LPL/Saunders C7700MX/670 power supply help

Rodrigo Silva

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Hello everyone, this is my first post here, always find great references in this forum in my Google searches.

I bought an LPL C7700 Enlarger that is in good general condition but missing the 12v power supply and correct bulb.

The device arrived with a 250W lamp connected directly to the electrical mains (127v). Which serves for some quick tests, but not for use since it generates too much heat, and can cause damage to the mixing box or filters.

I was able (with some difficulty) to find a 12V 10amp transformer and built a box with selector on/off that should work but my doubt is whether the original power supply has other components besides the voltage transformer.
I tried Google images but couldn't find any reference.
100w 12v lamp is on its way...

Has anyone here ever opened or had to make any repairs to confirm this information?
Thank you!
 

koraks

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I don't know to be honest, but I sure hope that your diy setup (which will work fine I'm sure) does have a fuse in there somewhere. It looks like you hooked up the primary of the transformer directly to the power socket, but it should be fused for obvious reasons.

If I were to diy a power supply for this application I'd use 12vdc (from a transformer, rectified and with a fairly large capacitor or a off the shelf smps unit) and put in some sort of voltage regulator to keep the voltage at exactly 12v. This will prevent any contrast or color issues in case you have voltage fluctuations (which is more a question of how big they are than if there are any).
 
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Rodrigo Silva

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To be honest, I forgot to buy the small housing fuse and it is something fortunately very easy and that I will add, but I am not sure is necessary since my configuration will be:
110V network - power stabilizer - timer - Enlarger (both stabilizer and timer have fuses).

I thought about using a modern 12vDC power supply which would be easier and cheaper than the transformer but I read some conflicting information about running these halogen bulbs in DC. (I m all about DIY but don't know much about electronics)

At the end of the day is just a light bulb and should be fine...
 

koraks

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I'd still add a fuse for good measure, but I can see the sense in your reasoning. It's not a big risk when it's hooked up this way assuming that the power conditioner is indeed fused and the timer as well (on the circuit it actually switches, which is not necessarily the case).

I don't see the problem in running these bulbs on DC but maybe the power conditioner you're using already offers sufficient stabilization. Depends on what's in the box.
 

Bikerider

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The equivalent bulb for use in UK (which has a 230v 50mh voltage supply) is 24v from the transformer and the bulb is 100w so you should not be too far from the target.
 

voceumana

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The power supply provided for us in the USA has a transformer, a power switch, and that is all, apart from the chassis, cable, and a connector for the enlarger lamp head. It did not include a fuse, but a fuse is always a good idea. Output is 12 VAC at 100 VA.
 

AgX

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koraks said:
If I were to diy a power supply for this application I'd use 12vdc.
Click to expand...

Why DC ??


DC only makes sense if you need to have a voltage regulation and want do it at the low voltage stage and then at DC.
Is that the best way to do it?
 
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Rodrigo Silva

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Good to know, thanks for the info!
I will add the fuse in my box just for extra safety and I should be able to have the enlarger up and running again for the weekend.
I'll let you guys know if worked.
 

koraks

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AgX said:
Why DC ??


DC only makes sense if you need to have a voltage regulation and want do it at the low voltage stage and then at DC.
Is that the best way to do it?
Click to expand...
Yeah, because it's easier and cheaper to do regulation on DC. If I were to design a power source for an incandescent enlarger light source, a stable voltage would be one of my requirements to prevent shifts in the spectrum and output of the bulb, as color temperature tends to drop quite steeply as you lower the voltage on the bulb. It's kind of a chore to do that on the AC side but very easy on DC. A simple voltage regulator and a pass transistor for the heavy lifting does the job.
 

AgX

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Actually one just could use in this case a 12V 10A regulated, lab power supply ...
 
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Rodrigo Silva

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Well now is a little too late for the DC options but is good to know that would work.
I could save cost and would be about half for something like meanwell smps lol
By the way I added the fuse, now just waiting on the bulb...
 

ic-racer

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Many stabilized enlarger power supplies use pulse width modulation of the incomming AC via thyristors or triacs. Not sure how a conversion to DC is going to be more elegant than that.

 

Majk369

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Hi,
Id like to ask if you could use a solid state LED supply that is 12v 100w DC, like this one:
I am asking since these are ubiquitous and i feel better having something new and solid state as opposed to a 30 yr old transformer that i dont know how it was stored/handled, since ive had some bad experience with such PSUs in the past.
 

koraks

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Hi @Majk369 and welcome to Photrio!

