2008-05-16
        Capacitor Discharge (Safety) Device:

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(Deutsch, hier klicken):


On a DIY-CDI prototype by MArio as 9.1MOhm was installed to discharge a 1.0µF capacitor. I tested voltage over time by a cheap DMM and made a chart click . Beginning with 300VDC, after 3 Minutes voltages is below 50VDC and 6 Minutes after disconnecting the capacitor voltage is below 12VDC.


Some thoughts and early discussion:

The big capacitors could be dangerous, see:
http://zekfrivolous.com/faq/sam/captest.htm#ctsdc

I'd like to add a simple discharge device.

In captest.htm you'll find this:
[...]

The total discharge time is approximately:
LV (TV and monitor power supplies, SMPSs, electronic flash units) - up to 1000 uF, 400 V. Discharge time of 1 second per 100 uF of capacitance (5RC with R = 2 K ohms).

[...]

Adjust the component values for your particular application.

[...]

I guess our 2 pieces of 1µF/275V capacitors are rather small LV (low voltage) types.
1/100s discharge time with a 2kOhm restistor? I guess we could rise resistance to keep current as small as possible which extends lifetime of the micro switches.

[...]

Capacitor Discharge Technique
The technique I recommend is to use a high wattage resistor of about 5 to 50 ohms/V of the working voltage of the capacitor. This isn't critical - a bit more or less will be fine but will affect the time it takes to fully discharge the capacitor. The use of a current limiting r150 KBesistor will prevent the arc-welding associated with screwdriver discharge but will have a short enough time constant so that the capacitor will drop to a low voltage in at most a few seconds (dependent of course on the RC time constant and its original voltage).

[...]

5Ohm*300V=1.5kOhm
50Ohm*300V=15kOhm
Which wattage?

[...]

For the main capacitors in a switching power supply, TV, or monitor, which might be 400 uF at 350 V, a 2 K ohm 25 W resistor would be suitable. RC=.8 second. 5RC=4 seconds. A lower wattage resistor (compared to that calculated from V^^2 / R) can be used since the total energy stored in the capacitor is not that great.

[...]
25W looks a little bit strong. Would a small 0.6W resistor also work?

Most of those switches are rated up to 50mA what's about discharge current?

Mini swithes pin pattern:
http://www2.produktinfo.conrad.com/datenblaetter/700000-724999/707627-da-01-en-Miniatur_Printtaster_rot.pdf (101kB)
http://www2.produktinfo.conrad.com/datenblaetter/700000-724999/707678-da-01-en-Miniatur_Printtaster_rot.pdf (94kB)
http://www2.produktinfo.conrad.com/datenblaetter/700000-724999/704369-da-01-de-Miniatur-Taster_grau_2polig_00418842xxx.pdf (267 KB)
http://www2.produktinfo.conrad.com/datenblaetter/700000-724999/700479-da-01-en-Drucktaster_T604.pdf (205 KB)
http://www2.produktinfo.conrad.com/datenblaetter/700000-724999/700460-da-01-en-Drucktaster_T602.pdf (150 KB)


Maybe all effort would be useless because DIY-CDI article says at the end of chapter "Disabling multi-spark operation":
[...]

Two 680kΩ resistors are connected in series across C2 to discharge it should the coil become disconnected from the circuit. This improves safety since a 1µF capacitor charged to 300V can produce a nasty shock.

[...]
Is this enough protection?


When we discussed this topic first at http://www.msefi.com/viewtopic.php?t=6380 (on Fri 15 Apr, 2005) a helpful member suggested:
[...]
I'd rather use a serial resistor (say 10k/5w if I calculate it well enough, but please check that it really discharges the cap good enough before relying on it!!!), it will limit the discharge current.
[...]


If you have any suggestions to this topic, especially to the rated resistor and/or switch please send a message to DIY-CDI Yahoo-Group .

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Deutsch:
   Kondensator-Entlade-(Sicherheits-)Vorrichtung:


Auf einer DIY-CDI Prototyp-Platine von Mario wurde ein 9.1MOhm reingelötet um den 1,0µF Konensator zu entladen. Ich überprüfte duie Spannung über duie Zeit mit einem billigen DMM und wertete das in einem Schaubild aus klick . Angefangen mit 300VDC, fällt die Spannung 3 Minuten später auf unter 50VDC und 6 Minuten nach dem Abklemmen des Kondesnators ist die Spannung unter 12VDC.

