I recently inherited an awesome 4foot 2x250Watt metal halide lighting unit for my marine tank.

The previous owner wanted to dump it because one of the lights was flickering and he could not really figure out why.

So after I got it; I stripped it down and realised the terminals on the coil transformer were overheated and the wires were not in good contact. I replaced the terminals, made sure there is good contact and re-stripped the wires and made sure they were still up to the job.

I figured out that one of the ignitors was faulty and I need to order a new one. When I could not find the same model I ran into a horde of different ratings, models, packages. In the end I found the one I need. Then looking at some circuits - everybody says there should be a capacitor ... I do not have one? But the one working light does ignite and sustain.

From this page I can say my circuit is 1 - but without the capacitor?

enter image description here

Looking at the lookup 250HQI/MBI it says I should have a 35 µF cap before the core transformer? I have read various articles and they say that the light won't ignite WITHOUT this capacitor but mine does - Also I have two ballasts and ignitors.

Should I put a cap in there to help the two ballasts?


I just bought two new ignitors.

One is a standard ignitor (<250W) for £5 (no cap shown in schematic) enter image description here

And a newer electronic ignitor for £20 (<1000W) (shows cap in schematic)

enter image description here

I wired this one in - Nothing happens! :( Going back.


4 Answers 4


There are several types of ballasts - and yes, most of them require a capacitor for High Pressure Sodium bulbs but not for Metal Halide (Mercury) bulbs!


Used only with 120volt input (like USA) and is the cheapest ballast to produce as it requires no capacitor or ignitor (unless strictly specified) but is a very un efficient one with a low power factor of 50% (can also be found in high power factor but not popular)


Is a reactor ballast (as above) with an extra coil attached to regulate voltage like 240volt down to 120volt - There are high power factor and low power factor types and typically only high power factor (>80%) ballast require a capacitor with a matched valued in order to operate properly. But the normal power factor ballast 50% does not need a matched capacitor; But you can put one in if you want; However, this does not provide regulation to the lamp, and may draw a higher current during open circuit operation.

These are slightly higher in cost but still cheaper than regulated ballasts. They draw more start current and are poorly regulated.


Combines the best features of the High Reactance and Magnetic Regulator Ballast. It has a high power factor of over 90%,starting current is low, costs less than a magnetic ballast and losses are low. But they cost more and weigh a lot.[I could not find anything about capacitor on these types.. needs more info here]


It is a three winding ballast similar in design to the older mercury ballast design, but provides regulation with line voltage variation of + or – 10% from nominal. It also has a capacitor in the circuit for wattage control and is a High Power ballast. This type has better wattage control and is safer as the 3rd coil is isolated but is the most expensive and prone to frequent loss.


Is similar to the Auto Regulator Ballast in performance. The Isolated Regulator Ballast electrically isolates the lamp socket and capacitor from the line. The Magnetic Regulator Ballast also isolates the lamp circuit from the line and additionally improves lamp wattage regulation, but may cause an increase in input watt. It provides good wattage control and is cheaper than the magnetic ballast and ballast losses are average.


Capacitors are used as a power factor correcting or current regulating device and provide the control necessary to ensure proper lamp and ballast operation. Different wattages, voltages, and ballast types require a variety of different capacitor values. The ballast I.D. label specifies the microfarad and voltage rating needed to operate properly. If the capacitor is incorrectly wired, improper operation of the fixture as well as other component failure could result.


You have two types of ignitor circuit that rely on the type of ballast you have. There is a super imposed and pulse ignitor. No mention of a capacitor.

  • Superimposed ignitor circuits is where the ignitor creates the high voltages without the ballasts help. - The ballasts in this circuit type have two wires(output) and is called untapped.
  • Pulse ignitor circuits use a tapped ballast that has a third (output) wire which is wired into the ignitor; But has a common place at the live side of the bulb. The ignitor will decide when to pulse high voltage that are generated at the ballast!

enter image description here

enter image description here

Which one is better? Not sure.. some say the super imposed last longer but make some humming noise as where the pulse ignitor can use silent electronic ingitors. Your choice.

So based on those types I identified that I am using a superimposed circuit(the old style) Since it originally came with no capacitor it must be an HX ballast on normal power factor and only super imposed ignitors up to 4.5kilovolts will work within that ballast.

So I cannot use the electronic ignotor I bought because my ballast does not have a high voltage coil. This more expensive electronic ballast is better because it provides a clean high frequency on high voltage start-up current that reduces lamp smouldering and increases its overall life.

