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# Smps

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#### andrie_20

##### New Member
Please me how to control the output voltage in my SMPS circuit. i already try to put potentiometer but when the load is applied the potentiometer will burn. by the way, i am trying to control 12V, 8.5A SMPS.

thanks

Using a Pot/Variable resistor to control upto 8.5A probably wont be too efficient.

All a Pot does it resist current, and drops a voltage across it.
Or in other words, uses up power and gives it off as heat instead of the load getting the power.

If you still want to use a Variable Resistor, you can work out what wattage you need using some formula's.

Power (in watts) = I (current in amps) X V (voltage in volts)
&
V (voltage in volts) = I (current in amps) X R (resistance in ohms)

12 volt is a constant. And i'm gussing its DC.
(AC can get tricky with power factor, because of capacitive or inductive loads. Dont worry about it if you dont know what it is)

So 12 volts DC.
Say if you have a 10Kohm Pot.
(I = V/R)
At 10K you will have 0.0012amps flowing (providing your load is 0ohms which will never happen, so everything changes because all loads have resistance, or its not a load. So now it becomes a series resistance circuit with voltage dropped across your pot and the load.)

But anyway say for instance there is 0.0012amps flowing (1.2 milliamps),
You will need a 0.0144Watt Pot (14 milliwatt)
(P = I X V)
So with a 1/4W Pot (250 milliwatt)
Your Pot will be able to dissapate the heat no problem

BUT if your resistance drops to 400ohms say
you will have 30 milliamps flowing. (I = V / R)
Therefore at 30 milliamps your power consumption by the Pot will be 0.36W (360milliwatt)

AND this exceeds the rating of a Pot rated at 1/4W
and will cook.

simple maths will tell you what size (in wattage) your pot needs to be to successfully disapate enough heat without cooking....

or if you want set voltages, just use a voltage regulator try the 78xx series (7805, 7809 7812).
And if you want variable voltages try a LM317 variable voltage reg.
Teh datasheets will show you the circuit. just google those part numbers.

What do you want to use the switch mode power supply for?
Is it out of a computer?

If you have a fixed output "SMPS" and want to change it's output voltage with a pot, you will have to find the "output voltage divider" in the feedback loop. A schematic of the SMPS would help point you in the right direction. In general, you would put the pot in series with the lower resistor, and that would allow you to adjust Vout down. There will be limits as to how far you can adjust it before you "screw-up" the loop and the supply will go unstable unless you "re-tweak" the compensation. Loop stability in a SMPS is also dependent on the load type... capacitive, inductive or resistive. If the supply is running in current mode you might be able to incease the voltage as well, as that type of control is "power limiting", but you would have reduced output current (the output power would remain constant).

To: jakeselectronics

thanks for the reply. i will this SMPS for lighting a halogen. i already try some of the voltage regulator you suggest but no output when we measure in the output. maybe my set-up of the voltage controller is wrong. can you show me the correct connection of the voltage controller?

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Here is a simple 5v regulator circuit (the 5v regulator can be replaced with any fixed-voltage regulator):