dc388c board not working properly

Hi,

Thanks for contacting Analog Devices!

Thanks for sharing the waveform. This is actually a very classic instability issue in TEC (thermoelectric controller) loops, especially with the DC388C.

What your waveform is showing?

From your image:

• Thermistor value oscillates between roughly 8.6kΩ and 8.8kΩ
• Oscillation is slow and periodic
• It continues for a very long time (15+ minutes shown)
• Current is hitting:
  • ~320 mA (cooling max)
  • ~280 mA (heating max)

-> This clearly indicates:

The control loop is saturating and oscillating (limit cycling)

Root cause (very important insight)

This is NOT a hardware failure.

This is almost certainly:

Loop compensation / tuning issue

Your system is behaving like:

• High loop gain
• Very slow thermal system
• Aggressive controller

Result:

• Controller overshoots temperature
• Corrects too aggressively
• Overshoots again
• Repeats → oscillation

Why this happens in your setup

The DC388C is designed as a generic platform, but:

-> Your laser + TEC + thermistor system is custom
-> Thermal dynamics are very slow and nonlinear

Mismatch occurs between:

• Controller compensation
• Actual thermal system response

Key symptom confirmation

These two observations confirm instability:

1. Current always at extremes

• Either max heating or max cooling
• Never settles in mid-range

This means:

Controller has no fine control → behaving like bang-bang

2. Oscillation does not decay

• Amplitude not reducing with time

This means:

Loop is underdamped or unstable

What you should do (actionable steps)

Step 1: Reduce loop gain (most important)

You must slow down the control loop

Try:

• Increase compensation capacitor (I-term cap)
• Reduce gain resistor

-> Effect:

• Slower response
• Less overshoot
• Oscillation reduces

Step 2: Increase integration time constant

For thermal systems:

• System is inherently slow
• Controller should be even slower

-> If controller is faster than thermal system → oscillation

 Regards,

Tech Support