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Old 12th January 2006, 12:06 PM
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Default Solid State Diodes - Choice

Solid State Diodes - Choice

Standard solid state diodes are reliable but retain a charge when they switch which causes a spike. This can cause direct RF radiation into circuitry and the electrical energy can also be reflected back into the mains transformer causing ringing.

There are now quieter switching diodes available. Get the data sheets from the web in .pdf. They usually give guidance notes as to suitability. Look for info such as, "Hyper fast with soft recovery, Ultra fast, Soft, Noise Free Recovery" etc.

Look for a low trr (reverse recovery time) in the 50ns to 100ns region. ( Compare that with the likes of BYV96D at 200ns which was considered fast switching a few years ago. )

Then check the current and voltage. Be generous and allow 3x the AC supply voltage and perhaps 5x the current drawn ( as the following capacitor will only charge on peaks which may be very small with a low impedance transformer. ) Finally, check the package will fit OK!

"Hexfred" is an International Rectifier tradename for their fast recovery and soft switching diodes. Other makers use other names such as "Turboswitch" for similar products though the technology may or may not be the same.

Schottkies are a different technology and do not have a recovery time. They do have other drawbacks but are, perhaps, almost perfect ( ditto valve rects come to that ).

Some useful high voltage single diode numbers are,

RURD4120 4A 1200V 70nS RS485-4918 74p each TO251
RHRP8120 8A 1200V 55nS RS543-9009 £1.84 each TO220
RHRP15120 15A 1200V 65nS RS183-7140A £1.85 TO220
STTA512D 5A 1200V 95nS RS217-8436 95p each TO220

UF4007 1A 1000V trr 75ns Axial
UF5408 3A 1000V trr 75ns Axial

STTA512D 5A 1200V trr 45ns Ultrafast Soft Recovery TO220AC (square 2 legs)

RHRD4120 4A 1200V trr <60ns Hyperfast Soft Recovery TO251 (small square 2 legs)

HFA08PB120 8A 1200V trr 28ns Ultrafast Ultrasoft Recovery TO247AC (square 2 legs)
HFA08TB120PBF 1200V 8A Hexfred 28nS TO220 package (Sq 2 legs) RS 541-2650 £5.24
HFA08TB60 8A 600V trr 55ns Ultrafast Ultrasoft Recovery TO247AC (square 2 legs) RS 244-9889 £2.69

Cree CSD04060A Schottky 4A 600V trr 0 (A is square 2 legs)
Cree CSD05120A Schottky 5A 1200V trr 0 (A is square 2 legs)

For making up schottky heater bridge assemblies try these single diodes:

1N5819 1A 40V Schottky Axial
1N5820 3A 20V Schottky Axial
1N5822 (RS 544 4988) 3A 40V Axial
STPS1045D (RS 486 0216) 10A 45V TO220AC package (square 2 legs)

Bridges made with these have very low forward voltage drop and will often run straight off 6.3Vac and with a 10000µF 25V capacitor give you very close to 6.3Vdc

For complete non-schottky SS heater bridge assemblies,

Any of the 25Amp -35 amp bridges rated at 100V will be suitable but have more forward voltage drop and often need to be run from higher than 6.3V. Try:

KBPC2502 25A 200V
KBPC3502 35A 200V
GBPC2502 25A 200V
GBPC3502 35A 200V

Heatsink not normally needed but easy to mount if bolted to the chassis

Many other SS diode and bridge types are becoming available so check for a new project.

Last edited by Richard; 7th May 2008 at 08:28 AM. Reason: extra info and tidy-up