[0-30 Vdc Stabilized Power Supply]

Hi Thomas,

Yes that's exactly what I had in mind, take note though, they are only rated for full power when bolted to another larger heatsink or chassis, eg: the 25w types are only good for 9w on their own and remember to derate if you run them hotter than 25 degrees.

[0-30 Vdc Stabilized Power Supply]

Hi Audioguru,

Thanks for the description of range switching.
I prefer to use just pots rather than the range switch you describe, I will get both a 10 turn and separate pots for coarse/fine & decide which I like best. This psu might be overkill for gain testing transistors, one slip and BOOM!

[0-30 Vdc Stabilized Power Supply]

Hi Audioguru,

I hadn't thought of a range switch, how would you implement this ???

Do you think current & voltage setting would be easier with fine & coarse pots instead of multiturn ones ? If the current setting range is from 2mA to 3A that's a big adjustment range, it would be difficult to provide accurate control at each end of the range. On the other hand, how much use is adjustment down to single milliamps ?

I think calibrating the meter scaling is surely easier than calibrating front panel markings & higher resolution too.

[0-30 Vdc Stabilized Power Supply]

Hi All,

I have an idea which may solve a difficulty raised earlier in this thread ie: you cannot see the current limit set point easily, particularly if you want to use a multi turn wirewound pot.

Expanding my theory for current metering, you could use another opamp buffer to read the voltage on U3 pin3 ( current limit set point ) & feed it through a momentary switch to the ammeter. This would need to be done with no current flowing to the load, otherwise the

[0-30 Vdc Stabilized Power Supply]

LOL,

Ok final question, would I need a proper darlington or could I use 2 x BC557 for a discrete one ?
Thanks for your help so far Audioguru.

[0-30 Vdc Stabilized Power Supply]

Aha, this is a theory test isn't it ? ;)

Ok here goes: High value resistors are needed to prevent current leakage through the existing LED, probably 100x Q3's bias pair values, hence a darlington to get enough current gain to turn on the 2nd LED. Correct ? ( My transistor theory was never very good :-\

[0-30 Vdc Stabilized Power Supply]

Hi Audioguru,

The meters are powered by 2 x SR44 watch batteries per meter so I figured I could use a separate isolated 3v supply ( another winding on the transformer + reg/rec circuit maybe, or take the guts out of a wall wart?). I'll try them battery powered initially then add a supply later.

For the ammeter drive, is opamp choice important ? A fancy opamp like the OPA445 isn't necessary for this circuit is it ? If supply voltage was a problem, couldn't I just add a small 78 type regulator to provide say +15 for the opamp & take the -5.6 from the existing rail ? The TL081 supplied in the kit seems an obvious choice.

Also, a cosmetic idea : Is it possible to add another LED which lights in CV mode ? I do like my test equipment to clearly show what it's doing

[0-30 Vdc Stabilized Power Supply]

Hi all,

I've been considering building one of these supplies & reading this forum with great interest. I will be ordering a kit from my local supplier shortly & then upgrading it to the spec you guys sorted out. ;)

I want to fit voltage & current meters & have obtained a couple of cheap ( 10UKP) DMM's with 3 3/4 digit displays (3999 count), Maplin order code N49AB. They are Mastech M320, and they're autoranging !! ;D

They're fine for the voltage display but the max current range is only 400mA. Is it possible to use some kind of low gain opamp circuit reading the voltage across R7 ( 1.41v @ 3A ) to provide a buffered & scaled signal to the DVM ?