will flicker, perhaps not noticably, but might cause dangerous strobing effects.

My copy of Horowitz and Hill is packed away, so I had to ask the Internet:

Quote:

"[A good approximation for a 6.5A supply is given by:]

C (filter capacitor) = i (load current in Amps) x t (period of the rectified AC) x 1,000,000 (conversion from farads to uF) / E (ripple voltage)

So; assuming 50 Hz AC:

C = 6.5 x .01 x 1000000 / your ripple voltage."

Unquote

In your case, you will need to know the current drawn, (very little), and the voltage at which your LED light extinguishes, so as to work out an acceptable ripple voltage, in order to calculate a capacitor size.

Or forget 24V and fit one of these into your housing, (And fix the base of the lamp to the wall behind if it is getting in the way mounted on the lathe). These lights are brilliant, very bright, sealed, so no swarf or dust can get into the light itself, and are powered directly from the mains, no PSU block needed. (These might flicker though, mine are used as down-lighters in the bathroom and I have no way of checking):

Edited By John Doe 2 on 19/08/2023 10:22:30

To answer the question :

For your Sealy 27W lamp the current draw will be over 1A so you need a 1.5A or greater bridge rectifier. I'd suggest a 2 or 3A item. If using individual diodes to make a bridge then you can use 4 x 1A diodes like 1N400x series. This is because each diode is only carrying current 1/2 the time.
For capacitor you only need a minimal amount for simple electronics a rule of thumb is1000uF per amp of load. So for your lamp 470uF is adequate. Voltage rating needs to be at least 35V. Even 100uF would probably be enough to stop strobing at 100Hz.

Hope this helps.

Robert.

Edited By Robert Atkinson 2 on 19/08/2023 10:38:34

I have just used a variable speed battery hand drill held under one of the down-lighters I referred to above:

There is some strobing detectable at certain speeds, but it is quite mild. I don't think it would be enough to "freeze" a piece that was mounted in a lathe chuck, making it appear stationary, but just to warn you.

Just a guess because the description of Robin's Sealey lamp doesn't say so, but it claims to operate from a 10V to 30V DC supply. The input voltage range implies the lamp contains a some sort of electronics, perhaps a constant current power supply, in which case it may not need any smoothing at all.

My main concern with powering it from 24VAC and a rectifier is that the peak DC output is nearly 34V, which might result in the lamp having a short life. (The blurb says 10V to 30V, not 10V to 36V)

Unfortunately the spec doesn't say how much current the lamp draws. However, 27Watts is probably the light equivalent, not the actual power consumption. As LEDs are abut 10x more efficient than Halogen, I guess the lamp consumes 2 or 3 watts, say 100mA, not enough load to stop a beefy 24VAC supply rated to run a halogen lamp charging a big capacitor up to 34V.

The lamp costs £18.60. I'd either:

• Be a cowardly lion and run the lamp from a 12V supply, or,
• Assuming 100mA is about right and, add a 56 or 63ohm 2W dropper resistor. This would take peak volts down below 30V
• Try the lamp on 24vac (or via the dropper). with no capacitor. If it works without flickering, down tools. If not, try adding a capacitor, something between 470uF and 2200uF. Having an oscilloscope means I could measure the ripple voltage and calculate the correct value, but I suspect it's not necessary. Suck it and see.

How long a slightly over-volted LED lamp will last is anyone's guess: I suspect it will be OK. As the lamp is moderately expensive, I'd fit a dropper resistor.

This stuff is much easier if you have a mutlimeter and oscilloscope because a few simple measurements remove most of the guess work.

Dave

Yes, sorry, I did specify "without smoothing" in my reply.

If the lamp in question has some sort of smoothing or regulator built-in, it might be OK as a stand alone - I was assuming a basic LED lamp without.

As you say, hard to be sure unless one has a 'scope or a circuit diagram.

PS, the down-lighters I mentioned above are £10.95 + VAT each, and don't need any other components.

Edited By John Doe 2 on 19/08/2023 12:47:45

Hi Robin

,if you’ve got an old type car battery charger (consists of transformer and rectifier giving about 17 volts),you could try your new lamp on this .

If it works ok, add your rectifier to your 24volt supply.

Maurice

Edited By Maurice Taylor on 19/08/2023 13:21:52

The light rating is 27W input power so 1-3Acdepending on voltage. Clearly intended for vehicle use confirmed by the R10 approval. It must have a built in regulator to work over this voltage range. While the peak voltage of 24V AC is out of the noinal range of the lamp, as mentioned by SOD, the loaded average votage is probably OK. To be safe just use half wave recification (1x 3A or higher rated diode) and a larger smoothing capacitor, say 1000uF or 2200uF 35V.

Robert.

The lamp pictured is not compliant with UK regulations. Additionlly a metal bodied lamp with a two core mains lead is problematic. The 220V rating ia also incorrect for UK. I'd need to examine it but I bet it has multiple safety issues. Note that despite being UK stock the contact info is in China so good luck with support or getting your money back.

Robert

Thanks for replies, all read attentively and (more or less) digested.

Thanks Robert, that's exactly what I was hoping for - a 'rule of thumb'. I have the diodes and some 1000uF 50V capacitors in my 'bits box' so I can lash something up and see how it performs.

John Doe 2 - I came across the formula you quoted (or something very similar), plugged in some numbers (guesswork) and came out with 450,000 uF. Obviously ridiculous, but not being sure which piece of 'garbage in' led to this 'garbage out' I gave up and posted.

SoD - yes, 10-30V not 36V. Not sure what happened there! Thank you for your suggestion of using a resistor to drop the peak voltage. I think I have a box of 5W resistors somewhere. I have an oscilloscope so now I have a reasonable value for the smoothing capacitor I can build a simple circuit and see what happens.

Alan Johnson 7 - it's a transformer. Until I started investigating this I hadn't realised that there is 240/24V transformer which is always energised when the lathe is switched on at the mains plug. It seems to run all the switchgear (interlocks etc) as well as the light. It takes only a few Watts when quiescent so has probably only eaten about £50 over the 5+ years the lathe has been left on at the wall 24/7.

Maybe it's simpler to junk the 24V lamp and go for something mains powered but I wanted to explore possibilities and learn something along the way.

Robin.

Low voltage lamps were used on machine tools and as inspection lamps fpr two reasons:
Safety. Even if the moving and more exposed parts that may be damaged became live 24V is not a hazard.
Lamp life. Low voltage fiiament are lower electricalresistance and thus thicker. This gives longer life and better vibration resistance.

Care is needed when buying low cost mains powered machine lights (or making our own). Even just looking at the photographs of many on ebay show they do not comply with UK safety regulations and probably are not safe in practice. Nor having a proper UK mains plug and only two core cable are big red flags.
Yes you can make a class II (double insulated) lamp with metal parts and no earth. But it requirescare in detail design, material choice and manufacture. Add in the requirement for vibration resistance in normal operation of a machine lamp and it is difficult to acheive a suitable standard. Much better to use low voltage.

Robert.