Timers on Fluorescent Grow Lights

by Michael E. Basham

For fluorescent grow lights, I've read that it is best to use mechanical timers as opposed to digital timers. Supposedly, the electronic ones can sometimes blow out the ballast in a compact fluorescent light (CFL). What are your recommendations on this subject?

Answer: Michael- Personally, I have always used fluorescent grow lights for the vegetative stage and HPS lights for flowering. When my plants are under the fluorescents I keep them on 24 hours a day, so there has never been any need for me to use a timer with my fluorescents....however, I do have a few friends that flower under T5 fluorescent lights (TEK lights), and per my recommendation they have always used digital timers and have never had any problem with them.

Now, the ballast in a T5 fluorescent light may be a little different than the ballast in another kind of fluorescent light (such as a CFL), and perhaps the problem truly lies with the ballast of a particular brand or type of fluorescent light....this I am just not sure of.

The reason I recommend using a digital timer over a mechanical one is simply this....eventually the little plastic pins used for setting a mechanical timer wear out, and so you come home one day to find your grow lights are still on when they should be off. This is a particularly horrible problem when you are half way through a flowering cycle....the whole garden gets light when they should be getting complete darkness, which causes some plants to go hermaphordite (which can cause pollination issues that were not a problem before that point), and causes other plants to be confused about whether they are supposed to be in vegetative growth or flowering....which delays the flowering process and delays the final harvest date of the crops in the garden.

In reality, whether you use a digital timer or a mechanical style timer, a significant electrical load is transferred in a split second's time (when the timer kicks on) to the ballast of the light you are running....so if a particular brand or type of fluorescent grow light had a ballast fail, then it probably would have failed whether using a digital timer or a mechanical timer. Thanks for the question Michael, and Happy Growing!

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Apr 08, 2011
by: downtheroaddave

Hi, i own a hydroponic store in the Uk and have some advice for timers. The initial inductive load when a light comes on varies between 1.25 amps and up to seven. Most timers of the generic type are generally rated around .4-.5 amps and that is what blows your timer. Contact relays remove this issue from the room and hold the power in the unit and as the timer turns on then a gate opens (simply) and hey presto no big inductive load to pop the timer. I always recommend them with a light or kit and these are the people who never have to replace timers.............

Aug 22, 2010
CFL's and electronic timers
by: Michael E. Basham

Food for thought: I recently purchased two Sunlite 105w daylight CFL's. On the packaging box it states, in part, "do not use with emergency exit fixtures, exit lights, dimmers, photo controls, or electronic timers." Either the manufacturer is covering their backside legally, or there is an issue here using CFL's with digital timers.

Added Response:I have not heard of any issues in particular with using CFL's with digital timers...my gut feeling is that the manufacturer is covering their backside with this type of warning.

Aug 11, 2010
by: Kevin

Actually, you need to pay attention to the specifications of the timer - while many timers say it can cope with a load of 5Amps/10Amps/13Amps, they do not say what inductive load it is capable of.

While this is not of concern at the lower end of the scale, i.e., a few hundred watts of flouro's but then you use that timer (the common cheap variety) to switch anything above a 400w HID will arc across the contacts as it closes and will micro spot-weld the contacts inside the timer and fuse it to the closed position thus rendering it permanently on or off depending on how the manufacturer has set it. For HID's above 400w you should be thinking of a mains relay that works via a timer - sometimes called a contactor. Putting a brief but heavy inductive load across a timer that cannot cope with it is a fire-risk.

Added Response: All very good points to keep in mind. Thanks Kevin.

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