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

Discussion in 'General Electronics Chat' started by sdowney717, Sep 18, 2014.

1. ### sdowney717 Thread Starter Member

Jul 18, 2012
392
13
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)

2. ### Papabravo Expert

Feb 24, 2006
10,163
1,796
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.

3. ### sdowney717 Thread Starter Member

Jul 18, 2012
392
13
Say these pumps draw 15 amps at 12vdc.
Can you share with me what number of diode would be appropriate?

Jul 18, 2012
392
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5. ### #12 Expert

Nov 30, 2010
16,343
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• ###### MBR30L60CT-D-107094.pdf
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Last edited: Sep 18, 2014
6. ### faley Member

Aug 30, 2014
94
13
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.

7. ### alfacliff Well-Known Member

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

Nov 30, 2010
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9. ### sdowney717 Thread Starter Member

Jul 18, 2012
392
13
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?

10. ### sdowney717 Thread Starter Member

Jul 18, 2012
392
13
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.

11. ### #12 Expert

Nov 30, 2010
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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.

12. ### faley Member

Aug 30, 2014
94
13
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.

13. ### sdowney717 Thread Starter Member

Jul 18, 2012
392
13
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?

14. ### faley Member

Aug 30, 2014
94
13
Why not forget the diodes altogether and go with the relay set-up that you posted?

15. ### #12 Expert

Nov 30, 2010
16,343
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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.