reducing voltage

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nomore

Member - Posts: 5

Joined: August 17, 2008

Location: Michigan, United States

Posted: August 17, 2008 at 10:38 PM / IP Logged

I've got a pump that requires 24volts dc and 0.2 amps. I have a misc adapter that is 30 volts dc and 1amp. I would like to know what I need to get this powerbrick down to 24 volts. The powerbrick is showing 35 volts with no load. I'm guessing thats normal to show a higher reading before a load is connected? Would a resistor lower this? Where would I need to place it at on the output wire (+ or -)?

Thanks in advance.

KPierson

Platinum - Posts: 3,527

This member consistently provides reliable information

Joined: April 14, 2005

Location: Ohio, United States

Posted: August 18, 2008 at 10:36 PM / IP Logged

I personally would just run it at 30VDC - 6VDC isn't a big difference and the pump should be able to handle it.

If you don't want to risk overheating the pump I would recomend finding a 24vdc power supply. A resistor will not acomplish what you are trying to do.

The other option is to use a 7824 - something like this: http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail?name=296-11115-5-ND

It will take the 30vdc and knock it down to a steady 24vdc - most voltage regulators requires a few caps to maintain a steady voltage so you'll need to read up on the devices datasheet.

Kevin Pierson

nomore

Member - Posts: 5

Joined: August 17, 2008

Location: Michigan, United States

Posted: August 19, 2008 at 6:33 PM / IP Logged

Remember the powerbrick is 30vdc but its putting out 35vdc when I measure it without a load connected. I'm guessing it drops down to 30 after a load is present. Would that be an issue?

I don't want to dump $20 or $30 for a new powerbrick. Just seems like too much money. Right now I'm using a laptop powerbrick which is 19 volts. The pump runs but at a slight reduced pressure rate. Enough that I want to bump the voltage higher.

I thought resistors can lower voltage? What are they used for then?

KPierson

Platinum - Posts: 3,527

Joined: April 14, 2005

Location: Ohio, United States

Posted: August 19, 2008 at 6:45 PM / IP Logged

Resistors resist current, not voltage. You can use two of them to make a voltage divider circuit, but your current output will be drastically reduced (they should only be used for voltage references, not voltage sources).

When you put a load on the power supply the voltage should drop. The problem with little power supplies like that is you have no idea how tight the tolerance and regulation is.

Kevin Pierson

nomore

Member - Posts: 5

Joined: August 17, 2008

Location: Michigan, United States

Posted: August 20, 2008 at 1:46 AM / IP Logged

Well I decided not to use that power supply. I had a spare transformer that I ripped out of an old furnace a few years ago that I just remembered having stored away. It was for the thermostat. Output was 24vAC and 39va. I simply desoldered the circuit board from the 30vdc powerbrick transformaer and soldered it to the secondary leads of the 24vAC transformer for the AC-DC conversion.

I was wrong when I said the other powerbrick was putting out 35v, its actually putting out 37v when I measured it before removing the circuit board. The transformer itself was at 30 I believe. So something on that board is bringing up the voltage some. Do the capacitors do that? Theres 2 big 50v capacitors (can shape), a smaller yellow one (non can shaped), 4 diodes, and a fuse. Even when I connected this board to the 24v transformer I still get 37v when powered up. The 24v transformer is outputting 24 just by itself.

Anyways, I still connected it to my pump, turned the pump on, then while it was on I measured the voltage, 30 volts! Still 6 more volts than the pump calls for but I guess it will have to do?! Perhaps the original one was putting this much out as well. I don't remember if I ever measured it. So whats bumping up the voltage like that? It's not the transformer cause its putting out the correct voltage. Btw, this all fit back in the original powerbrick housing.

I tried to find the amp rating on the 24v transformer but all it lists is 39VA?!?

KPierson

Platinum - Posts: 3,527

Joined: April 14, 2005

Location: Ohio, United States

Posted: August 20, 2008 at 5:48 AM / IP Logged

Are you measuing the output of the transformer as an AC voltgage and the output to the pump as DC voltage?

The 4 diodes form a full wave rectifier and convert the AC to DC - the caps smooth out the output (remember AC is a sine wave and DC is a constant voltage). Wihtout the big caps you would have a saw tooth looking output, the caps make it a straight line output (when viewed on an oscilloscope).

Most transformers are rated in VAs because they have multiple taps and can be hooked up in a variety of ways. To figure out the output current you would divide the VA rating by the output V - so 39/24 = 1.625 A at 24vac.

Kevin Pierson

nomore

Member - Posts: 5

Joined: August 17, 2008

Location: Michigan, United States

Posted: August 22, 2008 at 5:12 PM / IP Logged

KPierson wrote:

Are you measuing the output of the transformer as an AC voltgage and the output to the pump as DC voltage?

Yes AC after transformer (before it ties into the board), then switched to dc to measure output voltage to pump. This transformer has a primary and secondary. Thats it. Primary is 110v, secondary is 24v. I've had the pump going for a couple hrs or so and it seems fine. Was warm to the touch so was the powerbrick.

megaman

Copper - Posts: 385

Joined: June 24, 2008

Location: Montana, United States

Posted: August 22, 2008 at 5:41 PM / IP Logged

The DC ouput will be slightly higher than the rated AC output of the transformer because of the rectifier circuit, which converts the AC to DC. The AC ouput voltage is based on RMS value or about .707 of peak ouput. Some transformers will list an RMS and a peak-to-peak voltage. Because of the nature of the circuit you can divide the rated AC output by .707 and get the approximate Rectified DC output.

e.g. : 24V/.707 = 33vdc.

thoughts KPierson?

KPierson

Platinum - Posts: 3,527

Joined: April 14, 2005

Location: Ohio, United States

Posted: August 22, 2008 at 6:14 PM / IP Logged

I believe the output will be slightly lower, as the diodes in the rectifier will cause a slight voltage drop. What is the AC voltage at the secondary of the transformer? Do you know the actual voltage on the primary?

Kevin Pierson

nomore

Member - Posts: 5

Joined: August 17, 2008

Location: Michigan, United States

Posted: August 22, 2008 at 8:56 PM / IP Logged

KPierson wrote:

I currently have the pack put back together but do remember the transformer was showing 24vac on the secondary before the board. I just measured the plug that it was plugged into and its reading 124~125vac. So obviously thats what would be at the primary side. The dc output however is higher as mentioned earlier.

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reducing voltage

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