Best Grow Light Ballasts: Magnetic vs. Electronic


August 22, 2016

This article was originally published in 2015, Garden Culture Magazine US05, under the title, “Ballast Pros & Cons: Magnetic vs. Electronic”.

Almost every grow room has a pile of ballasts driving a bunch of lights. There’s a lot of confusion among growers about ballasts. Which ones are the best grow light ballasts? How do they work? What are they for? Let’s take a look…

What is a Ballast?

Grow lights are usually High-Intensity Discharge (HID) bulbs, which take a little work to get running. Whether the bulb is a metal halide used for vegetative growth or an HPS (High-Pressure Sodium) bulb used for flowering, they all require a very high voltage to ionize the material in the bulb to get it glowing and conducting electricity. This Strike Voltage can run as high as 4000 volts and is only on for a few seconds, or tens of seconds. Just long enough to get the gasses in the bulb glowing and conductive enough for the bulb to light.

Once the arc strikes, the bulb needs to be current-limited as it warms up, and finally held at a stable operating point when it’s good and hot. It’s the ballast’s job to strike the arc, limit current during warm-up, and maintain a stable operating point once we’re up and running. Kind of like a tightrope walker with a balance pole – the ballast’s role is to maintain a stable operating point for a naturally unstable arc light.

Core & Coil

How do the best grow light ballasts work?Iron core inductors are a simple and cheap way to make some of these things happen. If you wire an inductor in series with your HID bulb two things happen: First, the inductor charges up when current flows through it and dumps all that energy when the current stops. You can get a pretty good-sized spark this way. It’s the same technique we use to fire spark plugs in a car. Since the AC power line cycles 50-60 times per second, the inductor can provide us with the needed high voltage for starting the arc.

Since an inductor resists changes in current, it can also act as a current limiter and, if you choose your parts wisely, it can set the final operating point of the bulb. Not bad for a steel core wound with a mile of wire!

The bad news is that current and voltage move close to 90 degrees out of phase with each other across an inductor. Power is voltage multiplied by current, and if they are out of phase, you don’t get the use of all that power you’re paying for at the meter. Electrical engineers report this out of phase condition as Power Factor, with 1.0 being perfect. You can think of Power Factor as the percentage of power you pay for that doesn’t get used by the light. A PF of 0.85 means 15% of the power you buy doesn’t do any work.

Magnetic ballasts use a capacitor to move the current and voltage closer to being in-phase, but the net result is that with the power factor being quite a bit less than 1. Due to the resistance of all that wire and the magnetic losses in the core, a magnetic ballast’s efficiency is going to be around 85% or so. That means it’ll take 1170 Watts of power to light a 1000W bulb, with the extra 170 Watts just heating up the room. Not exactly what we want, but easy to build, and inexpensive to buy.

The good news is the only thing that ever really goes bad on a magnetic ballast is the capacitor, which can usually be fixed at the store where you bought it.

Electronic Ballasts

Electronic ballasts have four big advantages over core and coil ballasts:

  1. Variable operating point – Electronic ballasts can compensate for ageing bulbs, so you get the same light output even as the lamps age.
  2. Efficiency – Core and coil ballasts gobble up 1100-1200 Watts to run a 1000 Watt bulb, where a good quality electronic ballast will only need 1050-1100 Watts.
  3. Bulbs live longer – Electronic ballasts can adaptively maintain output power levels to keep bulbs brighter for a longer period of time. You change bulbs less often.
  4. Dimming – Electronic ballasts can be dimmed, and some advanced designs can also drive multiple bulb types and power ratings (400 / 600 / 1000).

Electronic ballasts are also:

  • Smaller and lighter
  • Able to restrike a hot bulb sooner
  • Lower in perceived lamp flicker
  • Can be quieter and produce less acoustic noise

As prices on power semiconductors came down in the 1990s, engineers started pushing forward with Switched Mode Power Supplies (SMPS), since a switching supply running at high frequency requires smaller transformers and inductors (the expensive parts) than an SMPS running at a lower frequency.

Buying Tips: Best Grow Light Ballasts

Designs for HID ballasts have steadily improved, and electronic ballasts are now a practical and accepted solution for running HID lights. A good electronic ballast has a Power Factor >0.98, almost perfect, and runs at better than 90% efficiency. Some advanced designs are now pushing 97% efficiency, at power factors approaching 0.995! There’s a world of difference between magnetic and electronic ballasts, and quite a difference between brands of electronic ballasts as well.


One question I see a lot on the forums and hear in hydro stores is about fans. Is it better to have a fan on the ballast or not? Heat is the ending of all electronic devices, and you can count on halving the life of a given device for every 18°F rise in temperature. So, a fan is good, right? Well, sometimes… Fans on electronics are like turbochargers on cars – if you can run without it, the machine will live longer.

If the ballast runs cool without the fan, a little forced-air cooling will make it live longer. If a ballast NEEDS a fan to keep from melting, that’s not so good. Fans suck in dirt along with air, and dirt makes things break. Put your hand on the ballast at full power. If it is warm to the touch and has a small fan, that’s fine. If it’s hot to the touch, or has big fans, and is still hot, that’s not what we want. If it needs a fan to stay alive, it’ll die when the fan does.

Cooler is always better for the life of the equipment, and for your power bill. Remember, you’re paying TWICE for all that heat: once to heat up the ballast, and again to have your air conditioning remove the heat from the room.

Electrical Noise

Acoustic noise is easy to check just by listening, but electrical noise is the kind that makes pH and TDS meters read wrong. It also alerts anyone with a radio that there’s a room full of lights and ballasts next door. When the local WiFi has problems precisely 12 hours on and 12 hours off, it doesn’t take an electronic engineer to sniff out the cause!

A simple and easy test is to take a hand-held AM radio and tune it between stations so you just hear static. Turn the lights on and tune up and down the band. Do you hear your ballasts blasting away? Most of them do, and it’s an easy check to make before purchasing a ballast. Quieter is better because if your ballast doesn’t interfere with your neighbour’s radio, cell phone, or computer he’s less likely to give you a hard time about your garden.

Electromagnetic interference (EMI) can come from the ballast itself as radiation, from the lamp cord, or from the power wiring. Every wire is an antenna at some frequency, and the longer your lamp wiring is, the more likely you’re going to hit the antenna jackpot. For lamp wiring, shorter is better.

Conducted EMI coming down the power line is more likely to cause problems with computers than radios, but there’s not much a grower can do about it without redesigning the ballast. In our own EMI testing, we found that all the electronic ballasts were noisy on the lamp wiring, but the worst offenders were the ones that had conducted EMI (power line) issues.


Magnetic ballasts are heavy, inexpensive, and less efficient than electronic ballasts. Electronic ballasts beat magnetic models in all areas of performance, but poorly designed units can make enough electrical (RF) noise to cause instrumentation issues for the grower, and radio interference issues for his neighbours. A quick radio check can save you a lot of trouble by telling a good one from a bad one, as can simply putting your hand on the ballast to see how cool it runs. You don’t need expensive instruments to check your ballast – just a cheap radio, and your five senses.

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