Fuel gauge adapter

[alec_t]

Glad it's working now and glad to have been of help. It's always good to have confirmation that a real-world circuit behaves as simulation predicts. Happy New Year!

 

[jelliott]

Bad news: I broke it. I'm almost too embarrassed to post this, but the way I had it wired--upper ground (as shown in Post #87) directly to battery monitor for the sake of accuracy, lower ground to chassis so the bulk of the current wouldn't go through the battery monitor--created a bit of a vulnerability when, while tinkering with other systems on the car, a short from the 48 V bus to the chassis occurred. With the master disconnect disconnected, the chassis was isolated from battery negative, and the short went right through the first IC in my fuel gauge adapter!

I've replaced the first IC, all three transistors, and most recently the second IC, and now it at least responds to the input, and Bus1 voltage matches output voltage, but it's still not working correctly and I'm at a bit of a loss as to how to proceed, besides starting from scratch and fabricating a whole new PCB, etc. I’m at home now, so I no longer have access to my brother’s car (but do have access to my multiple benchtop power supplies, good-quality multimeters, oscilloscope, and function generator), so I’ve simulated the gauge with a 220 ohm resistor and a pair of 280 ohm resistors in parallel for 140 ohms.

As I apply an input voltage of 5 V to 0 V to simulate the battery monitor’s output, the PCB starts out pulling a seemingly-appropriate (considering my slightly too-high “gauge” resistance) 80 mA. At this point, voltage at the IC end of the first IC’s resistors (relative to ground) ranges from 6.3 to 3.9 V, and the second IC’s resistors are at 0.16 to 0.11 V. But as I turn down the input voltage, the current drops only slightly, such that it’s still at 76 mA with an input of 2.7 V. (Resistor voltages largely unchanged at IC #1; IC #2’s pin 1 now 0.2 V, pins 18-10 now 12.4 V.) Then it drops very rapidly (not quite instantaneously though) to 30 mA at an input of 2.5 V. (Resistor voltages largely unchanged at IC #1 except pin 10 up to 4.5 V; all 12.5 V at IC #2.) Further reduction in input voltage reduces current to 24 mA (zero reading) before the input even gets down to 1.0 V. From there it is unchanged as input voltage continues down to 0 V. (All resistors at 12.5 V where they meet their respective ICs.)

Since the huge jump in current seemed to correspond to a change in state of IC #2’s first stage (pin 1), I tried disconnecting its corresponding diode from the IC, but circuit behavior seemed completely unchanged. I couldn’t think of anything else to try, so now I guess it’s up to the experts on this forum to save me from starting all over again with a blank copper-clad board!

 

[alec_t]

Sounds like the second IC isn't getting the right reference voltage. You should have 1.25V at its pin 4 and 2.5V at its pin 7. Check those pin voltages, also the voltages at pins 4 and 7 of the first IC and the values of R1 and R31 (which connect to pins 7 of respective ICs).

 

[jelliott]

Sounds like the second IC isn't getting the right reference voltage. You should have 1.25V at its pin 4 and 2.5V at its pin 7. Check those pin voltages, also the voltages at pins 4 and 7 of the first IC and the values of R1 and R31 (which connect to pins 7 of respective ICs).

I've got 1.27, 2.53, 0.0, and 1.27 V at IC #2 pin 4, IC #2 pin 7, IC #1 pin 4, and IC #1 pin 7, respectively. R1 and R31 measure 4.6k and 9.6k ohms, respectively (measured in-circuit), but have the seemingly-correct voltage drops (1.27 and 2.51 V) across them when the circuit is powered on.

 

[alec_t]

Well those measurements all seem spot on. I'll tinker with the simulation to find another suspect and get back to you.

 

[alec_t]

I get similar effects to those you found by disconnecting all the anodes of the IC2 diodes from Q1 base. So I suggest checking that area of the circuit for a missing connection. If that fails then check the IC2 diodes themselves haven't gone open circuit (an in-circuit test with an Ohmmeter 'both ways round' should do).

 

[jelliott]

All of the IC2 diodes have continuity to Q1 base, show 0.61-0.63 V drop across them when my multimeter is in diode-checking mode, and and show infinite resistance with the ohmmeter 'backwards.'

