How to make your old analog panel meters last forever (well almost forever).

We all have bushel baskets full of old analog panel meters. There are great devices for helping students learn positive from negative. For explaning simple physics of two terminal devices one can't beat the analog panel meter.

ammeter face

But the ammeters are problemmatic. If they aren't fused, students burn them out quickly. One can buy new meters (but at nearly $100 a pop who wants to do that?) or use multimeters (but they are confusing) to replace the old analog meters.

If the meter is fused, the fuse must be replaced multiple times before the student figures out why polarity is important andwhy one must pay attention to how much current flows in a circuit. Worse than the students are the instructors, commonly knowns as "TA"s or at our school," AI"s.They would dutifully replace a blown fuse with a good one. Not knowing what to do with the old fuse they would throw it back into the box of new fuses. After only a couple of lab periods the blown fuse would get reused and the meter would seemngly not work. So the meter is declared bad and goes in the broken meter shelf and a new one is pulled fromthe shelf. Soon every meter is labelled bad and the faculty is asking why all of our equipment is broken. So I pondered for years on how to resolve this dillema. I tried written signs saying "do not put old fuses in the new fuse box". I tried psychology and cajoled and threatened with big sticks. I promised great rewards of pizza and baggles if we got through the week with no bum fuses put in the good fuse box. I tried hardware solutions: inductors, positive coefficient resistors, relays and many other things but nothing really said "this works"! Then I discovered mosfets while designing a motor control circuit. A mosfet is a transistor with the wonderful property that it's internal "on" resistance is very low (like milliohms!) when sufficiently biased in the forward direction. So, by adding a mosfet (actually two - one for each direction of current flow), along with a low power comparator with hysterises one can easily construct a circuit that will protect your meter and prevent most inadvertent nisconnections from damaging the meter.

Here are some pictures of the meter with the completed protection circuit mounted. It is recommended the circuit is placed in a box to protect it from prying fingers. It is a very high impedance circuit and may be rather susceptible to static charge.

meter back viewmeter side view

So here is the schematic and bill of material for the meter protection circuit. We have a very talented electronic engineeer, Paul Smith (thank you Paul!), who designed a pc board for us. Should you want to produce boards yourself or just want to have a bunch made from our files, just let us know and we can send you the details.

schematic

 

# Ammeter Protector
# Bill of Materials - Other Components
# date: Thu Jul 14 12:05:30 2011
Item No.
Qty
Ref Name Description PN Source
1
2
9V_Wires Battery Connecor, 9V BS4T-HD-ND Digikey
2
6
C1, C2, C3, C4, C5, C6 Capacitor, ceramic, 1 uF, 16V, 0805 311-1365-1-ND Digikey
3
2
PZ1, PZ2 sounder 102-1196-5-ND Digikey
4
2
U1, U2 Mosfet  IRLL2705PBFCT-ND Digikey
5
2
U1, U2 IC, opamp  LTC1540CS8#PBFND Digikey
6
4
D1, D2, D3, D4 Diode, schottky, 70V  MMDL770T1GOSCT-ND   
7
3
R1, R2, R4 Resistor, 100K, 1/8W, 1%, 0805  RHM100KCRCT-ND Digikey
8
1
R7 Resitor, 10k, 1/2W, axial CMF10.0KHFCT-ND Digikey
9
1
R6 Resistor, 10M RMCF0805FT10M0CT-ND Digikey
10
1
R3 Resistor, 20K, 1/8W, 0805 311-20.0KCRCT-ND Digikey
11
1
R5 Resistor, 1.82M - Varies depending on meter 311-1.82MCRCT-ND Digikey
14
2
BT1, BT2 Battery, 9V P647-ND Digikey
15
1
Perf board

 

The circuit is simple enough that you could hand wire the circuit if you wanted. Our circuit has some nice bells and whistles: leds that light up showing whether the meter is connected backwards or whether the meter is connected frontwards. We also added some piezoelectric beepers that go off on overload. This is most effective at getting students to pay attention to the connections as the noise of a dozen beepers going of in the lab room is really aggravating. BUt if you just want to protect the meter (and save your ears) you can save a fair amount of money by deleting the beepers.

As time goes on I will add more details about the circuit. Hopefully there is enough here to get you started. Please feel free to ask questions. I will answer them as best I can.

 

Dan Beeker
Lab Coordinator,
Physics Department, Indiana University
debeeker@indiana.edu