How safe/effective is a basic magnetic ballast as far as current limiting goes? I pulled one out of a fish tank light and was wondering if I could use it for current limiting of a heating element that has a very low DC resistance (couple of ohms). What kind of resistance are they designed to work with? I know a mercury bulb is very low impedance when it's conducting after it ionizes but exactly how much? The ballast I have is rated for a single T8/T12 bulb at 320ma, the coils DC resistance is 27 ohms, I don't know it's inductance but I've worked out it should have an inductance in the area of 250mH if it's rated current is short circuit current (120V line), otherwise it's inductance going to be different depending on the resistance it's rated current is based on. Can anyone shed some light on these things for me? I would like to use it after a fuse for limiting total power to very low impedance loads.

24 hours and not a single response? Come on folks throw me a bone! I thought more users would have used these things before.

I have a couple in the fluorescent fixtures in the ceiling of my shop. They hum annoyingly. That's the sum total of my experience with them.

Sounds like a reluctance ballast to me, someone must have tried to use them for power limiting a native line load other than a mercury bulb? In case anyone is confused reactant/magnetic ballast means 'great big honking series inductor'

Can you explain the difference between reluctance and inductive reactance? I thought they were the same thing.

I think I might be starting to answer my own question; Reluctance is based on the permeability of the core itself. Depending on the output load and the self inductance and native DC resistance of the main winding and it's cores permeability it's going to have a very non linear current limiting response? Now I guess I'm trying to figure out what the current limiting properties of my ballast are with various current loads, particularly purely resistive loads without having to short anything out. I think I might need to browse the Yahoo groups ltspice library again and find better inductor models that model the inductor core, especially with ferrite or iron cores.

Originally Posted by Roff

Can you explain the difference between reluctance and inductive reactance? I thought they were the same thing.

Reluctance is resistance to magnetic flux, whereas inductive reactance is (part of) resistance to an AC current. The former is a magnetic thing, the latter electrical...

Originally Posted by Sceadwian

How safe/effective is a basic magnetic ballast as far as current limiting goes? I would like to use it after a fuse for limiting total power to very low impedance loads.

Common reactive ballasts not only limit current during steady-state operation, they produce a HV spike when first turned on for ionizing the gas mixture. Is the latter something your circuit can withstand? If not, you need to look elsewhere...

I don't know.

I would think they are made as cheaply as possible so are not designed for short circuit operation.

When struck the tube will have between 30V and 150V (on 120V a larger ballast will be an autotransformer to boost the voltage) across it (going from a small 4W tube to a huge 100W tube). A smaller fixture might be alright since 30V isn't a big deal but a larger fixture might overheat because the current will double.

However, it's possble for a switch starter to weld it's contacts together short circuiting the tube which I've heard can cause older ballasts to overheat and catch fire but modern ones will either be designed for short circuit operation or contain a thermal fuse which is probably non-resettable.

Originally Posted by Sceadwian

How safe/effective is a basic magnetic ballast as far as current limiting goes? I pulled one out of a fish tank light and was wondering if I could use it for current limiting of a heating element that has a very low DC resistance (couple of ohms). What kind of resistance are they designed to work with? I know a mercury bulb is very low impedance when it's conducting after it ionizes but exactly how much? The ballast I have is rated for a single T8/T12 bulb at 320ma, the coils DC resistance is 27 ohms, I don't know it's inductance but I've worked out it should have an inductance in the area of 250mH if it's rated current is short circuit current (120V line), otherwise it's inductance going to be different depending on the resistance it's rated current is based on. Can anyone shed some light on these things for me? I would like to use it after a fuse for limiting total power to very low impedance loads.

With very low impedance / short circuits it should work for a short time, as they are designed to allow the tube to start.

As Hero said, they should accept a long time short circuit without going up in flames, but perhaps they have an internal non resetable fuse. You may try it measuring its temperature and stopping if it gets too hot.

A mercury lamp (like a LED) has very low impedance, but it also has a fixed voltage drop (the voltage deppends on the length and diameter of the tube). So it needs the balast to limit its current.

You may estimate the voltage drop of the tube from its power and current. (V = P / I).

Saturn, the starter was a seperate circuit, the ballast is basically just an inductor, where is an HV spike going to come from?

Originally Posted by Sceadwian

Saturn, the starter was a seperate circuit, the ballast is basically just an inductor, where is an HV spike going to come from?

The HV spike comes from the inductor precisely because it's an inductor. The starter is there to allow current to flow through the end filaments of the fluorescent tube, before ionization of the gas mixture occurs. After the starter opens, the ballast (inductor) generates the HV spike (due to the rapid interruption of current) which ionizes the gas, and primary current through the lamp thus now flows through the ionized gas from one end to the other, and is then limited by the ballast (inductor) in steady state operation.

I haven't looked into the newer systems with electronic ballast, etc, so I don't know how those operate...

I was only interested in the ballast itself not the starter, the starter isn't directly attached the the ballast, I wasn't concerned with it. Though now I know how it works though thanks =)

Originally Posted by Sceadwian

I was only interested in the ballast itself not the starter, the starter isn't directly attached the the ballast, I wasn't concerned with it.

Fine. What happens when you turn your circuit off?

The nice thing about having a non-galvanic element (the lamp) in the loop is that it can take the punishment of the ballast-induced HV spike, whether the starter initiates it, OR when turning your circuit off. Can your ohmic load, though? Just something to keep in mind :-)

I haven't done anything with it yet, that's why I was asking. And what happened to it would entirely depend on what part of the current cycle it was turned off at, as at the 0 current crossing turning the circuit off would produce no kickback spike at all. I'll figure out what the maximum energy of a spike when being cut off at peak current, thanks for the warnings, I bet the voltage could be pretty nasty, enough to weld contacts rated for nominal current/voltage at least.