The Shop > Electronics & IC Programing
Siemens micromaster 420
awemawson:
For a depolarised electrolytic I'm not sure (*) that an incandescent lamp will reduce the current sufficiently to prevent damage - a depolarised electrolytic will present a very low resistance and you want a controlled current of a few milliamps - just maybe a few tens of milliamps. A cold filament of an incandescent lamp also presents a very low resistance until it heats up where upon the resistance increases. The damage to the capacitor is done well before that happens :bugeye:
(* read that as I'm certain that it won't !)
appletree:
Hi John the comment about florescent and LED was for a laugh , obviously no good, might try a couple of daffodils in series (Bluechip) as they are plentiful just now failing that tulips will be along soon. I have some of those nice gold anodised power resistors somewhere 100 ohms or so, if only I can remember where.
Phil
John Rudd:
Andrew,
You are correct....( as usual.... :scratch:).....
I would normally use said bulb if I were testing a power supply to limit the current and prevent blowing it up if there was a fault....a common use in servicing brown goods....where a variac isnt available...I've also made good use when repairing inverters, limiting the power available and preventing the things from destroying themselves...
In this instance where a smaller current is needed, then my approach is inappropriate....I apologise for giving misleading information.... :coffee:
Bluechip:
OK had a stab at re-creating the schematic. It's screwed together from essentially 3 bits:
[1] The full-wave doubler giving some 136V DC .and 68V DC
[2] A string of zeners to set the o/p voltage of the pass transistor.
[3] A 10mA current limiter, 0-20mA meter and a switch to discharge the cap.
NB
In accordance with my lifelong habit of not getting anything error free, the transistors are MJE 13005, not MJ 13005.
The 2 resistors with *** may need small changes as the load regulation of the transformer may give a higher o/p voltage.
If anyone builds it,
[1] Do the doubler first and check the volts are fairly near, although they won't be precisely the same.
[2] Make the Zener circuit, then set VR1 for 20V on the 16V range. [ you'll see why in a bit ]. Measure from Q1 emitter to 0V. Check the other voltages, make sure you ain't got a cranky Zener. They should all read about 4V high.
[3] Make the rest. Put a short circuit instead of the cap. and set VR2 to give 10mA on the meter.
[4] With a 10R resistor instead of cap. tweak VR1 again to read 16V from the -ve to +ve test terminals on the 16V range. Then check the other ranges.
The -ve test terminal actually sits at some +4V because of the Q2 limiter action, hence the 4 extra volts in [2].
Poke a GOOD biggish cap. in and set up. [ Not in that order ] :thumbup:
The meter should whip up to 10mA and stay there. When the cap's charged up it will swiftly drop to zero.
As this contraption is sort-of linear you can make a fair stab at finding out what a cap. really is.
CV = It ... :thumbup:
Capacitance x Volts = Amps. x Seconds
Or:
C = ( I x t ) / V
So:
Suppose we have 25V, Amps. = 0.01, time to when the needle starts to drop, 6 seconds.
We have (6 x 0.01) / 25 = about 2400uF.
Have done this with caps. marked 56,000 uF, 25V and then with my Peak capacitance meter and it's pretty accurate.
D.
EDIT S1 is a 1P 12W rotary switch, stopped off @ posn. 8 ..
S2 on mine is a ON [ON] 2P 2W just flick it to discharge the cap. :thumbup: DVM should drop to 0V, then cycle cap. again if it's not been used for a long time. The 'slow' drop from 10ma to 0 gets sharper as cap. improves.
appletree:
Thanks for posting the circuit Dave, I shan't be using it this time, but I am sure someone will find it useful.
Phil
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