End of the day I still don't know what capacitor I should use but now I know that I do not need one. In my configufration it is optional and will help increase the life of my ballast if it is heavily used but not much more. You can buy dry cell caps rated for you system but it only seems to be used in specific ballasts configurations.

Off course the right thing to do is replace these old ballasts with to be used with electronic pulse igntors - that are silent, more power efficient, have built in protection circuits for it self and the bulbs and come in a single package this is much lighter and smaller than all those mentioned above; but they are the most expensive to buy!

Ref1 Ref2 Ref3 And the rest of the internet...


Good info Pumpkin. I believe the input capacitor serves three main functions.

  1. reduce egress noise
  2. Improve THD and Power Factor egress
  3. ensure low ESR across the line at the transient firing rise time. So high ESR translates to lower firing voltages. Since new MH lamps have lower trigger voltages that increase with age, it appears you are ok .. for now. Then EOL or end of life is when the HD lamp gradually increases its power output and trigger voltage so that it exceeds the capability of the supply and fails to trigger.

Providing a low ESR for the line neutral loop would seem more important to me than the resonant values, but that may not be true for reactor types.

I would be looking at Film plastic caps with low ESR not the cheaper ones for induction motor starting.

It must be a massive tank. I guess you wont need a water heater now.. just a relay to regulate the lamp for temperature. ;) j/k

  • \$\begingroup\$ Halides are 6ft above live sand (for proper lighting exposure for fish and fauna) so there is a good gap between the tank and the lights. They do not heat the water much(or at all) my heater is still working full whack in winter. But the unit it self gets very hot. I installed low speed fan to help circulation and cooling. When I got it all the wires were burnt out and melted into a goo of plastic. Put ceramic blocks in and this new silicone 400C heat resistant 3mm2 core wire.. purrs like a cat now. Nice effect too. +1 \$\endgroup\$
    – Piotr Kula
    Dec 6, 2012 at 9:27
  • \$\begingroup\$ Good cooling will extend the life. Must be a beautiful setup. Yes the GE Silicone we used would prevent wires from burning on payloads during re-entry. \$\endgroup\$ Dec 6, 2012 at 9:37
  • \$\begingroup\$ Will post a pic in meta or something when I get a chance. It is a nice tank but it needs allot of work "investment" still :) \$\endgroup\$
    – Piotr Kula
    Dec 6, 2012 at 10:06
  • \$\begingroup\$ I have submersive LED's if interested. \$\endgroup\$ Dec 6, 2012 at 10:09
  • \$\begingroup\$ Awesome, you also got a tank? You on ultimatereef or another forum perhaps? \$\endgroup\$
    – Piotr Kula
    Dec 6, 2012 at 10:51

What I have learned about metal highlight in the field is that an M59-E is a "probe start"(with capacitor, but no electric ignitor) and a M138-H is a "pulse start"(with electric ignitor and capacitor). There are more types but generally 2 letters and 2 numbers is probe, and 2 letters 3 numbers is pulse.

(They will be extinct like mercury vapor and replaced with LED) (Your posts were a great read and i learned something, thank you)


Yes, you need to use a capacitor with the Blast, as new electronic ballast required to be use with a Capacitor, its use is like a big-Kick Capacitor. Big-Kick Capacitors are normal capacitors, the only thing with their name is from their usage, they are used for initial kick-start type application. If your lamp or ballast are week after several hour usage than this Capacitor will act as a fresh power for starting purpose.

Some electronic ballast may start new ballast and lamp without this capacitor but old ballast or lamp may required a proper rating of capacitor

  • \$\begingroup\$ I use a magnetic coil(weighs 2kg) and 2-5kv ignitor. The MetalHalide lamp starts normally (ignition) WITHOUT the capacitor - So why do I need it? PS I have two lights running like this. \$\endgroup\$
    – Piotr Kula
    Oct 30, 2012 at 13:21
  • \$\begingroup\$ Does not answer my question really ? I still not sure if i need to use the cap, and with which ballasts, ignitors? I did an edit with links to ignitors. I cannot work out what caps to use and they don't say anywhere if they are mandatory or not.... \$\endgroup\$
    – Piotr Kula
    Nov 2, 2012 at 11:45
  • \$\begingroup\$ You like my answer? I never knew it was that complicated but now I know about the difference ignitor types and various ballasts configurations. That answers my question because now I know what to buy.. and No. it was not a capacitor, -1!! \$\endgroup\$
    – Piotr Kula
    Nov 5, 2012 at 22:09

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