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Author Topic: how to adjust crowbar on pyramid power supply?  (Read 3025 times)
FOXBAT426
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« on: September 22, 2007, 09:07:41 AM »

what is the proper way to adjust the crowbar (overcurrent protection) on a pyramid phase 3 power supply. i don't have a manual - there is a knob on the left hand side of the face of the unit and right now the knob sits about 1/4 clockwise from the left most position. thx, john
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N3OX
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« Reply #1 on: September 22, 2007, 09:23:11 AM »

John,

The term "crowbar" is actually used for the circuit that protects against OVERVOLTAGE... but that's OK, I know what you're talking about.

It really is as simple as turning the current knob up.  

The thing you need to do is think of the "DC current adjust" as a limit with respect to radios.  Set the voltage at 13.8V and you can actually turn the current limit ALL THE WAY UP without worrying about damaging anything, because the radio will only draw its rated current.

You always want to keep the current limit set higher than any current your rig will draw.

I keep my current limit ALL THE WAY UP all the time.  That is generally safe to do.  The supply still has overcurrent protection if there's a short circuit... it's just set at the max current the supply will deliver.

Dan
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73,
Dan
http://www.n3ox.net

Monkey/silicon cyborg, beeping at rocks since 1995.
KE3WD
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« Reply #2 on: September 22, 2007, 09:26:01 AM »

The Pyramid Phase III is a 7A, 13.8VDC supply.  

A "Crowbar" circuit typically consists of a design that places an SCR (silicon controlled rectifier) across the output of a power supply and gates the SCR as a function of current drawn from the supply.  If the current exceeds a setpoint, the SCR is gated on and becomes a short circuit across the output terminals of the power supply and blows the main fuse for the supply, thus shutting down the thing before it can be damaged.  Theoretically speaking.  

That adjustment isn't critical in attended use such as in your ham shack, you may as well just turn it up all the way and not be too concerned, but if you like you can adjust it such that it will kick in when the current draw is a good bit above the highest amperage your rig can draw when in transmit mode.  This would likely mean having a supply of fuses onhand as when it trips it will blow a fuse.  

Not worth the bother IMO at the 7A level, just turn the knob to full or near full and operate.  


KE3WD
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KE3WD
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« Reply #3 on: September 22, 2007, 09:27:10 AM »

Crowbar is NOT a voltage sensing circuit when in a REGULATED VOLTAGE PS.  

It MUST sense current.  


KE3WD
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N3OX
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« Reply #4 on: September 22, 2007, 09:40:03 AM »

huh maybe the term is used for both overvoltage and overcurrent protection.

I thought it monitored the output voltage and if it exceeded some threshold (15V or so) it fired an SCR across the transformer secondary or filter caps, causing the power supply's fuse to blow.

I thought it was mostly there in case series pass transistors fail shorted and dump unregulated voltage on the output..




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73,
Dan
http://www.n3ox.net

Monkey/silicon cyborg, beeping at rocks since 1995.
AA4PB
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« Reply #5 on: September 22, 2007, 10:17:40 AM »

The current limit adjustment on the front of the supply is not a crowbar circuit. A crowbar circuit is triggered by over-voltage and shorts the output of the supply to blow the fuse and stop the over-voltage from damaging the load. Over-voltage is usually caused by a shorted pass transistor inside the power supply.

The adjustment on the front panel is a current limiter. If the load draws more current than the limiter is set to then the supply reduces the output voltage until the current drops to the set value. The supply does not "trip" off, it only reduces the output voltage to limit current to the desired maximum.

If the knob is turned all the way up it will be set to the maximum current that the supply is rated to handle. Unless you have some load where you need to limit the current to a lower value (charging a battery for example) you should turn it all the way up. For powering a radio and most other loads (that limit their current draw to a safe value) it should be set to maximum.
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KE3WD
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« Reply #6 on: September 22, 2007, 11:24:35 AM »

Thanks for the clarification on this supply.  

Couldn't find a manual online for it at all, and since the original poster mentioned "crowbar" it pointed at the adjustment doing just that.  

At any rate, like I said earlier, I would turn it up and forget about it at the 7A level anyway.  


KE3WD
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KA1MDA
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« Reply #7 on: September 22, 2007, 03:37:27 PM »

"rowbar is NOT a voltage sensing circuit when in a REGULATED VOLTAGE PS"

Maybe someone should call Astron and explain that to them. Seriously, a crowbar circuit IS a voltage sensing circuit, no matter what type of power supply it's in. The purpose of a crowbar is to short the output and blow the fuses if a pass transistor or regulator should fail and apply full voltage to the supply's output. It's called a crowbar because it does the same thing as throwing a crowbar across the output terminals. It has nothing to do with current limiting, which is something else entirely.

73, de Tom, KA1MDA
www.ka1mda.org
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K6AER
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« Reply #8 on: September 22, 2007, 06:12:44 PM »


Wikipedia is your friend and in this case has done a very good job discribing the function of a crowbar curcuit.

