L-Adapter

[Salas]

For having two types of output connectors you don't have to modify the BOM. How many VA depends on how much output current you plan to feed in loads.

 

[SKA76]

Can be limited to 2A each: 5V x 4A x 2? 40VA or better 50VA? Thank you.

 

[Salas]

The transformer has not to be overheated in case of prolonged 4A total, so 60VA. You also have to consider proper sinking for the main transistor if that will be the prolonged loading situation.

 

[SKA76]

Good to know that it is also important to factor in adequate heat dissipation. Thanks again.

 

[thimios]

I'm using two L adapters to power the DCG3 ,+/-17V.
What is the maximum A.C input voltage for L adapters?
I asked because I have a pulled R core transformer 2X30V.

 

[Salas]

Max AC input is for as much rectified + ripple DC level as the reservoir capacitors allow before raw DC - Vref DC exceeds the 40V LM334 max spec. The bridge diodes usually have very high PIV unless they are low PIV Schottky. The real concern is the extra heat on the pass transistor that is going to be created by too much a DC in - DC out difference. The more current hungry the load the more Watt to sink in combination with the more voltage across the L-Adapter. At least with the DCG3 as a load you know what to expect and plan for as most of its consumption is its constant current output stage bias.

 

[adiaz]

In my opinion, this design is very good and extremely reliable.

The only critical points that must be carefully selected based on the load are the transformer (total power/secondary voltage) and the required dissipation.

 

[thimios]

Max AC input is for as much rectified + ripple DC level as the reservoir capacitors allow before raw DC - Vref DC exceeds the 40V LM334 max spec. The bridge diodes usually have very high PIV unless they are low PIV Schottky. The real concern is the extra heat on the pass transistor that is going to be created by too much a DC in - DC out difference. The more current hungry the load the more Watt to sink in combination with the more voltage across the L-Adapter. At least with the DCG3 as a load you know what to expect and plan for as most of its consumption is its constant current output stage bias.

Thanks Salas!
I know about the high dissipation in the pass transistor but amn't shure if correct understand the raw DC- VrefDC.
As the expected rectified voltage is about 42v ,is it safe for lm334?Or the peak voltage must be taken in account?
We always speaking about the DCG3 as a load..

 

[Salas]

Vref voltage in this design is Vout plus Q1 Vbe. So the difference across the LM334 in your case is lets say 42V - 17.6V when all voltages have settled. But there is a ramp up mS time also until Vout charges to programmed level. So maybe precarious. See posts #35 and #70 as well.

 

[adiaz]

I've used this design, but with an LM317 in CCS instead of an LM334. And it works very well. I always leave at least 5V more on the input than on the output, and never more than 10V more, so as not to compromise dissipation.

 

[Salas]

LM317 also has 40V input-output max voltage differential. The LEDs Vref is biased at just 5mA in this design though and I don't know if that's enough for guaranteed regulation threshold in any LM317? Worst case 10mA is needed to make sure with datasheet's max spec but some say 3.5-5mA is usually sufficient.