Texecom 8 compact power supply on boat

[Andy J]

Hello,

I am fitting a Telecom 8 Compact burglar alarm system to a boat.

The boat has both nominal 12v DC supply (ie up to 14.2 v when engine running) and a regulated 12v DC supply for electronics. I dont want to power it with the inverter so 240v is not available.

My original plan was to remove the transformer, connect the nominal 12v supply to the transformer AC input on the PCB- the rectifier diodes should sort out the polarity and I see a voltage stabilizer on the PCB. I would use the small internal 12v battery as normal.

When I removed the transformer I briefly powered it up to check the voltage- it supplies 16 volts to the alarm with approx 13 volts supplied to the internal battery.

My question is whether to supply the alarm via the AC input as above or whether to remove the internal battery and supply the alarm with the regulated 12v supply via the back up battery terminals. I don't see any loss of functionality (apart from the power supply LED not lighting) if powering it via the back up battery.

Any advice appreciated.

Thanks!

Andy

 

[Lucien Nunes]

The 'nominal 12V' supply might not have enough overhead to charge the internal battery. This will depend on the circuit of the charging regulator, which is usually a linear type. A typical SLA float voltage will be 13.6-13.7 and if the charging circuit requires more than 0.5V overhead the battery will never be fully charged. If it does float OK with 14.2V input, note that it will only charge when the engine is running. The internal SLA won't charge from the external battery. In any case if the bridge recitifer is not bypassed, it will rob you of 1.4V and the alarm ground will not be at the same voltage as general ground (because there's 0.7V of diode between them)

The 'regulated 12V' might be a better bet if it's 24/7. The alarm might not have much of a reservoir capacitor on its main power rail, as it's intended to be held stiff by the battery (which will be removed). So make sure it doesn't lose the plot with inevitable small glitches in the distributed voltage when other loads are switched on.

A further option is to fit an isolated DC-DC converter from the nominal 12V to downstream of the rectifier. This will make it resistant to any disturbances on the supply side and make its internal battery function normally and independently, so the alarm cannot be disabled by sabotaging the domestic circuits. You mentioned 16V - presumably this is smoothed output of the rectifier? If so, a 15V-output DC-DC with an output trim resistor added to set it to 16V might suit (they all adjust by at least 10%). You can get DC-DC's with 9-18 or even 9-36V input range, which has the added benefit of allowing the alarm internal battery to remain on float at the proper voltage even when the engine is stopped, so that its full backup duration is always available. Obviously you would have to choose the wattage of the converter to suit the maximum load of the alarm circuit including charging.

 

[Andy J]

The 'nominal 12V' supply might not have enough overhead to charge the internal battery. This will depend on the circuit of the charging regulator, which is usually a linear type. A typical SLA float voltage will be 13.6-13.7 and if the charging circuit requires more than 0.5V overhead the battery will never be fully charged. If it does float OK with 14.2V input, note that it will only charge when the engine is running. The internal SLA won't charge from the external battery. In any case if the bridge recitifer is not bypassed, it will rob you of 1.4V and the alarm ground will not be at the same voltage as general ground (because there's 0.7V of diode between them)

The 'regulated 12V' might be a better bet if it's 24/7. The alarm might not have much of a reservoir capacitor on its main power rail, as it's intended to be held stiff by the battery (which will be removed). So make sure it doesn't lose the plot with inevitable small glitches in the distributed voltage when other loads are switched on.

A further option is to fit an isolated DC-DC converter from the nominal 12V to downstream of the rectifier. This will make it resistant to any disturbances on the supply side and make its internal battery function normally and independently, so the alarm cannot be disabled by sabotaging the domestic circuits. You mentioned 16V - presumably this is smoothed output of the rectifier? If so, a 15V-output DC-DC with an output trim resistor added to set it to 16V might suit (they all adjust by at least 10%). You can get DC-DC's with 9-18 or even 9-36V input range, which has the added benefit of allowing the alarm internal battery to remain on float at the proper voltage even when the engine is stopped, so that its full backup duration is always available. Obviously you would have to choose the wattage of the converter to suit the maximum load of the alarm circuit including charging.

Brilliant thanks. Yes that all makes sense.

There is also solar charging but I don't think that would have enough overhead to charge the SLA. I think the DC-DC step up converter is the best route to go down and I will match it to the AC voltage.

Many thanks for your reply!

Andy