I am attempting to mod a power supply...
I am trying to work out the amps coming through so that I can buy the correct POT (Using R = V / I)
Am I on the right track?
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M$3 Answers
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M$
The schematic of that power supply may be very similar to this...
The current passing through the circuit all depends on the load present under a given voltage.If it is a reasonably safe power supply, it will have a fusing system, or a manner of overload protection- never assume there is one though.
An easy way to determine how much current it provides, you could put a large, high wattage potentiometer in line with an ammeter, and short the power terminals to each other. the pot will simulate a load, and the resulting current will show on the ammeter. slowly reduce the resistance to get an idea of how much it can provide, while keeping an eye on the temperature of the components. THis is kind of a touchy method to finding out how much it can supply, but short of designing one yourself or fully reverse engineering the power supply, it give you a ballpark to work with.
But start with easy stuff first. If it's a 220v device, chances are it has a 110v mode too. Look for a switch. Reading the label on the back is your best bet, as it will also tell you the wattage of the device, which when divided by your voltage, gives your current (before converting and inefficiencies though).
the capacitor's probably just there to filter out noise. And31v seems a little odd- what was the power supply from? And what are you trying to control- the voltage, or the current? is the supply going to be all purpose or dedicated to a specific task?
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M$
That is a switching flyback power supply. It is 120 VAC input. 94V-0 is a flammability rating.
Is there a TL431 on the back side of the board ?
Here is a picture of the circuit board. As you can see it is fused. I can read the fuse and it says T2.5AH 250v (my apologise before, I thought it was a 240v device).
The big cap says 400v and 68uF the small black cap says 50v 270uF.
Its from a printer/scanner and I want to turn it in a variable power source for different projects. So in some cases I want to be able to turn down the voltage and current to power just a single LED. I know its probably over kill using this but hey, its a challenge :)
Thanks for your help
Right off the bat, have you been able to power this board up? It doesnt look like a 220 or even a 110 powered device- the leads look polarized, and it has a note at the top saying 94v-0.
To adjust the voltage, you'll need to determine what kind of current you want, and that will determine how big of a pot you need. As an example- If you want 1 amp of current avaiable, you determine how much wattage that is. In this case it is 31x1=31 watts. so you need a pot that can handle that much power. THis would likely come in the form of a 50watt, which is big and ugly. Even then, you need to control the max current, which would be done with a separate resistor.
So to do it, and set it up for 1 amp of current (max) at 31v, we find out that we need 31ohms of resistance. In this case we would use a 50watt resistor connected to the positive terminal. From the resistor, we connect to one end of the pot. THe other end of the pot connects to ground (negative terminal). Your varied voltage is provided by the middle pot lead (the sweep). THis forms a voltage divider. You still need another pot or resistor connected to the middle lead to control the current out.
The size of the pot is up to you. If it is several kohm, the current will decrease dramatically along with the voltage, since the pot is resistive- whereas if the pot is only a couple ohms (50 or the like) the current will not decrease as bad while the voltage changes. A negative side of having a low ohm pot as a voltage divider is this- say you have a 31 ohm resistor, and a 50 ohm pot. you have 81 ohms of resistance between positive and negative, which means you're draining at least 380mA at all times, even when nothing else is connected. A far from efficient system.
This is a very wasteful method to making a power supply. It will generate a hefty amount of heat. it also dumps a lot of power needlessly through the pot. If this is just for playing around with, and getting familiar with electronics, then enjoy. But If you want a supply you can use for more practical projects, i'd use the lm317 like opossum mentioned- the circuit could be powered by a much lower voltage, waste next to no power, be much simpler and cleaner to assemble; and best of all there's no mystery components that could cause undesired operation.
And for the record, i dont care what the documentation says- LED's are current sensitive devices, not voltage- i've run them at 84 volts and not had problems. just keep an eye on your current and everything's gravy.