A crowbar or crowbar circuit is an electrical circuit used to prevent an overvoltage condition of a power supply unit from damaging the circuits attached to the power supply. It operates by putting a short circuit across the voltage source, much as if one dropped a tool of the same name across the output terminals of the power supply. Crowbar circuits are frequently implemented using a thyristor (also called an SCR) or a trisil or thyratron as the shorting device. Once triggered, they depend on the current-limiting circuitry of the power supply or, if that fails, the blowing of the line fuse or circuit breaker.

A crowbar circuit is distinct from a clamp in that, once triggered, it pulls the voltage below the trigger level, usually close to ground. A clamp prevents the voltage from exceeding a preset level. Thus, a crowbar will not automatically return to normal operation when the over voltage condition is removed; power must be removed entirely to stop its conduction.

The advantage of a crowbar over a clamp is that the low holding voltage of the crowbar lets it carry higher fault current without dissipating much power (which could otherwise cause overheating). Also, a crowbar is more likely than a clamp to deactivate a device (by blowing a fuse or tripping a breaker), bringing attention to the faulty equipment.
The term is also used as a verb to describe the act of short-circuiting the output of a power supply.

As for adjustment, you set your trip voltage to a level above your operational voltage like 16 volts on a 13.8 VDC power supply. You adjust the trip variable resistor with the voltage source set at 16 volts until the crowbar circuit trips. Leave the adjustment alone and reconnect it to the power supply output.
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AA4PB
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« Reply #9 on: September 22, 2007, 07:13:59 PM »

To avoid confusion, I point out again that the control on the front of the power supply is NOT the trip voltage for the crowbar. The crowbar trip adjustment is internal and should never need to be adjusted unless crowbar circuit compontents are replaced.

The control on the front panel is a current limiter adjustment - see my previous post.
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KE3WD
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« Reply #10 on: September 22, 2007, 10:01:39 PM »

Agreed that the control in question is not the trip point for the crowbar circuit.  


But also agreed that the crowbar circuit is not typically set up to monitor current, although it could be.  Most use simple zener or even voltage divider circuit to trigger the SCR.  Cheaper to implement.

Couldn't find an online schematic for this supply, took the original poster's word for functionality of the thing is all.  If he said the pot controlled the crowbar circuit, then the advice given still is solid, turn the friggin' thing up to the pin and don't worry, be happy.  


!
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WA9SVD
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« Reply #11 on: September 22, 2007, 11:52:55 PM »

 KE3WD on September 22, 2007    Mail this to a friend!      The Pyramid Phase III is a 7A, 13.8VDC supply.

A "Crowbar" circuit typically consists of a design that places an SCR (silicon controlled rectifier) across the output of a power supply and gates the SCR as a function of current drawn from the supply. If the current exceeds a setpoint, the SCR is gated on and becomes a short circuit across the output terminals of the power supply and blows the main fuse for the supply, thus shutting down the thing before it can be damaged. Theoretically speaking.

That adjustment isn't critical in attended use such as in your ham shack, you may as well just turn it up all the way and not be too concerned, but if you like you can adjust it such that it will kick in when the current draw is a good bit above the highest amperage your rig can draw when in transmit mode. This would likely mean having a supply of fuses onhand as when it trips it will blow a fuse.

     Beg your pardon, but a "crowbar circuit" in a power supply has NOTHING to do with current draw.

    The "crowbar" is a circuit to protect against OVERVOLTAGE.  For a typical 13.8 Volt Power Supply (regardless of the current rating) the crowbar circuit will typically be set to trip at an output voltage between 14.5 and 15 Volts.  If the output VOLTAGE exceeds the trip point, then an SCR placed across the output will conduct, effectively creating a short circuit, and (hopefully) causing a fuse (or circuit breaker) to blow before damage can occur.  It has absolutely nothing to do with the current rating of a supply, or the current drawn from a supply.
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N0IU
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« Reply #12 on: September 23, 2007, 05:22:39 AM »

I am surprised that no one has bothered to ask John why he feels the need to adjust the crowbar circuit? It is set at the factory to a specific point for a reason. I am going to go out on a limb and say that if the power supply is not performing as expected, then the problem is probably not a mis-adjusted crowbar setting.

Scott N0IU
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AA4PB
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« Reply #13 on: September 23, 2007, 12:00:20 PM »

Because the front panel control he's asking about IS NOT a crowbar circuit adjustment, its a current limiter. The correct answer has been given numerous times - turn it full up for normal applications like powering a radio.
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N1WOM
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« Reply #14 on: September 24, 2007, 02:51:58 PM »

I noticed that in John's original post that he never mentioned which Pyramid Phase III P.S. he had yet everyone seems to assume that it is a 13.8 VDC, 7 Amp supply.

The Pyramid Phase III 13.8 VDC, 7 Amp supply is a fixed amerage, fixed voltage supply with a 5 Amp continuous, 7 Amp surge rating.
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