 

[alec_t]

So the diodes seem ok. Is the continuity to Q1 base very low resistance?
The big step in current suggests that the second IC is the problem and for some reason is not sinking any/enough current; all/most of the current is being sunk by the first IC. Still looks to me like a missing or high resistance connection, since you've replaced the chip. I seem to remember the +ve supply relied on a terminal being screwed down? Can you confirm the chip supply voltage at what should be full-scale on the gauge?

 

[jelliott]

Continuity to Q1 base is 0.20-0.24 milliohms (same as if I touch the ohmmeter leads together).

Supply voltage does rely on a screw (in the car); on the bench it's supplied via an alligator clip to the +12V end of R14. Voltage at pins 3 and 9 of IC2 is 12.491 V, essentially identical to the 12.488 I see if I stick the voltmeter probe in the power supply directly.

 

[alec_t]

Hmm. Can you confirm pin 2 of IC2 is properly grounded? This has to be IC2-related, but I'm running out of ideas!

 

[jelliott]

I initially measured 0.62 ohms to ground from pin #2, but after cleaning some connections in the experimental setup I got 0.33. Circuit behavior is unchanged.

 

[alec_t]

0.33 still seems rather high. 80ma would drop ~26mV across that, which could (depending on circuit layout) affect the reference voltage seen by the chip. But that would probably only shift the meter current over the whole range rather than produce a step in it. Odd. I'll have another play with the sim when I get time, and try to get back to you in a day or so.

 

[alec_t]

I've checked that the 0.33 doesn't have any significant effect on the sim.
Can you check the voltage at IC2 pin 6? It should be exactly the same as at pin 7. Absence of the pin 6 voltage, or a high resistance (dry joint?) between pins 6 and 7, would by my reckoning (I can't simulate it) produce the symptoms you describe.

 

[jelliott]

IC2's pins #6 and #7 both measure 2.53 V. (And as for the 0.33 Ω measurement to ground, my meter reads 0.20 when I touch the probes together, so the actual resistance is probably considerably less than 0.33 Ω.)

 

[alec_t]

Attached is the response from the sim if the internal comparators in IC2 only get a small fraction of the reference voltage they should receive via pin 6. It's close to what you describe. Can't think of any more external suspects, so perhaps there's an internal problem in IC2

 

[jelliott]

I went ahead and replaced IC2 (again) and the behavior is unchanged.

 

[alec_t]

Oh, that's a shame. Clutching at straws now; are you using leaded solder or lead-free (I'm thinking voltages measured on the circuit board may not be the voltages actually reaching the IC pins if there's a dry joint somewhere) ?

 

[jelliott]

Oh, that's a shame. Clutching at straws now; are you using leaded solder or lead-free (I'm thinking voltages measured on the circuit board may not be the voltages actually reaching the IC pins if there's a dry joint somewhere) ?

The IC1 and the transistors were replaced using solder marked "Lead-Free Silver-Bearing Solder." IC2 was replaced (both times using solder marked "Rosin-Core Solder" which I assume has lead in it.

However, I've been making a point to take all these recent measurements from the component side of the board (i.e. the actual IC pins) to avoid any such misleading measurements.

 

[alec_t]

"Rosin-Core Solder" which I assume has lead in it.

I'd assume that, too. I've not used lead-free and hope never to have to. By all accounts it can be problematic (difficult to tell a good joint from a bad joint, growth of tin whiskers etc).

I've been making a point to take all these recent measurements from the component side of the board (i.e. the actual IC pins) to avoid any such misleading measurements.

Good. But that eliminates my last suspect . If any other possible cause of the problem crosses my mind I'll let you know. In the meantime all I can suggest is apply 5V (= 'full') to the circuit input, and double-check that every pin voltage (both ICs) is what it should be.

 

[jelliott]

IC pin voltages at 'full' conditions (input = 5.0 V, 12V bus = 12.5 V):

1.1 6.13 V
1.2 0.01
1.3 12.50
1.4 0.01
1.5 2.64
1.6 1.28
1.7 1.28
1.8 0.01
1.9 12.50
1.10 3.97
1.11 4.50
1.12 4.56
1.13 4.68
1.14 4.81
1.15 5.13
1.16 5.78
1.17 6.09
1.18 6.29

2.1 0.15
2.2 0.02
2.3 12.50
2.4 1.28
2.5 2.64
2.6 2.54
2.7 2.54
2.8 1.28
2.9 12.50
2.10 0.10
2.11 0.11
2.12 0.11
2.13 0.11
2.14 0.13
2.15 0.14
2.16 0.15
2.17 0.15
2.18 0.16