Hi,
I have a problem, In the following circuit,
I have a voltage regulator with a selector that chooses between different resistor values to make different voltages.

But if ever the selector malfunctions or when changing, it will be disconnected, so the circuit will be open and the voltage regulator will send the maximum voltage, which may burn the thing connected at the output.
I haven't figured out how to solve this problem,
do you have an idea?
Thanks

 

Hi Eth.
You can get Make before Break rotary switches.
E

 

A make-before-break switch would cause the voltage to go lower instead of higher by putting two resistors in parallel while switching.

Make-before-break

 

Hi Eth.
You can get Make before Break rotary switches.
E

Great minds…

 

Great idea !
but I can't find a mbb switch 6positions + a common pin on aliexpress, would you have a link?

 

Connect a link from the highest value resistor to negative, as shown by the blue link, and recalculate the lower value resistors based on being in parallel with the highest value resistor.

This limits the maximum output voltage to the highest value if the switch goes open.

 

Hi Eth,
Look here for the switch type.
E
https://www.ebay.co.uk/sch/i.html?_...=make+before+break+rotatary+switches&_sacat=0

 

Another way is to put the resistors in series (below) and recalculate the resistor values to get the desired voltage steps:
That will also only go to the max voltage setting when being switched.

But a make-before-break switch is needed if you don't want the voltage to rise at all when being stepped.

 

do you have an idea?

Install a resistor from the adjust pin to ground and recalculate all of your resistor values to account for the parallel resistance. Choose a value for the non-switched resistor for the maximum voltage you allow.

But if ever the selector malfunctions or when changing, it will be disconnected, so the circuit will be open and the voltage regulator will send the maximum voltage, which may burn the thing connected at the output.

A make before break switch won't help with the malfunction you're concerned about.

 

I went looking for a good switch and now have a question. Digikey.com has MBB switches but some are "shorting" and "non shorting" and I don't know what that means.
I have some wonderful slide switches that are MBB but they are no longer made.

 

all your solutions just lock the voltage at a maximum value. But that wasn't really the point, I'm choosing between 3.3 and 15v.
The problem is that if I'm on 3.3 and it suddenly jumps to 15v, it'll burn out whatever I was powering.

The MBB seems to be a good solution.

I went looking for a good switch and now have a question. Digikey.com has MBB switches but some are "shorting" and "non shorting" and I don't know what that means.
I have some wonderful slide switches that are MBB but they are no longer made.

From what I understand shorting is just another word for MBB

 

I have a suggestion.
Use the series resistor configuration as in post #8.
Then use single slide switches to select the desired voltage. The lowest voltage selected takes priority.

 

Use the series resistor configuration as in post #8.
Then use single slide switches to select the desired voltage. The lowest voltage selected takes priority.

But again, it needs to be a MBB switch.

 

Simple basic rules for operating an adjustable bench supply...

Never adjust the voltage with a sensitive load attached. (bad things can happen even with a pot)
Never turn on or off the supply with a sensitive load attached.

You might want to consider placing crowbar protection ahead of your sensitive loads.

 

Add a pushbutton switch or a toggle switch that set the output to 1.25V, then turn the knob.

 

all your solutions just lock the voltage at a maximum value. But that wasn't really the point, I'm choosing between 3.3 and 15v.
The problem is that if I'm on 3.3 and it suddenly jumps to 15v, it'll burn out whatever I was powering.

There is no simple, cost effective way to make all circuits immune from a power supply failure or user error.

The MBB seems to be a good solution.

That still won't address your concerns.

Any component could fail and cause the output voltage to rise. While highly unlikely, a resistor could become open circuit. A solder joint could fail. The regulator could fail. A user could set the output to an incorrect voltage.

In your failure/mistake prone world, the only solution is to put over voltage protection in everything that will ever be connected to your power supply.

 

Here's an example sim of the post #15 solution but automated since, given that Murphy is always on the job, someone will, at some point, forget to push the button before switching voltages :
When all the switch contacts are open, as during switching with a BBM switch (shown here with a 1s break between switching), the transistor pulls the output to the minimum (≈1.3V).
The sim shows three switch positions and, of course, you need a diode for each contact position (five for the TS's circuit).

This protects against any voltage going above the desired set point for a normal operating circuit, but as noted by dl324, to protect against all possible failures, a OV crowbar circuit is needed, (which will likely need a double-pole voltage-selector switch to set a variable OV trigger point if you want to protect against anything above the desired set voltage).

 

How about this radical solutuon: Change it to a potentiometer and add a voltmeter.

 

How about this radical solutuon: Change it to a potentiometer and add a voltmeter.

Still can have a voltage spike problem if the pot track has a speck of dirt and the wiper open-circuits momentarily as its being rotated.