Voltage regulator Question ?

[warpie]

What is the benefit of using only positive regulators? In production it might make sense but in one-offs?

 

[Newmarket]

Argua

What is the benefit of using only positive regulators? In production it might make sense but in one-offs?

also often more convenient in One Off / DIY situations wrt MOQ, cost and availability.

 

[JohnRoberts]

What is the benefit of using only positive regulators? In production it might make sense but in one-offs?

I have never done that in production. I can only imagine two scenarios that make sense.

1-Long long ago, before there were standard (inexpensive) off the shelf 3 terminal negative regulators, stacking two positive regulators in series was the easy way to use canned off the shelf designs.

2- For a DIY project using parts on hand, and no negative regulators handy. Use what you got.

JR

 

[user 133392]

I have never done that in production. I can only imagine two scenarios that make sense.

1-Long long ago, before there were standard (inexpensive) off the shelf 3 terminal negative regulators, stacking two positive regulators in series was the easy way to use canned off the shelf designs.

2- For a DIY project using parts on hand, and no negative regulators handy. Use what you got.

JR

There is a third scenario which is when the load current is greater than the 1A capability of the 79XX and LM337 regulators.

"Stacked" positive regulators - using the µA723 and all NPN pass transistors - were used in most bipolar open frame linear supplies of the last century. I can't recall seeing one that didn't.

Douglas Self addresses higher current bipolar supplies in Small Signal Design using the positive-output LT1083 as an example.

 

[JohnRoberts]

I recall using the ua723 back before 3 terminal regulators became popular in the 1970s. As I recall it gave a decent regulated output.

At my first electronic technician job in the late 60s (MITIL) I worked on a DC-DC switching power supply for the Navy DSRV that used a LM100 linear voltage regulator tricked to work as a switcher. As I recall we blew up so many LM100s that we substituted LM300s, the cheaper voltage tolerance and operating temperature version. We were debugging an unproved design.

JR

 

[mushanokoji]

Wrong wrong wrong mate
Yours schematics is wrong and if you put 44VDC thru LM 7818 is propably dead
Why +/- 44VDC after bridge recrifier?
The way you design the circuit is you multiply the Voltage, positive and negative while deceasing amperage
You had the luck that caps din't explode
Use just one bridge recrifier, you can use after that caps with 35V Voltage, 2200uF
For lower nosie use LM317 and 337 instead od 7818, 7819 and that's it
If you want schematic just pm me

 

[C H]

I don't think the dual rectifier is the issue. As others have noted, that transformer has most likely got very poor regulation as the wire is so small, hence high DC resistance, and the output voltage is quoted for full load. 44V no load seems quite normal, however the bridge is arranged.

 

[Matt Syson]

The '723 is/was indeed a very handy IC with sufficient access terminals to do quite a variety of useful 'tricks'. The real problem with supplies is getting rid of HEAT and it is very noticeable that the serious supply manufacturers use T03 can devices with only 3 Amps or so expected of them so less reliance on tight thermal control measures (per device).

 

[PermO]

My 2x18V transformers arrived, it will be fine.
Same dimensions, 42mA per winding instead of 31mA per winding on the 24V, so slightly thicker wire I guess.
I'll put some load resistors before the regulators to shave off a Volt.

Also got some different brand 7918.

I'm not worried it will now drop below the treshold of the reulators as the load on the PSU wil be 10% - 20% max.

It will be an interesting test, so I will try and put a full load on and see what happens.



Will be continued...

 

[warpie]

PermO, I just noticed that in the original schematic you use an RC filter with Fc≈48Hz. Is it not a bit too high?

 

[PermO]

You mean the 1 Ohm / 3300uF combination ?

 

[warpie]

You mean the 1 Ohm / 3300uF combination ?

Yes

 

[PermO]

Note to self;

Always use a RC calculator !

I will make the resistors 10 Ohm so it will be 4.8 Hz

I'm really glad I started this topic as I am learning a lot of new things here, thanks people !

 

[warpie]

I've done this before and it worked great. It would be desirable to have less voltage on both inputs. And to use silpads under both regulators before you bolt them to a heatsink.

What do you mean by having less voltage on both inputs?

 

[Philbo King]

It means that each regulator dissipates less power.
P(diss)=(Vin-Vout) x I(load)

 

[warpie]

It means that each regulator dissipates less power.
P(diss)=(Vin-Vout) x I(load)

Sorry but I don't follow. Do you mean that with dual bridge rectifiers there will be higher voltage drop and therefore the Vin will be smaller?

 

[Philbo King]

I mean that any voltage regulator, regardless of rectifier type, will burn less power if the input voltage is lower, as long as it has enough voltage to operate.

 

[abbey road d enfer]

These transformers are self-proclaimed "inherently short-circuit proof".
I don't know the exact details, but I noticed that they have a rather high DC resistance, which imposes a quite high open-load voltage. Actually, when loaded, the voltages are correct.