Pump Running Indicator

[sailorbob]

I have a boat with a pressure water system. 12V DC drawing about 5 amps when the pump is running. It has an interesting mode that occurs when the water tanks are almost empty. The pump comes on, but because there is now some air in the lines, it can't build up enough pressure, so the motor stays running. I want an indicator light (prefer an led) that comes on when the motor is running. The pressure switch and the pump are in the same housing so I can't access the pressure switch contacts, so I can't drive an indicator that way. What I need is a cheap and dirty circuit that I can put in the panel and mount the led beside the circuit breaker so there is a visible indication when the pump is running. I don't think the pump motor is all that sensitive to voltage, so there wouldn't be a big issue if whatever is inserted in the line to sense the current (a diode?) caused a small voltage drop. Ideas?

 

[Chippie]

If you put a 6 amp diode in series with the motor, you could use the 0.6v drop across the diode to switch a transistor on and off as the pump runs....

 

[KMoffett]

Wind twenty to forty turns of 16 Ga magnet wire, in one or two layers, on a 1/4" form. Wire this in series with one of your pump leads. Place a small reed switch through the middle of the coil. Use this reed switch to switch on an LED. You may have to experiment a little depending on the reed. I was able to trip a very small reed with a single layer, 12 turns, of insulated 14 Ga solid Romex wire with a 6.5A current. More turns of smaller wire will work at a lot lower current.

Ken

 

[sailorbob]

Thanks. OK, the diode and transistor idea sounds possible. The complication here is that getting electronic parts in small quantities here is difficult. I'm up against minimum order quantities if I mail order and my choice in town is The Source (by Circuit City), formerly Radio Shack. I checked their website and they seem to have a 6 amp diode and a selection handful of general purpose transistors. No reed switches alas.
I'm a computer guy, not a circuit designer. Chippie, could you give me a hint of what that transistor circuit would look like?

edit: I don't really trust their product specs, so I'd likely be better off to put 2 diodes in parallel. They won't share equally, but they will share .....

 

[kinarfi]

Where are you at? How sure are you about the 5 amp draw of the motor?

 

[sailorbob]

Where are you at? How sure are you about the 5 amp draw of the motor?

Canada. About 100 km NE of Toronto.
There's an ammeter on the DC panel. Ran the pump with nothing else running.
The breaker is 15 amp. Two 6 amp (their likely optomistic rating) diodes in parallel should handle the load???

 

[sailorbob]

I was poking around on the net and found this circuit.
Circuit - 12v Lamp Current Indicator - Circuits designed by David A. Johnson, P.E.
It kind of looks close to what I need.
It uses the .6V voltage drop across the diode as suggested.
It should work with a dc motor (the pump) instead of a lamp. Right?
Obviously I need better than that 3A 50V diode.
The question is, why is the load limit on the lamp 1 amp when the circuit has a 3 amp diode?

 

[sailorbob1]

Somebody thinks I've done something wrong and I don't know why. Anything I've tried to post seems to be going into what they call the 'moderation queue' and outgoing PMs seem to be blocked. That includes the one to admin. Incoming seems OK. For all of you that responded, thanks for your help. I think I've got the problem solved. I cribbed a reed switch from one of those battery powered door alarms and wrapped about 20 turns of stiff wire around it. It appears to switch on my workbench with an automotive driving lamp as the load (say about 4 amps). I'll know for sure when I get it wired into the dc panel with an indicator led. Thanks guys.

P.S. Would one of the moderators look into why I'm being blocked. Thanks.

 

[kchriste]

All Noobs have to have their posts vetted by a moderator at first.

 

[sailorbob1]

I hear you, but it's been 36 hours now. Seems 'a bit' excessive. IMHO, it also would seem more appropriate to moderate my first posts, not allow three and then start moderating.

edit: It wouldn't even let me send a PM thanking another user for his help, which would make me appear ungrateful .....

 

[kchriste]

When you first posted, a Mod was probably online so you didn't notice the vetting. On subsequent postings, the Mods weren't online and thus took more time to see your posts.

 

[sailorbob1]

When you first posted, a Mod was probably online so you didn't notice the vetting. On subsequent postings, the Mods weren't online and thus took more time to see your posts.

We're getting off-topic here, but I don't think so. This will be post #3 from this 'new' account and I've had no difficulty posting from it. I suspect post #4 will be added to the moderation list. And I'm still missing that original post and it's now getting close to 48 hours.

Moderators: I'm deleting this second account right after making this post. I only started it to get someone's attention to my problem.

edit: Maybe not. I don't seem to be able to find the delete account function. Moderators, feel free to delete this second account, but keep the first one. Thanks.