L6203 LED indication issue used for peltier heating/Cooling

[Vaibhav Pawar]

Posted on March 01, 2018 at 13:31

We are using L6203 IC in production with annual consumption of 400 Nos. The product where it is used in biomedical instrument used for blood analysis and diagnosis. IC L6203 is used for controlling temp of peltier from 3 set poins (25,30 and 37 �C) and temp can be changed to any values either 25,30,37 �C from its earlier set value ( either heating or cooling )

Temp is sensed using LM35 temp sensor and using analog circuitry it is assigned to EN, IN1 and IN2 pin of L6203 to achieve set temp.

Attached schematics and in design, D6(red) is indication of heating and D7 is indication of cooling.

As per our observation when it is heating , D6 (red) is glowing.

when is is cooling , D7(green) is glowing.

But after stabilizing temp to set value, 4 conditions are observed  ( only D6 is ON, only D7 is ON, Both LED ON, Both LED OFF ), but temp is controlled correctly as per provided settings.

So please guide us 'why LED indication is different when set temp is achieved'? Otherwise L6203 is operating correctly to heat or cool as per settings. Only problem is of indication.

#h-bridge #motor-driver #schematics #stepper-motor-driver #l6203

[Dvorak.Peter]

Posted on March 03, 2018 at 23:22'why LED indication is different when set temp is achieved'

When both leds are off the Peltier driver is disabled by the state of the 2 off schematic inputs. U7-5  U7-10

[Vaibhav Pawar]

Posted on March 05, 2018 at 16:05

@dvorak.peter, Thanks for reply. When temp is already achieved we are seeing different conditions as below in different PCB's:

  only D6 is ON, only D7 is ON, Both LED ON, Both LED OFF

Can you suggest for this? Why it is glowing 'either of D6 or D7 or both ON/OFF' randomly? 

[Dvorak.Peter]

Posted on March 05, 2018 at 17:13

On the enclosed schematic the equal  values for R8 and R16 are strange. It may put the logic input into undefined region.  R16 should be 2 kohms or lower.

For troubleshooting you need more complete schematic.

Peter

[Chris Lynch]

Posted on March 06, 2018 at 10:02

I'm not familiar with the device, but here's an educated guess:

I'm assuming you have the following control states: Cooling, Heating, Do Nothing (set point reached), Measure sensor

Lets say the temp is currently 25 Deg C and you want to reach 30 Deg C.

So the following happens:

Heating: D6 is ON and D7 is OFF, the output of the H bridge allows the peltier element to heat.

Measure sensor: 29 Deg C: Negative error so state remains: Heating 

Measure sensor: 31 Deg C: Positive error and state changes to: Cooling

Cooling: D7 is ON and D6 is OFF, the output of the H bridge 

allows the peltier element to cool.

Measure sensor: 30 Deg C: 

Set point reached do nothing

Do Nothing: D6 and D7 are off, t

he output of the H bridge is disabled. 

Measure sensor: 29.9 Deg C:

 Negative

 error and state changes to: Heating

Measure sensor: 30.1 Deg C: P

ositive error and state changes to: Cooling

Its possible that the oscillation between the last 2 states allow both leds to be on?

And the other states account for either led or neither to be on?

[Vaibhav Pawar]

Posted on March 06, 2018 at 09:23

We tried with 2K value at R8 and R16 but still same issue. Attached schematics. J2 is connected to LM35 temp sensor.

[Dvorak.Peter]

Posted on March 07, 2018 at 16:38

Chris Lynch wrote:

Its possible that the oscillation between the last 2 states allow both leds to be on?

And the other states account for either led or neither to be on?

I agree.

The LM35 connection violates design guidelines in 

http://www.ti.com/lit/ds/symlink/lm35.pdf

  

section   8.1.1 Capacitive Drive Capability

The design should add 75 ohm resistor in series with C6. 

( 

R8 and R16

 should remain 10k )

Peter

[Vaibhav Pawar]

Posted on March 20, 2018 at 06:26

After adding resistor in series with c6, still same problem is observed ie after attaining temp LED indication is different everytime.