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Protecting a Signal Generator while Driving Transistors.

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Ash Small

Sun Aug 21 2011, 05:55PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

Well, my DMM no longer seems to want to measure capacitance, so time for a new one. (I'm watching a couple of second hand Flukes on Ebay)

In the meantime I can't measure any capacitors, so I'm wondering if I can replace C1 in this circuit:

with an electrolytic, as these have values printed on them clearly. (I've removed the two 100Î© resistors, R2 and R3)

This may involve reverse charging the electrolytic to a few volts briefly. I've read somewhere that this isn't usually a problem if the electrolytic is of a sufficiently high voltage.

Can anyone advise me on this. I have electrolytics ranging from 10V to 400V.

(When this circuit is running, voltage across C1 is usually around 3V to 6V Pk-Pk, sometimes a bit higher.)

Mattski

Sun Aug 21 2011, 10:43PM

Registered Member #1792 Joined: Fri Oct 31 2008, 08:12PM
Location: University of California
Posts: 527

I've certainly used electrolytics as DC blocking caps with small reverse voltages applied, some googling indicates that 1.5V is about the maximum reverse voltage each can handle. As I think this is a chemistry related thing it won't matter if the capacitor has a higher voltage rating.

You can connect two electrolytics in series (but in opposite direction) which apparently creates a non-polar capacitor:

Ash Small

Tue Aug 23 2011, 01:21AM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

Mattski wrote ...

.As I think this is a chemistry related thing it won't matter if the capacitor has a higher voltage rating.

.

I'm not certain about this.

Electro-chemistry is mostly about Amps per square inch, or whatever.

While a 10V electrolytic presumably has the same surface area as a physically larger 400V electrolytic, the 400V electrolytic presumably has to cantain more dielectric. This would 'presumably' be more resilient to 'a given amount of reverse current' than a 10V electrolytic of the same value?

EDIT:

Mattski wrote ...

You can connect two electrolytics in series (but in opposite direction) which apparently creates a non-polar capacitor:

I'll try this if I can find a suitable pair.

Ash Small

Tue Aug 30 2011, 11:53PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

I'm on the verge of giving up with this, I've tried everything I can think of, but I'm still getting severe ringing.

I've added the anti-parallel electrolytics, and they made a difference, I've added a 'conventional' snubber across the primary, and that made a difference, but as soon as I turned up the power the ringing was back.

I've tried connecting the Darlington directly to the outputs of the sig. gen., and that made a difference too, but as soon as the power was turned up the ringing was back.

It's time to start again. The only thing I've learned is that a 'bog standard' TV LOPT will arc at 150kHz, albeit with smaller arcs than at 32kHz.

I might 'split' this project now. Part of my motivation was to investigate resonance in these circuits, but my 'main' project has now taken the form of an H-bridge, which obviously works on different principles to 'flyback topology', so I think I'll still carry on with the 2N3055 flyback thing, but I'll also start working on the H-bridge circuit (maybe experimenting with a centre-tapped, push-pull circuit on the way).

I might even take notice of 99.99% of people on here and start 'messing around' with some TL494's (and maybe some 555's)

EDIT: To clarify, when I turn the power up, the ringing gets so bad that the waveform goes positive when it should be negative, rendering the circuit useless. I'll give it another go, but I'm stripping the breadboard and starting again.

Proud Mary

Wed Aug 31 2011, 12:28AM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

Ash Small wrote ...

While a 10V electrolytic presumably has the same surface area as a physically larger 400V electrolytic, the 400V electrolytic presumably has to cantain more dielectric. This would 'presumably' be more resilient to 'a given amount of reverse current' than a 10V electrolytic of the same value?

"...polar capacitors do not tolerate a voltage reversal. Incorrect polarities of
up to 1.5 V are, however, permissible for short periods of time as the formation of a damaging oxide
layer on the cathode only starts at voltages of this magnitude. (This is because the cathode
foil is covered by an air-oxide layer that corresponds to an anodized dielectric layer with a breakdown
voltage of approximately 1.5 V)."

Source: Epcos
Aluminum Electrolytic Capacitors
General technical information
Date: December 2010

Note that the 1.5V short-term reverse voltage limit applies to all aluminium electrolytic capacitors, regardless of their capacitance or voltage rating.

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