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Power supply only producing 11 volts

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Ausant

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Hi all, pretty new to this. Just wired a computer power supply sock can charge my rc batteries. However it will only produce 11v on the 12v wires. I have a 10w 10ohm resister on the 5v to produce a load, and apparently this will help raise the voltage on the 12v side of things. At 11 volts I get a low voltage error on my charger. How can I get it to 12 or a little more? Thanks!!!!
 
How can I get it to 12 or a little more?
Not something to try unless you're reasonably experienced at electronics. You would need to identify the resistor combination which feeds back a voltage to the SMPS control IC and alter one of the resistors.
 
What do you get on the 12 V supply with no 12 V load connected?

Do you know the 12V amp rating of the supply? it's very possible that it's too low.
 
You need to find out what is the minimum or typical input voltage of your charger. It is a circuit, it is not just a piece of wire.

Three Li-po cells produce an average output voltage of 11.1V but the battery is fully charged to (4.20V x 3=) 12.60V.
So the charger circuit might need a minimum input of 14.6VDC or more.

Some of my Li-Po batteries have two cells. They produce an average output voltage of 7.4V but the battery is fully charged to (4.20V x 2=) 8.40V. The charger is powered from a 12VDC wall-wart.

Some of my Li-Po batteries have one cell. They produce an average output voltage of 3.7V but each battery is fully charged to 4.20V. The charger is powered from four AA alkaline cells (6V) or four AA Ni-MH cells (5.2V).
 
Alec_t:
By 'alter' the resistors do you mean remove, replace or modify?

KeepItSimpleStupid:
I get 11v and the amp rating is 18amps.

Audioguru:
I have Li-fe batteries. Transmitter is 9.9v and receiver is 6.6v. The lowest voltage the charger will allow is 10v, however as you mention the charger needs probably at least 12 volts to charge the 9.9v battery. This is my issue, I need to raise the voltage by 1v. I put additional load on the 3.3v 'circuit' by using a small globe (i wanted to do this anyway for a power on indicator) but still no difference.

Surely there is a simple way to get the results I want?

Thanks for replies!
 
Ok, what did you do with the 3.3V sense lead? https://en.wikipedia.org/wiki/ATX It should be tied to 3.3V.

It is a possibility that the 3.3 V supply is higher than usual, causing the 12 V supply not to be right.

With a load on the 5 V supply as you had it, measure all of the voltages, like 3.3. 12, -12, 5 standby
Then connect 3.3 to 3.3 sense and do it again.

Try putting the load on the 3.3 V supply or both the 3.3 and the 5V separately, of course, but make sure 3.3 is attached to 3.3 sense.
If you have two resistors, that would be really great. One for the 3.3 and one for the 5.
 
all pc power supplies i worked with used common feedback. they would produce precise voltage for 3.3V rail (or in the past for 5V) while other voltages are not regulated separately. so if 3.3V is lower, all voltages would be lower. 3.3V sense is just another 3.3V wire but with no load so motherboard can use it as reference to check voltage drop on 3.3V wires that carry current (the more current, the larger voltage drop). do not connect this to GND (what made you think that?)
 
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3.3 V needs to be connected to 3.3 sense. Usually a diode is placed in the sense line, so if it's not connected it is usually a diode drop more.
With these power supplies, one supply matters and it will usually be the one with the highest current rating. Once that supply is in regulation, the others fall into place as long as they are not overloaded.
 
Ok, so connect orange 3.3v to brown 3.3 sense wire? Thanks for your help. Will let u know if it makes a difference. Cheers!
 
pin #1 (orange) to pin #13 (brown)

The power supply connector seems to indicate two possible colors on Pin #13.

The voltmeter readings before making the connection above might confirm that the 3.3 V supply is not correct. The 3.3V supply may have to be the one that's loaded.
 
Ok I have load on both 3.3v and 5v. I have the 3.3v sense connected to another 3.3v wire. I don't have meter with me at the moment but it still is not enough and still get low voltage error. I might have to try another power supply.
 
By 'alter' the resistors do you mean remove, replace or modify?
Replace or modify. Usually there are two resistors forming a voltage divider between the output voltage (+3.3 or +5, depending which is to be regulated) and ground for feeding back a control voltage. If the grounded resistor is decreased in value, or the non-grounded resistor is increased in value, the voltage fed back will be lower and hence the output voltage will increase to compensate.
 
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It actually could mean that the 12 V rail is governing the bulk of the power supply. I think I would put at least a minimum load on the supply with the highest product of V & I. It's unclear whether all of the other supplies require a load of some sort.

Without a load, the supply thinks that the voltage is higher than it really is. The manufacturer's of the supplies don't want to put the minimum load as part of the supply because it messes up efficiency.

Generally, the schematics of multiple supplies that I have see, agreed a bit ancient such ss the DEC PDP-11 and the Amiga generally put the effort into regulating one of them.

The 5V supply has always been a target because it's specified at usually +-10% or so, but now the logic supplies are less tied to a specific voltage than they used to be.

It's hard to guess what's required, but before I gave up totally, I'd put 10% of a full load on each supply.

One key will be the voltages of all of the supplies. Are they ALL in regulation.

Just a note about remote sensing. Usually in a lab supply that sports remote sensing, there will be a sense lead for (+) AND a sense lead for (-). This compensates for the resistance in the leads, usually up to a diode drop or 0.6V. You don;t want the supply to go way out of regulation so the sense lead sees 0.6V more when unconnected. This is to avoid things like putting 40V on rail that's supposed to be 5V.

5V supplies were typically designed with crowbars which would short the supply if the voltage went over a certain value. This was called overvoltage protection or OVP.
 
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