2
\$\begingroup\$

We have an old board that we do not want to redesign because it is near end of life. However, there is a TO-3 5V regulator that we can not source and when I do find it, it is for $13.

I am wondering if someone makes an adapter to solder a TO-220 5V regulator to a TO-3 adapter as that would be the easiest solution. The next easiest is to just make a small pcb that does this which we may end up doing.

\$\endgroup\$
  • 1
    \$\begingroup\$ @GeorgeHerold, he has T0-3 footprint requirement. \$\endgroup\$ – Chupacabras Sep 13 '17 at 20:03
  • 1
    \$\begingroup\$ @GeorgeHerold If I am reading this question correctly PCB is designed for a TO-3 footprint and OP has TO-220 parts and is looking for a way to fit them. Not the other way round. \$\endgroup\$ – Warren Hill Sep 13 '17 at 20:05
  • 5
    \$\begingroup\$ The adaptor PCB is likely your best bet, however, the thermal characteristics of a TO-3 are very different from a TO220.. I am assuming you have considered that.... \$\endgroup\$ – Trevor_G Sep 13 '17 at 20:11
  • 5
    \$\begingroup\$ Also, if you are making a PCB anyway.. you might want to consider making it a whole little switch mode regulator rather than just a package adaptor. \$\endgroup\$ – Trevor_G Sep 13 '17 at 20:15
  • 3
    \$\begingroup\$ If the pin-out suits, you can bend the outer legs of the TO-220 to fit in the TO-3 pins, and bolt the TO-220 tab down to one of the TO-3 mounting holes. \$\endgroup\$ – Peter Bennett Sep 13 '17 at 20:38
5
\$\begingroup\$

I'm pretty sure I've seen this before: Just cut the middle leg off the TO-220, bend the remaining leads 90° at the right distance, and mount the TO-220 to one of the existing TO-3 mounting holes. It isn't hard to get the two remaining TO-220 legs to to line up with the TO-3 pins reasonably well.

\$\endgroup\$
5
\$\begingroup\$

Looks like someone makes a switch-mode replacement for a TO-3 5V linear regulator. Will this work for you?

LM323K TO-3 SMPS Replacement

http://www.ezsbc.com/index.php/psu5.html#.WbmuWBNSyuU

If not, you can make your own using an off-the-shelf design from Linear, Maxim, etc.

\$\endgroup\$
  • \$\begingroup\$ This is pretty much exactly what I was looking for, but for some reason couldn't find it through googling. \$\endgroup\$ – tooclosetosee Sep 18 '17 at 18:44
3
\$\begingroup\$

One thing to strongly consider is the voltage regulator current capability. Many TO-220 package regulators are limited to 1A maybe 1.5A at most. A TO-3 regulator may have been used in the original design due to greater current capability. In the past I've used TO-3 parts that were rated for 3A.

By the way I really like the idea that was proposed elsewhere in this question/answer posting to make a circuit board adapter that uses a small SMT regulator to create a high efficiency buck switching regulator to replace the antiquated and inefficient linear regulator. A high frequency PMIC type part with a small size inductor, a few caps and few resistors should easily be able to fit the outline and maybe even rid you of an existing heat sink.

\$\endgroup\$
2
\$\begingroup\$

Installing a TO220 into a TO3 design is simple with a different size fastener for a smaller hole size and 5 in-lb max torque until you get into thermal design ( essential for any EE to learn)

This biggest issue is max Pd depends on the heatsink on the board. If none, then you may have to add one. Consider that the TO220 has a thermal resistance from junction to case of 60'C/W and with 3W it will fry.

So you must define;

  • Pd Max ( Vdrop*Imax)
  • Tj max ( usually 85'c ) for high MTBF
  • T amb max ( maximum internal ambient )
  • Air flow or convection restrictions ( helps or hurts Rth )
  • space available around one mounting hole that is grounded for heatsink footprint and height
  • fastener size ( may need smaller size )

Then compute the max Rjc + Rca for a TO220 + heatsink to meet your above requirements.

Then consider one of these options in order to complete this simple exercise to prepare an ECN with assembly and BOM changes.

Use thermal resistance @ natural and add 5'C/W for Rjc of the TO220 (j=junction, c= case) and you do the math.

enter image description here Both use case ground, so bending the leads to reach the other 2 holes is easy at right angles with 2 needle nose pliers.

Then like Ohm's Law for voltage, compute the junction temp rise and heatsink Rca @ natural needed to not exceed 85'C ( or higher if approved) at max interior ambient temp.

Pd*(5'C/W + Rth_ca(heatsink)) =< (85'C-Tmax) (internal ambient)

  • solve for Rth_ca
\$\endgroup\$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.