Where to find, or how to make a MBB Form D relay

[gazzarose]

Hi guys,

My friend has just rang me asking me to help him design something for work. Its a bit hush hush because of the application, but essentially he has some video recording equipment in a case which runs on 12V, but has a 240/12v in the case. It also has an extra power input so that it can be run off a large external battery so it can be used away from power. Obviously the easiest way to change between power supplies would be to plug in the battery and then remove the 240 supply, but were trying to make it as idiot proof as possilble, as well as reliable as possible, so were thinking about a way to be able to leave both supplies connected so that if the 240v power were to be remove it would automatically revert to battery backup. The problem is that the equipment has to have a constant supply, other wise everytime theres a dip in power, ie the time between a relay changing state, the recorder would need to be reset.

So we need a make before break relay. I've found loads of references online, but nowhere that actually sells them. Is there an easy way to make a reliable MBB relay from a few other relays. We don't need massive current handling, only about 6amp so normal automotive relays would be fine.

Any help with this would be greatly appreciated.

Gareth

 

[JimB]

The standard way of doing this is to use two diodes no relays required - simple!

JimB

 

[gazzarose]

Hi, thanks for the reply.

We were playing with that idea last night. The only reason we were swaying towards a relay is so there would be no extra voltage drop because we've already used a diode on the battery, so that if multiple batteries are connected togeter, they run in parrallel and not bak feed into each other. I'm not sure what the effect the extra 0.7v drop would have on the run time when running off batteries. Also if we just used diodes would it draw all its current from the adapter or would a share come from the battery?

Thanks again

Gareth

 

[Reloadron]

If you know the load could you just run the system off a large enough UPS?

Ron

 

[Mr RB]

Buy a 13.8v regulated supply and run it through a diode to the battery, so the battery is floated at 13.2v or so.

 

[gazzarose]

If I put a small battery charge charging a small battery inside the equipment case in parrallel with an external battery each through a diode so the power supply wouldn't charge the battery, would the external battery discharge at all? Im guessing if the charger was floating the small internal battery at around 13 the 12v battery would be ignored unless the charger was turned off which would let the small battery drop to the same voltage as the external battery, at which point both batteries would discharge together. I can't let the external battery be charged from the charger because the client has specified the cables that will be used between the battery and the equipment (which can only handle around 10amp) , as well as specifing the need to connect additional batteries in parrallel to give a higher run time. The problem is if a discharged battery was connected alongside a discharged battery there would be a large current rush into the discharged battery throught cables that are too small. The specified batteries are 12v 100AH gel batteries, so would be more than cabable of melting small cables. We are trying to design it as idiot proof as possible.

Gareth