Needle protection for small current instruments

[harp]

Just want to know is there some common method to protect needle from crashing to end, if the meter is galvanometer or microampermeter with both side movement, needle in center?

If it comes to fast and high unbalance on bridge, the needle was smash and bent. Some electronic circuit might help but it should not interference with measurement if value is in ok range.

[eccerr0r]

Re: Needle protection for small current instruments

I'd thought that standard operating procedures would be useful here - don't connect it directly, add resistance/current limit the meter and slowly take away resistance as you get closer and closer to balancing the bridge.

[stj]

Re: Needle protection for small current instruments

i didnt know the coil could generate enough force to damage the needle!

[harp]

Re: Needle protection for small current instruments

I drive a differential circuit from one input divider, and to operate must have steep/fast difference input from NTC... so, because I dont know what temperature will be, some protection be needed before I upgrading with standard w.bridge and uA.

[eccerr0r]

Re: Needle protection for small current instruments

Draw real circuit and actual/expected component values at operating conditions.

Usually whetstone bridges are used at low power because it's a technically very wasteful circuit, alas it balances out a lot of problems. At low power this should not cause meter issues - if it is, you may need to rethink how to design the circuit for measurement of your unknown.

Thermistors have slow slew rate so I'm not sure what you're looking for or what you're trying to accomplish.

[redwire]

Re: Needle protection for small current instruments

The standard way to protect an analog panel meter is having a diode clamp across the coil. Silicon for 0.7V or Schottky for around 0.3V limit. Usually a 1N4148 or 1N4004 is enough, and use two back to back if negative pegs can happen.
AC panel meters, like an AC ammeter can already have a rectifier inside, so they are harder to protect.

For shipping analog panel meters, you short them out. This lessens the needle banging around because it acts like a generator when the needle gets moved. It used to be normal to get one with a jumper wire across it.

[harp]

Re: Needle protection for small current instruments

"At low power this should not cause meter issues - if it is, you may need to rethink how to design the circuit for measurement of your unknown"

yes... if it is... thats why I ask here, before if it is

I have device who work very well but has no fine visualization, like to see if temperature go on low side, or slightly up before hit threshold.

Working condition of probe with ntc 10k is about 10-200 deg.C so it is 20k to about 100R final resistance and huge diference in half seconds (sensor is very light mass, almost without inertion), so fast voltage change has also big difference in readings.

Maybe it has not bent needle, but it is not like to see (in fine measurement device) where needle bumping arround along one side to another... that condition will neither help the needle in any way... I have only one uA meter and I like to save it if I can

Idealy, I was able to see a clear movement around center, and the needle can never reach and touch either side, like logaritmic gain... but dont know how to do it.
Or, around center needle has big and fast response and on sides it is very small and slow... etc.
I wish to circuit stay as simple as possible, just protection not science fiction.


EDIT
and just for clarification, device is usually using for detecting very small diferences in temperature, about 0.1 deg.C (but fast/sharp edged) arround any ambiental temperature in range, and it is always possible that when measuring probe come on and touch 200 deg.C also...

[redwire]

Re: Needle protection for small current instruments

Don't put a 0.1uF cap across an op-amp output, they will want to oscillate. Instead place it across the 470k feedback resistor to filter out noise.

The circuit doesn't make total sense, you have an AC amplifier due to the 10uF cap but want DC response for temperature.

[harp]

Re: Needle protection for small current instruments

It is a magic from noneducated technician
I design the circuit to cover my need, but I dont know indepth how it work.

Basicaly, on opamp input is 0v, so any positive voltage change will detect as output.
I use it in detecting the short element on pcb, it is my version of "poorman thermal camera", and it works ok, but I think how to upgrade with visual microammeter...

Look on 7:00 how it work
https://www.youtube.com/watch?v=d7jXHcYf9wk

Any hint how to proper protect it?

[harp]

Re: Needle protection for small current instruments

Don't put a 0.1uF cap across an op-amp output, they will want to oscillate. Instead place it across the 470k feedback resistor to filter out noise.

For visualization I use 7- seg. temperature module and jumping signal from ntc. It work nicely, I can read actual temperature and alarm start with detecting of steep rising edge of ntc...
In some settings of selected threshold, sensivity, I noticed little interference with output buzzer, it is not sharp cutoff beep, more like fast pulsing. Then I recal what you say, and moved capacitor which resolve problem completly, now cutoff is very sharp. Thank you more again

[eccerr0r]

Re: Needle protection for small current instruments

seems like asking for fast risetime but limit current - two conflicting requirements as risetime requires current to achieve. You probably need another op amp to drive the meter and use the gain of the op amp for risetime and the voltage rail or comparator as the limit for ensuring it doesn't go over the scale limit of the meter...

[harp]

Re: Needle protection for small current instruments

I am finishing my visualization with W1209 module, and I am very happy with it. It is just I want to be. My previous idea was to do it with analog meter, but in this case I must have exactly circuit approved by expert...

I found some indices how to achieve wanted analog control, but not experiment with it, I attached it for future use to other.