Exceeding DC Amperage of Loads

Thread Starter

eafrank

Joined Aug 31, 2015
5
I have a 12 VDC bilge pump (rated for 2.5 amps) in a bucket wired to a 20-watt, 17 volt 1.16 amp solar PV module to demonstrate solar PV water pumping applications. I will set up a similar system with two modules wired in series, and two modules wired in parallel. On each system a Craftsman AC/DC Clamp-on Ammeter is used to measure the current so students may observe the effect of wiring modules in series, and wiring in parallel. The question is, when solar irradience level is high enough (and in southern AZ we can exceed 1000 w/m2), if we exceed 2.5 amps (no inline fuse installed), are we risking damage to the pump (load), or damaging the conductors, or both?

Thank you for your assistance.

Ed
 

crutschow

Joined Mar 14, 2008
23,494
It isn't how much current the panel can generate, it's the voltage that's of interest.
You don't want to exceed the motors maximum voltage rating. If that's a possibility then you need a voltage regulator to control the voltage.
The motor will take only the current it needs as long as its voltage rating isn't exceeded.
 

Thread Starter

eafrank

Joined Aug 31, 2015
5
It isn't how much current the panel can generate, it's the voltage that's of interest.
You don't want to exceed the motors maximum voltage rating. If that's a possibility then you need a voltage regulator to control the voltage.
The motor will take only the current it needs as long as its voltage rating isn't exceeded.
 

Thread Starter

eafrank

Joined Aug 31, 2015
5
So connecting the 12 volt pump to a 17 volt source (or potentially 24 volt source) will damage the pump? Hmmm. Would a 17 volt PV source damage a 12 volt battery if it was connected to it (temporarily, with out a control charger)? If that is the case then a 30 volt DC submersible pump would be damaged by a 34 volt module?
 

shteii01

Joined Feb 19, 2010
4,647
When two modules wired in series, they will have double the voltage, so to power 12 volt pump you need two 6 volt modules in series, 6+6=12 volt.

When two modules wired in parallel, they will have double the current, so to power 12 volt pump you need two 12 volt modules.

Do you grok the problem?
 

crutschow

Joined Mar 14, 2008
23,494
Exceeding the maximum voltage rating of a motor can cause high currents to flow so should be avoided.
In your case, connecting a 17V, 1.6A panel to a 12V, 2.5A motor likely won't cause a problem, since the current is inherently limited by the panel. But you should monitor the motor voltage to be safe.

Solar panels are inherently current limiting (they mostly look like a constant current for a given illumination) so connecting a 17V PV directly to a 12V battery shouldn't hurt unless the panel can deliver more current then the battery can handle.
 

shteii01

Joined Feb 19, 2010
4,647
So connecting the 12 volt pump to a 17 volt source (or potentially 24 volt source) will damage the pump? Hmmm. Would a 17 volt PV source damage a 12 volt battery if it was connected to it (temporarily, with out a control charger)? If that is the case then a 30 volt DC submersible pump would be damaged by a 34 volt module?
There is some room for variation. 12 volt pump should survive a few volts above and below 12 volts. It is when you overvolt/overcurret by a lot (like double or tripple) is when you actually kill the equipment.

Like crutchshow said, if you use regulator, then you don't need to worry about the the higher voltage, you can feed 34 volts into properly selected regulator and power 12 volt pump without any trouble.
 

Thread Starter

eafrank

Joined Aug 31, 2015
5
When two modules wired in series, they will have double the voltage, so to power 12 volt pump you need two 6 volt modules in series, 6+6=12 volt.

When two modules wired in parallel, they will have double the current, so to power 12 volt pump you need two 12 volt modules.

Do you grok the problem?
Right now, I use a 17 volt module to power the 12 volt pump. It is for a very short amount of time.
 

shteii01

Joined Feb 19, 2010
4,647
Right now, I use a 17 volt module to power the 12 volt pump. It is for a very short amount of time.
Like I said, a few volts is ok. I think it will survive 17 volts for a while, a few weeks at least.

Here is classic 12 volt regulator, LM7812: http://www.mouser.com/ds/2/405/lm340-n-405278.pdf
Feed it voltage between 14.8 and 27 volts, and it will output 12 volt and about 1 A of current. The current is a little low. There probably better regulators out there.
 

Thread Starter

eafrank

Joined Aug 31, 2015
5
Like I said, a few volts is ok. I think it will survive 17 volts for a while, a few weeks at least.

Here is classic 12 volt regulator, LM7812: http://www.mouser.com/ds/2/405/lm340-n-405278.pdf
Feed it voltage between 14.8 and 27 volts, and it will output 12 volt and about 1 A of current. The current is a little low. There probably better regulators out there.

I know that due to changing sun conditions, the output voltage level will fluctuate over the course of the day. Thanks for your reply!
 

shteii01

Joined Feb 19, 2010
4,647
I know that due to changing sun conditions, the output voltage level will fluctuate over the course of the day. Thanks for your reply!
Right, but that the thing, since we are dealing with the sun, the change is gradual (slow in other words), as long as the panel putting out 14.8 volts or more (the regulator I linked has maximum input of 27 volts), the regulator will produce steady 12-12.5 volts. You can add some complexity, put the battery between regulator and pump, now the regulator is charging battery (though you need 13 or 14 volts to charge something like car battery) and battery is feeding the pump, when sun goes down, the panel output voltage drops below what regulator needs, regulator sort of turns off, but battery is still feeding the pump. Battery becomes a buffer (and a maintenance item) for the pump.
 
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