is it possible to build a voltage attenuator? - Page 1
Forum section: electronics / beginners

author: FriedMule
author_rank: Frequent Contributor
timestamp: February 27, 2021, 07:38:20 am
content: In my journey out in the big and fascinating electronic universe, am I playing with mosfets and other componentsHave I decided to investigate by myself, what happens around threshold voltage. But my power supply is of course in no way capable of adjust voltage in mV or maybe even in uV voltage steps.Is it possible to build something where I can adjust voltage in minute steps?

author: mvs
author_rank: Frequent Contributor
timestamp: February 27, 2021, 07:53:43 am
content: You can bild/use resistance decade box.

author: larsdenmark
author_rank: Regular Contributor
timestamp: February 27, 2021, 08:02:23 am
content: Sure:https://youtu.be/onqsjDJq4I0It may be easier to set up a voltage divider using some variable resistors. If you want to automate this using a microcontroller you can get a 12 or 16 bit DAC.

author: Siwastaja
author_rank: Super Contributor
timestamp: February 27, 2021, 08:02:51 am
content: If you don't need any current (just voltage to bias something that does not consume current, like MOSFET gates), plain old voltage dividers are easy to build. A high-quality potentiometer goes a long way.

author: ogden
author_rank: Super Contributor
timestamp: February 27, 2021, 08:10:31 am
content: Right. Use multiturn potentiometer paired with multimeter. Single turn pot won't let you do small adjustments. BTW such experiments will benefit from multiple multimeters. Hopefully you have two.

author: beanflying
author_rank: Super Contributor
timestamp: February 27, 2021, 08:32:31 am
content: Apart from joining those of us with several DC Voltage Calibrators or KVD'sThe quick and fairly dirty option is a 16 Bit (or better) DAC, an Arduino and maybe add in a better Voltage Reference for the DAC to improve accuracy/stability.You will find a heap of projects out there try "arduino 16 bit dac" or similar should get you some reading.

author: FriedMule
author_rank: Frequent Contributor
timestamp: February 27, 2021, 04:12:31 pm
content: No need for automation. :-) So you'll recommend a voltage standard (expensive and professional made) compared to DIY (cheap and budged together)?A voltage divider with a multi turn pot sounds easy to build, but does that not also demand a super low ripple voltage?

author: larsdenmark
author_rank: Regular Contributor
timestamp: February 27, 2021, 04:39:20 pm
content: It is always nice to have a voltage standard, but they are expensive and you are probably not going to use it every day (I am not using the one I have very often).The accuracy you need to achieve depends on the purpose of the measurement. If you are going to replicate the data in the data sheet you need a good (calibrated) measurement of the voltage and of course a low ripple.If you just are going to find out what how a certain mosfet will behave in a circuit you make you get get away with less. You then just have to replicate the scenario you are aiming for.If you are just aiming to see what happens at certain voltages at the gate at steady state where there is no current flowing you can simply use a simple voltage regulator and a voltage divider. You probably need to be able to measure the voltage at the gate, source and drain (unless one of them is ground) as well as the current going from source to drain. If there is a high current you may also need to measure the temperature.If you want to measure the dynamics of the mosfet (i.e. what happens when you go from e.g. 0 V to 5 V on the gate) you'll need more instrumentation.

author: TimFox
author_rank: Super Contributor
timestamp: February 27, 2021, 05:07:38 pm
content: To measure the behavior of a MOSFET around threshold, since the gate circuit draws negligible current, a simple battery, appropriate voltage divider, and good voltmeter suffices for the gate circuit. The drain circuit needs a power supply with appropriate rating for voltage and current, but high resolution is not required. Cheap DMMs will suffice for voltage and current at the drain.For the gate circuit, a 10-turn pot by itself gives a resolution better than 1/100 of the voltage across the pot. Two cheaper pots in series with each end can set the full range of the pot to the voltage range you need. The measurement resolution will be determined by the voltmeter. The battery will have negligible ripple and its stability will not matter because of the voltmeter. 20 V across a 10 kpot will draw only 2 mA from the battery.

