| Author | Message | ||
     Jeff Young New User Username: jeyjey Post Number: 7 Registered: 10-2010 |
My coolant level amplifier has a blown diode. (It's the one between 12V and Gnd, which is the upper one in the workshop manual Fig M29.) What's an appropriate replacement? Will a standard signal diode (for instance, an IN4148) do the trick, or do I need a higher-current rectifier diode? Thanks, Jeff. (Message edited by david_gore on 26 October 2010) | ||
| Richard Treacy Grand Master Username: richard_treacy Post Number: 2245 Registered: 4-2003 |
See the Technical Library (link below), then go to Jaycar and spend less than 50 cents each. Best replace all the diodes while you are at it. If you need hundreds, they can be bought for 5 cents each elsewhere, but Jaycar outlets are conveniently located all over Australia and online ordering is fine, likewise RS Components for that matter. http://rrtechnical.info/miscellaneous/diodes.html EM513, 1N4001, 1N4004, 1N4007 etc. RT. | ||
| Jeff Young New User Username: jeyjey Post Number: 8 Registered: 10-2010 |
Richard -- Thanks. The other diode in the circuit I believe has something to do with the amplifier (I think it's a zener to control voltage to the emitter of the transistor connected to the probe). So I'd need to replace that one with a zener, right? The one that's burnt out though doesn't seem to have anything to do with the amplifier circuit. I assume it's a flyback diode for the test circuit relay? I have some IN4148's (and it's a bank holiday weekend here so I can't get anything else till Tues). Would they work for the flyback diode (assuming that's what it is)? Or should I wait till I can get a 1N400x? Thanks again, Jeff. (Message edited by david_gore on 26 October 2010) | ||
| Richard Treacy Grand Master Username: richard_treacy Post Number: 2246 Registered: 4-2003 |
Note: the IN4148 is a 200mA switching diode and is not suitable. It may work for a while if you are lucky, but it is not designed for rectifier use. This application requires a low-power general-purpose rectifier diode, such as the EM513 rated 1 amp at 1600V. A 1N4001 (pretty much superseded by 1N4007 and EM513) is fine with a rating of 1A at 50V. A IN4148 would certainly be unsuitable for most diode applications in your car. | ||
| Richard Treacy Grand Master Username: richard_treacy Post Number: 2247 Registered: 4-2003 |
There is a 6.3V Zener there somewhere. Best advise the exact number and location . Always, it is best to replace the transistors on the coolant amplifier board whenever the board is removed. Don't forget to lacquer it with clear to prevent moisture problems once repaired. | ||
| Richard Treacy Grand Master Username: richard_treacy Post Number: 2248 Registered: 4-2003 |
On reflection, I may be thinking of the Zener on a 20k SZ diode board. If you are referring purely to your coolant amplifier, I believe all the diodes are EM513 etc. Don’t be fooled by Crewe’s jazzy diode symbols. A suitable output transistor (PNP) is a trusty BD138, BD140 etc. For the others, a good-old BC107 (NPN) and BC177 (PNP) are fine, along with practically any other signal transistors (BC546, 7, 8, 9 NPN; BC177, 8, 9, BC447. etc PNP). These components cost next to nothing.  | ||
| Jeff Young New User Username: jeyjey Post Number: 9 Registered: 10-2010 |
Thanks, Richard! It was a bit of work to get the dang thing out, so I guess I should do the transistors at the same time. Since I'm ordering those, I'll get the correct diodes too. (I found the BC107, a pair of BC327-25's and 1N4007's at RS.) Jeff. (Message edited by david_gore on 26 October 2010) | ||
| Jeff Young New User Username: jeyjey Post Number: 10 Registered: 10-2010 |
One more question, Richard. You mention lacquer; is that for just the bottom of the board (with the solder joints) or the top too? And what's the handiest way to do it: by brush? Or mask the contacts and spray? Lastly, any particular kind? Solvent-based? Water-based? Polyurethane? Any of the above? Thanks, Jeff. (Message edited by david_gore on 26 October 2010) | ||
| Richard Treacy Grand Master Username: richard_treacy Post Number: 2249 Registered: 4-2003 |
Mask the connector points and spray both sides generously. The solvent type is not important once it has dried. BC327 (PNP 50V 500mA 650 mW) is fine of course, but I would suggest a BD138/140 or the like for the output PNP. Isn't it amazing how inexpensive these noble components are ! R. | ||
| Jeff Young Experienced User Username: jeyjey Post Number: 11 Registered: 10-2010 |
Cool. Parts should be here tomorrow. I'll let you all know if I'm successful. ;) Jeff. | ||
| Jeff Young Experienced User Username: jeyjey Post Number: 12 Registered: 10-2010 |
Success. I haven't varnished it yet, but here's the finished board:   Jeff. | ||
| Jeff Young Experienced User Username: jeyjey Post Number: 13 Registered: 10-2010 |
I decided for safety's sake to have a look at my ice warning amp as well. Since the flyback diode in it hasn't been toasted, I could actually read the number off it, and stragely enough it is a Zener (BZX61C24). That means if you ever apply > 24V to your electrical system, it will provide a dead-short to ground. Is this some sort of surge protection to make sure the fuse blows before the transistors cook? (Judging from my coolant amp, it's not going to save you having to rip the thing out as not cooking the transistors will probably be at the expense of cooking the Zener.) Jeff. |
