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Meter overload ... ?

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thomas
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Post Number: 189
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Posted on Sunday, 08 July, 2007 - 10:44 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

Hello Friends,

What is the best (simple) way of attenuating an output to prevent meter overload? Supposing one typically has a 1V swing at an output -- which may, however, very occasionally swing to 5V. Supposing the meter is rated 1 mA. One then requires a 1k series resistor to give full scale deflection at 1V. So 5V (in fact anything above 1V) is an abnormal situation where one wishes to "bypass" the meter ... ?

With kind regards,
Thomas.
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joe
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Post Number: 330
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Posted on Sunday, 08 July, 2007 - 11:21 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

Wouldn't a zener and a resistor wired as a shunt regulator do this ?

Anything upto the regulator voltage would pass, anything over would be capped.

http://en.wikipedia.org/wiki/Zener_Diode
Do one thing each day that scares you – work here !
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obiwan
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Post Number: 1847
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Posted on Monday, 09 July, 2007 - 12:58 am:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

I think you either want a shunt or a non-linear voltage amp.

I'm not sure exactly what it you're doing, but those would be my two guesses. Either something to drop down the voltage (shunt), or some way of keeping full scale on the meter, without over loading it (non-linear amp).

I can provide you with one or two really cool non-linear amps, if that's what you want.

I was amazed at how they work. Sine wave in, square wave intermediate, and sine wave out. Just amazing it could restore the signal input.

But you'd need a log scale on your meter.
Do Not Hit The Fly That Lands On The Tigers Head.
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gajjer
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Post Number: 21
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Posted on Monday, 09 July, 2007 - 06:59 am:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

Hi
1V is a bit low for a zener, but a silicon diode like a 1N4148 will start to conduct around 0.6 - 0.7V.
Find a meter/resistor combination that gives 0.6V full scale. Put the diode across this combination. Anode at the +ve end. Then put a resistor in series with the combination to give the sensitivity that you want.

If you apply 5V it will hit the end stop but not as violently.

cheers
gaj
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petelobus
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Posted on Monday, 09 July, 2007 - 08:23 am:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

It depends on the precision you want to clamp with (which depends on the overload capability of the meter) and the amout of added complexity you are prepared to tolerate. From memory, niether a Zener, nor an ordinary diode turns instantly from non-conduction to conduction, and both have significant thermal effects (seem to remember that 5v1 zeners are almost thermally neutral). A clamp circuit using an op-amp with a zener in the feedback loop would be more precise, but harder to implement. Given a deal of thought I could probably come up with a circuit that I used to use. Alternatively what about a PIC (with onboard A/D), loads of software, and a meter, or bargraph display?

Ian
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thomas
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Post Number: 190
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Posted on Monday, 09 July, 2007 - 10:23 am:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

Thanks, folks. Ah yes, it is coming to me now. Possibly with a Zener or diode(s) wired to a suitable potential divider I can adjust things fairly precisely. I was already thinking Zener, but hadn't thought of suitably accurate control. Thomas. P.S. Ian what's a PIC? ;-)
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steerpike
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Posted on Monday, 09 July, 2007 - 12:19 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

Thomas - I've been doing a similar thing, I've found the diode shunts to be very temperature sensitive & hence not really useful; I'm still looking for an ideal solution. A log amplifier would work, but it's more complex than I want to use.

OBIWAN, please show me your non-linear amps.
I'm looking for a circuit to extend the range of a moving coil meter - something that lets me see the low end more clearly, yet doesn't change the FSD. (actually a VU meter that I want to get showing -40dB up to +8dB)
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joe
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Posted on Monday, 09 July, 2007 - 01:20 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

For better thermal stability, what about somthing like a TL431 adjustable shunt regulator with it's output to a potential divider, or perhaps even a ZXRE125EF.
Do one thing each day that scares you – work here !
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obiwan
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Post Number: 1849
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Posted on Monday, 09 July, 2007 - 05:14 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

Steerpike, I'll send them to you.

