Variable Output Voltage

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skyglin

Joined: 28 Mar 2004
Posts: 23

13 Aug 2004 20:36 Variable Output Voltage

Hi, I need to design a circuit that can output different voltage outputs(variable, say from 3v to 10v) based on certain TTL inputs from a microprocessor. The circuit should be low output impedance. I was thinking of using a digital potentiometer connected to a voltage source and using microcontroller to control its resistance. I don't know if this is a good design because the output impedance varies(isn't it??).

skyglin

yjkwon57

Joined: 31 Jul 2004
Posts: 221
Helped: 21

13 Aug 2004 20:57 Re: Variable Output Voltage

I think you can accomplish this function by using one OP amp and several resistors. The OP amp is used as an analog adder. Each bit coming out of the macroprocessor is connected to the negative input of the OP amp through a resistor. The value of the resistor affects the voltage level of the OP amp output, i.e., determines the effect of each bit from the microprocessor on the OP amp output voltage.

FoxyRick

Joined: 09 Jan 2004
Posts: 597
Helped: 35

13 Aug 2004 21:03 Re: Variable Output Voltage

Hi,

You could do it this way - add an opamp to buffer the output voltage from the digipot. How much current do you need from the output? If you need more than the opamp can give, use a power opamp or add an emmiter-follower with a transistor after the opamp.

Any reason you don't want to use a standard DAC? There are a multitude of ways to do this.

Cheers,
FoxyRick.

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Big Boy

Joined: 20 Jan 2004
Posts: 253
Helped: 10

14 Aug 2004 1:17 Re: Variable Output Voltage

Take a DAC output, buffer it with op-amp and transistor

Code:

Source-only buffer

+12V
|
|\ | C
DAC output>---|+\ B|/
| >---| NPN transistor
-|-/ |\
||/ v E
| |
----------+--> (Image of DAC output
high current)

skyglin

Joined: 28 Mar 2004
Posts: 23

14 Aug 2004 23:21 Re: Variable Output Voltage

Hi, the max voltage I need is about 1A, this will be used to drive a DC brush motor.
Big Boy, can you explain to me how does your circuit work, i understand the op-amp part but not the transistor, what is its purpose? and we don't need biasing for the transistor?

skyglin

FoxyRick

Joined: 09 Jan 2004
Posts: 597
Helped: 35

14 Aug 2004 23:41 Re: Variable Output Voltage

Skyglin,

This is pretty much what I said - the circuit can be added to a DAC or your digital potentiometer output.

The operation is that the opamp buffers the DAC/digipot output so that the output is not loaded - its output impedance then does not matter.

The transister is used as an emitter-follower. That means that its emitter follows whatever voltage is at its base, less a diode drop (0.6V). The emitter then is the new output - the transister can be chosen to provide far more current than the opamp or DAC/digipot can.

Note that the emitter is also connected back to the inverting ( - ) input of the opamp. This way, the opamp will make sure that the transistor's ouput is exactly the same as the opamps positive input. So, the transistor's diode drop is accounted for. ( An opamp adjusts its output so that the sum of its inputs is zero - remember one is + and the other - )

If you need an amp over the range 0 to 10V, you will need an opamp that can swing its output up to about 11V (shouldn't be a problem) and a transistor that can handle 1 amp and 12W dissipation (worst case taken to be 12V drop across it with 1 amp current).

Also, make sure the transistor's current gain (hFE) is enough so that the opamp does not have to supply too much current. For instance, if the opamp you use can supply 20mA (check the datasheets), the transistor must have a gain of at least 50 to supply one amp without overloading the opamp.

Be aware that if you stall the motor, it may draw a rather larger current.

Another thought - the usual way to vary the speed of a DC brush motor when driven by a microcontroller is by PWM (Pulse-Width Modulation). Many micros have a PWM output built-in. All PWM does is provide the full voltage in quick pulses; varying the width of the pulses controls the 'power' given to the motor, hence its speed. This is probably much simpler if you can do it this way. You just need a power mosfet on the PWM output pin to give the required voltage/current from the supply.

What micro are you using?

FoxyRick.

djalli

Joined: 10 Nov 2001
Posts: 887
Helped: 15
Location: 1600 Pennsylvania Avenue, Washington DC 20500

15 Aug 2004 6:18 Re: Variable Output Voltage

skyglin wrote:

There are several IC chips out there that convert frequency of signal to voltage. But I am ataching an article which seems more serious to me this one. Have a look.

djalli

Sorry, but you need login in to view this attachment

skyglin

Joined: 28 Mar 2004
Posts: 23

15 Aug 2004 23:19 Re: Variable Output Voltage

FoxyRick, that is really a detailed explanation, thank you!!

Since you mentioned PWM, I took a look on the web for it and I agree it is a very efficient way of motor speed controlling using microcontroller. I realized that the switch of the transistor is very fast, do we must use a POWER MOSFET for PWM?

Regarding the microcontroller, I haven't decide which one to use yet but planning to use AtMega32 from Atmel, don't know if it has PWM.

skyglin

FoxyRick

Joined: 09 Jan 2004
Posts: 597
Helped: 35

16 Aug 2004 10:44 Re: Variable Output Voltage

The ATMega32 has 4 PWM outputs Very Happy

I don't use them myself so can't give specific code, but in general PWM outputs are easy to set up in code. Search the web or EDAboard - there's lots about this around.

PWM is certainly the better way to go for motor control. It gives better low-speed starting and running than just lowering the voltage does.

The ouput pin is just connected to the Power MOSFET - this is better than a bipolar transistor because it is much faster and effectively only dissipates heat during switching, so you can get away without a heatsink if you use the right MOSFET.

Cheers,
FoxyRick.

Kevin Weddle

Joined: 10 Jul 2004
Posts: 101
Location: San Antonio, TX

16 Aug 2004 16:30 Re: Variable Output Voltage

What you need is a regulator that is controlled by the microprocessor. The regulator can be standard. The control will involve a decoder that will be used to operate a transistor. The transistor that is selected will be biased to give you the voltage. This transistor is the control transistor. It is connected base to the output through a resistor divider. The collector goes to the base of the main current transistor. I would use the ouptut of the decoder to reverse bias the junction of the control transistor. This may require another transistor. So you can see that the solution is readily at hand, but will require a little engineering.