Dc to DC

[Lenp]

I have inherited a project that seem to be technically a kluge but here it is.

We have well sampling pumps, used for water analysis, that run on 12 VDC. Because of well depths and the head pressure, the manufacturer supplies a DC to DC converter to step the 12VDC up to 15VDC. This provides a faster flow rate at depth and compensates for voltage drop in the down hole cable. Maybe the pump should have been redesigned but it was not.

Several of these DC-DC converters have cooked themselves because of really poor design. Mostly the fault is inadequate cooling. These are small enclosures, the pump draws about 8 Amp under full head and they are used outside in possibly the 100° summer cool. The last one got so hot it desoldered its power transistor from the board due to a poor wiring job on the cooling fan.

So repair and repackaging these power supplies with better cooling is one option but I would like to find a more robust design.
DC-DC converters are available but at the required current the pickings are a bit scarce. I considered a 12VDC to 120VAC inverter then to a 15V switch mode power supply but that is getting a bid excessive. I don't want to start from scratch, the quantity and time does nopt warrant it. I am looking for an off-the-shelf unit that I can package if needed!

All thoughts considered...

Thanks,

Len

[Bob Scott]

Do you have a copy of the schematic for the 12-15VDC converter?

I'm thinking that if the secondary is isolated, then you may be able to rewire it as a 12V to 3V converter and wire the secondary in series with the existing 12V. Same power output with less power load on the circuit?

[Lenp]

Hi Bob,

No, unfortunately there are no drawings, and, different circuit boards were used, I guess, depending on whoever the supplier was that week. The boards leave no clue as to their source.

I think the converter circuit itself is OK, It looks like they bought bare boards and packaged them in-house with cobbled up packaging without thought to proper cooling. The board has heat sinks for the power hungry critters but it is so restricted in the enclosure there is minimal airflow. One design uses a small CPU cooler fan at the end of the enclosure with a few holes for the air to 'leak' out of. Another uses a larger top mounted fan but the exhaust holes are not sized to allow airflow equal to the fan. I think the enclosure's static pressure goes up but there is really no flow! Another 'design' used hot glue to hold the fan in place. It got hot, the fan dropped, the blade jammed and it let the smoke out of the board!

If all else fails we may order some new converters from the manufacturer and immediately repack them with increased cooling to make this problem go away.

Len

[richfloe]

There are voltage boosting units designed for use with amateur radio SSB transceivers. These radios generally transmit with 100 watts as long has you have the engine running and the radio is supplied with 13.5v or more from the alternator. The power output drops dramatically when running from the battery alone and the voltage drops to around 12v.

My memory isn't what it used to be but I think there was one made by W4RRY. MFJ Enterprises sells one called an MFJ-4416 and I recall a project in QST magazine (an amateur radio publication) showing how to build one. These units take an input voltage of 11v or more and boost the output to 13.5v or more and are adjustable. If the manufactured units wont quite get up to 15v, you could make a small change in the windings of the one in QST magazine to provide more boost.

These units would provide a constant output voltage even though the input voltage may vary some.

How that helps!

Rich

[Externet]

Hi Len.
To start, where is the original weak 12VDC supply coming from ?
If not from a car battery but from mains, that could be boosted by a variac or use a different supply intended to yield 16V instead of 12. No failure-prone mikimouse boosters would be needed nor redesign of the pump motor.
Just think of the pump being truly 16V and its current supply being the wrong one to be tackled.

If there is some room to add a few turns yielding a few volts on top of the existing secondary transformer windings, added in series to the 12V winding, would solve it too.

If from a car battery, well, add a 4V or 6V one in series to it.

Miguel

[Lenp]

Thanks guys,

Yes, the power is from a vehicle battery since these monitoring wells are in the middle of a field. I looked at the MPJ unit and it seems, without making some modifications, that he voltage tops at 13.8, whish probably is too low. The OEM voltage booster outputs a nominal 15VDC and since I have no clue as to the limits of the pump I prefer to stay near the 15V mark. The pump is comppletely sealed so The type of motor is unknown. It may be a simple DC motor, which would tolerate more voltage, or a more advanced system with circuitry to run a brushless motor which may not and I don't want to change the well casing into a smokestack!

Adding an extra battery will work, using a 3-4V rechargable unit, but charging it in the field, if needed, could present additional complications and delays in testing. Charging both the 12V vehicle battery and a 4V booster battery while they are both in use from running engine could be a diode lovers nightmare

I did find a 12-15V booster http://www.digikey.com/scripts/DkSearch ... ow&cur=USDr about $130 that has real promise. CUI makes several models so maybe one of these is the simple answer.

As usual, thanks for the opinions and comments!

Len

[Bob Scott]

I did find a 12-15V booster http://www.digikey.com/scripts/DkSearch ... ow&cur=USDr about $130 that has real promise. CUI makes several models so maybe one of these is the simple answer.

As usual, thanks for the opinions and comments!

Len

That looks like a good solution. Just one more observation:

I believe that a lead-acid battery's discharge voltage is close to 12.6V if it is not under charge at the same time. Boosting that to 15V will cause increased current and power drain. The voltage booster unit's advertised efficiency is 82.5%. So power consumption from your battery will be (15 / 12.6)^2 / 0.825 = 1.72, or 172% faster than if the pump were working at 12.6V.

You'll have to change out or charge the battery 1.72 times as often if it is left out in the field.

[Lenp]

Hi Bob,

You are right, but, these tests are not done continuously. These wells are remote monitoring wells used to assess the ground water conditions. The tests are done on a monthly cycle, and the vehicle is parked close to the well, and left running, so battery life not an issue. The pump is dropped into the well, instrumentation is set up and the testing takes about an hour's total run time for the pump then they move to another well.
I will check with the lab to see if they prefer a dedicated voltage booster, or as I said earlier, a DC to AC inverter and a separate 15V power supply since they may have additional uses for an inverter.

Thanks all!

Len

[Lenp]

BINGO!
I think this is the answer, and, they are on order now. Look at the price!!!
http://www.ebay.com/itm/DC-DC-10-32V-To ... OU:US:3160

[Externet]

Very nice find, Len !

Opens many possibilities to many projects.
Ships from China, but if the seller wants to stay in business, will have to comply with ebay rules and behave.

Now I will look for one that ranges input from 6V up and delivers 14.0V
Thanks, Miguel.

[Lenp]

Follow Up:
I got the DC/DC inverters from China and they are as advertised.
The output kicks in at about 10.25V input, and it is adjustable up to about 31V
I ran a test, 18V @ 8A and the heat sinks warmed but did not appear to be overheated
When I package this, I will add a fan to the enclosure for increased reliability

They appear to be well made, have brass standoffs an LED and a 10T trimmer... All for $9.00!

Len

[gerty]

"They appear to be well made, have brass standoffs an LED and a 10T trimmer... All for $9.00!"

I hope you bought a few spares, should this one disapear ...

[Lenp]

Indeed I did!
There are several listings for the same unit on eBay. Some are for $1 plus $8 shipping, others are $8 plus $1 Shipping.
All are from China and at the same delivered price. It may be the same seller...Who really knows?
Side note... U.S. parts cost, excluding the board about $20.00!
Len