Added 858D measurements

content/posts/2018-06-25-Saike-858D-Measurements.md

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+---
+title: "Saike 858D Rework Station - Measurements"
+date: 2018-06-25T13:20:00+02:00
+author: eNBeWe
+type: post
+categories:
+  - Allgemein
+  - Bastelkram
+tags:
+  - 858D
+  - DIY
+  - Arduino
+---
+
+The next part in the series about reverse engineering the Saike 858D rework station ([Overview][1]) brings a few 
+measurements, I did in my device. 
+It presents me with the problem, that I am not sure anymore if I want to keep the transformer, 
+currently used in the station.
+
+<!--more-->
+
+I soldered some wires to the connector of the transformer, allowing me to hook up my Oscilloscope to the transformer output.
+
+## 10VAC Rail
+
+![10V Rail Without Load](/posts/images/858D/10VAC_No_Load.png)
+
+The first measurement is the 10V rail in idle state, without the heater or fan running. 
+The "sine wave" is not really pretty but I don't really care, given the fact that it will be rectified anyway.
+The peak output voltage is 16.6 volts, and the RMS output is 12 V instead of 10 V. 
+This means the transformer produces a voltage a bit higher than rated.
+
+![10V Rail With Fan Running](/posts/images/858D/10VAC_Fan_Running.png)
+
+If the fan is running, the 10V rail degrades a bit more and slightly drops in voltage.
+Again, given the fact that it it fed through a rectifier and then a voltage regulator, that is almost fine.
+
+I don't really like that we use a voltage of about 12V as the input for the 7805 voltage regulator. We power the microcontroller, the display, the EEPROM and a bit of other stuff from the regulator so I would guess the current is not too high, probably in the few hundreds of mA.
+
+## 30VAC Rail
+
+![30V Rail Without Load](/posts/images/858D/30VAC_No_Load.png)
+
+The second rail of the transformer should supply 30VAC. In reality it overshoots quite a bit and delivers 34VAC RMS, peaking at 47 volts. The diodes in the full-bridge rectifier have a 1 volt drop each, so the rectified voltage is probably somewhere close to 45 volts.
+
+![30V Rail With Fan Running](/posts/images/858D/30VAC_Fan_Running.png)
+
+With the fan running, the voltage drops a bit, peaking at 43 volts. This is the voltage, the control circuit for the fan has to regulate to not overload the fan.
+The fan is labeled with a consumption of 0.35A, which means the transistor has to sink about 4 Watts (11 volts at 0.35A) of heat. As a result, with the fan running, the transistor almost instantly heated up to about 50°C.
+
+## Fan output
+
+![Fan voltage sweep](/posts/images/858D/Fan_Sweep.png)
+
+Additionally, I measured the voltage at the output terminal of the fan.
+At the minimal potentiometer position, the fan has a voltage of 11.4 volts applied to it.
+At the maximal position, the voltage rises to 28.2 volts, more than 4 volts over the fan specification
+
+## Conclusion
+
+I am not really happy with the transformer in the device. The rating is dodgy at best, and lots of energy is wasted in heat in regulation circuits.
+
+At the moment I tend towards replacing the transformer with a 24 volts switching power supply and a small buck converter for the 5 volts logic circuts. The fan regulation could then be done through simple PWM.
+
+
+ [1]: {{<relref "2018-06-20-Reverse-Engineer-a-Saike-858D.md">}}
+