Digital panel meters with VCC < 5V?

Hello Folks,

Just happened again: A design needs a digital panel meter to display a voltage. Something in the usual $35 range. Problem is that Vbat must be allowed to drop to 5V or a tad less and that is also the supply of the usual panel meter. They seem to come only in 5V, 9V and 4-20mA loop for their supply. The loop versions cost a lot more. Sure, I could generate

5V, 9V or whatever but it's a hassle.

Is there any mainstream (non-expensive) digital panel meter that can live with less than 5V? I mean, there are plenty of handheld meters out there that use a couple AA cells. It just wouldn't be practical to disect these and try to mount them into bezels or cutouts.

Regards, Joerg

The Vbat low seems tantalizingly close enough does it need to be regulated tho ? if so would an ultro low dropout regulator do (100mv) ? the meter 5v must have a +-10% on its spec at least.

Colin =^.^=

Hello Colin,

An LDO is what I will use if I must accept a 5V meter. But LDOs aren't the most stable circuits so I'd rather not if I can avoid them. They are often fickle when it comes to source impedance.

Specsmanship for many meters leaves something to be desired. Often there isn't any spec for VCC tolerance on the usual two-page pdf. Then you have to contact the source, hoping it isn't just some sales outlet.

Regards, Joerg

Hello Frank,

They do for many of the 5V versions. That is ok in this case.

Regards, Joerg

regulated

must

I had a similar problem once, needed to power a 68hc11 in a handheld unit, but the power requirment went up so needed larger bateries wich meant I cld only fit 4 Nicads instead of the 6 smaller ones I planned, I managed to get away with it but had to use a p-ch mosfet and op amp as an alomst zero drop regulator, good luck. Ive used LDOs but at low currents where they are more stable. is this an LED meter ?

Colin =^.^=

go there, they have a whole list of panel meters LCD/LED in the 5 to 9 voltage range using different configs in the range of

12.95 .. 16.00 bucks or so.

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Try

- the spec sheet says the Vcc is 3.5 - 5.25 volts _and_ one side of the measured voltage can be connected to the power supply ground (I've just bought two of these from Newark, but haven't actually put them in service yet...)(they are based on a Maxim MAX 138)

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I suppose if a panel meter is rated for 5V it may work too with 4V. But you need to try and find out.

Not very practical indeed. You'd also need to check if the test lead and negative battery terminal share a common ground.

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Hello Winfield,

Thanks. These are nice. The only downside is that these seem to be in the $50 class. Mouser has quite a few of them.

Regards, Joerg

Hello Colin,

Yes, those last minute spec changes. I try not to get grumpy when that happens but see it as a challenge.

This would be an LCD meter, non-backlit. LED consumes too much. However, the 5V versions consume much more than the same meter in 9V. Guess they just roach on a converter. If we could obtain the schematic this would all be a cake walk. But mostly they don't disclose that.

Regards, Joerg

Hello Jamie,

Thanks. They do have good prices. Much better than Digikey and the other big ones. The specs, ahem, I guess they took a digital camera and photographed the single sheet that comes with the meter. But for test rigs that may be ok.

Regards, Joerg

Joerg? What are you leaving out here? You have a DPM application, and you're afraid your 5V battery will droop below the +5V input spec of your DPM?

There's something I'm missing here - or, Joerg, are you funning with us? ;-P

Cheers! Rich

I know you want to avoid the hassle of generating the voltage, but you might want to re-think that, if you want a guaranteed supply to the panel meter when the Vcc dips. I don't know your Vcc range - only that it dips below 5V. With that in mind: I've used a Tl499 boost converter/regulator down below 2 volts to get a rock solid V out. The thing works on Vin from 1 to 10, and has a resistor programmable Vout of 2.9 to 30, and provides up to 100 mA. You need an inductor, 3 resistors and 2 caps. You can set Vout to whatever V the dpm needs, allowing you to choose the lower cost units.

Ed

Hello Rich,

The rest of the system has a battery that can drop to 5V or a bit less. So even an LDO would let go if I'd try to regulate to 5V. If the meter could take 4.5V it would be fine.

Regards, Joerg

Hello Peter,

Thanks. That's exactly what I was looking for. The SP200 has a backlight which can't be used here but I bet it's readable without.

Regards, Joerg

Hello Ed,

Yes.

I might. But instead of the TL499 you can also use an inverter as an oscillator, create twice VCC and regulate down. Maybe with a couple of inverters to buffer for enough current but that won't be a big deal since they come in a "six pack".

Regards, Joerg

Yes - easily readable in normal room lighting without the backlight.

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Hello Peter,

Thanks, that is good to know. This isn't always the case. For example the LCD display on the phone here in my office isn't readable until the backlight comes on.

Regards, Joerg

Don't laugh too loud, but... Why not stick a AA, C, or D cell in series with the meter power supply? OK, this is a bad idea if you then have to regulate the meter supply with a linear regulator, or if this makes the meter supply too high when the system battery is fully charged. Also, if there is a general spec of "no batteries or parts that have to be replaced" then you lose. If it's an LCD meter, though, it probably draws very little current and a battery may last a long time.

Matt Roberds