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Components and Circuits For discussions about component types, alternatives and availability, circuit configurations and modifications etc. Discussions here should be of a general nature and not about specific sets.

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Old 27th Nov 2022, 4:32 pm   #1
TonyDuell
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Default Voltage Detector IC?

The attached photo is of a component from the logic board of a Toshiba hard disk drive (probably over 30 years old, ST412 interface). It's a 3-lead TO92 package, this one is dead if only because the middle pin has now dropped off.

The writing on the device reads

<logo that looks like SWS> 74
T520D

I can find nothing about it with Google.

I'll put the device in 'context'. There's an 8049 microcontroller on the board too. The reset/ pin has the standard circuit of a capacitor to ground and a resistor + diode in parallel to +5V. The 3 pins of this device are connected to ground, +5V, and the reset line.

My guess is that the last pin is an open-collector or open-drain output It holds the reset line low until the 5V line reaches a particular voltage (say around 4.75V), then releases it, the capacitor charges via the resistor and then the microcontroller starts up.

Anyway, any ideas about it, and what I could replace it with?
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Old 27th Nov 2022, 4:40 pm   #2
Cruisin Marine
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Default Re: Voltage Detector IC?

If you are stuffed, an NPN with a 3.9v zener feeding the base perhaps ?
Assuming it is open collector of course.
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Old 27th Nov 2022, 4:46 pm   #3
TonyDuell
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Default Re: Voltage Detector IC?

You need a resistor in series with the zener too, otherwise you will burn out either the zener or the b-e junction of the transistor. But anyway it would work backwards, it would hold the reset/ line low when the 5V rail was over the threshold. There also isn't muck space to make a discrete-component circuit.

If it is what I think it is, there are possible replacements are RS, but on backorder until an unspecified date.
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Old 27th Nov 2022, 5:14 pm   #4
RogerEvans
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Default Re: Voltage Detector IC?

I have used these reset controllers in the past with micros that seem overly sensitive to contact bounce the usual R, C and reset switch. I don't off the top of my head remember the part number but a quick search on microprocessor reset controller came up with the MAX810 and similar. They come with variants for 5V and 3.3V.

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Old 27th Nov 2022, 5:17 pm   #5
Neutrino
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Default Re: Voltage Detector IC?

The logoミツミ reads Mitsumi. It looks like a PST520 for System Resetting. The D suffix indicates 4.2V Typical.
https://www.radiomuseum.org/tubes/tube_pst520.html
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Old 27th Nov 2022, 5:30 pm   #6
TonyDuell
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Default Re: Voltage Detector IC?

Thanks. I am pretty sure that's right (lthough how I was supposed to guess the 'PS' is beyond me). It is what I thought it was, so now to find something that will replace it.
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Old 27th Nov 2022, 6:04 pm   #7
duncanlowe
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Default Re: Voltage Detector IC?

Also often known as voltage supervisors. Essential in circuits that might get into a bad state if the voltage gets a bit low, but not low enough to trigger a clean reset. They are very common in modern equipment, though can't guarantee any pin compatibilty of course.
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Old 27th Nov 2022, 6:10 pm   #8
TonyDuell
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Default Re: Voltage Detector IC?

With only 3 connections which are long-ish wires on a TO92 package, I am sure I can kludge something in. As I said, RS seem to list things from other companies what would appear to be suitable but they're on backorder until an unspecified date.

I guess it's used in the hard drive to disable the thing (by resetting the processor and the control logic) if the power goes low to prevent data corruption on the disks. Makes sense.
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Old 27th Nov 2022, 7:00 pm   #9
buggies
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Default Re: Voltage Detector IC?

Couldn't see a 4.5 MCPxxx for sale but maybe 4.35V ? - cheap enough:-
https://www.picaxestore.com/cartquic...ct/view/id/203
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Old 27th Nov 2022, 9:27 pm   #10
Cruisin Marine
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Default Re: Voltage Detector IC?

You need a resistor in series with the zener too, otherwise you will burn out either the zener or the b-e junction of the transistor.
Yes, of course, I was just mentioning a rough idea, not the final cct.
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Old 27th Nov 2022, 10:53 pm   #11
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Default Re: Voltage Detector IC?

Here's the datasheet of the Mitsumi device if it's of any use.

A search for 'supervisory and reset ic's' should find a viable alternative.
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Old 28th Nov 2022, 4:32 am   #12
TonyDuell
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Default Re: Voltage Detector IC?

Thanks for the datasheet.

A lot of the PSU supervisors with open collector/drain outputs seem to have internal pull-ups which I don't want here. But it looks like an MCP120 from Microchip would do. What a surprise, RS have them on backorder until $deity-knows-when.
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Old 28th Nov 2022, 4:48 am   #13
Mark1960
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Default Re: Voltage Detector IC?

Ds1233 is another possible, though it does have a pullup on the output.

Why is a pullup a problem? I donít think the RC determines the reset time on this one.
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Old 28th Nov 2022, 4:54 am   #14
TonyDuell
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Default Re: Voltage Detector IC?

The output of the IC goes to a conventional RC reset circuit hung off the 8049 reset/ pin. The extra pull-up will affect the time constant of that circuit which is something I want to avoid.
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