Just sent you a private message.
Do Not Hit The Fly That Lands On The Tigers Head.
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steerpike
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Post Number: 232
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Posted on Monday, 09 July, 2007 - 05:18 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

In my search, I found this - it may be useful to you Thomas:

http://www.maxim-ic.com/appnotes.cfm/an_pk/3611

Figure 1a and Figure 1b looked useful simple circuits to compress the scale of a meter.
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gordon
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Posted on Monday, 09 July, 2007 - 10:19 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

A lot of analogue multimeter circuits just have a couple of back to back silicon diodes to protect the meter movement from overload.

Your meter may have a FSD of 0.2 volts for a typical 1mA analogue meter, so maybe a single schottky diode might be suitable for overload protection.

Not sure what the maximum overload without damage for an analogue meter would be, but probably at least 2-3 times FSD, depending on design.
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terrym
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Posted on Tuesday, 10 July, 2007 - 11:31 am:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

Schottky diodes are too temperature sensitive to use as meter shunts. "been there, done that" and had to recall several tens of units of a product to replace them with standard 1N4001's.

TM
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steerpike
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Post Number: 236
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Posted on Wednesday, 11 July, 2007 - 05:21 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

Obiwan Kenobi, thanks for the diagrams. I'm going to give the log converter a try - the diode clamps I found to be just too temperature dependant - merely touching the diodes heats them enough to bring in a 3dB error.
I did find a temperature compensated variant of your circuit in an ancient ETI (around 1983), which I like the look of - IF i have enough LM324s lying around. (this was supposed to be a spend-no-money project!)
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obiwan
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Posted on Wednesday, 11 July, 2007 - 07:56 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

I'm glad to hear that circuit "might" work out. I wish I could remember how we compensated it at the medical company.

I do know it was a simple method, like a single thermister and maybe, maybe another opamp. It was a 4 page schematic, and I'm missing that one page. So I'll look for it again, it'd be nice to have it.

But the compensation shouldn't be that hard.

Seems to me, the thermister connected to the other leg of the opamp. So as the temp of the diode goes up and brings in another resistor, the thermister went down and brought it back into where it should have been.

Make sense? But I'll look around for the other page.
Do Not Hit The Fly That Lands On The Tigers Head.
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steerpike
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Post Number: 237
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Posted on Thursday, 12 July, 2007 - 12:41 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

I did some theory calcs last night, and i came up with an error of 5% over a temp variation of 15C to 30C, That is an insignificant error on a small moving coil meter, so I might get away without any special compensation.
There are several ways to redice the error, but i wanted to do it all with parts already in my box!
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epithumia
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Posted on Thursday, 12 July, 2007 - 05:56 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

Here's a slightly different tack:

If a regulated PSU is available, how about using a rail-rail op-amp. Say the PSU is 5V. Give the op-amp a gain of 5 so a 1V input drives the op-amp to the PSU rail. Adjust the meter's series resistor for FSD at 5V.

Only as accurate as your PSU of course, but I don't know the application.

Epi
If you need me, Neil and me will be hanging out with the Dream King. - Tori Amos
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obiwan
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Posted on Thursday, 12 July, 2007 - 06:15 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

I think one way you can adjust the error is by using something like a LM334Z, the temp sensor.

That is programmable. So I think you can program the current to offset any number of diodes in that circuit.

We used a regular resistive thermistor, but the 334 might work better, as it's programmable.

But I know how you feel, wanting to do it with what you have. The problem is, you don't have enough stuff!!
Do Not Hit The Fly That Lands On The Tigers Head.
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gajjer
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Post Number: 26
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Posted on Thursday, 12 July, 2007 - 09:16 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

How true those words are.

You can never have enough stuff.!!

I like it.

Also, perhaps - the stuff you have is never quite the stuff you need.


Who would have thought such a simple problem would generate so much debate.

I like the saturating op amp myself.

cheers
gaj

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