Equivalent Replacement Diodes - Expert Electronics Advice Please!

[Nedreud]

I need some expert electronics advice on suitable equivalents for an obsolete diode. Maybe the likes of @myPinballs or @lukewells would know?

On my Bally GOLD BALL rectifier board are 4 big fat green diodes. They form the bridge rectifier for the 11.9 VDC that gets sent to the regulator board to be turned into the regulated +5 VDC for the MPU.

The part number is MR751 and they are, to the best of my knowledge, Schottky fast recovery high current rectifier diodes. Why they are needed when other Bally power supplies just use standard one-piece bridge rectifiers I don't know. Could be something to do with the unique combo Solenoid/Regulator/Lamp driver board used only in GOLD BALL and GRAND SLAM?

Here's the spec for the MR751: http://www.mouser.com/ds/2/308/mr750-d-303637.pdf

Would these modern parts be suitable equivalent replacments? http://www.mouser.com/ds/2/115/ds26007-14753.pdf

Both parts are billed as high current, fast switching, high surge capacity and seem to have very closely matching electrical characteristics. Schottky's are bit of mystery to me as to why you would need to use them for rectification over more common diodes like 1N4007.

Many thanks in advance,

Peter

 

[Paul]

When current flows through a diode there is a small voltage drop across the diode terminals. A normal silicon diode has a voltage drop between 0.6–0.7 volts, while a Schottky diode voltage drop is between approximately 0.15–0.45 volts. This lower voltage drop can provide higher switching speed and better system efficiency.

Ripped from Wikipedia

As for equivalent - I'll leave it to the experts as i havent got time to read all the datasheets at the mo

 

[Wiredworm]

Ripped from Wikipedia

Baaaah, you ruined the illusion. Right then I had you down as 'the man' and the guy that knows this stuff and then you had to mention wikipedia...... ;-)

 

[abaxas]

Ermm, those 751 things just look high(ish) surge 6amp diodes.

Obviously something can/does/may eat a few amps for a short period.

 

[Paul]

Baaaah, you ruined the illusion. Right then I had you down as 'the man' and the guy that knows this stuff and then you had to mention wikipedia...... ;-)

I did know there was a difference, and what the difference was... however didnt have a clue about the reason for it Learn something new every day

 

[Wiredworm]

Just winding you up Paul. I had no clue so it's great to learn more. :-D

 

[Moonraker]

Like you say Peter, the characteristics are similar although the original diodes are beefier (100A extra surge capability, quicker response). If they are working fine then I would leave them as is but if they must be replaced then the modern ones are worth a try.

 

[lukewells]

I'm not sure that schottky diodes are actually relevant to this power supplies design.

If we calculate the expected DC output voltage when using 4 regular rectifier diodes, arranged for full wave rectification, we get the following :-

Transformer Vrms = 14.2 (according to the schematic)

VDC = (Vrms x 1.414) - 1.4

So in theory we get 18.7VDC unloaded out with standard rectifier diodes

Any chance you want to measure what the actual AC rms voltage is on the 14.2v winding unloaded, as well as the current VDC output after the diodes ?


Personally if it was me, I would try some common garden 6A20 diodes as I normally have them to hand. I can't see any specific reason for the use of Schottky in this particular circuit.

The lower voltage drop of the schottky diodes results in increased effieicncy, but thats really more relevant when you are designing a PC power supply that needs to supply 40amps of current on a rail. I imagine the +5v circuit in a Bally is pulling 2-3amps continuous.

 

[Nedreud]

Thanks, @lukewells, for a very detailed and insightful answer. I too was wondering why Schottky diodes are used? I understand the lower voltage drop and efiiciency, etc., but the design of the +5VDC that they feed remains unchanged. On PARAGON this circuit is feed with 12VAC and uses a VJ248 8A 200V bridge rectifier, whilst on VECTOR it's 14.2VAC feeding a bridge of four discrete 1N5401 diodes. Checking the specs on-line these are 3A 100V Standard Recovery Rectifiers.

A quick gander on rec.games.pinball and there are numerous suggestions to replace the 1N5401s with 6A4 (6A 400V) or 6A2/6A200 (6A 200V). Your suggestions to use 6A20 therefore seems ideal.