# What results from Forward voltage drop of diode? And multiple bilge pumps

#### sdowney717

Joined Jul 18, 2012
706
I understand it need that much voltage to conduct.

Does this mean it looses that voltage on the output?
So if you had 12vdc going in, and FVD is 1.5v, then you have 12-1.5 = 10.5 vdc only after the diode?

I am interested in a bilge pump boat circuit where there are 2 pumps, 2 automatic float switches, and 1 manual switch.

The 2 schematics I was given are here, I can use two 12 vdc cube 40 amp relays, but what could be used for the diodes in the other schematic?

FS =float switch
BP = bilge pump

(The cathodes are not connected)

#### Papabravo

Joined Feb 24, 2006
19,565
You are correct. The forward drop of the diode reduces the voltage seen by a load in series with the diode. A Schottky diode may be used to reduce the required forward voltage drop fro the same amount of current. The forward voltage drop will increase the more current you pull so watch the specifications carefully.

#### sdowney717

Joined Jul 18, 2012
706
Say these pumps draw 15 amps at 12vdc.
Can you share with me what number of diode would be appropriate?

#### #12

Joined Nov 30, 2010
18,224

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#### faley

Joined Aug 30, 2014
88
Odd circuit. Usually float switches control relays and the relays carry the load. Also, it's not uncommon to see an alternator employed to control the primary. (I assume one pump is low level and the other is high- or are they not co-located?) Why the diodes? If the relay were to be electronically controlled, I could understand a free-wheeling diode across the coil.

#### alfacliff

Joined Dec 13, 2013
2,458
the diodes are there to prevent fs1 from turning on both p1 and p2, and visaversa.

#### sdowney717

Joined Jul 18, 2012
706
Wondering about startup motor surge, and what minimum you think is ok on the diode ratings?

So put multiple diodes together to reduce the burnout risk?

#### sdowney717

Joined Jul 18, 2012
706
Odd circuit. Usually float switches control relays and the relays carry the load. Also, it's not uncommon to see an alternator employed to control the primary. (I assume one pump is low level and the other is high- or are they not co-located?) Why the diodes? If the relay were to be electronically controlled, I could understand a free-wheeling diode across the coil.
These are 2 emergency use backup pumps. I have a primary low level, so these are higher level pumps. All the pumps on boats I have seen just wire float switches directly inline with their pumps.
primary is Rule 2000 GPH
backups are Rule 3700 GPH

Keeping voltage as high as possible to pumps keeps the GPH up as high as possible.

#### #12

Joined Nov 30, 2010
18,224
I figure 8 times the run current for a start surge. That's why I keep listing parts with surge capabilities in the hundreds of amps.

#### faley

Joined Aug 30, 2014
88
the diodes are there to prevent fs1 from turning on both p1 and p2, and visaversa.
Thanks. The drawing of the DPST relay layout through me. Going to a single contact. Makes sense that way, sort of- just a shorter contact life. Thanks alfacliff.

#### sdowney717

Joined Jul 18, 2012
706
Would putting multiple diodes in parallel lower the forward voltage so the pumps get more power?
How many diodes could you put together?
Would that add a safety factor in case a diode failed?

#### faley

Joined Aug 30, 2014
88
Why not forget the diodes altogether and go with the relay set-up that you posted?

#### #12

Joined Nov 30, 2010
18,224
Yes, parallel diodes will share the load and have less voltage drop. You can save about a tenth of a volt for every parallel diode, up to about 4 diodes.
You can put 10,000 diodes in parallel, but that won't fix anything if one of them fails in the shorted condition.