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Hints, Tips and Solutions (Do NOT post requests for help here) If you have any useful general hints and tips for vintage technology repair and restoration, please share them here. PLEASE DO NOT POST REQUESTS FOR HELP HERE! |
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1st Feb 2017, 10:12 pm | #1 |
Dekatron
Join Date: Apr 2012
Location: Wiltshire, UK.
Posts: 14,007
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Extending leads to power-transistors/voltage-regulators
A piece of kit I was working on today uses a TO220 series-pass transistor in its voltage regulator (providing 28VDC to subsequent stages). As originally designed this transistor was sat directly on the PCB and had a rather small finned heatsink attached - and there was plenty of evidence to suggest it was living a rather stressed life.
I wanted to move it away and relocate it so it could be bolted to the rather substantial equipment-chassis. But how to do? Looking in my junkbox-of-bits I came across a 3-wire "PCB Header" ribbon-cable and socket; a quick check showed that this was a nice tight fit on the 3 legs of the TO220 transistor, and the wires were long enough to reach back to the holes on the PCB where the series-pass transistor originally lived. Duly rewired, the transistor now runs a lot cooler and should have a much-extended life! |
1st Feb 2017, 10:23 pm | #2 |
Nonode
Join Date: Jun 2010
Location: Spalding, Peterborough, Cambridgeshire, UK.
Posts: 2,859
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Re: Extending leads to power-transistors/voltage-regulators
Saw something very similar in my 15-20 or so year old Hitachi Scope a few months back.
Certainly a worthwhile mod you made. Rob
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1st Feb 2017, 10:23 pm | #3 |
Dekatron
Join Date: Jan 2005
Location: Oxford, UK.
Posts: 17,863
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Re: Extending leads to power-transistors/voltage-regulators
I've seen this done on Aiwa cassette decks, with the main series regulator power transistor bolted to the rear of the metal cabinet, several inches from the PCB.
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2nd Feb 2017, 7:11 am | #4 |
Dekatron
Join Date: Jun 2015
Location: Biggin Hill, London, UK.
Posts: 5,215
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Re: Extending leads to power-transistors/voltage-regulators
Molex KK connectors (aka Maplin 'Minicon') work well for this too. I've seen it done in commercial units (certainly HP and Samsung) and at least one Heathkit did it too.
But a word of warning. If you do this for the 3 terminal regulators of the 7805/7905 type and the 'common' pin goes open-circuit (due to bad contacts in the connector for example) then the output voltage will rise essentially to the input voltage. A good way to kill ICs all over the unit! I think I would want to fit a good crowbar circuit on the output of the regulator, or ensure the common connection was made by other means (e.g. via the tab/chassis for a +ve regulator). |
2nd Feb 2017, 7:30 am | #5 |
Dekatron
Join Date: Jun 2016
Location: London, UK.
Posts: 3,687
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Re: Extending leads to power-transistors/voltage-regulators
Heathkit IM-2410 does this. Good idea if you ask me. I will be doing the same in things I build too.
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2nd Feb 2017, 9:47 am | #6 |
Dekatron
Join Date: Nov 2010
Location: Oxford, UK.
Posts: 4,992
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Re: Extending leads to power-transistors/voltage-regulators
Doing it this way adds two non-soldered transitions - wire to KK crimp, and crimped contact to regulator pin. And these connect to a part that will be subject to thermal cycling.
I well recall a definition I read 40 years ago "a connector is a source of trouble joining two pieces of equipment" Just my pennyworth, but I solder on pigtails and either use silicone sleeving (Hellerman or similar) or small bore heatshrink over the joints. Craig PS In space instrumentation, transparent heatshrink has to be used so that every joint (wire splice etc) can be inspected. |
2nd Feb 2017, 11:18 am | #7 |
Octode
Join Date: Jul 2009
Location: Carmel, Llannerchymedd, Anglesey, UK.
Posts: 1,509
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Re: Extending leads to power-transistors/voltage-regulators
Just another thought - if the wiring to a 3-term voltage reg. is a bit too long before it meets the necessary bypass cap., it can lead to a bit of self-oscillation.
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2nd Feb 2017, 12:07 pm | #8 |
Dekatron
Join Date: Feb 2007
Location: Lynton, N. Devon, UK.
Posts: 7,088
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Re: Extending leads to power-transistors/voltage-regulators
It's a good modification from a thermal and reliability point of view, but it needs to be done with care!
TonyDuell and Craig_Sawyers make valid points, to which I'd add another: Adding a few inches of wire can, in some circumstances, introduce enough inductance to make the circuit oscillate! Some TO220 regulators (negative ones, and some low-dropout positive ones) need capacitors right on the leads. MOSFETs can go unstable unless gate resistance is present right at the gate lead. And even bipolars, in linear circuits, can do funny things... A voltage regulator is a heavy negative feedback circuit, with lots of opportunity for it to hoot! Last edited by kalee20; 2nd Feb 2017 at 12:08 pm. Reason: Post crossed with Les! I'm working with dodgy connection! |
2nd Feb 2017, 12:29 pm | #9 |
Dekatron
Join Date: Jun 2013
Location: East Sussex, UK.
