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Registered Member #61406
Joined: Thu Jan 05 2017, 11:31PM
Location:
Posts: 268
I'm using a 555 IC to switch one P MOSFET and another N type. If one side becomes conductive it goes through a voltage divider to switch on the opposite side. I have used diodes to stop both sides switch on a straight leg, and some on the voltage divider. It is run with 12V for the 555 and 18V for the bridge. I get about 3V at the AC terminal with enough current to warm 11.2ohm 10watt resistors.
The problem when I measure the 555 output and ground or positive I get 49.1khz which the values workout to,but measuring the AC terminal I get 3000-500khz.
Registered Member #2939
Joined: Fri Jun 25 2010, 04:25AM
Location:
Posts: 615
Plasma wrote ... The picture is pinouts not usual diagrams.
And thats a problem. If you persist in making it difficult to understand your circuit, no-one is going to bother helping you. There are plenty of free tools that will allow you to create nice clean schematics - please pick one and use it.
Registered Member #2099
Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1714
>> The AC terminal is were the motor usually connects to.
Where is the motor usually connected on a H bridge driven at 49 kHz?
No AC terminal is labeled in the schematic.
Don't H bridges generally have two AC terminals? Then when you talk about AC voltage, you need to be clear about whether it's terminal-to-terminal, or one terminal to ground.
Regarding the apparent "frequency multiplication", it would help to see a photograph of the real circuit, including your probe connection. Probe connections feature at least two points of contact that matter, except perhaps for clip-on current probes. Please tell us about the measuring instrument and how you get frequency from it.
Registered Member #27
Joined: Fri Feb 03 2006, 02:20AM
Location: Hyperborea
Posts: 2058
How is it contructed? If there are wires going all over the place you could have ringing, feedback and other problems and if so even the act of measuring the signal may affect it.
Registered Member #61406
Joined: Thu Jan 05 2017, 11:31PM
Location:
Posts: 268
Bjion I cleaned up the cabling, which stop it swinging majorly, but on the DMM dial selecting hz displays 270khz, changing it to Ac volts and pressing the hz button displays 28-38khz. I added a bypass,ripple capacitor on the 18V power supply.
Suleiman Yep I have +12 to the 555 output for the P MOSFET and 555 output to ground for the N MOSFET. Testing that the DMM shows 7v for the latter and 6v for the former on the DC setting.
Kludgesmith The blue wires are between the diode and lower MOSFETs, measured on DC setting 1.9V, Ac setting 3.5V open circuit and about the same for a 3.3kohm load.
Registered Member #3637
Joined: Fri Jan 21 2011, 11:07PM
Location: Buffalo, NY
Posts: 1068
You're trying to measure frequency of a gate drive circuit using a DMM instead of an oscilloscope?
That might be why you're getting errant readings. It might be getting confused by ringing or harmonics present giving you weird answers. An oscilloscope ...really is what you should be using here.
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h-bridge is multiplying frequency at gate
