How much higher can the output current be compared to the input current for a fast switch?

Thread Starter

StevenStrange616

Joined Dec 9, 2020
5
Hello,

I'm currently in search for a fast switch that can pass 20A-25A and 20V-50V pulse train at 25% duty cycle and ~ 2kHz-10kHz through a purely resistive load. I found this "Fast Square Wave Pulser" from BEHLKE that claimed to be able to do so. From their instruction manual, the power supply requirements are : 15VDC @ 0.4mA/kHz and 120VDC @ 0.2mA/kHz.

Now, assuming my pulse train is at 25% duty cycle and 10kHz, the current requirement would be about 40mA and 20mA (6mW and 2.4mW). I don't know how the switch can provide (20A*20V) = 400W!

Could someone please help clarifying this?

Here is the link if anyone is curious: http://www.behlke.com/pdf/datasheets/fswp_instructions_v1.pdf

Thanks in advance!
 

andrewmm

Joined Feb 25, 2011
1,468
Hello,

I'm currently in search for a fast switch that can pass 20A-25A and 20V-50V pulse train at 25% duty cycle and ~ 2kHz-10kHz through a purely resistive load. I found this "Fast Square Wave Pulser" from BEHLKE that claimed to be able to do so. From their instruction manual, the power supply requirements are : 15VDC @ 0.4mA/kHz and 120VDC @ 0.2mA/kHz.

Now, assuming my pulse train is at 25% duty cycle and 10kHz, the current requirement would be about 40mA and 20mA (6mW and 2.4mW). I don't know how the switch can provide (20A*20V) = 400W!

Could someone please help clarifying this?

Here is the link if anyone is curious: http://www.behlke.com/pdf/datasheets/fswp_instructions_v1.pdf

Thanks in advance!

Is this a high side switch to a load referenced to ground ?

If so a nice fet is where you could look
 

BobTPH

Joined Jun 5, 2013
3,315
It is a switch! Switches do not create the power, they turn it on and off.

Here is the second sentence in the document you linked:
In conjunction with an external high-voltage supply and a suitable control signal source, high-voltage square-wave pulses can be produced.
My bolding.


Bob
 

LowQCab

Joined Nov 6, 2012
538
Please,
always state exactly what the problem is that you are trying to solve .
There is almost no such thing as a "purely resistive load" at a frequency of 10khz .
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Thread Starter

StevenStrange616

Joined Dec 9, 2020
5
Please,
always state exactly what the problem is that you are trying to solve .
There is almost no such thing as a "purely resistive load" at a frequency of 10khz .
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I'm trying to heat up a metal wire using resistive heating method. Though I'm not sure how the property changes with pulse heating so purely is a poor word choice.
 

LowQCab

Joined Nov 6, 2012
538
A frequency of ~10khz is not necessary, ~100hz would be just fine,
that is, as long as your Heating Element Wire is larger than, lets say, ~28-AWG or so .
Super fine wire may require a higher frequency because it can cool-down and heat-up very fast .

With very small Current requirements,
there is no real advantage in adding all the complexities that
come with designing a PWM Temperature Control Circuit .

--------------------------------------------------------------------------------------------------------
120 Volts on a bare Wire which may be touched by a person, or by another metallic object,
IS EXTREMELY DANGEROUS .
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"" 15VDC @ 0.4mA/kHz and 120VDC @ 0.2mA/kHz ""

This specification makes no sense ...... Watts divided by Frequency ?????
Why would you choose a Fixed Pulse Width, and a Variable Frequency, for a PWM Scheme ?
A Fixed Frequency, with a Variable Pulse Width, makes much more sense .

15V X 0.0004 Amps = 0.006 Watts or 6mw .
120V X 0.0002 Amps = 0.024 Watts or 24mw .

Are you sure that this is measured in "MilliAmps" and not "Amps" ????

15V X 0.4 Amps = 6 Watts,
120V X 0.2 Amps = 24 Watts,
These sound like more believable numbers .......

With a Heating Element,
the only measurement that counts is WATTS, using Ohms Law.
This is where you must start,
knowing the Maximum Wattage that the Element can comfortably handle,
and knowing the general Required Wattage needed in a particular application, or for a particular usage .

