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Electronics => Repair => Topic started by: Alex42 on September 22, 2018, 02:35:13 pm

Title: Value of current sense Resistor in Korad KA3005D PSU
Post by: Alex42 on September 22, 2018, 02:35:13 pm

Hello
I have a broken KA3005D (5 A) PSU. The current sense resistor and the diode under it is burnt. I am looking for the value of the resistor (wire coil resistor). I an discussion about the KA3003D (3A version) PSU i found a schematic that shows a 50 mOhm resistor. When i replaced the resistor i found out that i need a 500 mOhm resistor. A mutch higher value then i expected. If i take a smaler value the displayed current is much to low. Everything else seems to work.

Can someone meausure the value of his KA3005D current sense resistor and post it please.

Sincerely
Alexander

Title: Re: Value of current sense Resistor in Korad KA3005D PSU
Post by: Kleinstein on September 22, 2018, 03:44:56 pm

500 mOhms would be much to higher for a 5 A current sense resistor. This would cause 2.5 V drop and 12.5 W of heat. 50 mOhms would make sene and even this is more on the high side.  So chances are there is another fault, that somehow reduces the gain for the current reading and maybe also limiting.

Title: Re: Value of current sense Resistor in Korad KA3005D PSU
Post by: DaJMasta on September 22, 2018, 04:43:42 pm

If your current sense resistor is burned, then it wouldn't be surprising that that kind of overcurrent would have damaged the sense amp as well, if not something else.  I would try to probe the outputs to some of those parts to check, and probably check anything else connected to those output terminal traces.

Title: Re: Value of current sense Resistor in Korad KA3005D PSU
Post by: Alex42 on September 23, 2018, 08:19:10 am

Thanks Kleinstein and DaJMasta

I agree with you that the 500 mOhm ist far to high. After inspecting the sense amp (U6) and the corresponding resistors i assume that it is destroyed and the voltage is not amplifed. Will have to build a mounting so that i can probe the board under power without destroying it.

Thanks for your super quick responses.

Title: Re: Value of current sense Resistor in Korad KA3005D PSU
Post by: bilibili on November 17, 2018, 03:55:28 am

I've measured this resistor with my LCR meter, it shows 95.x mΩ.
so the actual value maybe 100 mΩ

Title: Re: Value of current sense Resistor in Korad KA3005D PSU
Post by: t1d on November 17, 2018, 05:18:37 am

Quote from: bilibili on November 17, 2018, 03:55:28 am

I've measured this resistor with my LCR meter, it shows 95.x mΩ.
so the actual value maybe 100 mΩ

That makes sense, is a common value for a sense resistor and sounds about right...

Title: Re: Value of current sense Resistor in Korad KA3005D PSU
Post by: Alex42 on November 17, 2018, 07:57:15 am

Thanks bilibili

Funny this weekend i have borrowed a RND KA3005P (seems to be the same PSU under a differend lable + USB) to measure the resistor.. As mentiond before i have the replacements for the OPs and latches in the current path. At least the OPs are dead. Its difficult to measure on the board because it has a protective coating on it.

Thanks for all your help

Title: Re: Value of current sense Resistor in Korad KA3005D PSU
Post by: Alex42 on November 17, 2018, 12:47:12 pm

I measured the RND KA3005P current sense resistor and it is slightly under 100 mΩ. I measured the voltage drop over the resistor with different constant loads.