LM358 failed after 30 minutes and gets hot in 4-20mA pump circuit

[djzulu]

I have a current pump circuit that converts 0-5V to 4-20mA.  When the circuit was online for 30 minutes the LM358 got hot and finally blew. I thought that the cause could be the possible high temperature environment and replace the LM358 with the high temperature variant LM2904.

After changed the opamp to the LM2904 I found that after 30 mins of operation the LM2904 would blow and get hot. There are some high voltages in the proximity and I wanted to add TVS protection, it seems that with TVS if the voltage is below the threshold adding the TVS would have no affect unless a TVS event occurs.

Would the attached circuit cause the LM2904/LM358 to fail without a transient event ?

What is the best way to protect the circuit with TVS diodes ?

P0-0 = +24V DC

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In addition I have included a schematic showing the TVS devices.

[srb1954]

You stand a better chance of getting an answer if you post a better quality schematic that is sufficiently readable so we can actually see the component values and other details.

[djzulu]

I've updated the Schematic.

[bdunham7]

What package are you using?

[djzulu]

All components are SMC, the LM2904 is SOIC-8.

[bdunham7]

At first glance it doesn't look like you're actually over thermal limits unless your ambient is pretty high, like 50-60C--then you'd be getting close.  However, the way it fails does seem like a thermal issue.  Reducing the supply voltage would help if that is feasible with your design.  24V is pretty close to the design limit as well.  What compliance voltage is required for the 4-20mA output?

[djzulu]

The initial PCB had the LM358 op-amps, the card was in a terminal box that could have experienced temperatures up to 60C in summer. After a few failures we realized that the thermal limit was too low and opted to use the high temperature version of the LM358, the LM2904 which is rated to 125C. After replacement of the boards in winter when the temperatures were close to freezing we experienced failures. The failure mode was interesting, the boards would operate fine, some would run for weeks and others would fail almost immediately, once failed the LM2904 would get too hot to touch.

There are some coils in the vicinity that have secondary voltages in the 30-50kV range, to my best bet there is a transient event that is blowing the LM2904/LM358 and causing an internal short which is raising the temperature.

Note we have other op-amps on the board, i.e MCP6002 that have never failed, these appear to be just as exposed to transients are the LM358's are.

The 4-20mA voltage lop does not have a required voltage, anything between 12 and 24V would work, we are measuring a voltage over a 475ohm shunt resistor.

[bdunham7]

You might just try putting a ~6V zener in series with your supply to drop the voltage down to 18V.  If your failures stop, that tells you where the problem is, if they continue, then you know where it isn't.

[Swainster]

When specifying TVS diodes, you need to allow some headroom for component variation - perhaps something like 10% i.e. for the 5V regulator, your Vrwm should ideally be above 5.5V, which would bump your TVS diode up to the next available size. That said:

1) You seem to straying quite close to the datasheet limits of lm358/2904, and as such should be careful with part substitution - not all manufacturers' lm358/2904s will have exactly the same limits.
2) Seems like this circuit is subject to proper industrial conditions, and mounted on the end on a cable. This adds a whole extra dimension to the design requirements. Hard to know where to start...
3) Current loop transmitters are typically supplied from the far end of the cable (receiver end) - is this the case here?

[djzulu]

I am not sure why I am at the limits of the LM358, it does sound plausible because only the lm358's are failing. The maximum supply valve voltage from the datasheet is 32 volts and I am supplying the LM358 from the board with 24 volts.

The supplier is different to loop powered 4-20 milliamp circuits, we are powering the op-amps at the board. The signal cables are typically 400 m long and I'll terminated with a shunt resistor to ground. The shunt resistors value is 360 ohms (475R was error), this provides a measuring range of 1.44V to 7.2V.

The input BUF is from an op-amps power by 5V.

[magic]

Simply driving 20mA DC output causes the chip to dissipate 17V·20mA = 340mW.

Are you sure it isn't oscillating at a few hundred kHz on top of that?

[djzulu]

The lm358 is that 4 milliamps for the most part, the 20 milliamp output is a seldom event and it's a peak output when it occurs.

I have not put an oscilloscope on the output but we will try that.

[Andy Watson]

The signal cables are typically 400 m long and I'll terminated with a shunt resistor to ground.

Are these complete loops or is the remote ground connected to a common point that is assumed to be 0V?

[djzulu]

The remote connection is connected to ground 0v at the shunt resistor. We monitor the voltage over the shunt resistor using an ADC.

[djzulu]

You might just try putting a ~6V zener in series with your supply to drop the voltage down to 18V.  If your failures stop, that tells you where the problem is, if they continue, then you know where it isn't.

I do not understand how a Zener in series would reduce the voltage, I can reduce the voltage at the power supply from 24V to 18V and try to fail the components.

