When buying parts for boards, BOMs will indicate specs for caps, resistors, etc. My question is: Are those specs meant to indicate the exact specs that should be bought, or are they mean to set out minimums?
For example: In a BOM i am working on ordering parts for, the values for the Caps are listed as 120u/25V.
The capacitance is 120u. Does that mean ONLY 120u, or is that a minimum?
25V -- is that ONLY buy a 25V cap, or does that mean 25V is the minimum?
Another example:
resistor spec is: .25W Film, 4.99K
Does that 4.99K value mean ONLY buy 4.99K resistors, or is there a range from that that is acceptable?
I am early going in this (obviously) and i have always assumed these specs meant ONLY. But I see discussions where people are talking about variations pretty freely, and so it makes me think that the specs are looser than might otherwise appear.
Bones
Lincoln, Nebraska
For example: In a BOM i am working on ordering parts for, the values for the Caps are listed as 120u/25V.
The capacitance is 120u. Does that mean ONLY 120u, or is that a minimum?
25V -- is that ONLY buy a 25V cap, or does that mean 25V is the minimum?
Another example:
resistor spec is: .25W Film, 4.99K
Does that 4.99K value mean ONLY buy 4.99K resistors, or is there a range from that that is acceptable?
I am early going in this (obviously) and i have always assumed these specs meant ONLY. But I see discussions where people are talking about variations pretty freely, and so it makes me think that the specs are looser than might otherwise appear.
Bones
Lincoln, Nebraska
Voltage rating for capacitors & power rating for resistors are a minimum - so the part doesn't blow up or burn out in use.
Value depends on what it is doing in the circuit.
Often a higher value of capacitance is good but not if it is operating as a frequency filter (tone circuit). A power supply filter on the other hand, bigger is often better.
Likewise, resistance may or may not be critical - this is part of the learning process.
A resistor kit is a good way of getting lots of values very cheaply, easier than messing about ordering piecemeal.
Best to post your circuit and the values you wish to substitute and let the experts advise.
Value depends on what it is doing in the circuit.
Often a higher value of capacitance is good but not if it is operating as a frequency filter (tone circuit). A power supply filter on the other hand, bigger is often better.
Likewise, resistance may or may not be critical - this is part of the learning process.
A resistor kit is a good way of getting lots of values very cheaply, easier than messing about ordering piecemeal.
Best to post your circuit and the values you wish to substitute and let the experts advise.
Like russc said resistor watts are maximum, capacitor voltage are minimums and inductors are usually denoted in maximum current.
Values like capacitance 120u or reisistance in ohms is usually presented along side a deviation percentage. So a 1% resistor at 100 ohms. That means that the resistor can be any value within 99-101 ohms.
Cheaper parts might have a bigger deviation percentage. Most circuits take this deviation in account when designing the circuit, hence trimming pots here and there to adjust for part differences.
Values like capacitance 120u or reisistance in ohms is usually presented along side a deviation percentage. So a 1% resistor at 100 ohms. That means that the resistor can be any value within 99-101 ohms.
Cheaper parts might have a bigger deviation percentage. Most circuits take this deviation in account when designing the circuit, hence trimming pots here and there to adjust for part differences.
Rating vs Value
General rule you can follow is to use the value indicated in the circuit or BOM and if the part isn't available, you can try to find substitutes in a higher rating.
For example, if a BOM specifies 4.99K 1/8W resistor, you should find 4.99K resistor but can substitute with higher rated resistors at 1/4W, 200mW, 400mW etc.
For capacitors, if a BOM specifies 10uF 25V, you should use a 10uF part and can substitute 35V, 50V or higher rated caps if you happen to have some on hand or if you can't find one at 25V. Once you understand this and later understand the implementation, you can start to decide if you want to substitute different values.
Just be careful that the footprint (lead spacing, cap circumference, etc) will work for your substitution (will physically fit) if you are working with a pre-designed pcb.
Cheers,
Stephen
General rule you can follow is to use the value indicated in the circuit or BOM and if the part isn't available, you can try to find substitutes in a higher rating.
For example, if a BOM specifies 4.99K 1/8W resistor, you should find 4.99K resistor but can substitute with higher rated resistors at 1/4W, 200mW, 400mW etc.
For capacitors, if a BOM specifies 10uF 25V, you should use a 10uF part and can substitute 35V, 50V or higher rated caps if you happen to have some on hand or if you can't find one at 25V. Once you understand this and later understand the implementation, you can start to decide if you want to substitute different values.
Just be careful that the footprint (lead spacing, cap circumference, etc) will work for your substitution (will physically fit) if you are working with a pre-designed pcb.
Cheers,
Stephen
Not a dumb question at all. Finding the proper parts, or an acceptable substitute for an existing part
(that will electrically and physically still work), is one of the main tasks of an engineer.
Such problems are getting more and more common and difficult recently, due to supply chain problems.
(that will electrically and physically still work), is one of the main tasks of an engineer.
Such problems are getting more and more common and difficult recently, due to supply chain problems.
This will keep you busy, see chapter 7.
https://pearl-hifi.com/06_Lit_Archive/02_PEARL_Arch/Vol_16/Sec_51/4420_The_Art_of_Electronics.pdf
https://pearl-hifi.com/06_Lit_Archive/02_PEARL_Arch/Vol_16/Sec_51/4420_The_Art_of_Electronics.pdf
Absolutely correct. The first thing we were taught in circuits class about designing transistor circuits was
to design them as a voltage amplifier, not as a current amplifier. Always use the worst case current gain,
and ensure that the circuit is noncritical of it. This requires use of an emitter resistor, however.
to design them as a voltage amplifier, not as a current amplifier. Always use the worst case current gain,
and ensure that the circuit is noncritical of it. This requires use of an emitter resistor, however.