Pyrotronics MFS-4 Pull Station Question

Hey everyone,

Today I received my Pyrotronics MFS-4 pull station, and while it was NOS, it didn’t come with any mounting screws or official documentation, to my dismay.

The wiring of these stations is similar to the MFS-2, however these stations have a large black resistor placed across a couple of terminals. My MFS-2 had (or still has, I can’t remember) a similar resistor soldered onto some of the terminals.

On this model, The switch wires meet on one of the same terminals with said reistor, while the other end of the resistor just has that soldered onto the terminal.

My question is, in order to use this on my 4004 system, do I need to cut this resistor off? I know these were meant to be used on a old Pyr-A-alarm system as the branding says so on the station as well as on the sticker.

I’m just wondering whether this needs to be changed or cut somehow because I’m not using a Pyr-A-alarm system, and it’s not on the end of line either, and even then it wouldn’t be the correct resistor type.

Just wanted to ask before I wire anything because I don’t want to damage either the station and/or the panel because of the resistor on it.


I’ve never run into an issue with the resistors on the MFS-2 units. However, if it concerns you, using only the two terminals connected to the switch will take the resistor out of play.

Ok! Thanks!

If you’re interested to know, it’s possible the resistor was there for high voltage applications. Back in the 1940s-1960s(ish) the fire alarm systems used AC power directly. There weren’t any logic circuits on the control panel, so to supervise, everything was wired in series with large high voltage resistors, so if the wiring was complete, relays would stay energized and keep the trouble bell off. If a wire broke, the power to the supervision relay would be lost and so its contacts would switch positions, turning the trouble bell ON. If a pull station activated, the voltage across the loop would increase to a level capable of activating the alarm relay which would switch the horns on. Very basic setup and not ideal but since relays were more reliable (and less expensive) than transistors and DC circuits, they were the standard.

On those old high voltage Pyro panels (250VDC zone) you had to put a resistor (I want to say 80K ohms but I don’t remember - it was 25 years ago) in series with contact type devices so the zone didn’t blow out. The EOL for those old high voltage panels (FIU or ZIU) was 90K.

Wow, that is extremely high voltage! Yeah, that pretty much explains it right there.

Ok, now even though this is starting to make sense, does the same principle apply to the MFS-4 units as far as wiring goes?

I have pictures in another topic of the terminal block of said pull station and where this resistor is hooked up. From what I have heard, it will not affect the operation of the device, even if it’s not the EOL or damage the panel because it won’t do anything because the voltage isn’t high, correct?

The resistor certainly would not cause any type of damage to the panel. The IDC is literally designed to be shorted. It’s likely that the resistor on that pull station won’t even conduct at 24 volts DC since it looks to be designed for the much stronger 250 Volts AC/DC.

Even if the resistor does conduct electricity, the worst that can happen is it causes the panel to falsely indicate an alarm/trouble on the zone it’s hooked up to. If this happens, just cut the leg of the resistor soldered to the screw terminal so it doesn’t have a path to conduct electricity.

Thanks for the help! With that I’m going to go ahead and hook everything up.

no problem. Let us know if there’s any problems!

I metered the resistors on a few of my MFS-2 stations, and they all read 10KΩ. That’s a low enough resistance to be overcome by 24V easily, so keep that in mind while wiring the station.

If you install a wire jumper in parallel with the resistor, you can use both terminals it is soldered to and not have to worry about it whatsoever. The minimal resistance of the jumper will mitigate whatever resistance is provided by the resistor, effectively bypassing it.

If the resistor was installed across the screw terminals to allow for supervision, wouldn’t the installation of a jumper in parallel with that resistor just end up being a short across the terminals?

Not necessarily, if the circuit is wired correctly. Consider the diagrams below. The pull station switch contacts, shown in orange, are soldered to the #1 and #2 terminals. The 10K resistor, shown in blue, is soldered to the #2 and #4 terminals. Nothing is connected to the #3 terminal.

By attaching a wire jumper between the #2 and #4 terminals, the resistor is effectively bypassed. An additional jumper between terminals #1 and #3 allow all four terminals to be used for the four wires of the IDC circuit, which is what I think the OP is trying to do.

The simplest solution, which I have be suggesting from the start, is simply to ignore the #3 and #4 terminals and use the #1 and #2 terminals for all wiring connections. This takes the resistor entirely out of play.

Nick is correct. I’m only going to be using the first two terminals to take said resistor out of play. Once I have the two pulls hooked up, I’m going to plug in the 4004 and see what happens. If it does take the resistor out of play, the 4004 should only present the battery trouble as the only fault on it (I’m not going to be using batteries on this system as I don’t really have a use for them anyway).

I have, however, noticed a fault in your diagram. On the MFS-4 model (which I believe was intended to be the successor to the MFS-2), the resistor is placed and soldered across terminals one (1) and three (3), instead of terminals 2 and 4, and the switch leads are soldered to terminals 3 and 4. On these plastic stations, a push button switch mechanism is utilized instead of a toggle switch mechanism which the MFS-2 (metal, toggle switch reset, resets with a Super Cast key and made by the Autocall co.) utilizes. The MFS-4 also resets with the standard hex key that the Pyrotronics MS series also takes. I’m not sure whether this was done due to the design change, but the diagram you made probably doesn’t affect the wiring setup as it appears to be similar. And since these newer stations were probably intended to replace the MFS-2 series stations on the Pyr-A-Larm systems, the resistance is probably the same, at 10K Ohms.

Ah, I see what they did. The terminal numbers were just reversed, so if you flip the numbers in my diagram (starting with 4 at the top and 1 at the bottom) everything matches up.

Oh, I see how that works, that makes sense. I was under the impression that the terminals, switch, and resistor were set up in such a way that the resistor jumps the 2 contacts of the switch (like on the BG-12LX and its variants).

Now, if firefreak57 only uses terminals 1 and 2 on your diagram, the resistor is not even part of the circuit, just like you said.

What I don’t understand however is how you would wire this pull station to use the resistor (like what kind of system was this built for, and how was it intended to be wired in that system, and why the resistor was placed like it is)