Einige zugrundelegenden Gedanken und frühere Diskussionen:

Der große Kondensator könnte gefährlich sein, siehe:
http://zekfrivolous.com/faq/sam/captest.htm#ctsdc

Ich würde gerne eine einfache Entladevorrichtung in die Schaltung integrieren.

Auf captest.htm findest du das:
[...]

The total discharge time is approximately:
LV (TV and monitor power supplies, SMPSs, electronic flash units) - up to 1000 uF, 400 V. Discharge time of 1 second per 100 uF of capacitance (5RC with R = 2 K ohms).

[...]

Adjust the component values for your particular application.

[...]

Ich denke dass 2 Stück der 1µF/275V Kondensatoren ziemlich klein sind, also LV (low voltage) Typen.
1/100s Entladungszeit mit einem a 2kOhm Widerstand? Ich denke man kann den Widerstand anheben um den fließeneden (geschaltenene!) Strom so klein wie möglich zu halten - das würde die Lebensdauer der Micro-Schalter erhöhen.

[...]

Capacitor Discharge Technique
The technique I recommend is to use a high wattage resistor of about 5 to 50 ohms/V of the working voltage of the capacitor. This isn't critical - a bit more or less will be fine but will affect the time it takes to fully discharge the capacitor. The use of a current limiting r150 KBesistor will prevent the arc-welding associated with screwdriver discharge but will have a short enough time constant so that the capacitor will drop to a low voltage in at most a few seconds (dependent of course on the RC time constant and its original voltage).

[...]

5Ohm*300V=1.5kOhm
50Ohm*300V=15kOhm
Für wieviel Watt soll er ausgelegt sein?

[...]

For the main capacitors in a switching power supply, TV, or monitor, which might be 400 uF at 350 V, a 2 K ohm 25 W resistor would be suitable. RC=.8 second. 5RC=4 seconds. A lower wattage resistor (compared to that calculated from V^^2 / R) can be used since the total energy stored in the capacitor is not that great.

[...]
25W schaut ein wenig stark aus, würde ein kleiner 0.6W-Widerstand auch reichen?

Die meisten der Schalter sind bis zu 50mA dimensioniert - wie hoch ist der ENtladestrom?

Mini-Schalter- PCB Loch-Muster:
http://www2.produktinfo.conrad.com/datenblaetter/700000-724999/707627-da-01-en-Miniatur_Printtaster_rot.pdf (101kB)
http://www2.produktinfo.conrad.com/datenblaetter/700000-724999/707678-da-01-en-Miniatur_Printtaster_rot.pdf (94kB)
http://www2.produktinfo.conrad.com/datenblaetter/700000-724999/704369-da-01-de-Miniatur-Taster_grau_2polig_00418842xxx.pdf (267 KB)
http://www2.produktinfo.conrad.com/datenblaetter/700000-724999/700479-da-01-en-Drucktaster_T604.pdf (205 KB)
http://www2.produktinfo.conrad.com/datenblaetter/700000-724999/700460-da-01-en-Drucktaster_T602.pdf (150 KB)


Vielleicht sind alle Anstrengungen unnötig, denn der DIY-CDI Artikel sagt am Ende von Kapitel "Disabling multi-spark operation":
[...]

Two 680kΩ resistors are connected in series across C2 to discharge it should the coil become disconnected from the circuit. This improves safety since a 1µF capacitor charged to 300V can produce a nasty shock.

[...]
Ist das ausreichen Schutz?



Als wir das Thema das erste Mal auf http://www.msefi.com/viewtopic.php?t=6380 diskutierten, schlug ein hilfreiches Mitglied folgendes vor:
[...]
I'd rather use a serial resistor (say 10k/5w if I calculate it well enough, but please check that it really discharges the cap good enough before relying on it!!!), it will limit the discharge current.
[...]


Wenn du irgendeinen Vorschlag zu diesem Thema hast, speziell zur Dimensionierung des Widerstandes und des Schalters sende bitte ein Posting an die DIY-CDI Yahoo-Group .

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