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Enlarger Regulated Supply

Regulated supplies avoid voltage variations which are responsible of density and color changes on prints.

Some types of regulated supplies:

Conventional Supply

Here is a project for a 12V/10A regulated supply for my Focomat V35 enlarger but which can be used with every 12V light bulbs up to 120W. This model has been replaced by a switching supply (see below).
If your timer is intended to switch main supplies, it's possible that it cannot switch the higher current involved with a 12V supply, check at the Jobo 200 page how I solved the problem.

Set the 5K variable resistor to 0 Ohms and once the circuit connected to a true bulb, increase progressively this value to obtain 12V.
Check and adjust this voltage from time to time.

And the final product:

Switching Supply

A stabilized switching supply is much more efficient than the above circuit.
You can find commercial models at reasonnable price.
The filament of the bulb has a much lower resistance than a hot one, there is a current spike, also called inrush current, that can go as high as 10x the nominal lamp current, I strongly advise you to:

The chosen model  is ESP 150-12S distributed by Traco in Switzerland which allows 150W under 12V. The voltage is adjustable at +- 10% and current is limited to a 105% of the nominal max current, thus around 13A. This limitation avoids the current peak and lengthen the lamp's life without slowing noticeably the switching on of the lamp.

Magnetic Stabilizer

Special magnetic transformers act as voltage stabilizers for the mains voltage.
Their principle is to saturate their magnetic core so that changes in the primary do not reflect as much on the secondary.
Nonetheless, they are less effective than the stabilzed supplies.

Some manufacturers:

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