Defect of ZEQ25 Polar Scope LED Power Supply [SOLVED]

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kenkitson
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文章: 51
註冊時間: 週二 07 1月, 2014 15:08

Defect of ZEQ25 Polar Scope LED Power Supply [SOLVED]

文章 kenkitson » 週三 20 5月, 2015 00:50

Dear friends,

I just bought a new ZEQ25 mount, but the weather was bad these days so I decided to just try its function at home.

After a few trial, I found that my LED for the polar scope stopped working. I tried to light the LED with 1.5V battery, but it was not working either. Then I proceeded to replace the faulty LED with a healthy one. After replacement, it worked for a few times, then failed again.

After checking, I found that the power supply for the LED ("Reticle" Port") gives normal voltage (e.g. 5V) for the LED. However when the "+" side of the "Reticle Port" touches the main frame of ZEQ25 (which is metal), the voltage went to 12V. This might causes the LED to blow up.

If the "+" side of the "Reticle Port" accidentally touches the bolts of the mount (via the connecting wire), it shuts down the main circuit and sometimes spark occurs. This might be caused by the 12V dc power short-circuit through the metal frame.

I tried to search online for this defect and found in cloudy nights forum that some ZEQ25 users also experienced LED failure a couple of times. (see: http://www.cloudynights.com/topic/40911 ... ns/page-57)

That's why I suspect that this is a defect in the design of ZEQ25. The power supply for LED somehow shorts to the metal frame of the mount and causes a 12V short ciruit. 12Vdc is not harmful to human, but it definitely can damage electric circuit elements.

Anyone else had the same experience as I did?
附加檔案
IMG_8240.JPG
The resistance between the bolt and the LED housing is very low (~1 Ohm or lower). Technically it is a short circuit.
(44.7 KiB) 已下載 4465 次
IMG_8235.JPG
Voltage went up to 12V when measured across the “+” of the “Reticle” Port and the metal of the mount
(49.27 KiB) 已下載 4465 次
IMG_8233.JPG
Normal voltage level (5V) when measured across the &quot;+&quot; &amp; &quot;-&quot; terminal of the &quot;Reticle&quot; Port
(52.97 KiB) 已下載 4465 次
IMG_8232.JPG
&quot;Reticle&quot; Port for polar scope LED power supply
(38.24 KiB) 已下載 4465 次
最後由 kenkitson 於 週一 25 5月, 2015 23:15 編輯,總共編輯了 1 次。

saikwansky
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文章: 112
註冊時間: 週三 22 1月, 2014 14:59

文章 saikwansky » 週三 20 5月, 2015 21:31

in fact I am an experienced user on Zeq25 Gt, I had 2 times failure on the led, that was why I order 2-3 spares in one time.

I like this mount very much, it is lightweight but enable me to carry up to full set of 90mm APO without problem. While I used in guding mode, I can exposure long enough to 7-8mins in one shot without startrails.

in non guiding mode, I can exposure up to 100s

I heard a lot of complaints on this mount, but at the price point and with this performance, what can you say?

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Nosneb
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文章: 2814
註冊時間: 週五 12 5月, 2006 13:31
來自: 中華人民共和國 香港特別行政區

文章 Nosneb » 週四 21 5月, 2015 08:30

機身螺絲至機身各部份的金屬位電阻值有約1歐姆是正常的. 這是負接地(negative ground), 可以減少外來電磁波干擾
不過LED似乎是線路設計上錯誤地將降壓電阻置於LED的回路(-)上, 可以試試量一下插頭的負極和機身之間是否有電阻值以求證
我都有呢款mount, 以後使用時都要小心留意了 :wink:

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鄧登凳
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文章: 9493
註冊時間: 週一 03 8月, 2009 17:15
來自: 3rd planet of solar system

文章 鄧登凳 » 週四 21 5月, 2015 21:11

LED燈殼和控制箱殼都是膠做的, 不明為何會碰到機身的外露金屬部份??

kenkitson
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文章: 51
註冊時間: 週二 07 1月, 2014 15:08

文章 kenkitson » 週四 21 5月, 2015 22:38

@saikwansky:

I agree that ZEQ25 has a high performance/price ratio and this is why I bought this mount at the first place. And I am relieved to hear that I am not the only person who experienced LED failure.

Now I know I did not get a defective unit and don't need to get a replacement. I believe it is a general design issue of the circuit board.

