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I need a circuit that will close a load circuit when an oxide capacitor is charged to specif voltage

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tarakan2

Sun Nov 04 2012, 03:03AM

Registered Member #3859 Joined: Sun May 01 2011, 03:47PM
Location:
Posts: 179

Hello
I am about to design an interesting circuit but I need to design a simple circuit that would close a load circuit when the voltage on an electrolytic capacitor that is being charged through a resistor reaches a certain vaule and than open the load circuit once the capacitor is empty.

I have power MOSFETS and 5.1V zeners lying around but I failed to build this circuit the way I thought it will work.
It seems like there is not enough current for a voltage divider with a zener diode to do the job.

How can I solve this problem so the current consumption of the circuit when the capacitor is charging is minimal?
The circuit has to be powered by this capacitor.

This circuit should not work like a smooth voltage regulator. It has to accumulate and than discharge.

Thank you.

(The resistor and the power circuit is not shown on the drawing.)

Steve Conner

Sun Nov 04 2012, 08:41AM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

You need a SCR, not a MOSFET.

Sulaiman

Sun Nov 04 2012, 10:43AM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

Yes, a thyristor would be a good choice,
you need to choose a thyristor with a sensitive (low current) gate
and of course high enough voltage and current rating for your application
and the holding current must be less than the charging current

What is the capacitance, charge current, maximum voltage and discharge current ?

You may also be able to trigger a normal (gate current = milliamps) thyristor
by using a neon or small gas discharge tube or a shockley diode (SIDAC, DIAC)

Depending upon the voltage and current discharge
just a neon/GDT/SIDAC/DIAC may be sufficient.

As a general rule, when asking for help, give as many details as possible
as members may have alternative solutions.

Experimentonomen

Sun Nov 04 2012, 11:44AM

Registered Member #941 Joined: Sun Aug 05 2007, 10:09AM
Location: in a swedish junk pile
Posts: 497

It can also be done with a mosfet and comparator.

genious 7

Sun Nov 04 2012, 07:59PM

Registered Member #2887 Joined: Sat May 29 2010, 11:10PM
Location: Panama City, Panama
Posts: 107

The problem with the circuit is that as soon as the Mosfet turns on (when the voltage on the gate reaches the threshold voltage), a current flows through it until the gate voltage is below threshold voltage. Thus, you are not actually discharging the capacitor fully, just the tiny bit for the voltage to descend.

If you want it to discharge fully, you can either use a SCR (which will stay on until the current = 0 as in fully discharged cap) or add some hysteresis to the mosfet.

tarakan2

Sun Nov 04 2012, 08:16PM

Registered Member #3859 Joined: Sun May 01 2011, 03:47PM
Location:
Posts: 179

Thank you.
So what should I type into the search engine.
I want an existing circuit.

Now I understand it. A Thyristor is switched on by something like a Zener diode and it stays on for as long as there is voltage across it. When there is no voltage in the capacitor, the thyristor would be reset waiting for a new input from the zener. This circuit can also use a MOSFET driven by a thyristor.

tarakan2

Sun Nov 04 2012, 10:50PM

Registered Member #3859 Joined: Sun May 01 2011, 03:47PM
Location:
Posts: 179

So what I am building is called a Crowbar Circuit.
I want to use a most power-efficient drive circuit that I can so current is not wasted on the zener-resistor pair when zener starts to conduct current.

http://pinside.com/pinball/forum/topic/overvoltage-crowbar-circuit-for-gottlieb-system-3-machines

Assuming that I am going to build this circuit, is there any way to use a less powerfull zener, less powerful resistor with more resistance so the current that is going through would not be 6.2 V /100 Ohm = 0.062 A ?

What value on the datasheet resembles that analogy to the gate voltage of a MOSFET and a base current of a bipolar transistor?

I know how transistors and MOSFETs work but I am not accustomed to Tyristor circuits.
Is tyristor a current-controlled device or a voltage - controlled device?
I assume it is current-controlled.
Gate trigger voltage is like threshold voltage on a transistor?

UDATE:
So there are latching MOSFETs, field-effect driven tyristors or MCTs out there.
It would be a great part to use. Are there any commercially availabe or I have to use a small MOSFET to drive a load-current rated tyristor?

This would be great because I prefer voltage-controlled devices over current-controlled. Theya are harder to burn and easier to work with.

macona

Sat Nov 10 2012, 02:04PM

Registered Member #3272 Joined: Mon Oct 04 2010, 11:40PM
Location: Beaverton, OR
Posts: 101

What you have been talking about is not a crowbar. A crowbar directly shorts the rails like in the link you posted causing the fuse or circuit breaker to pop. With a SCR it stays latched conducting as long as there is current flow though it. I dont think that is what you want, or is it?

Sounds like what you want is a voltage clamp, when the voltage in a circuit exceeds a certain set point the transistor begins to conduct dumping the extra voltage into a resistor to prevent it from exceeding the set point. Circuits such as these are used un regenerative motor drives to dump extra buss voltage during braking. They are also used in inverter style welders to kill voltage spikes in the main buss from IGBT switching. Is this what you want?

It helps to be a little more descriptive of what you want to accomplish.

Zeners dont conduct till the voltage reaches their threshold so if the thing is dumping energy somewhere, who cares about 60ma?

Sulaiman

Sat Nov 10 2012, 02:11PM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

tarakan2, you still have not mentioned;
the voltage
and capacitance
and desired peak output current
and how frequently.
these things affect design or component choice.

Steve Conner

Sat Nov 10 2012, 02:20PM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

SCRs are very commonly used in overvoltage crowbar circuits. They work fine, there is no need to go looking for weird exotic things like MCTs.

Yes, the SCR is current controlled. The gate terminal looks like the base of a transistor. For more sensitive triggering, you could drive it with an emitter follower or even another smaller SCR. A SCRlington if you like. :D

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