author: Siwastaja
author_rank: Super Contributor
timestamp: February 27, 2021, 05:17:33 pm
content: A 7805 powered by a 9V battery - or a 7812 or 7815 powered by two 9V batteries in series -, followed by a good multi-turn potentiometer is very simple-to-build yet surprisingly handy biasing instrument. Because it floats, you can just connect it to any nodes of a circuit.Adjustable voltage regulator built from LM317 or similar + good multi-turn pot is also very easy to make and works down to the Vref of the regulator (for example, 1.25V). Such supply can also provide some current unlike the simple resistor divider.

author: FriedMule
author_rank: Frequent Contributor
timestamp: February 27, 2021, 09:41:01 pm
content: I like the idea of using two multi torn pots, it'll let me adjust the resistance and the division.About the battery, I know it's a simple thing to implement and will work greatly, I may just be more into a permanently powered solution.While I am at it, would it be possible to make a combined voltage-adjuster and a current adjuster, say I want to test a current-driven component at breakdown point?EDIT: Added a schematic that will not work but I just got the thought that some components is reacting at negative voltage, so the schematic shall just illustrate a variable voltage from negative to positive. :-)

author: TimFox
author_rank: Super Contributor
timestamp: February 27, 2021, 10:02:20 pm
content: That circuit would be useful if you need a voltage range from negative to positive, but the danger is that if you adjust both pots to low resistance, the current from the voltage sources becomes very large.You can always connect an extra variable resistor in series with the output of the divider and measure the voltage across the load simultaneously with the current out of the extra resistor. Fancy lab equipment is useful if you need unattended operation, or very stable voltages, but otherwise these simple circuits are limited by the accuracy of the voltmeters, and well-calibrated high-resolution voltmeters are easier to find than equal quality voltage sources.

author: WattsThat
author_rank: Frequent Contributor
timestamp: February 28, 2021, 12:25:37 am
content: TimFox explained the danger with that connection scheme. Just for the sake of clarity, connecting a potentiometer in that way means it is no longer an adjustable voltage divider, it becomes a reostat. Each device has its uses and some times, abuses.A potentiometer is a ratiometric device which means the output is in proportion to the setting and it’s input voltage, assuming there is little to no current drawn through the wiper. This can be a desirable or undesirable behavior, depending upon the application.If you want an output that can go negative and positive, you need two batteries and one pot. Put the batteries in series, battery 1 (+) goes to one end of the pot, battery 2 (-) to the other end of the pot. Output is the center connection of the two batteries and the pot wiper. You get a continuous output range from negative battery voltage to positive battery voltage.If you’re driving a load where the current changes, use a rail to rail op amp as a voltage follower so you can change the load and not have the change in current affect the output voltage. You can then place a second pot in reostat mode in series with the op amp output to act a current limiting resistor. Using a buffer opamp means you don’t need to take into account the resistance of the divider that is sourcing the voltage.Without it, for every change, you must calculate both settings and change two knobs instead of one.If you need a schematic, ask. I’m on an iPad and too lazy to get to the computer.

author: FriedMule
author_rank: Frequent Contributor
timestamp: February 28, 2021, 01:01:53 am
content: Yes it may sort of work but not as intended (in my description) and lol maybe better to test "magic smoke" :-)I have looked around and found this article that may sound as a fine power supply base?https://www.eevblog.com/forum/projects/ultra-low-noise-lab-power-supply-add-on-for-your-existing-lab-supply/The solution does not necessarily have to be simple, but also no need for an entrance-product to NASA, my goal is to be able to be as precise you can be with reasonable jellybean components and without oven stabilized something something. :-)The idea of putting a couple of extra resistors in series, would be an easy and great solution. They may have to be fairly high current (2W-5W)?I am not sure if this will work as both a voltage and current adjuster? I do not know how to combine positive and negative current circuit. I just added it to illustrate my idea of placement in the circuit.The last image is of a current limiter with nearly no voltage drop, it would maybe be a great feature.

