One for electrics experts........ Imagine the simple 12 Volt On/Off relay.... Ignore any other connections, only concerned with the supply and earth connections to Open (or Close) the internal 'switch'. When in operation, is the internal 'switch' using only a minimal amount of current? Or in other words, would it take long to drain a normal 12Volt battery? AL
Al Short answer is Yes, the 'internal switch' ( activated by the coil when current is flowing) has a large resistance and consequently draws only a relativity small current If you can measure the resistance of the coil using a multimeter, you should be able to determine the exact amount of current flow . ie: measurement of coil resistance shows it to be 1000 ohms. and I = V/R Therefore the amount of current flow will be 12/1000=0.012Amps (on a 12v system) Theoretically time taken to drain a 12Amp hour battery would be 12/.012 hours=1000 hours or, you could just put an ammeter in series with the coil circuit when it has current flowing, and measure the current flow that way! Your choice. Hope that was helpful.:biggrin:
Thanks John......Very helpful..... I am currently modifying the home made alarm on my bike, so it can do another 'trick' without upsetting anything else. To do it, an extra earth wire is required in a completed circuit state (IE working), when the alarm is On, but broken when the alarm is Off........and the only way I can do it is with a relay. Bearing in mind the alarm is only On for say, maybe three hours at most; none of the other components use current unless the alarm is triggered (apart from two flashing LEDs, which are minimal drain). Although my electrics are fairly good, I wasn't sure about the sort of drain a relay would incur if it was just being used to switch the earth wire to a connected state. Thanks again, AL.
Al, if you have the relay reference number and manufacturer you should be able to check the spec for the relay from the manufacturers technical data sheet. If it is a standard Tyco/Bosch type relay used commonly in car and bike applications they have coils which are often approx 75 ohms. You can measure this by measuring the resistance across the coil pins which are usually 85 and 86, this is one of the tests you can use to check a relay is working although most folks just change them for a new one rather than faffing about on them with a meter. It should tell you on the tech spec sheet in any case and they also usually state the current required to close the switch.
Thanks Denzil..........I haven't got the relay yet.......I was asking the question initially to see if the idea was viable and now, of course, I can source a low-ish consumption one. AL
Just a thought - Could you not use a variation of the Fiam horn relay with 87a pinout? I believe this terminal provides power on the secondary circuit with no load on the primary until the primary is energised at which point, power on the secondary is interupted. Note that not all Fiam relays have this extra terminal and most applications do not require it anyway. hope this helps
Isn't that what is referred to a latching relay?.Thanks, I will investigate........ Note that I only want the relay to complete a 'break' in an earth wire......... I could revise my home made alarm to work without the relay, but it will mean changing several connections and re-wiring, so I was looking for a simple alternative. AL
Crystaljohn maths is correct but the numbers a little out. A typical automotive style relay has a resistance of approximately 100 ohms plus a fully charged battery is going to be over 14v. This will give a much faster initial discharge rate which will slow as the battery discharges. A 12AHr battery would be discharged in around 100 hours in this scenario, but more importantly the ability to effectivly start the engine would happen much sooner.
These numbers on the relay are a standard for ISO mini relays 87 is a normally open contact (it closes when the relay energizes) 87a is a normally closed contact (opens when the relay energizes) Changeover is a good description, I like that. A latching relay is one that locks in the state until some positive action is taken to reset it. Anyway enough of my blethering I only wanted to point out if you are interfacing, relays and electronics (its an alarm correct?), if electronics are involved in the "switch" circuit, then get a relay that has a voltage suppression circuit included or the induced back EMF when switching of the relay could spike the circuit and kill the electronics. I'll get ma coat
Danger mouse Yep, appreciate number are not realistic. Just used them to illustrate the method of calculation
This gets more interesting with each reply..........thanks all.... So the simple question is, will a normal / standard 4 pin make and break relay drain a good battery enough to prevent starting of the engine, if it was in operation for say, three hours? From the answers, I guess 'No'. The alarm system I have is nothing special in the way of electronics and it has a loud siren, but I was investigating adding the bike horn into the system..........however, the way the horn is wired means that I can only get it to operate with an additional earth wire which is normally in an Open state (otherwise the alarm movement sensor will trip the horn when the bike is moving) and Closed by a relay when the alarm is switched On. I can deal with it without a relay, but it will mean completely re-wiring the alarm, so that it's power supply is permanent and put into operational state when switched on by completing the earth circuit; rather than having a switched power supply......So it is much easier to add the relay. AL
