Evaporative cooler controller board problem

[flinx]

I have the controller board from an Evaporative cooler. there is a 5 volt regulator on the board L7805cv. Connected to the Vin pin 1 is a resistor that has blown off it's outer layer and the value is unreadable (it measures 118 ohms). Across ground and pin 3 is a 4.7k resistor.

I have searched for a schematic for the controller board and of course came up with nothing. Any ideas on how I can guess the value of the bad resistor?

I'm going to pull the regulator and test it.

[flinx]

Re: Evaporative cooler controller board problem

5 volt regulator shows 4.94 volts with and without a load.

[budm]

Re: Evaporative cooler controller board problem

Can we see good clear pictures of the board top and bottom sides?

[flinx]

Re: Evaporative cooler controller board problem

I knew someone was going to ask that. standby

[flinx]

Re: Evaporative cooler controller board problem

back side
https://www.dropbox.com/s/pwjoo5dv67...10601.jpg?dl=0

front side
https://www.dropbox.com/s/36b0c6lpib...6_HDR.jpg?dl=0

best I can do at this time.

the blue wire on the back is there because part of the trace came of when I removed the regulator.

[budm]

Re: Evaporative cooler controller board problem

It looks to be a resistor to drop down the Voltage feeding the input of the 5V regulator so the regulator does not to have dissipate too much power due to no heattsink for the regulator.
What Voltage drop do you get across that input resistor with the load connected so we can calculate how much current is being draw and see if the resistor Wattage is under rated or not.

[flinx]

Re: Evaporative cooler controller board problem

It looks to be a resistor to drop down the Voltage feeding the input of the 5V regulator so the regulator does not to have dissipate too much power due to no heattsink for the regulator.
What Voltage drop do you get across that input resistor with the load connected so we can calculate how much current is being draw and see if the resistor Wattage is under rated or not.

I cannot answer that since I would have to take it back to my brothers house, go up on the roof, install it in the cooler and power it up.

If you think it is there to drop the voltage, what would happen if I attached a largeish heat sink and just jumper the resistor?

[budm]

Re: Evaporative cooler controller board problem

You need to find out what the input Voltage is and the how much current is drawn through the regulator, for example: if the input is 15V, the output is 5V and the current draw is 0.50A, that means the Voltage drops across the Regulator will be 10V, the power dissipation will be 10V x 0.50A = 5Watts of power so you need very good size heatsink, also the output of the transformer will vary with line Voltage so at higher line Voltage the input Voltage feeding the regulator will be higer so even more heat is generated.

[flinx]

Re: Evaporative cooler controller board problem

ok, well I'll see what I can do. The big problem is that I injured my back a few days ago and would have to have one of my brother go up and re-install it and make a measurement.

My other option is that I have a local number to a place that might carry parts for this brand of cooler. I would have to go there on monday and see if I could look at another board (if they have one).

A third option is the heat sink. there may be room inside this for one that could be pretty dang big (relatively speaking).

The large capacitor on the board is a 2200uf 25v the positive side feeds in to the voltage regulator through the burned resistor, so I would expect the voltage to be less than 25vdc.

[flinx]

Re: Evaporative cooler controller board problem

ok, well I took a chance. I found a schematic of the control unit (but not the pc board). I gave the board power and checked the voltage going to the burned resistor. I got a reading with the resistor in the circuit of a little more than 22vdc.

With the resistor clipped I got a reading of about 23.17vdc.

The amp draw was 6.15 milliamps going in to the regulator with the thermostat control panel not attached to the board.

The thermostat control panel is at my brothers house. These are 1/4 watt resistors so the amp draw shouldn't be very high.

[flinx]

Re: Evaporative cooler controller board problem

I did some calculations on possible current draw, and decided to replace the resistor with a 1200 ohm 1 watt resistor.

The amperage could be as high as 15 milliamps or as low as 6.15 milliamps leaving the regulator to have to deal with as much as 7 volts (over the 5 needed). Then I added a heat sink I pulled from an old motherboard (used some thermal compound on it).



We will see what happens tomorrow.

[flinx]

Re: Evaporative cooler controller board problem

I had my brother re-install it this morning.

The display now powers up, when you tell it to start cooling, it runs through a cycle where it starts the water pumps, says wait on the display, when that goes away you can turn the temperature setting down and the fan motor powers up, then about 1 second later the display goes blank and everything shuts off. If the motor is set for low speed the motor will run for about 3 seconds before everything shuts down.

I had my brother check the voltage being supplied to the resistor with the unit on and display working (fan motor off). The voltage is 19.2. I then had him check the voltage across the resistor and that is 13.4, leaving 5.8 for the regulator. With a current draw of about 11 milliamps.

Am I right in thinking that I may need to decrease the resistor and allow more voltage for the regulator? Say change out the 1200 ohm for a 560 ohm which would give the regulator about 13 volts to play with, with a current draw of around 11 milliamps a 1 watt resistor would work fine and the heat sink I installed would take care of the rest.


EDIT: I checked the specs and the dropout voltage is 2v so I will need to change the resistor.

[flinx]

Re: Evaporative cooler controller board problem

Changed the resistor to a 470 ohm. Unit now works fine. Nothing is getting hot on the board.

For future reference, the control board is from a Phoenix manufacturing or Aerocool EBP-115 evaporative cooler.

[Th3_uN1Qu3]

Re: Evaporative cooler controller board problem

Good job. The original resistor was likely much lower in value, hence why it blew during an overload. You might want to use a 220 ohm 2W resistor, if the pump drops out during undervoltage.

If it works, leave it as it is.