Resistor vs Transformer update

There are several power resistor manufacturing technologies in use, so it is possible for the dimensions to be different. You should be using the same resistor technology to preserve the intent of the original design, especially if it calls for flameproof self-fusing metal oxide types.

Measured output on a transformer, will vary with load, the supply voltage (which is not exact), and the tolerance of the part itself. 123v for a nominal 120, is closer, than the incoming supply accuracy itself will normally acheive...

Best Wishes

Ah then the mystery is solved. Now the hard work of finding what technology is used in the originals.

I am also having a problem in replacing the varistor. The original had a limit at 130V , so I assume the new one should be about 250V but my supplier also needs a resistance value. The load which is mostly resistive has an actual power value of around 300Watts. Before I spend hours checking on datasheets of MOVs , has anybody had a detailed experience with this problem.

Hans

I fully agree that the modification of the internal components is not the solution to this problem, especially when undertaken by an amateur and for such a critical application as a fire alarm system. A properly rated step-down transformer is the only method that I think would be acceptable if there were a suspected failure of the system and it was investigated for insurance purposes. A separate 120 VAC line could be run to all the devices, or individual transformers could be used.

An open circuit output voltage of 123 volts with 232 volt mains indicates the transformer has a regulation of 123-116 = 7/120 or about 6%, which is fairly typical for small transformers.

Circuit protection varistors are typically red or green in color, and usually have markings indicating the peak voltage and the energy rating. They will usually fail by becoming leaky and then shorting, so the circuit needs to be protected by a suitable fuse.

A power resistor of a given wattage can be arbitrarily small if the temperature can be arbitrarily high. The same amount of heat will be generated. If more power will be dissipated inside the sealed enclosure due to changed components and higher input voltage, there is a real danger of overheating and eventual failure. Yes, this is literally "playing with fire" in many ways.

Paul E. Schoen

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They had some strange code writing which was totally unrelated to their voltage characteristics and were blue just like the capacitors.

Hans

You should check on the circuit board. On capacitors usually a C followed by a number. In the case of a varistor , you should see a VDR.

lemonjuice

Varistors are voltage dependent resistors so that means their resistance varies from a maximum value with no voltage to a minimum value just before they blow out. He is probably refering to the resistance before it burns . Quite easy to calculate that. FROM P = VI => P= V^2/R . Substitute yr 300 watts and 250V and you have yr resistance.

lemonjuice

I think these are Metal Oxide Varistors (MOVs) which are essentially an AC zener diode. When voltage exceeds the peak value, they conduct and dissipate energy as heat, but also each spike degrades them a bit. They are highly capacitive and should read near infinite resistance if good. When they go bad, they short out, so be sure to have a properly sized fuse protecting the PCB traces.

I'm getting a very bad feeling about the way this project is going!

Paul E. Schoen

"Weinberger Hans"

Before you kill someone.

........ Phil

"Weinberger Hans":

Just tell your "parts grocer" you want blue ones.

........ Phil

Something is very wrong here.

If the units are designed for 120V in and you get that 120 from 240V mains by using a step-down transformer, then why do you want to change the MOV's?

That is, the unit will be working on 120V, so by changing the MOVs to 240V units all you'll be doing is making sure that 240V transients can get into the unit, which is _not_ a good thing.

"John Fields" "Paul E. Schoen"

Where the *rated* primary and secondary voltages are known, the regulation percentage follows immediately from a knowledge of the off load primary and secondary voltages.

Duh !!!!!

Only transformers of 5 VA or less have such poor regulation.

A 50 VA tranny is normally speced at 10% and a 160 VA 6%.

......... Phil

--- What I'm amazed at is that the OP's boss (who must be technically even less competent than the OP) has let him have such a long leash.

Essentially, a novice has been given the responsibility for making critical technical decisions which can adversely affect human life and the protection of property with, apparently, no technical supervision whatever.

Pretty scary.

-- John Fields Professional Circuit Designer

"John Fields" "Paul E. Schoen"

Bet he has exactly one, Asian sourced, 120 volt alarm system.

.......... Phil

Paul . Their equivalent circuit is 2 back to back zener diodes... thats why their behaviour is what I outlined above. They are protective devices , so if they short at excess voltages the whole circuit will blow up which isn't the intention why they are used.

Don't worry too much about the guy ... remebember an expert is one who has made every possible mistake in a very narrow field.

lemonjuice