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regulator booster

Thanks Martin, I think I'll be under 50ma, it's just some parts of the 5v circuit in a cd player.

In case I'm not would you be able to spec the settings for a higher current?

Regards

Pete
 
Sure, its just a matter of choosing a suitable transistor* and using rather smaller R values to get enough current to drive the transistor without excessive voltage dropout. Compensate for the low R value by making the cap bigger.

Something like 1K/ 22-47uF (eg tantalum) or even larger C will work well in for most purposes up to a few hundred mA. - when the extra dropout voltage x current might start becoming a bit wasteful (hot!)


*say BD139 or BD140, or go straight to a proper power type like the D44H11 /D45H11 for -ve rails - or in fact any old medium power transistor you have to hand.
 
Hi,
I would like to implement these in my NAPST which has four LT1085CT's in it at the moment.
I am a little confused as to what is the ultimate build on these things.
I see the schematic and photo on post #164, which I understand is the way it has been implemented, but recently, after a query to Teddy (I'm sure he won't mind me quoting him), he said,
"In my opinion C2 should be connected between the ground and the emitter of Q1. I would also add a resistor in parallel to C1 to have at least a 2v dropout on the pre regulator".
I'm not sure what to do here so I am hoping that one of you knowledgeable techo's here will be able to help.
Which method will work better and what resistor value should be used in parallel to C1 if used?
Would the BC547C be fine for this use in this area?
Thanks,
Pete.
 
Sure, its just a matter of choosing a suitable transistor* and using rather smaller R values to get enough current to drive the transistor without excessive voltage dropout. Compensate for the low R value by making the cap bigger.

Something like 1K/ 22-47uF (eg tantalum) or even larger C will work well in for most purposes up to a few hundred mA. - when the extra dropout voltage x current might start becoming a bit wasteful (hot!)

Hello Martin,

I have used a BC547C/BD139 homemade Darlington with 56K for R of and 1uF for C (i.e gyrator from TeddyReg) in front of an LM317 to supply around 200mA for an SAA7220 and it seems to work very well.

I have also used the BC547 and values shown on this thread with LM317's and 7805s for other applications of up to 40mA, and again these seem to work very well,

However, I now need a couple of regs for about 55mA and 60mA and am a bit anxious about going with just a BC547. I would like a bit more 'headroom' in the current capacity.

Could I use an MPSA14 Darlington (collector current of 500mA) for this application and would I use the same R and C values as used in the BC547/BD139 Darlington?

I think I could also use a BC547C/BC639 (collector current of 500mA) for the Darlington and I have the same question about R and C values.

Joe
 
Thanks Martin.

I went back back and double checked the R and C values I used on the BC547/BD139 combo for the SAA7220 and it was 33k and 1.0uF MKT. So I will start with that and the MPSA18 and see how it goes.

Joe
 
You might find that even with a the darlington, to push 50ma plus will leave quite a large voltage drop appearing across 33K. Try 10K to begin with.
 
And would the 1uF cap be okay? Film or tantalum? I note the Flea gyrator uses 3.3uF tantalum with a 1Ok.
 
Yes it will be fine. And so would a 10uf electrolytic etc.

For the flea we were ver concerned with noise and a film cap was justified for that design. But in practise any reasonable cap will work well enough, and it broadens the scope somewhat eg if you need minimum dropout voltage, 1Kohm plus 10 or 100uf works very well :)
 
Hello, I'm planning on building one of these for 7812 regulator but am striggling to understand the terminology. Can anyone help? some people have mentioned the reg needs to go in reverse? or is it the other transistor?
 


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