hi,
I just bought the components to build an amp, to find out most of the resistors are too big compared to the holes in the PCB.
Is it a problem to mount them 2 cm distant from the PCB surface (basically by soldering the tips of the metal wire that sticks out from the resistors to the PCB), e.g. picking up noise or I don't know what.. ?
I just bought the components to build an amp, to find out most of the resistors are too big compared to the holes in the PCB.
Is it a problem to mount them 2 cm distant from the PCB surface (basically by soldering the tips of the metal wire that sticks out from the resistors to the PCB), e.g. picking up noise or I don't know what.. ?
It is acceptable practice to mount the resistors vertically, (one short lead and the other bent back down the body)allowing for a 20mm component to fit a 10mm sized spacing..
For the ones that connect directly to active devices, I would put the end that is closer to an active device all the way into the hole, and let the lead that's farther from the active device be the long one, in case the parasitics might have an effect, otherwise, and since if you must make an antenna, it might as well be as isolated as possible.
What about having the resistor floating in mid air above the other components? I.e. orizontally mounted, but 2 cm from the pcb..
95% of the time there is no problem but its messy and looks untidy. Given that resistors are so cheap why not just get the correct size/wattage.
Resistors change temperature when they pass current.
That temperature change gives rise to thermal gradients in the device.
If the two ends of the resistor are at different temperatures, then the soldered, or mechanically fixed ends, can generate different voltages.
This error in temperature difference between the ends is better managed by placing the resistor horizontal to the vertically moving air flow that cools it.
Resistors tight against a vertical PCB can become inadvertent voltage generators if they are oriented incorrectly.
Some times a vertical to the PCB placement can help eliminate these inadvertent generation of unwanted voltage differences.
The actual mounting is not important. The thermal gradients leading to similar end temperatures can be important and especially so in precision work.
That temperature change gives rise to thermal gradients in the device.
If the two ends of the resistor are at different temperatures, then the soldered, or mechanically fixed ends, can generate different voltages.
This error in temperature difference between the ends is better managed by placing the resistor horizontal to the vertically moving air flow that cools it.
Resistors tight against a vertical PCB can become inadvertent voltage generators if they are oriented incorrectly.
Some times a vertical to the PCB placement can help eliminate these inadvertent generation of unwanted voltage differences.
The actual mounting is not important. The thermal gradients leading to similar end temperatures can be important and especially so in precision work.
should be able to do it in 2 mm - not cm - just bend the leads close to the body in a U so they lie along the R body, then put 90 degree bends to fit holes - can be symmetric, adds less loop area - as long as the leads are mechanically clear of other parts, don't short to anything
Isn't it beneficial to allow some space between the resistor and the pcb to allow air circulation? That would aid cooling I assume.
I see. So it seems advisable to mount them close to the pcb.
What about the heat transfer to nearby components? A hot resistor will "move" more heat towards a closely located electrolytic cap and increase its temperature when away from the pcb (so air is the main means), or when it is close to the pcb, with the aid of pcb conduction?
Hi,
In and amplifier just mount them vertically. The long lead
should be connected to the lowest source impedance point.
e.g. all connections to the rails and earth should be long lead.
The feedback resistors - long leads to the output and earth.
rgds, sreten.
There is no cooling advantage to close to PCB mounting,
quite the opposite. Airflow is better away from the pcb
and the extra leg lengths add to the cooling area available.
In and amplifier just mount them vertically. The long lead
should be connected to the lowest source impedance point.
e.g. all connections to the rails and earth should be long lead.
The feedback resistors - long leads to the output and earth.
rgds, sreten.
There is no cooling advantage to close to PCB mounting,
quite the opposite. Airflow is better away from the pcb
and the extra leg lengths add to the cooling area available.
Last edited:
Power dissipation in most of the resistors in a power amp is minimal, a few tens of milliwatts down to microwatt levels. I've never considered cooling resistors to be an issue apart from obvious resistor locations that do dissipate high power.
Sorry, but my comment was general - in fact asking about power resistors that get hot to the touch. I didn't phrase that well! 🙂
As I asked before, do you think that placing a resistor close to the pcb will transfer more heat to nearby components (thus increasing their temperature), or if you place it away from the pcb? I assume that in the second case air is the main transfer means, whereas in the first case the pcb comes into play, as AndrewT noted.
EDIT: I had not seen screten's comment which is opposite to AndrewT's opinion. Still, my question remains!
there are reasons, applications for both
sometimes big, high temp rated power resistors (multi watt metal oxide or wirewound) that get really hot are purposely mounted with long leads to avoid scorching the pcb
but for smaller R the heat conduction through short copper leads to the pcb trace can be part of the power rating
sometimes big, high temp rated power resistors (multi watt metal oxide or wirewound) that get really hot are purposely mounted with long leads to avoid scorching the pcb
but for smaller R the heat conduction through short copper leads to the pcb trace can be part of the power rating
Last edited:
Hi,
Yes the issue is the pcb is a poor conductor of heat, its gets as hot as the
resistor and gets scorched when resistors get hot due to some other fault.
rgds, sreten.
Yes the issue is the pcb is a poor conductor of heat, its gets as hot as the
resistor and gets scorched when resistors get hot due to some other fault.
rgds, sreten.
My statement is NOT an opinion.
It is not my opinion.
It is a comment stating a FACT.
"The conduction to the PCB MAY be better for cooling than air circulation alone."
Longer lead outs reduce the quantity of heat transferred to the soldered pads. This route for cooling is quite important and especially so in smd assembly.
A copper plane, on the component side, could also reduce the temperature of "touching" components that are generating their own heat.
I stand by my original statement in post9. And repeated in post16 with a correction from that to than.
A copper plane, on the component side, could also reduce the temperature of "touching" components that are generating their own heat.
I stand by my original statement in post9. And repeated in post16 with a correction from that to than.
Last edited:
AndrewT, I did not imply that you post is not based on facts on scientific evidence. Just used another word for "post". 🙂
Hi,
Well its not generally true, whatever you "stand" by.
Generally what your saying is simply wrong.
Generally the small pads and tiny links to them in
the rear of a PCB for resistors offer no significant
thermal path at all compared to leg dissipation,
and short legs will do nothing except be worse.
rgds, sreten.
Common sense rather than dogma should be applied.
Last edited:
- Status
- Not open for further replies.