@Nosneb:

I checked the resistance between the -ve terminal and the mount is ~38kOhm, which is too high to be the normal resistor for volt drop purpose. My guess is they are not connected.

I'll try checking the circuit to see what's wrong. At the moment, the temporary solution is to provide a separate power supply for LED or insulate the LED power output from the mount.

@Tang sir:

The LED case is not plastic, but metal covered with black paint. At the screw thread, the paint falls off and expose the metal surface.

And there is a metal ring at the bottom of the LED, which touches the LED case, and then connects to the metal case of the polar scope (and the mount). I think during plug-in, the +ve terminal might accidentally touch the metal ring and create a short circuit.

The control box is plastic and so I am not sure why the circuit is shorted to the mount. Will try to find out soon.
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IMG_8294.JPG
The metal ring at the bottom of LED touches the metal case, and then short to the mount
(47.35 KiB) 已下載 4129 次
IMG_8295.JPG
The LED case is metal covered with black paint, but the paint at the screw threads fell off and so exposed the metal surface
(66.42 KiB) 已下載 4129 次
IMG_8291.JPG
The resistance between &quot;-ve&quot; terminal of LED port and the mount is ~38kOhm
(58.42 KiB) 已下載 4129 次

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鄧登凳
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來自: 3rd planet of solar system

文章 鄧登凳 » 週五 22 5月, 2015 11:21

I did not notice that the LED casing is metallic. Is the cap plate mounting the connection socket metallic or plastic?

Are you sure that the original LED is a 5V one? Please check the supplied voltage when you set the polar scope lighting intensity to max. 12V LEDs are commonplace.

kenkitson
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文章: 51
註冊時間: 週二 07 1月, 2014 15:08

文章 kenkitson » 週一 25 5月, 2015 23:14

ISSUE SOLVED:

Dear friends,

The issue was solved after spending some time studying the mount and the control circuit. Here is the explanation:

Mount shorted to ground:

One of the control circuit is placed on top of the tension adjuster screw opening. Perhaps during mount assembly, too much pressure was applied to fix the circuit board into position and the paint near the screw opening was scratched off. Thus some metal surface is exposed.

Unfortunately, there is a capacitor positioned on top of that metal surface, and the ground pin of that capacitor touches the metal surface, causing the mount to short to the ground potential. I believe this is not the original design intent, and shall be considered as a design/assembly issue.

LED damage:

I've checked the supply voltage range by the mount, which is from 0V (off), ~2V (min intensity) to ~9V (max intensity). The maximum current allowed for this type of surface-mount LED (SMD LED 0603) is around ~20mA. Thus current larger than that may damage the LED. (ref: http://www.vishay.com/docs/81602/vlmw11.pdf)

After inspecting the PCB, I made a rough schematic of the LED circuit. There are 2 LEDs in ZEQ25, one for power on/off indication (LED1), one for the polar scope (LED2). From the schematic, you can see the LED1 and LED2 are connected in parallel to 12V power supply.

LED1 is connected in series with a 20k resistor to limit the current with no brightness control. For LED2, it is connected in series at the -ve pin with a 10k resistor and a NPN transistor. The transistor is used for controlling the applied current and so the brightness of the LED.

Therefore, the +ve pin for LED2 is connected to 12V, and if the -ve pin of LED2 is shorted to the ground via the mount, the 10k resistor and the transistor will be bypassed. As a result, a large current can pass through LED2 and damage it.

Solution:

To resolve this issue, I placed some insulation tape on the exposed metal surface. Now my mount is no longer shorted to the ground and I can use the original power supply for my LED without worry. :D
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IMG_8425.JPG
Insulation tape is applied to the exposed metal surface to insulate the mount from the circuit ground.
(330.8 KiB) 已下載 3790 次
IMG_8426.JPG
If the -ve pin of LED2 is shorted to the mount, which is at ground potential (shown as red in the schematic), then 12V is directly applied to LED2 without current control.
(343.73 KiB) 已下載 3790 次
IMG_8422.JPG
The -ve pin of the capacitor near the screw opening touches the metal surface on the mount, bringing it to ground potential.
(460.86 KiB) 已下載 3790 次
IMG_8420.JPG
The paint at the screw opening fell off, exposing the metal surface.
(367.69 KiB) 已下載 3790 次

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