Yes, a supply like that will work in principle, but there are some caveats.

I'd always recommend a hefty safety margin in the power rating. If you intend to supply a 100W bulb, then take a supply rated for 150W at least. Otherwise it'll run hot and may not survive for long, although an enlarger is a fairly benign working environment in this respect.

Also keep in mind that if you switch the entire power supply on and off, there will likely be a delay when turning it off as the output capacitors discharge through the lamp; you'll see the bulb fade out within a period of a dozen to a few hundred milliseconds. This will create problems with short exposures. In some Durst enlargers, this doesn't matter as the actual timing of the exposure is done with a shutter, but only on higher end models like the L1200 (I think? Someone can chime in here no doubt). Alternatively you'd have to switch the 12V DC output, but the contacts in a typical darkroom exposure timer aren't rated for this amount of current (around 8A for a 100W lamp) and the switching relay will burn out quickly.

So it depends a bit on the exact enlarger and what your setup is and/or how you intend to modify the situation, if this particular unit will work reliably.
 

Majk369

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Thanks a lot, i havent thought about the switching problem at all, but it makes sense of course. The enlarger i intend to use this for doesnt have a shutter, so ill probably have to either source the original PSU and hope it works, or build something myself.
 

koraks

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In principle it's not ver difficult electrically to build or modify a timer to use a MOSFET switch to switch the 12V DC side of the power supply. I don't know about your aptitude with electronics, but with a handful of off the shelf modules (or discrete components if you're not afraid to design a circuit & solder it together) you could get pretty far.
 

Majk369

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One more question:
would running the halogen bulb on 12V DC rather than AC (as I guess that the original PSUs are outputting AC) have a significant impact on the intensity and color temperature of the light?
 

koraks

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The bulb will give the exact same amount and quality of light provided it is fed with the same DC voltage as the RMS AC voltage it runs on in the original setup.

Btw, a friend of mine recently did some testing and perhaps even modified a big CLS1000 power supply by replacing the problematic original unit with a fairly simple AC dimmer circuit, the kind that uses simple phase control dimming and that can be fed with a PWM signal to set the output 'voltage' (in fact the phase dimming duty cycle). This is generally a cheap and in principle effective setup. I personally am not a huge fan of phase control dimming, but that's a bit of a personal pet peeve. It might be worthwhile considering it given the low cost and simplicity. The obvious drawback is the lack of galvanic separation between the bulb and the power grid which you may (should) take precautions for.
 

Majk369

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After some time of looking into this I have designed an actual timer for the enlarger built around arduino nano. The thing is that when I want to build a switching circuit for the high current line going to the bulb, there is one question that I have. Do the capacitors have a way of discharging (since they are not going to be able to discharge via the bulb) when the switch opens? Or is there a risk of them keeping the charge and possibly damaging the bulb when the mosfet becomes conductive again? The PSU i ended up using for now is the original one so we are talking about 12V AC.
 

koraks

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Majk369 said:
Do the capacitors have a way of discharging (since they are not going to be able to discharge via the bulb) when the switch opens?
Click to expand...

Depends on the circuit you have in mind; without knowing this, I can only take a wild guess and say that if you have a circuit where you open a switch, nothing can then charge or discharge anymore. This would be the case of you use e.g. an N-channel MOSFET as a low-side switch for the bulb.
What capacitors are you talking about btw? The filter caps on the power supply?

Majk369 said:
we are talking about 12V AC.
Click to expand...

Ok, so we're NOT talking about a MOSFET switched circuit but probably something with a relay (?)
Keep in mind that switching a powerful 12V bulb involves pretty high inrush currents and you'd have to use a pretty hefty relay for it to survive very long. Relay contacts will burn/weld at such high currents.

Alternatively you could use something like a thyristor circuit; there are 'Arduino' modules for this as well where you apply a trigger or even a PWM to an input and the module will switch any (50/60Hz) AC load. The module will take care of things like zero-cross detection and timing + applying the thyristor gate pulse.
In this scenario there won't be any significant capacitance in the circuit, it being AC. Be sure to select a module that can switch the current your bulb requires (plus its significantly higher initial current when the filament is still cold); most thyristor modules will be intended for 230V/115V operation and lower currents; you're working with a substantially lower impedance.

So in short, everything depends on how you're going to approach this. I get the feeling you're not really sure about how to do this; how about an off-the-shelf solution? There are various enlarger timers on the market; I'm sure there's something that would fit your needs.
 
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