[Kean]

The remote connection is connected to ground 0v at the shunt resistor. We monitor the voltage over the shunt resistor using an ADC.

Are you sure there isn't any potential difference (e.g large transients) between the "grounds" at each end of that 400m cable?
Do you see the same temperature rise and/or failures when the output is just terminated with a shunt resistor and no remote measuring equipment?  (with or without the long cable)

[Kean]

You might just try putting a ~6V zener in series with your supply to drop the voltage down to 18V.  If your failures stop, that tells you where the problem is, if they continue, then you know where it isn't.

I do not understand how a Zener in series would reduce the voltage, I can reduce the voltage at the power supply from 24V to 18V and try to fail the components.

A reverse biased zener in series between P0-0 and the opamp power supply pin will provide a voltage drop.  You'd want to have a local decoupling capacitor for the opamp (you don't show any in the schematic).

[bdunham7]

I do not understand how a Zener in series would reduce the voltage, I can reduce the voltage at the power supply from 24V to 18V and try to fail the components.

OK, if you can just reduce the power supply voltage some other way for testing, that's good enough.

Yikes, a 400m cable?  What form exactly is the cable--a twisted pair?  What kind of environment is it in? Do the failures occur in testing without that cable?  Can you connect an oscilloscope and record what goes on at the output of your current source? 

[free_electron]

are you sure they are not oscillating at very high frequency ( MHz ). where is your power supply decoupling ?
Opamps can be unreliable when the input difference gets large. put two diodes antiparallel between the inputs. that way there can never be more than 0.6 volts of delta.

[wraper]

The initial PCB had the LM358 op-amps, the card was in a terminal box that could have experienced temperatures up to 60C in summer. After a few failures we realized that the thermal limit was too low and opted to use the high temperature version of the LM358, the LM2904 which is rated to 125C.

That was quite pointless, it's exactly the same IC but with tighter binning. It's just a guarantee that IC will meet specs in wider temperature range. Maximum junction and storage temperatures are exactly the same for all variants.

[djzulu]

The remote connection is connected to ground 0v at the shunt resistor. We monitor the voltage over the shunt resistor using an ADC.

Are you sure there isn't any potential difference (e.g large transients) between the "grounds" at each end of that 400m cable?
Do you see the same temperature rise and/or failures when the output is just terminated with a shunt resistor and no remote measuring equipment?  (with or without the long cable)

Under steady state conditions there is no temperature rise, the temperature seems to spike just before a failure.

[djzulu]

are you sure they are not oscillating at very high frequency ( MHz ). where is your power supply decoupling ?
Opamps can be unreliable when the input difference gets large. put two diodes antiparallel between the inputs. that way there can never be more than 0.6 volts of delta.

We will add the diodes and a decoupling capacitor to the +24V rail, as the failure occurs with a lab power supply we do not believe that this is the issue, we do not know the reason for the failure.

[Swainster]

Being able to duplicate the fault on the bench is very interesting - it would take most of the environmental conditions out of the equation. Presumably this was with the receiver closely connected to the transmitter?

Talking of the receiver, is this a standard loop receiver or did you roll your own? If it is an off the shelf part, is it supplying the loop power? If this is the case then supplying power to the output of an lm358 will quickly burn it out.

Ignoring the bench failure for the moment, with a long enough cable, especially in an industrial environment, if the supplies at either end are not fully floating with respect to each other (e.g. if they are partially or fully tied to local ground), then significant common current will flow in the loop conductors. This could burn out the transmitter (and has been known to melt wires, at least in communication cables).

[djzulu]

Talking of the receiver, is this a standard loop receiver or did you roll your own? If it is an off the shelf part, is it supplying the loop power? If this is the case then supplying power to the output of an lm358 will quickly burn it out.

The received is a simple shunt resistor 360R, we measure voltage of 1.9 to 7.2V over the resistor. I have one card that has been running for 2 months with a shunt on the output in the lab.

Ignoring the bench failure for the moment, with a long enough cable, especially in an industrial environment, if the supplies at either end are not fully floating with respect to each other (e.g. if they are partially or fully tied to local ground), then significant common current will flow in the loop conductors. This could burn out the transmitter (and has been known to melt wires, at least in communication cables).

If I short the outputs this would drive 20mA through the LM358, would this rule out cabling or are you concerned about transient conditions too ?

[Swainster]

Shorting the lm358 will rule out current being driven into the output, though it will heat up the op-amp a fair bit - not all lm358s are rated for constant short at 24V (are any of them?). Typically constant short to gnd for lm358 assumes room temperature and an equal split supply I.e. only shorting about 15V to ground. I guess you would already be pushing abs max power for the package at 15V, never mind at 24V