Looking for least expensive variable 12VDC 8A power supply

[brettpim]

My main microscope is a Reichert Zetopan but in my early microscope enthusiasm, the biology department at my university was getting rid of some microscopes so I also have a Wild M20 and Leitz Ortholux. I am planning to donate these to my local field naturalist club but I need to get them working. The Wild is missing a power supply. The bulb is a 12V 100W halogen. I thought I could build myself an inexpensive variable power supply with an inexpensive 12V 100W LED power supply and dimmer . But the power supply gives up as soon as I get the dimmer up to about half way (voltage is about 5.8V). I have two questions

1) (for my own education) what is going wrong with my set up

2) what are some inexpensive options to get a 12V 100W variable power supply for this bulb?

[jfiresto]

Incandescent bulbs have a very low cold resistance (roughly 1/10th of when hot) and are among the most challenging loads to power with solid state electronics. Given its compactness, I expect you have a PWM dimmer and that you will need to oversize the power supply to keep the supply from fold-back limiting and not recovering, before the filament heats up and its resistance rises.

[brettpim]

Thanks for explanation John. How long does it take for an incandescent bulb to get "hot" and come up to its working resistance?

[jfiresto]

At the start, the filament looks like a short circuit. It could easily take a 1/10s for it to fully heat up, but it really depends on how much current the power supply and/or dimmer can provide before they detect an overload and shut down, and then for how long. (Some supplies you have to turn off.) I am somewhat familiar with driving 10~100W of incandescent, 12V automotive lighting,. Personally, I would probably find a dimmable, electronic transformer specifically designed for halogen lighting. Here is an example a contractor might use that suggests what I have in mind.

Better still, I might keep my eyes open for a used, 100–150W, 12V halogen light source, such as a Schott KL 1500, and wire your 100W lamp to that. (I picked up a fairly clean one off ebay.de with a single, fiber optic light guide for less than 50 euros.)

[brettpim]

Thanks John. Am I correct in thinking that, because the output is 12VAC, I cannot use the PWM dimmer? What kind of dimmer would I use and do I wire it to the 120VAC input side of power supply or the 12V output side of the power supply?

[DavyC]

Brett, you could try another approach. A cheapo enclosed 12v 10 amp DC power supply (Amazon). A cheapo 12V 20Amp PWM controller (Fleabay). Make sure the PWM is 'beefy' enough.
(Plenty of heatsinks is a good sign). So,.... AC in, to the Amazon power supply, The 12VDC out, to the input of the PWM controller. Output of controller to your Bulb. There should be a 'Pot' on the controller to vary the power to the bulb.
You might have to tweak things if you are up to that. Controlling the DC side of things is the way I would go. Good luck!

[jfiresto]

Thanks John. Am I correct in thinking that, because the output is 12VAC, I cannot use the PWM dimmer? What kind of dimmer would I use and do I wire it to the 120VAC input side of power supply or the 12V output side of the power supply?

The halogen transformer I linked, and probably others like it that for home/office interior lighting, expect the dimmer on the primary side: "Light dimmer should be installed on power input wire leading to transformer." I am not sure how this one conveys the 100/120hz chopping at the primary to the, I expect 10kHz+ AC output.

I have a silly question: can the 12V 100W supply directly drive the microscope illuminator, for at least a short time? The DC power supply may need to be able to do that, even when you add an intermediate PWM dimmer.

It really depends on the design and I do not know the cost cutting people resort to create low cost power supplies. It costs more to insure electrical safety and I am not one to take chances.

[Pau]

2) what are some inexpensive options to get a 12V 100W variable power supply for this bulb?

At the used market you can find dedicated 12V 100W power supplies for microscopes, not the most interesting option but likely the easiest one.

[brettpim]

Pau,

The used market here on the forums?

thanks
brett

[brettpim]

DavyC

Can you please link to an example of the kind of power supply you mean, because the cheapo 12V 10A DC power supply I indicated in original post did not work, so I want to make sure I consider the correct things.

thanks
brett

Brett, you could try another approach. A cheapo enclosed 12v 10 amp DC power supply (Amazon). A cheapo 12V 20Amp PWM controller (Fleabay). Make sure the PWM is 'beefy' enough.
(Plenty of heatsinks is a good sign). So,.... AC in, to the Amazon power supply, The 12VDC out, to the input of the PWM controller. Output of controller to your Bulb. There should be a 'Pot' on the controller to vary the power to the bulb.
You might have to tweak things if you are up to that. Controlling the DC side of things is the way I would go. Good luck!

[chris_ma]

Can you please link to an example of the kind of power supply you mean, because the cheapo 12V 10A DC power supply I indicated in original post did not work

13 USD for a 100W is really pushing it, simply not worth risking an injury or a fire because of cheap electronics!
also, if you have a 100W halogen bulb I'd want at least a 150W PSU.

I'd get a MeanWell 12V 150W, can be found for around 30USD over here.

[DavyC]

Brett, reading your first post I missed the links you used and just read your query. I took it that you were using a dimmer on the AC side of things.
These intelligent leading/trailing edge dimmers can be load sensitive.

The reason your meter wouldn't give accurate readings is down how it samples. You would need a scope to give a clearer picture.
Chris was more on the money with his 150w supply, as you need to downrate these supplies by around 20%, as figures quoted are best case scenario.

Budget option was key for you, but as the guys pointed out, a shortcut is often the longest way round.
I am not a fan of these cheap switch mode supplies. Cheap components/specs and they die in a heartbeat.

No cheap easy answer unless you hit lucky with a microscope supply

[brettpim]

Thanks Chris and DavyC. The example and explanations clears things up. I found a pair of used Mean Wells for a decent price.

[physicsmajor]

Looks like you likely have a solution, but for anyone else that needs a rather high amperage 12V power supply consider a repurposed computer tower PSU. Computer tower PSUs dedicate the vast majority of their output to the 12V rail. The main 12V rail on a 550W supply I have lying around from a decommissioned tower is rated for 41A (12V * 41A = 492W of this 550W PSU is dedicated to this rail), and I would expect it to survive the startup of a halogen bulb. People are practically giving away old towers on Craigslist, so you can get these inexpensively. When underutilized, some will run without the fan active.

Take care to use appropriate wire gauge when hooking up supplies that can deliver this kind of current...

[chris_ma]

computer power supplies certainly are a cheap good option, for those who are new to this you usually have to add a jumper to one of the contacts for it to start up. plenty of info about this on the internet.