What is the "Cold", (Room Temperature ), Resistance of your Heating Element ( Wire ) ?
The Resistance, ( in Ohms, or mOhms ( Ohms/1000) ),
will change to some small degree as the Temperature of the Wire Rises or Falls .
The Resistance Calculation can be found on-line if you know the length of Wire, and the metal alloy used .
If you already have the Wire in question,
you can probably measure it with a reasonable quality Digital-Multi-Meter .
If this is part of a manufactured product,
the manufacturer will know the general WATTAGE RANGE necessary for particular usage scenarios,
and what the expected temperatures will be at a given WATTAGE,
for a Wire of a particular Length and Cross-Sectional-Area,
( usually measured in AWG ( American Wire Gauge) in the USA ) .

For accurate Wire Temperature Control,
you will want to regulate the CURRENT, and not the Voltage level Supplied .
( With PWM Control ........
"Average-Current", and, "Average-Voltage", numbers are multiplied together to arrive at "Average-Watts" )

Preferably, ( especially if the Wire is exposed ),
the Peak Voltage should be kept below roughly ~30 Volts for personal Safety.

Current, or, Voltage Regulation, can be accomplished with PWM, or with a Linear-Control Circuit .
PWM is more complex and expensive, but can be very efficient with power utilization,
Linear is more simple, and generally requires a Heat-Sink to dissipate the larger amount of wasted Energy.

What is the purpose of this device ?????????????
Maybe a "Hot-Wire" Foam-Board Cutting Table ?
Maybe a "Mass Air Flow Sensor" ?
Maybe a Coffee Mug Warmer ?
Maybe Heated Winter Gloves ?
Does this Heating Element need to have "Closed-Loop" Temperature/Current Feedback Control ?
Does this application need Metering to indicate Load Level, or Status ?
Is this device Battery Powered ?

All the details matter .
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Thread Starter

StevenStrange616

Joined Dec 9, 2020
5
@LowQCab

Thank you for your detailed response and my apologies for the late response. A snowstorm knocked the power in our area.

For the experiment, current, frequency, pulse width, etc... are the parameters we are interested in studying. At the moment,
The DC current: 15A @ 20V
DC Pulsing Current: 20A-50A peak.
DC Pulsing Voltage: Unknown
Frequency: 1kHz-50kHz
Cold resistance of the load + fixture: 130mOhms
Resistance of the load+ fixture at 9A DC: ~2 Ohms
Power Supplies: Current controlled
The experiment: to recrystallize different types of metal

As BobTPH pointed out, the power requirements I listed were for turning on and controlling the switch, which I mistook for the actual power supply requirements. An external high voltage power supply is required to heat the element.

The switch itself seems to fit our requirement. However, the concern we have is the coaxial connection for the high voltage input/output to the load. The connections are built-in coaxial connectors. I have not been able to find a coaxial cable has current rating above 12A. I have no experience in assembling coaxial cables. Would it be possible to replace it with a normal low AWG wire with a coaxial connector? I don't expect the voltage to be higher than 100V.
 

LowQCab

Joined Nov 6, 2012
538
OK, hold everything ........
You are Quoting Numbers and Nomenclature that you have absolutely no clue about.

You must ........
1) DEFINE ALL WORDS for yourself, and for everyone else.
You will remain in a state of confusion on any subject that contains mis-understood words.

2) State the over-all purpose of the device,
including what parameters must be controlled,
and whether those parameters must be automatically, or manually controlled,
and what parameters must be monitored, and/or, displayed.

3) State any limitations on Power-Supply-Specifications, or any other "required" Hardware.
List any required Hardware with Part Numbers and/or, Full Specifications.

4) State the Proposed Theory that has prompted the need for a device with "XYZ" Specifications.

5) State the "Expected Results" of the implementation of this Proposed Device.

6) State any and all "Rules" or "Limitations" regarding
the construction or operation of this device.

All of this information will create a "Frame of Reference"
that can be understood by YOURSELF, and other people.
This will enable "Communication" and "Mutual Understanding".

It's a good start to have the general
Impedance Range vs Temperature of the "DUT" (device under test).
2- Thumbs Up......
Everything else so far has been meaningless blather with no Frame of Reference whatsoever.
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