author: FriedMule
author_rank: Frequent Contributor
timestamp: February 28, 2021, 03:09:22 am
content: A schematic would be fantastic:-) I like your comment and the op-amp is something I didn't think of. :-)

author: ogden
author_rank: Super Contributor
timestamp: February 28, 2021, 07:07:18 am
content: Use single pot. One end to positive, other to negative or GND, wiper as output. Never ever implement circuit of two pots you demonstrated because you can destroy pot(s) and/or power supply if not current-protected.

author: Siwastaja
author_rank: Super Contributor
timestamp: February 28, 2021, 08:06:33 am
content: Don't try to adjust the output impedance of a divider.Use simple divider only when the input impedance of whatever it is driving is very high. Then you can choose the divider resistor values for negligible power dissipation. Say like 10kOhm for a 9V battery.If you need to output more current, reducing the resistance only helps so far. Maybe you can go to like 0.5-1kOhm, but it's still very high and you are already dissipating significant power.Instead, buffer it; simplest way is to use an adjustable regulator like LM317 so you get voltage reference and feedback at the same time.

author: WattsThat
author_rank: Frequent Contributor
timestamp: February 28, 2021, 08:08:27 pm
content: I agree with the lm317 idea, add a lm337 for negative output but there are limitations with this, especially below the internal voltage reference of 1.25 volts.This all comes down to how much current do you need? An op amp could be good for perhaps as high as 50-100mA, depends on model. More than that and the linear voltage regulators might be better. Again, it’s application dependent.

author: Ian.M
author_rank: Super Contributor
timestamp: March 01, 2021, 08:51:57 pm
content: The whole concept of using a LM317 for a bias generator circuit is fatally flawed due to two major problems:Its 10mA minimum current to guarantee to maintain regulation, which will rapidly drain a PP3 9V battery.Its very limited ability to sink current, of only the difference between its Adj divider current and the min. load current at which it actually looses regulation.(1) can be alleviated by using a LM317L, which only needs 2.5mA min. load current to regulate, but that worsens (2).Therefore 'WattsThat' s suggestion of a potential divider plus an OPAMP buffer is almost certainly the best idea. I question the necessity to be able to reverse the output polarity by seamlessly transitioning through zero, as that limits the maximum output voltage considerably. 0-10V + a reversing switch would IMHO be a better arrangement. 0-10V is convenient as its achievable from a 2x PP3 battery supply even when the batteries are nearly fully discharged. You'll need a decent voltage reference IC or high quality linear regulator to keep the voltage across the multi-turn potentiometer constant as the batteries discharge. The reference needs some headroom, so I would suggest a 2.5V reference and a non-inverting buffer with a gain of slightly over four, or a 3.3V reference and a gain of slightly over 3, to allow the potentiometer to be calibrated with an additional trimpot in series with its top end so the markings on the multi-turn dial correspond to the output voltage 0-10.0V. To improve its sinking capability at low output voltages, put a red LED which also acts as a power on LED in series with the bottom end of the potentiometer + reference, to lift that point by a little over 1V, and take the OPAMP negative supply from the cathode (battery side) of the LED. Use a dual OPAMP, with the second channel used as a unit gain buffer for the anode of the LED, its output being the return connection for the external circuit. The power switch should also disconnect (or possibly short) the output.

author: Zero999
author_rank: Super Contributor
timestamp: March 02, 2021, 11:08:30 am
content: How about an op-amp and voltage reference IC, such as the NCP4300?The first op-amp, connected to the reference could be set to an amplifier with a vairable gain, from say 1 to 2, giving a course output voltage range adjust of 2.6V to 5.2V. A potentiometer could be added to the output, with the other op-amp used as a buffer, to reduce the impedance.https://www.onsemi.com/pub/Collateral/NCP4300A-D.PDFThere's also the LM10, which will work at lower voltages, but it's much more expensive.https://www.ti.com/lit/gpn/lm10