circuit that can set a specific resistance for testing?

When I am toying around with new projects I often need to test things out with resistors of different values. Often I find that I don't have the value I need, so I am constantly buying new ones just to test.
I do have a potentiometer but it's a manual process of turning the knob a little bit and then reading the resistance.

I was thinking of a hardware that would help me, so I would be happy if anyone knows if this exists to buy today.
My vision is a PCB with a display, a few buttons, powered by its own battery, and then an input and a output, as well as a configurable resistor.
I could then connect the input and output and on the display I would see what resistance the circuit is offering right now. I could use the buttons to go from lets stay 1 ohm to 1k ohm in incriments of 1 ohm at a time (or 5 ohms)
With this I could experiment using different resistor values before I find the perfect one and buy it.

So let me know, does this exist?

I have seen that there are programmable resistors, but they seem to be offered only in very high resistances. Now if course I could make a circuit that is always reading the resistance of my manual potentiometer and displaying it, but then I would need one that isn't so sensitive so that the smallest of turns of the knob doesn't bump it up 20 or 30 ohms

Hi.

Your search term is "resistor decade bank".
I suppose they are what you called a programmable resistor.
But they should come in a very wide range from 0 (zero) to mega Ohms.
The positions of the switches are in immediate relation to the resulting value.
And you could build your own version to your needs and specifications, out of the above example.

jontaa:
When I am toying around with new projects I often need to test things out with resistors of different values. Often I find that I don’t have the value I need, so I am constantly buying new ones just to test.

For little money you can get kits of 30 values times 20 resistors. Highly recommended. It typically gives you a whole range of about 10Ω to 1M. Exact kits vary.Of course you still may not have the value you need but you will quite certainly have one that’s close, or create the correct one from two others in parallel or series.
In 99% of the cases resistor values are not that critical you need such small steps, and if it is you are probably better off to use a trim pot (and you would need them to compensate for the tolerance of your fixed resistors in the final circuit). You can, by the way, get trim pots as well in such multi-value assortments.

Often I find that I don't have the value I need, so I am constantly buying new ones just to test.

Resistors are cheap, always buy more than you need, you will soon have a good selection.

Same applies to other cheap components like LEDs.

I dont see you can use electronics to do this a s it would involve it being connected tot eh resistors and therefore possibly change (or bee damaged by) the external circuit.

What I have done is to get three 12 position switches and wire resistors in series; so the first has 10 1 ohm r’s, the second ten ten ohm r’s and the third 10 * 100 ohm. (I’d add an extra decade and go 10 * 1k and also maybe 10 * 100k)

The standard procedure used by professionals is to buy a kit of through hole resistors and box of coin enelopes
or a plastic tray component cabinet and take the time to label the envelopes and sort them out by value.

What you are describing is what I commonly have called a Decade Resistance Box. You may find a good used one at a reasonable cost, really good ones made by for example Gen Rad (General Radio) carry a hefty price tag. Consider just the switch contact resistance when you want 1.0 or 0.1 ohm steps. I have seen some very expensive ones toasted because of over current too. Anyway a decade resistance box is what you are describing as already mentioned. Just don't have sticker shock when you see what a good one cost. :slight_smile: You can also find capacitance and inductance boxes.

Ron

Before you go off and do any of the excellent suggestions, determine the power dissipation your resistors need.

Paul

Paul_KD7HB:
Before you go off and do any of the excellent suggestions, determine the power dissipation your resistors need.

Paul

You really want to place stock in that. I have seen some very expensive precision decade boxes fried as a result of over current. There are boxes like the old Clarostat 240D designed for higher currents but even those can get fried.
Ron

You might try looking for a resistor substitution box, also.

Well of course, resistor substitution boards - not quite as easy to use as the old substitution box - are reasonably inexpensive.

Aliexpress link

But the OP wants it to be programmable. A "digital potentiometer" could have some application for a limited control range and given the limitation of its working voltage range which defines what voltages you can apply.

Using multiplexers would give a quite limited number of settings and is also limited by (the allowable voltage range and) the 60 Ohm internal "switch" resistance for a 74HC4067.

Lower on-resistance parts are also available.
https://www.digikey.com/en/products/detail/nxp-usa-inc/NX3L4051PW118/3679433

If you need support for higher voltages, SSR Relays can be used, altho I don't think you can get the 8:1 multiplexing of the earlier parts.
https://www.digikey.com/en/products/filter/solid-state-relays/183?s=N4IgTCBcDaIA4DcAuIC6BfIA

I found some 4-channel parts, but they are really pricey!
https://www.digikey.com/en/products/detail/panasonic-electric-works/AQS221N2SX/4757819

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