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Choke not working

Choke not working

Choke not working

(OP)
I've constructed a circuit to charge a capacitor bank.  A choke is in series with the 12V secondary of a transformer (pre-bridge rectifier) to limit current to about 5mA. The choke is 5.75H, 330ohm, which, at 60Hz, should have an impedance of about 2200ohm, right? 12V/2200ohm=5.4mA.

However, the 400uF of capacitors are charged in the first few cycles, instead of about 10s. Additionally, when I replace the caps with a 100k potentiometer, the current is definitely not limited to 5mA as resistance is decreased.

I measured both the inductance and resistance with meters, so I know the choke isn't defective.

Also, the active current limiter I use (2 BJTs, 2 resistors) as an alternative to the choke works as designed.

What is going on here??? Thanks for your help.

RE: Choke not working

Is the choke in the DC path? Or AC?

Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

RE: Choke not working

(OP)
AC - straight off the transformer secondary, before rectifier.  Rectifier is bridge-type, no center tap on transformer.

RE: Choke not working

Quote:

...about 10s.

Where did 10 seconds come from?

 

RE: Choke not working

The math seems OK. Around 10 seconds to charge 400 uF to around 10 V.

A 12 V choke with a 330 ohms resistance seems to be a very small one. My bet is that the core saturates heavily. Use a "better" choke. One with a larger core.  

Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

RE: Choke not working

Does your choke data sheet show a rated current? I bet Skogsgurra is right, it's probably saturating.

RE: Choke not working

(OP)
Choke rating is 40mA, transformer rating is 950mA on secondary.

One thing I had previously dismissed was another bridge rectifier coming off the secondary, prior to the choke, which supplies a DC bus for control circuitry.  Schematic is attached.  (The zener diode is actually a 3-terminal voltage regulator; capacitor is after regulator)

 

RE: Choke not working

(OP)
I just realized how strange this circuit may appear.  Perhaps it helps to know that this is a low voltage version of a pulsed power charging circuit; the high voltage equivalent we use charges 400u in about 10s (approximate), using a choke to restrict in-rush current.

RE: Choke not working

(OP)
Aha!  The common ground, in combination with the first bridge rectifier, is the culprit.  There is a current path I hadn't noticed before, that uses both rectifiers.  

I can either lift the charging caps from ground, or change the first rectifier to a single diode.

Thanks for everyone's help!

RE: Choke not working

I fail to spot that path. Can you describe it?

Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

RE: Choke not working

(OP)
On the negative half of the AC wave the current flows out the bottom of L2, then through D6, C1, and D3--not D7 like it "should".  The common ground provides that path.  The choke therefore only works only on the positive half of the AC wave.

RE: Choke not working

Quite! So it is saturated from DC, all the same. Didn't see that.

The same is true for the positive half-wave.

Looks like you have to separate those two circits.

Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

RE: Choke not working

Spotted the the parts in parallel = not needed, but worried about D9 as it is fed  directly by (BR1)=D1 to D4 !

 Reminds me of voltage multiplier  circuit based on Bridge rectifiers isolated  by caps in ac feed .

RE: Choke not working

Sorry oops ..
       ( just spotted zener (D9) is really a regulator comment )

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