Posts: 3,326
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Re: Extending leads to power-transistors/voltage-regulators
Be careful because if the wires/plug are accidentally pulled off the transistor then as it's 3 pin and reversible it might get refitted the wrong way round. I prefer direct soldering and sleeving for reliability.
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2nd Feb 2017, 12:37 pm | #10 |
Dekatron
Join Date: Jun 2015
Location: Biggin Hill, London, UK.
Posts: 5,215
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Re: Extending leads to power-transistors/voltage-regulators
I was once told that a correctly-made crimp connection was more reliable than soldering. However I don't have the right crimp tooling for the KK contacts and a poor crimp is very unreliable. So I crimp them with pliers to hold them and then solder them. So in my case, if I use a KK socket to connect to a TO220 package, there's just the mechanical contact from KK contact to device pin.
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2nd Feb 2017, 7:02 pm | #11 |
Dekatron
Join Date: Apr 2012
Location: Wiltshire, UK.
Posts: 14,007
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Re: Extending leads to power-transistors/voltage-regulators
Properly-made crimped connections are indeed more reliable than soldered ones, as the likes of the FAA will confirm; [I would not pass-for-flight any wiring repairs involving soldered joints].
Point taken about the contact between the flying-connector and the legs of the series-pass transistor: but having used classic 'network analysis' on this specific case: Collector-connection disconnects - no unregulated supply presented to transistor, so no output. Safe. Base connection disconnects - no base-drive so transistor doesn't turn-on. So no output. Safe. Emitter connection disconnects - no path from regulator to load, so no output. Safe. Summary: For any single loss-of-continuity between the flying-connector and the series-pass transistor, the outcome is fail-safe for the downstream circuitry. |
2nd Feb 2017, 8:01 pm | #12 |
Dekatron
Join Date: Jun 2015
Location: Biggin Hill, London, UK.
Posts: 5,215
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Re: Extending leads to power-transistors/voltage-regulators
Yes, an open-circuit on any pin of a bipolar pass transistor should be 'safe' (no output). In fact for multiple faults (2 or 3 connections opening) it's safe.
But as I mentioned, I've seen the same connectors used (commercially) on those 3 terminal regulators of the 7805 type. If the common pin of one of those goes open then the output does go up to the input voltage (if either the input or output connections opens then the output falls to zero, of course). |
2nd Feb 2017, 11:01 pm | #13 |
Dekatron
Join Date: Jun 2016
Location: London, UK.
Posts: 3,687
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Re: Extending leads to power-transistors/voltage-regulators
Good points. I've had a 7805 soldered in circuit go phut and dump the entire input voltage to the output. Bang went a lot of TTL ICs. Increasing that risk is probably pretty bad.
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3rd Feb 2017, 4:49 am | #14 |
Heptode
Join Date: Apr 2008
Location: Eastbourne, East Sussex, UK.
Posts: 671
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Re: Extending leads to power-transistors/voltage-regulators
With Molex KK and other similar connectors they are designed to mate with their matching headers made from similar metals. The tinning on a transistors leads is not the same, so electrolytic action can occur causing high resistance and voltage drops in the connector. It's also a problem when using mating different brands of similar pitched connectors. I have come across several items of equipment where faults have occurred due to this.
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3rd Feb 2017, 10:00 am | #15 |
Dekatron
Join Date: Nov 2010
Location: Oxford, UK.
Posts: 4,992
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Re: Extending leads to power-transistors/voltage-regulators
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3rd Feb 2017, 10:58 am | #16 |
Octode
Join Date: Feb 2009
Location: Bletchley, Buckinghamshire, UK.
Posts: 1,223
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Re: Extending leads to power-transistors/voltage-regulators
Some equipment had gold-plated plugs in tin-plated sockets (or vice-versa). They always failed eventually due to corrosion, and the only cure was to replace both the connectors so the mating surfaces were of the same metal.
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4th Feb 2017, 10:45 am | #17 |
Dekatron
Join Date: Nov 2010
Location: Oxford, UK.
Posts: 4,992
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Re: Extending leads to power-transistors/voltage-regulators
Gold plating is sometimes problematic. Through-hole space qualified components come with gold plated leads to prevent lead corrosion compromising solderability. However, if you solder onto gold plating, the gold alloys into the solder and gives long-term reliability problems.
To overcome this, component leads are "de-golded", which basically means tinning the leads. The gold comes off in the tinning, and leaves a nice bright lead for soldering. After de-golding parts are stored in a desiccator under dry nitrogen until used. Done under highly controlled conditions https://www.youtube.com/watch?v=KSdP5gSm0pQ |