Rectifier help .... 870

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Rectifier help .... 870

Postby doc280 » November 5th, 2018, 3:00 pm

I am attempting to track down what why my main brush motor is not working. The main brush motor works with a 9 volt and in a different Roomba.

I have tested as much as I can of the circuit board and all looks good.

In the picture: circled in blue are the two Rectifiers, positive and negative, going to the motor. Does anyone know what the ohms reading for each should be?

Also circled in red is the largest Rectifier on the board. Checking ohms on this Rectifier I am getting an open circuit, anyone have an ohms value for this one?
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Re: Rectifier help .... 870

Postby RTC » November 6th, 2018, 1:09 am

Each diode should read near 0 ohms in the forward direction (with about 0.7 volts drop across it) and open in the reverse direction (measured in circuit you may measure something smaller because of other components).

Diodes are best tested (in or out of circuit) using a digital meter with a diode mode (which will beep and display the voltage drop in the forward direction) instead of using a plain ohmmeter.
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Re: Rectifier help .... 870

Postby doc280 » November 6th, 2018, 11:59 am

Thanks for the reply. I hope I am understanding a little of of what your saying, I have no knowledge really of any of this.

If I place my meter in the mode pictured below I do get lower and higher readings, when I reverse the leads.

The two diodes in the main brush circuit I am getting the following values.

Positive diode : 0.544
0.188

Negative diode : 0.016
0.016 could this be the culprit?
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Re: Rectifier help .... 870

Postby TechGuy » November 6th, 2018, 12:35 pm

Charging battery directly: 400 Series, 500 Series,.H-Bridge Repair How to Desolder
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Re: Rectifier help .... 870

Postby doc280 » November 6th, 2018, 1:13 pm

Thanks.

I pulled both diodes, they are the same part numbers, however the negative one is testing as a bad (opened) diode.

I have some new ones on the way.

Results will have to wait until the new ones cross the pond, from the UK.
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Re: Rectifier help .... 870

Postby RTC » November 6th, 2018, 3:57 pm

Can you read part numbers on them. Generic 1N4XXX equivalent parts may be available near you that should work.
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Re: Rectifier help .... 870

Postby doc280 » November 6th, 2018, 5:57 pm

Yes, they are SR240. Doing a web search I located one place which wanted $2.40 for one and $7.00 to ship. On EBay none local, all are in China or the UK. Being on the east coast I chose the UK and paid $4.53 US. for ten of them and got free shipping.
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Re: Rectifier help .... 870

Postby RTC » November 6th, 2018, 9:23 pm

The SR240 is a 40V 2A Schottky Barrier diode.

Schottky Barrier diodes are ultrafast switching, low forward drop ( <0.1V instead of the 0.7V of a silicon diode ).

So unfortunately, no there is no reasonable substitute.
Last edited by RTC on November 7th, 2018, 1:00 am, edited 1 time in total.
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Re: Rectifier help .... 870

Postby doc280 » November 6th, 2018, 9:49 pm

I am really feeling positive this is the issue, so I can wait. The Royal Mail has always gotten things here within a week, if customs does not interfere.
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Re: Rectifier help .... 870

Postby RTC » November 7th, 2018, 1:00 am

If you are curious Schottky Barrier diodes do not have the ordinary P-N semiconductor junction, but instead a metal-semiconductor junction. This produces their special ultrafast, low forward drop characteristics.

They are probably being used here in a switching regulator power unit.
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