Regulator sos

[Jonathan Hayward]

Got a call from the studio saying the console (we just recapped) lost a rail.
Get down there to find someone had jammed a domestic 5 amp fuse (instead of a three)and a screw into the fuseholder. Grrr. Nobody knows who put it there.

Anyway, I thought I had lots of the right regulators 7818KC (TO3) spare but now can't find one for love nor money (farnell RS Rapid Craplin etc). So.
Does anyone have any 7818KC they'd part withor know of a UK supplier who does?

Or should I build a new board with an LM338K, if so, any good schematics around for it?

Thanks.

Jonathan

 

[gyraf]

try looking for a LM340K-18..?

 

[Jonathan Hayward]

Thanks Jakob, that is the same thing, lots more results!
However, still only seem to find 15V version, so just in case found some LM338K at Bowood-electronics.co.uk (for 1.75 each!)

 

[hitchhiker]

hi Jonathan,

I have a 7818 here but I'm on west coast of Canada?

Lance

 

[Jonathan Hayward]

Thanks hitchhiker, that's a kind offer, I'll bear it in mind if I can't get the thing working with LM338Ks. Time constraints mean I have to get it done asap though.

 

[Mark Burnley]

Hi Jonathan,

Aarrggh! Screws inserted in fuseholders....insanity! But you see things like this all too often....

Okay, for the 7818KC try MODE Components. If you phone, you'll speak to Chris who has a great knowledge of discontinued and hard-to-find semiconductors. He usually dreads my calls because I'm always looking for ridiculously rare semis, but he gets me what I need 80% of the time- and if not the exact replacement, a pin compatable sub. Prices are very good too.

:thumb:

Mark

 

[Jonathan Hayward]

aaarg, I misunderstood the problem on the phone . It's actually the negative rail that's gone (luckily studio is closed for refurb!). Anyway.
The 7818KC was used in conjunction with an MJ2955 'upside down' to give me the negative rail (18V about 3 Amps max I think)
I briefly tried the LM338k running 'upside down' but couldn't get it to work. I strongly suspected they're not supposed to.

My question is, should I actually persist with getting the LM338 to work negatively, or is there a circuit available using more common semis to give me my 18V 4 amp (preferably) supply (multiple negative adj regulators for eg)

Cheers.

 

[gyraf]

Look at the 78xx regulator's data sheets - they show how to boost output current in simple ways by adding a couple of transistors..

Jakob E.

 

[Rob Flinn]

Jonathon

Send me you email address & I`ll send a copy of a circuit that I used for my Trident (poached from an AMEK design) using an LM338 for the neg rail.

 

[gyraf]

http://info.hobbyengineering.com/specs/ON-MC7800-D.pdf

see page 14 - "Figure 10. Current Boost Regulator"

Jakob E.

 

[Jonathan Hayward]

Thanks Jakob and Rob (check yer PMs).
Jakob, in that application - more or less the same as the existing one in the PSU, would I get away with using a lower-current 7818 (say a T0-220 package) with the MJ2955?
That would solve my problem, as at the moment, the desk uses a (To-3) 7818KC which is proving elusive.

 

[gyraf]

in that application - more or less the same as the existing one in the PSU, would I get away with using a lower-current 7818 (say a T0-220 package) with the MJ2955?

Yes - that's the idea here. The datasheet is for the 1A version.

Jakob E.

 

[Guest]

Cricklewood have a t03 7818K
http://www.cricklewoodelectronics.com/Cricklewood/customer/home.php?cat=265
But you are paying an extra £1.70 (from 30p to £2) for a T03 package and an extra .5amp (I think the K is 1.5amps - even though there web site says on some and nto others)

 

[pstamler]

If you can't solve the problem any other way, a 15V regulator with its ground terminal floated to 3V would do the job. That should take about five diodes in series.

Peace,
Paul

 

[Rob Flinn]

Jonathon

There`s no PM from you ?

 

[Jonathan Hayward]

I think I must've been in too much of a hurry to click send accurately!

Thanks Rob, send to

[email protected]

I'm keen on the variables as their noise figures look better, it's actually for an Amek too.

Jonathan

 

[Jonathan Hayward]

Blimey, I'm losing it, that was supposd to be a private message to Rob, good job I didn't slag you all off eh?
Thanks Jacob and paul.
Simon, thanks for tracking that down, I will need them anyway in case the pos rail goes too.