MC1466L Precision PSU, Current drift problem

[Zener57]

HI, working on a Lab PSU based on the motorola MC1466L still available from china.

The circuit is basic motorola spec similar to this circuit
http://www.sentex.ca/~mec1995/circ/ps4002/ps4002.html

although I have 2x 2n3773 pass transistors with 0.33 ohm emitor stabilizer resistors
Driven by a 1000 hfe darlington so as not to pull to much from the IC.
and extended output to 0-50V 0-2A

I'v used the best parts I can for this as I want it to be a precision psu, buying digital pannel meters for current and voltage, taking care of layout and extra everything where I can.

Post build and test

Voltage: 0-50V calibrated and stable as far as 50K 10 turn pots can be

Cuurrent: 0-1999ma calibrated ok

Problem: I switch on and set output voltage to say 10v, short output, and set current to say 40ma

Then slowly the current starts to drift (Fortunately down!), after half an hour it's down to 10ma!!!
so so much for mc1466l precision Ic, enen my op-amp based psu's don't do this.

So it's somthing warming up then ?, I'v studied the motorola pdf and it goes on about temprature compensated circuits in the chip etc , sposed to be 0.01% on voltage & current.

The voltage seems pretty stable, but from a precision psu if I set 40ma I want it to be still pretty much 40ma after half an hour surley not 10ma.!

I bought 3 of these chips from china and they all show the same issues.
The real worry here is, I sprayed the chip (short spray) with switch cleaner, as i had no freezer
and the output climed back up to 40ma!

So the chips are crap or is this normal?
I'm evan suppying the chip with a regulated 25V!, not half wave like in the data sheets.


Any one any experience this with the MC1466L based psu?
Or any pointers
Many thank's

[amspire]

I think the drift is probably the IC. On page 3-100 of the datasheet, there is the specs for the current limit control  - 1ma + 0.2%. The 1mA is to allow for the current through the current control resistor as it bypasses the current sense resistor.

The lousy standard of the PDF's for this device makes it hard to interpret the stability figures - I just cannot read the reference schematics easily.

Anyway, this IC deliberately adds a offset voltage to both the voltage amp and the current amp so that both the voltage and current can adjust down to 0.  At 40mA out, you only have 4.4mV across the sense resistor and this offset can be as high as 40mV.  To go down to about 10mA, that is roughly a 3mV drift in offset voltage. More then I expected, but it could be that you cannot expect too much of this current limit. The 0.2% regulation figure will be measured near the top end of the current range (if only I could read the component values) rather then the bottom end of the range.

Richard

[Zener57]

Hi Richard,  thanks for the reply,

I must say I expected more from this IC, as it's allways being associated with the words precision current & Voltage regulator, and Lab PSU etc.

As for excellent 1ma + 0.2% current regulation, I thought I was going to get stable current settings acurate to 0.2% of a milliamp!

Hmm, oh well if I let it warm up for an hour before I use it..?
May be Its back to descrete components design after all..

Thanks Richard

[jaycee]

Gary Lecompte aka Chemelec has also produced a discrete version of the MC1466 - that might help you.

TBH the operating principle of the MC1466L could be just as easily implemented using two opamp IC's and a precision voltage reference such as a TL431. This is pretty much what the Mastech power supplies do.

[Bored@Work]

I must say I expected more from this IC, as it's allways being associated with the words precision current & Voltage regulator, and Lab PSU etc.

Don't forget the MC1466 is more than 45, maybe even 50 years old. What passed as "precision" then is not necessarily what passes as "precision" today. And "precision" even back 45 years was just a marketing world.

Then of course no one knows where your MC1466 come from. E.g. if they are NOS, second source, or clones.

[sorin]

http://www3.telus.net/chemelec/Projects/MC1466/MC1466.htm