schottky diodes in series

[drewc]

If I put 2 schottky diodes in series (1n5822) will I have an increase in the current from 3A to 6A and will the voltage forward increase from 0.52v to 1.04v?? I require to have a voltage cutoff of 0.9volts at 3A and was hoping this would do the trick. I am a novice so please forgive my ignorance.

 

[Oznog]

The Vf will double for a particular current.
The current of components of any type put in series is the same. If you put 3A through the string, 3A goes through each diode. Thus 2 3A diodes in series can handle a max of 3A.

Vf of a Schottky varies from about 0.3v at low currents to about 1v at its max current.

 

[Dr.EM]

The voltage drop should be one diodes drop + the other ones in series, so yes, if they are 0.52v each, the total is 1.04v. However, the current rating does not doube in series, it doubles in parallel. However, if the voltage drop is even slightly different between the diodes, one diode will conduct all the current and will probably burn out. The options to fix this are using low value resistors on each one to share current, or to use a larger 6A rated diode (which is better).

 

[eng1]

Can you give more details? In my opinion, it's strange that you want to have a 0.9 V drop exactly... isn't that the maximum value you can accept @ 3A?

 

[lord loh.]

To increase the current, you can put several diodes parallel...And to increase the voltage drop put them in series... So a combination of both may do the trick...

Putting 3 in series and this combination with a similar (3 in series) one in parallel...

This shall give you a .9v drop and 6A current...( assuming .3v to be the cutoff value of the diode).

    D1--D2--D3
-<             >--
    D4--D5--D6

 

[Oznog]

But diodes don't do usually do so well in parallel, not without some added resistance on each leg.

The question needs to be asked- what are you trying to do? Why do you need to drop voltage in the first place? There is probably a much better solution, especially at 6 amps.

 

[drewc]

Thanks for the replies. What I am trying to do is discharge a 6 pack of Nmh batteries (1.2v each 7.2 v total voltage) down to a safe voltage of 0.8-0.9v each. If I can get them all to equalise around the same voltage I can them charge them equally to round 1.2v each and then discharge at an equal rate in a radio controlled car. If I can use two diodes to pass 3A in series with a resistor and then cut off at 0.8v to 0.9v I can make six circuits and then have very similar voltages across each cell. 3A discharge initially is fine , but I realise this will decrease with voltage drop