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# Sizing load ratings of resisters in divided voltage circuit

## Sizing load ratings of resisters in divided voltage circuit

(OP)
My problem: I need to lower maximum voltage in a nominal 24vdc circuit. The circuit drives a pair of pumps in parallel each rated at 8-24vdc and the pump set has a caveat not to run at over 24/25VDC (the caveat I believe is because of unbalancing the impellers at the speed 24+ will create.) The circuit can reach 29+ when charging. The original manufacturer's solution was an 18-36vdc input/regulated 24vdc output controller. Those controllers are not manufactured anymore and to date I have found nothing to replace it.

I plan to utilize a divided circuit in the meantime to keep the pumps from receiving more than 24V. My problem is I draw a blank at rating Resister R-1 in the voltage divider circuit. Each of the two pumps is rated at 11 Ohms (2.1A)and that brings me to roughly a 2.5 ohm resister for R-1 to keep the voltage below 24vdc when charging at 29vdc. Will R-1 carry the same, more, or less load than the pumps?

Thanks for any input. If anyone can direct me to a reasonably priced, damp protected controller that will take the spec'd input and keep the voltage down to a regulated 24vdc that would be appreciated as well.

### RE: Sizing load ratings of resisters in divided voltage circuit

Your resistor, in the absence of a schematic, will see all the current your pumps draw. The issue will be that the ≥5 V drop you assume at 29V input will more or less be there with a 24V input, and you'll nominally be driving the pumps with only about 19V. And, you'll be dumping a truckload of power into the dropping resistor.

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### RE: Sizing load ratings of resisters in divided voltage circuit

This company have been in the low voltage / battery motor controller business for a long time: https://www.4qd.co.uk/product-category/controllers...

The current capability looks a little high, but that won't be a problem (it will just run cooler). Give them a call, explain what you're trying to do.

### RE: Sizing load ratings of resisters in divided voltage circuit

In the link provided by Scotty, the SST:031 is the most economical and provides overvoltage protection.

Bill
--------------------
"Why not the best?"
Jimmy Carter

### RE: Sizing load ratings of resisters in divided voltage circuit

(OP)
Thanks to you and the other posters. These pumps are electronically commutated so no brushes. I will contact the linked company to see if they offer another solution.

### RE: Sizing load ratings of resisters in divided voltage circuit

Are these Brushless DC motors? (BLDC)
How many wires come out of the motor, 2 or 3?
If these are brushless DC motors, they are actually three phase AC motors with an included inverter that converts DC power to three phase AC for the motor.
If so, you can not just apply 24 Volts DC to the motor, you need a new inverter.

Bill
--------------------
"Why not the best?"
Jimmy Carter

### RE: Sizing load ratings of resisters in divided voltage circuit

An old tried'n'tested solution used by many battery charger manufacturers is to have a string of series diodes in the output, and to selectively bypass these with relays or contactors to maintain a reasonably stable output voltage. This technique is applied in chargers delivering several hundred amps, and it is simple and robust. Before we get to that though, I think we need to know a bit more about the load itself.

### RE: Sizing load ratings of resisters in divided voltage circuit

#### Quote (OP)

These pumps are electronically commutated so no brushes.

Bill
--------------------
"Why not the best?"
Jimmy Carter

### RE: Sizing load ratings of resisters in divided voltage circuit

(OP)
The pumps are unique so I will try to post a link to them. The versions I use, "Strong" are two wire pumps. From my understanding the Vario, Solar and Basic all reach their maximum output at roughly 12vdc. The Strong model can increase output with the higher speed that comes with higher voltage but at above 24/25 volts there is a danger of unbalancing the rotor, hence the need to control the maximum speed through voltage.

The pump:

http://www.hvacquick.com/catalog_files/Laing_D5_Va...

The original controller which is not available any longer: (I used the D-24 S-24 model)

http://www.digicanmc.com/pdf/V-Infinity/VDZ-200%20...

I am currently running the pumps at 13.8vdc through a jury rigged 24/12 converter. Increasing the range to 19-24 can only be a positive. The string of diodes ScottyUK mentioned sounds quite workable for the moment and could be reworked later to accommodate charging/non-charging states.

### RE: Sizing load ratings of resisters in divided voltage circuit

Hi Bill -

Yeah, I noted the brushless motor type - I was assuming that the inverter was having problems with the slightly higher supply voltage as the inverter is an integrated module within the pump itself.

rbrake,

Plenty DC-DC converters out there. Vicor has always been my 'go to' brand for DC-DC stuff because they have bombproof reliability and most of the places I use them can justify a premium to get that reliability, but there are a lot of other brands available which are definitely cheaper and may be just as good - available through Digikey and the other usual suppliers.

This looks like a pretty close fit and the price isn't bad: https://www.digikey.com/product-detail/en/xp-power...

### RE: Sizing load ratings of resisters in divided voltage circuit

hi rbrake

the pump cataloge and the controller you sent were so helpful and clarified your problem more, well i add my suggestions for solving this problem you have

1- see these links with dc-dc isolated converters within specifications you need

https://www.amazon.co.uk/Waterproof-Converter-Step...

http://www.chargingchargers.com/converters/convert...

Alibaba, and aliexpress have also, of you searched you would get many, also rs.com

so, yes its available anywhere, and i don't recommend using didoes as this wont be reliable solution,

2- have you tried to repair?, if this is possible will be very cheap solution, and reliable if made right

i hope i helped, regards

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